A Practical Guide to the Determination of Human Exposure to Radiofrequency Fields : Recommendations of the National Council on Radiation Protection A (NCRP Report No. 119)

NCRP REPORT No. 119 A PRACTICAL GUIDE TO THE DETERMINATION OF HUMAN EXPOSURE TO RADIOFREQUENCY FIELDS Recommendations o...

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NCRP REPORT No. 119

A PRACTICAL GUIDE TO THE DETERMINATION OF HUMAN EXPOSURE TO RADIOFREQUENCY FIELDS Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued December 31,1993

National Council on Radiation Protection and Measurements 7910 Woodmont Avenue 1 Bethesda, Maryland 20814-3095

LEGAL NOTICE This Report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its reports. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this Report, nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or implied, with respect to the accuracy, completeness or usefulness of the information contained in this Report, or that the use of any information, method or process disclosed in this Report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or prwess disclosed in this Report, under the Civil Rights Act of 1964, Section 701 et seq. as amended 42 U.S.C. Sectwn 2000e et seq. (Title VZI) or any other statutory or common law theory governing liability.

Library of Congress Cataloging-in-PublicationData National Council on Radiation Protection and Measurements. A practical guide to the determination of human exposure to radiofrequency fields : recommendations of the National Council on Radiation Protection and Measurements. cm.-(NCRP report ; no. 119) p. "Issued December 31, 1993." Includes bibliographical references and index. ISBN 0-929600-35-5 1. Radio waves-Health aspects. 2. Radiation dosimetry. I. Title. 11. Series. [DNLM: 1. Radio Waves. 2. Radiation Protection-standards. 3. Radiation Dosage. WD 605 N277p 19931 RA569.3.N375 1993 612'.01448-dc20 DNLMIDLC for Library of Congress 93-45910 CIP

Copyright 0 National Council on Radiation Protection and Measurements 1993 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.

Preface This Report is the third in a series ofNational Council on Radiation Protection and Measurements (NCRP) reports concerning radiofrequency electromagnetic (RFEM) radiation which constitutes an extension of NCRP interest in the subject of nonionizing radiation. The first report, NCRP Report No. 67, Radiofrequency Ebctromagnetic Fields-Properties, Quantities and Units, Biophysical I n t e n tion, and Measurements, dealt primarily with quantities and units associated with RFEM fields. The second report, NCRP Report No. 86, Biological Effectsand Exposure Criteria for Radiofrequency Electromagnetic Fields, dealt primarily with the biological effects of such fields. This, the third report in the series, addresses the practical measurement of RFEM fields. This Report was prepared by Scientific Committee 89-2 on Practical Guidance on the Evaluation of Human Exposures to Radiofrequency Radiation (formerly Scientific Committee 78). Serving on the Committee were:

Richard A. Tell, Chairman Richard Tell Associates, Inc. Las Vegas, Nevada

Howard I. Bassen Center for Devices and Radiological Health Rockville, Maryland

Members David L. Conover National Institute for Occupational Safety and Health Robert A. Taft Laboratories Cincinnati, Ohio

Jules Cohen Carl H. Durney Jules Cohen and Associates University of Utah Washington, D.C. Salt Lake City, Utah Ronald C. Petersen AT&T Bell Laboratories Murray Hill, New Jersey

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PREFACE

NCRP Secretariat Thomas M . Koval, Scientific Stuff Assistant (1983-88) William M . Beckner, Scientific Staff Assistant (1988-93) Cindy L. O'Brien, Editorial Assisant The Council wishes to express its appreciation to the Committee members for the time and effort devoted to the preparation of this Report. Charles B. Meinhold President, NCRP Bethesda, Maryland December 1,1993

Contents Preface ...................................................................................... iii 1 Introduction ......................................................................... 1 1.1 The Study of Radiofrequency Hazards ......................... 1 1.2 The Radiofrequency Spectrum ...................................... 2 1.3 The Increasingly Crowded Radiofrequency Spectrum ....................................................................... 1.4 The Interaction of Radiofrequency Energy with Tissues ............................................................................. 1.5 Environmental Aspects ................................................ 1.6 The State of Radiofrequency Safety Standards ........... 1.7 Public Awareness ........................................................... 1.8 Overview of this Report ................................................. 2 Basic Concepts .................................................................... 2.1 Explanation of Terms and Units ................................... 2.1.1 Glossary ................................................................ 2.1.2 Units ...................................................................... 2.1.3 Vectors .................................................................. 2.2 Electromagnetic Fields .................................................. 2.2.1 Electric Fields ....................................................... 2.2.2 Magnetic Fields ................................................... 2.2.3 Static Fields .......................................................... 2.2.3.1 Static-Electric Fields .............................. 2.2.3.2 Static-Magnetic Fields ............................ 2.2.4 Quasi-Static Fields ............................................... 2.2.5 Interaction of Fields with Materials ................... 2.2.5.1 Nonmagnetic Materials .......................... 2.2.5.2 Permittivity .......................................... 2.2.5.3 Energy Absorption .................................. 2.2.5.4 Electric-Flux Density ............................. 2.2.5.5 Magnetic Materials ................................. 2.2.6 Wave Propagation ................................................ 2.2.6.1 Modulation .............................................. 2.2.6.2 Amplitude Modulation ........................... 2.2.6.3 Frequency Modulation ............................ 2.2.6.4 Spherical Waves .................................... 2.2.6.5 Plane Waves ............................................ 2.2.7 Near Field .............................................................

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CONTENTS

2.2.8 Far Field ............................................................. 2.2.9 Interaction of Fields with Objects ..................... 2.2.9.1 Planar Conductors ................................ 2.2.9.2 Planar Dielectrics ................................. 2.2.9.3 Standing-Wave Ratio ............................ 2.2.9.4 Nonplanar Objects ................................ 2.2.10 Poynting's Theorem (Power-Conservation Theorem) ............................................................. 2.2.11 Antennas ............................................................. 2.2.11.1 Near and Far Fields ............................ 2.2.11.2 Radiation Patterns .............................. 2.3 Dosimetry ........................................................................ 2.3.1 Electrical Properties of Tissue .......................... 2.3.2 Plane-Wave Absorption as a Function of Frequency ........................................................... 2.3.2.1 Planar Models ....................................... 2.3.2.2 Other Models ......................................... 2.3.3 Polarization ......................................................... 2.3.4 Specific Absorption Rate Characteristics ......... 2.3.5 Dosimetry Concepts as Applied to Radiofrequency Protection Guides ............................. 2.4 Concepts of Measurements ............................................ 2.4.1 Electric Field Measurements ............................ 2.4.2 Magnetic Field Measurements .......................... 2.4.3 Specific Absorption Rate Measurements .......... 2.5 Generalizations and Frequently Used Relationships .................................................................. 3 Procedures for Evaluation of Exposure ....................... 3.1 General Objectives ......................................................... 3.2 Protection Guide Criteria .............................................. 3.2.1 Single-Value Protection Guide .......................... 3.2.2 Frequency-Dependent Protection Guide ........... 3.3 Data Necessary for Exposure Evaluation .................... 3.3.1 Frequency Spectrum Coverage ......................... 3.3.2 Variability with Time ........................................ 3.3.3 Near- Versus Far-Field Conditions .................. 3.3.4 Probe Characteristics ......................................... 3.3.5 Time and Spatial Averaging ............................. 3.3.6 Effects of Secondary Sources ............................. 3.3.7 Uncertainty Factor ............................................. 3.4 Data Analysis and Exposure Evaluation ..................... 3.4.1 Limited Area Survey ......................................... 3.4.2 Area Survey ........................................................

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CONTENTS

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4 Instruments and Measurement Techniques ................. 77 4.1 Introduction ................................................................... 77 4.1.1 Broadband Survey Meters ................................... 78 4.1.1.1 Desirable Characteristics of Broadband

Survey Instruments ................................ 86 4.1.1.1.1 Isotropic response ................. 86 4.1.1.1.2 Frequency response .............. 86 4.1.1.1.3 Absolute accuracy ................. 87 4.1.1.1.4 Out-of-band response ............ 88 4.1.1.1.5 Dynamic range ..................... 88 4.1.1.1.6 Meter output units ............... 89 4.1.1.1.7 Response to the parameter being measured ................... 89 4.1.1.1.8 Electromagnetic interference ........................... 89 4.1.1.1.9 Probe burnout alarm ............ 90 4.1.1.1.10 Probe overload/burnout rating ..................................... 90 4.1.1.1.11 Peak hold .............................. 90 4.1.1.1.12 Static-charge sensitivity ...... 90 4.1.1.1.13 Battery operation ................. 91 4.1.1.1.14 Response time ....................... 91 4.1.1.1.15 Stability ................................. 91 4.1.1.1.16 Spatial resolution of the instrument .......................... 91 4.1.1.1.17 Multiple signal addition ...... 92 4.1.1.1.18 Modulation response ............ 92 4.1.1.1.19 Readability ............................ 92 4.1.1.1.20 Ease of adjustment and use . 92 4.1.1.1.21 Portability ............................. 92 4.1.1.1.22 Durability .............................. 92 4.1.1.1.23 Recorder output .................... 92 4.1.1.1.24 Response to other environmental factors .......... 92 4.1.1.2 Peripheral Equipment for Broadband Survey Instruments ................................ 93 4.1.2 Narrowband Systems ........................................... 93 4.1.2.1 Antenna Types ........................................ 94 4.1.2.2 Spectrum Analyzers ................................ 101 4.1.2.3 Field-Strength Meter .............................. 106 4.1.2.4 Automated Measurement Systems ;....... 110 4.1.3 Quasi-Narrowband Systems ................................ 117 4.2 General Measurement Techniques and Pitfalls ........... 120 4.2.1 Characterizing the Source ................................. 120

4.2.2 Leakage Fields ..................................................... 122 4.2.3 Antenna Fields ..................................................... 126 4.2.4 Precautions for Ensuring Measurement

Accuracy

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127

........ 128 ................................................. 129

4.2.5 Precautions for Protection of the Operator

4.3 Special Measurements 5 Recommended Areas for Further Research and

.

Technical or Engineering Development ........................ 131 Appendix A Hazard Evaluation Procedures for Common Sources ............................................ 133 A.1 Amplitude-Modulation Radio Broadcast ................... 133 A.2 Frequency-Modulation Radio Broadcast .................... 136 A.3 Very-High-Frequency and Ultra-High-Frequency Television Broadcast ................................................... 139 A.4 Terrestrial Microwave Radio (Point-to-Point Radio Relay) ...........................................................................142 A.5 Satellite Communication-Earth Stations ............... 144 A.6 Hand-Held Portable Radios (Including Cordless and Hand-Held Cellular Telephones) ........................ 147 A.7 Mobile Radios (Vehicle Mounted-Including Citizens Band and Cellular Radios) ......................................... 149 A.8 Diathermy Equipment (Microwave and Shortwave) .................................................................. 151 A.9 Electrosurgical and Electrocautery Units ................. 153 A.10 Hyperthermia Equipment ........................................... 155 A . l l Magnetic Resonance Imaging Systems ...................... 157 A.12 Radar ......................................................................... 158 A.13 Marine Radar ...............................................................163 A.14 Police and Sports Radar ............................................ 165 A.15 Microwave Industrial HeatingIDrying Equipment ... 166 A.16 Radiofrequency Induction Heaters ............................. 168 A.17 Radiofrequency Dielectric Heaters (Heat Sealers) ... 171 A.18 Anti-Theft Devices ...................................................... 174 A.19 Microwave Door Openers ............................................ 176 A.20 Microwave Intrusion Alarms ......................................177 A.21 Microwave Ovens .................................................... 178 A.22 Visual-Display Terminals .......................................... 180 A.23 LORANIOMEGA Navigational Stations ................... 184 Appendix B. Radiofrequency Exposure Determination: Examples ............................. 186 B.l Radiofrequency Dielectric Heater Exposure Survey: A Sample Problem ......................................................... 186 B.2 Point-to-Point Microwave Radio: A Sample Problem ....................................................................... 188

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CONTENTS

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B.3 Amplitude-Modulation Radi.0. A Sample Problem ...... 194 B.4 Multiple Frequency-Modulation Radio: A Sample Problem ......................................................... 196 References .............................................................................. 200 The NCRP ................................................................................. 209 NCRP Publications ................................................................ 217 Index ........................................................................................... 229

1. Introduction 1.1 The Study of Radiofrequency Hazards

The study of radiofrequency (RF) hazards is a relatively new area of investigation in the United States. Engineering analyses of potential electromagnetic (EM) radiation exposure environments and the absorption of this energy by humans, animals, and synthetic human and animal models (phantoms) have become necessary due to the proliferation and distribution of sources of RF energy in the workplace and the environment. Also, developmentsin this new technical area have been stimulated by both the formulation of safety guidelines and standards against which radiation exposure can be judged and a continually increasing public awareness and concern over the possibility of health hazards from exposure to RF radiation. Engineering measurement surveys and analyses of external RF fields can provide insight needed for the determination of the potential safety or danger associated with a particular exposure situation. Several United States and foreign documents have been written on the subject of evaluation of RF hazards (most notably: ANSY IEEE, 1992a; Kulikovskaya, 1970; Marha et al., 1971; 1981; Mastrantonio and Russo, 1989; Minin, 1974). There has existed for some time a need for a guide in the form of a compendium of general knowledge useful to those health and safety professionals concerned with evaluating RF hazards. However, there is no single reference manual or guide designed for use by environmental health and safety personnel who are not electrical engineers or physicists specializing in EM measurements. This Report, A Practical Guide to the Determination of Human Exposure to Radiofrequency Fields, has been prepared to fill this void. It is a handbook for guiding those responsible for the evaluation of RF and microwave hazards. This Report is designed to give practical information on how to evaluate RF exposure, i.e., the physical characterization of EM fields. This Report provides a comprehensive collection of information on various RF radiation sources, and a straight-forward "how-to" guide for estimating the exposure associated with these sources by providing a framework ("cookbook") for health and safety personnel to assess individual sources of RF and microwave radiation. It is

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1. INTRODUCTION

intended to serve as a convenient reference for practical everyday information on the subject, and to supplement more specialized EM field measurement documents. It also provides an overview of RF radiation safety as it exists today. This Report includes extensive information on levels of RF exposure for both occupational workers and the general population. References to many experimental studies of this subject as well as extensive theoretical data and results of exposure assessments and internal absorbed energy are provided. Evaluation of RF hazards has been traditionally approached from two viewpoints: the EM environment in the immediate vicinity of a radiator, wherein the specificradiator is the single most predominant source of exposure; or the environment somewhat removed from the immediate, overwhelming influence of a single RF source, wherein the resulting exposure is caused by multiple separate sources. Multiple source environments are principally implicated when describing the exposure of most of the general population. A major thrust of this Report is toward the engineering evaluation of single source environments, inasmuch as these account for the highest intensity exposures, but significant information dealing with the multiple source environment is also given.

1.2 The Radiofrequency Spectrum Figure 1.1provides three ways of viewing the different aspects of the EM spectrum, e.g., photon energy, wavelength and frequency. The International Telecommunications Union (ITU, 1982) defines Radio Frequencies \

mm Waves

Mkcrcwaws

UV

Infrared

-

I Non.lon$zingRad$l~on

lonirlng Radiation

a-

V~sibleLight

fi 'Fliuvl

300 MHz

300 GH;

700 nm

X rays, rays

loo nm

Photon Energy lev)

00

Poww

I

I

I

I

300krn

300m

I

I

I I

30cm

UHF I Mcrowave

I I 3 0 0 ~ I

I I

I I

300nm

300pm

I

I I

I I

Frequency

I-+-

l2

(HZ)

1kHr

lMHz

Fig. 1.1. The EM spectrum.

lGH2

lTHr

lolS

I

ulo1a1 -

1

1.3 CROWDED RADIOFREQUENCY SPECTRUM

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the upper limit of the RF spectrum as 3,000 GHz. However, the upper limit of the RF spectrum is defined in this Report as 300 GHz with the range being 3 kHz to 300 GHz. The conventional definition of nonionizing radiation is radiation with photon energy insufficient to directly ionize atomic or molecular systems with a single quantum event. Since photon energy of 10 to 12 eV is required to cause ionization in an atom, the dividing wavelength between ionizing and nonionizing radiation is approximately 100 nm (12 eV), a considerably shorter wavelength than that associated with the upper end of the RF spectrum (3,000 GHz = 0.012 eV). For convenience, most of the spectrum is partitioned into specified RF bands as illustrated in Table 1.1. It should be noted that in some countries, e.g., the former Soviet Union, different band designations may be used.

1.3 The Increasingly Crowded Radiofrequency Spectrum

Innumerable sources of RF energy make use of the RF spectrum for communications and for industrial processing (usually heating). The most familiar are radio and television (TV)broadcasting services and microwave ovens. Additionally, shortwave broadcast and telecommunications, satellite communications (SATCOM), mobile (cellular) telephone and two-way police, fire and taxi radios make continual use of this spectrum. Thousands of noncommunications types of equipment also use the spectrum, including medical diathermy equipment, radar and VLF radio-navigational aids. In addition, there are large numbers of nonintentional radiators, such as RF heaters, dryers and plasma etchers. The overall types and number TABLE1.1-Radiofmquency band designations. Frequencgr (MHz)

Band

SELF ELF VF VLF LF MF HF VHF UHF SHF EHF SEHF

Description

Sub-extremely-low frequency Extremely-low frequency Voice frequency Very-low frequency Low frequency Medium frequency High frequency Very-high frequency Ultra-high frequency Super-high frequency Extremely-high frequency Supra-extremely-highfrequency

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1. INTRODUCTION

of both intentional and nonintentional radiators are continually increasing. Although the frequency spectrum, as such, cannot be exhausted, it can support only a finite number of unique RF signals without overlap and interference for communications purposes in any given geographic region. One author has even referred to the spectrum as an exhaustible, invisible resource (Levin, 1971). Certainly the presence of severe band congestion, in at least parts of the spectrum, exemplified by interference due to crowded conditions, is symptomatic of a burgeoning use of the spectrum by modern technology. Human exposure to emissions from RF sources is thus generally increasing. With this increased contact, or proximity, comes the possibility of exposure to higher levels, as well as exposure to new and as yet unused frequencies.

1.4 The Interaction of Radiofrequency Energy with Tissues

Radiofrequency energy absorbed by biological tissues may cause a number of effects. By far the most clearly understood mechanism of interaction is that of molecular agitation (heating). Figure 1.2 illustrates the alignment of polar water molecules with the applied electric field. Rapid movement of the molecule, as the electric field reverses direction millions of times per second, combined with frictional forces, results in the production of heat. This phenomenon of dielectric heating is the principle of microwave ovens used for cooking. It is this same potential for heat generation in humans, albeit usually at significantly lower intensity irradiation than in a microwave oven, that prompts the development of exposure limits and the Water molecule Negative charge this end this end Direction of wave

+ + Fig. 1.2. An illustration of the alignment of the polar water molecule in an alternating electric field.

1.5 ENVIRONMENTAL ASPECTS

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evaluation of potential RF hazards. The biological effects literature does describe a number of interactions thought to be nonthermal, although in all except heating and the RF hearing phenomenon, the mechanism of interaction is unknown. In some countries, including the former Soviet Union, RF exposure limits appear to be based solely upon the reports of behavioral effects that occur at levels below which tissue heating is expected. It is not within the scope of this Report to argue such considerations, and the reader is directed to the literature for further insight (NCRP, 1986). A major activity of several organizations, including the NCRP, has been the critical assessment of the world literature on biological effects of RF radiation. For example, the NCRP has reviewed the pertinent biological-effects literature and has developed its own recommended exposure criteria (NCRP, 1986);the Environmental Protection Agency (EPA, 1984) has reviewed the available literature to support development of federal regulations; and the C95-1 committee of the American National Standards Institute (ANSI) in developing its RF protection guide (RFPG), has examined the biologicaleffects literature (ANSI, 1982).The ANSI has continued this review process, resulting in the issuance of a new standard (ANSIflEEE, 199215). In addition, the International Radiation Protection Association (IRPA) has reviewed the literature (IRPA, 1988; 1990; WHO, 1993) and developed exposure criteria for both ELF and RF fields. 1.5 Environmental Aspects

A principal concern in hazard evaluations is the potential hazard associated with exposure in close proximity to a particular source of RF energy. In general, the principal exposure of an individual is due to the single nearest source, be it a hand-held radio near the operator's face, a dielectric heater or a nearby radio station. But as the inventory of EM sources increases, so do the general ambient levels of RF energy. In this sense, one speaks of the environmental aspects of RF fields, and considerable information is available on this subject. In recent years the experimental verification of wholebody resonances in scale models of humans has shifted the area of classical concern downwards in frequency from the microwave region of the spectrum (300 MHz to 300 GHz) to include 30 to 300 MHz (VHF) and lower frequencies as well. Of particular interest is the fact that within the adult-body resonance range of 70 to 100 MHz, where whole-body absorption is proportionately greater than a t other frequencies,environmental measurements have shown that RF field levels are generally of the greatest magnitude.

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1. INTRODUCTION

1.6 The State of Radiofrequency Safety Standards

To protect people from excessive exposure to EM fields, exposure limits and standards have been developed throughout the world. Specificattention to these standards, from the viewpoint of measurement, is given later in this Report. During the last several years in the United States, major advances have been made in the field of RF dosimetry for humans and animals, in the knowledge of RF exposure levels in the environment and in the knowledge of biological effects associated with exposure to RF energy. These new insights have provided the framework for reevaluation of some existing exposure limits and the development of new ones. Sound RF hazard evaluations must be constructed on the basis of the latest knowledge in these dynamic and changing areas.

1.7 Public Awareness

An added impetus for health and safety personnel to perform RF hazard evaluations has been the rapid growth of public awareness of possible detrimental effects of RF energy. Numerous articles in have in many the popular media, e.g., Brodeur (1989a; 1989b; 1989~1, cases acted as a stimulus for the public's concern for new and existing RF-emitting products. In many of these instances a need has existed for an accurate and knowledgeable analysis of the anticipated levels of RF exposure, or actual measurements, to allay or confirm the concerns of the public.

1.8 Overview of this Report This Report is directed to individuals who are concerned with the evaluation of potential RF hazards, but who are not electronic engineers or physicists specializing in EM radiation measurements for human hazard assessment. The Report should find application in federal, state and local environmental and health agencies. Personnel responsible for implementing andlor evaluating actual or potential hazards existing near RF emitting devices or the environmental impact of broadcast and communications systems, e.g., pointto-point microwave radio, and radar installations should find the Report a convenient source of information. It provides a technical overview of the subject of RF hazards and should be useful to managers and nontechnical individuals involved with health and safety

1.8 OVERVIEW OF THIS REPORT

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by identifying present or future potential RF problem areas in the programs for which they have responsibility. It is envisioned that this Report could be used as the framework for a curriculum of instruction on RF nonionizing radiation measurement and hazard evaluations. The content of the Report is as follows: 1. Section 2 provides an overview of the basic concepts useful for a working understanding of the material that follows. It describes the field parameters of interest for measurement or calculation of RF fields, provides an introduction to the concepts of RF dosimetry that create the link between exposure fields and absorbed energy in biological systems and introduces the reader to the subject of antennas and propagation of EM waves. 2. Section 3 is a discussion of approaches for analyzing measurement data obtained in the workplace or environment. Various pitfalls that can lead to erroneous conclusions from RF measurements are described with the intent to help minimize the probability of such occurrences being created by measurement personnel. Finally, guidance is given on how to compare measurement data with exposure limits. 3. Section 4 addresses measurement instrumentation and techniques so that the reader may become acquainted with the different types of instrumentation available for quantifying RF exposures. Discussions are given for both broadband survey type of measurement instruments and for narrowband instruments necessary for resolving the field strength at discrete frequencies of multifrequency exposure fields. Some emphasis is placed on helping the operator select the most appropriate instrument for the job. 4. Section 5 discusses recommended areas for further research. 5. Appendix A is a quick reference source for assessing the relative significance of exposures from various RF sources and it provides a description of methods for performing practical measurements and computations. 6. Appendix B provides examples and recommended approaches for RF exposure assessment for selected common sources.

2. Basic Concepts A number of concepts are important to the understanding of any work that involves EM fields. The purpose of this Section is to summarize the most important of these concepts for the specific applications that are described in this Report. Where practical, concepts are explained without the use of complicated mathematical expressions so that they may be understood without an extensive background in electrical engineering or physics. This material is not intended to encompass all of EM theory, but rather to provide a convenient summary.

2.1 Explanation of Terms and Units 2.1.1

Glossary

The following list contains terms that are used throughout this Report. The list is more an explanation of terms than precise definitions. The definitions of many of these terms can be found in documents such as NCRP Report No. 67 (NCRP, 1981).

antenna:A structure that is designed to radiate or receive EM fields efficiently. Individual antennas, or antenna elements, are often used in combinations that are called antenna arrays. dielectric constant: Another name for relative permittivity. electric dipole: Two equal charges of opposite sign separated by a small distance. electric field: A term that is often used to mean the same as electric field strength. electric-fieldstrength: A vector force field that is used to represent the forces between electric charges. Electric-field strength is a unit defined as the force per unit charge on an infinitesimally small charge at any given point in space, and it is usually represented by the symbol E. The unit of electric-field strength is volt per meter (V m-l). electric field intensity: Another term for electric-field strength. (The term "field strength" is preferred.)

2.1 EXPLANATION OF TERMS AND UNITS

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electric-flux density (displacement): Usually designated by D. The electric-flux density is a vector quantity equal to the product of the electric-field strength and the permittivity. The total electric flux passing through a closed surface is equal to the total charge enclosed by the surface. The unit of D is coulomb per square meter (C m-'). electric polarization: Separation of charges in a material to form electric dipoles, or alignment of existing electric dipoles in a material when an electric field is applied. A vector quantity, usually designated P, the unit of polarization is dipole moment per cubic meter or coulomb per square meter (C m-7. emission: Fields generated a t a given distance from an RF source. Emission should not be confused with exposure, i.e., emission does not depend on the presence of a person. energy density (volume):EM energy in a given volume of space divided by that volume. The unit is joule per cubic meter (J m-3). energy density (surface):EM energy incident on a surface per unit surface area. The unit is joule per square meter (J m-4. exposure: External fields incident on occupied areas. The quantity of exposure depends on the duration and the strength of the field(& far field: The EM field a t a point far enough away from the RF source such that the fields are approximately plane-wave in nature. field point: A point at which the electric or magnetic field is being evaluated. frequency: The time rate at which a quantity, such as an electric field, oscillates. Frequency is equal to the number of cycles through which the quantity changes per second. Frequency is expressed in hertz (Hz). The unit for Hz is reciprocal seconds (s-'1. impedance, wave: The ratio of the electric field strength to the magnetic field strength of a wave. For a plane wave in free space, the wave impedance is equal to the square root of the ratio of the permeability to the permittivity of free space and is equal to 377 ohms. For a plane wave in a material, the wave impedance is equal to 377 times the square root of the ratio of the relative permeability to the relative permittivity of the material. lossy: A material characteristic which specifies attenuation or dissipation of electrical energy. magnetic field: A term that is often used to mean magnetic flux density. However, in common usage, magnetic field is also used to mean magnetic field strength. There seems to be no clear pattern of usage for this term. To avoid confusion, the specific terminology, i.e., magnetic-flux density and magnetic-field strength is used in this Report.

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2. BASIC CONCEFTS

magnetic-field strength:A vector field, usually designated H, that is equal to the magnetic-fluxdensity B divided by the permeability of the medium. Magnetic-field strength is the component of the magnetic field that is measured. Its unit is ampere per meter (A m-'1. H is a useful quantity because it is independent of the magnetization current in materials. magnetic-flux density: A vector force field that is used to describe the force perpendicular to the velocity of a moving charged particle. Magnetic-flux density is defined as the force per unit charge on an infinitesimal moving charge at a given point in space according to the equation F/q = v x B where v is the velocity of the particle, q is its charge, F is the vector force acting on the particle, B is the magnetic-flux density and x denotes the vector product. The magnetic-flux density is the product of the magnetic-field strength and permeability. The unit is tesla (TI. near field The EM field close enough to the RF source such that the field is not plane-wave in nature. The spatial variation of the strength of the EM wave is usually more rapid in the near field than in the far field. permeability: A property of a material that indicates how much magnetization occurs when a magnetic field is applied to it. The unit is henry per meter (h m-I). permittivity: A property of material that indicates how much polarization occurs when an electric field is applied to it. The unit is farad per meter (F m-I). phase velocity: The velocity of a point of constant phase on a single frequency wave. plane wave: A wave in which the wave fronts are planar, E and H have constant values in the planes of the wave fronts, and E, H, and the direction of propagation are all mutually perpendicular. polarization of EM wave: Orientation of the incident electric- and magnetic-field vectors with respect to the absorbing object. When associated with an antenna, polarization generally refers to the direction of the electric-field vector. power density(S):The power incident on a surface per unit surface area. Poynting vector: A vector equal to the vector product of E and H. It is usually designated as S and has the unit watt per square meter (Wm-'). The surface integral of S represents the instantaneous power transmitted through a closed surface. propagation coefficient:A quantity that describes the propagation of a wave. Usually designated k, it is equal to the radian frequency divided by the phase velocity, and has the unit of reciprocal meter (m-I).

2.1 EXPLANATION OF TERMS AND UNITS

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radian frequency: The angular rate at which a quantity is oscillating. The radian frequency is equal to 2nf, where f is the frequency in hertz (Hz). radiation: The propagation of EM energy in the form of waves. reflection coefficient: The ratio of the magnitude of the reflected wave to the magnitude of the corresponding component of the incident wave. relative permittivity:The permittivity of a material divided by the permittivity of free space. specific absorption rate (SM):The time rate at which RF energy is absorbed in an incremental mass divided by that mass. Average SAR in a body is the time rate of the total energy absorbed divided by the total mass of the body. The unit is watt per kilogram (W kg- I). spherical wave: A wave in which E and H are uniform on the surface of a sphere. E, H and the direction of propagation are all mutually perpendicular. An idealized point source radiates spherical waves. standing-wave ratio (s): The ratio of Em,, to E,,, where Em, is the maximum value of the magnitude of the electric-field strength anywhere along the path of a wave, and E,, is the minimum value of the magnitude of the electric-field strength along the path of the wave. A similar definition holds for other quantities that have wave properties. vector: A quantity having both a magnitude and a direction. Velocity is an example of a vector; the direction of motion is the direction of the velocity vector and the speed is the magnitude of the velocity vector. velocity of propagation: Velocity a t which a wave propagates. It is equal to the distance that a given point on a wave, such as the crest or trough, travels in one second. The unit is meter per second (m s-'1. wave impedance: (see impedance, wave). wavelength: The distance between two adjacent crests of a wave (or the distance between two adjacent troughs or any other two corresponding points). The unit is the meter (m). 2.1.2

Units

Listed in Table 2.1 are the SI basic units. Table 2.2 lists several pertinent derived SI units. The SI system is the internationally agreed upon system of units adopted by the Eleventh General Conference on Weights and Measures held in Paris in 1970. SI is the

12

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2. BASIC CONCEPTS

TABLE2.1-The SI basic units. Quantity

Common symbol

Unit

Symbol

Length Mass Time Electric current Temperature Luminous intensity Amount of substance

I

meter kilogram second ampere kelvin candela mole

m kg s A K cd mol

m

t

I T cd n

abbreviation for Systeme International d'Unites (International System of Units) (NBS, 1986; NCRP, 1985). 2.1.3 Vectors

Since vectors are used extensively in the description of electric and magnetic fields, this Section contains a brief explanation of vectors and vector notation. A scalar is a quantity that has magnitude only. In contrast to this, a vector is a quantity that has magnitude and direction. A familiar example of a vector quantity is the velocity of a particle. The direction of movement of the particle is the direction of the vector; the speed of the particle is the magnitude of the vector. Vedors are represented graphically by directed line segments, as illustrated in Figure 2.1. The length of the line represents the magnitude of the vector, and the direction of the line represents the direction of the vector. In this Report, vectors are represented by bold face type. Thus, A is a vector quantity. The magnitude of a vedor is represented by the same symbol in italics. Thus, A is the magnitude of the vector A. A summary of vector calculus, or even vector algebra, is beyond the scope of this Report, but the basic vector addition and multiplication operations are described here because they are important for understanding the EM field characteristics that are described later. Because vectors have the two properties, magnitude and direction, algebraic vedor operations are more complicated than algebraic scalar operations. Addition of any two vectors A and B is defined as where C is the vector along the diagonal of the parallelogram shown in Figure 2.2. The negative of a vector A is defined as a vector having the same magnitude as A, but the opposite direction. Subtraction of any two vectors A and B is defined as where - B is the negative of B as defined above.

Quantity Capacitance Charge Conductance Conductivity Current density Electric field strength Electric flux density (electric displacement) Energy Energy flux density (power density) Frequency Impedance Inductance Magnetic field strength Magnetic flux density Permeability Permittivity Power Reactance Resistance Resistivity Voltage (potential difference)

Symbol

Common

C 4 G u

J E D

W S

f

CC

Z L H B E

P X R P

V

TABLE2.2-Some

Symbol

Expression in tenns of other units CV-I As

Expression in of SI base u m-2 kg-' s4 sA rn-' kg-' s3 m-3 kg-' s3 m-2 A

n-' or AV-' n-lrn-'

F C S S m-' A m-2

Js-' VA-' VA-'

s-I m2 kg A m2 kg s-2A m-' A kg e-'A-' m kg s-2 Am-3 kg-' s4 m2 kg s-3 m2 kg s-3A m2 kg s-3A m3 kg s-3A m2 kg s-3A

m2 kg s-2 kg s-'

m kg s-3 Am-2 s A

Nm J s-'m-' VA-' Wb A-'

n n

WA-'

Wb m-Z

flm V

F m-' W

h A m-' T h m-'

n

Hz

J W m-2

V m-' C rn-'

&rived SZ units.

Name farad coulomb siemen siemen per meter ampere per square meter volt per meter coulomb per square meter joule watt per square meter hertz ohm h e ~ y ampere per meter tesla henry per meter farad per meter watt ohm ohm ohm meter volt

14

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2. BASICCONCEPTS

Fig. 2.1. A vector quantity represented by a directed line segment.

(a)

Fig.2.2. Vector addition.

There are two types of multiplication with vectors. One is called the scalar product or the vector dot product. If A and B are any two vectors, the vector dot product of A and B is defined as:

A - B =ABcos0

(2.3)

where 0 is t h e included angle between A a n d B, a s shown i n Figure 2.3. The dot product of two vectors is a scalar. As indicated in Figure 2.3, A - B is also equal to the projection of A on B times B. This interpretation is often very useful. Note that when two vectors are perpendicular, their dot product is zero because the cosine of 90 degrees is zero (also the projection of A on B is zero). The other kind of multiplication is called the vector product or the vector cross product, and i t is defined as:

2.1 EXPLANATION OF TERMS AND UNITS

1

15

Fig. 2.3. Vector dot product A . B.

where C is a vector whose direction is perpendicular to both A and B and whose magnitude is given by:

C

=

AB sin 8

(2.5)

As shown in Figure 2.4, the direction of C is the direction a righthanded screw would travel if turned in the direction of A into B.

Fig. 2.4. Vector cross product A x B = C (Cis a vector perpendicular to the plane containing vectors A and B).

16

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2. BASICCONCEPTS

The cross product of two parallel vectors is always zero because the sine of zero degrees is zero.

2.2 Electromagnetic Fields 2.2.1 Electric Fields

The subject of EM fields is based on the phenomenon of the forces that electric charges exert on one another. The mathematical statement of the force F on one charge q, due to the presence of another charge 9, is called Coulomb's law:

where r is a unit vector from q, to q,, r is the distance between the two charges, E , is the permittivity of free space and c is a constant of proportionality. The unit of charge is C, and the unit of permittivity is F m-l (see Section 2.1). When both q, and q2 have the same sign, the force in Equation 2.6 is repulsive. When the charges have opposite signs, the force is attractive. When more than one additional charge is present, the force on one charge is the summation of all of the forces acting on it due to each of the other individual charges. Since it is not always convenient to keep track of all the charges in a complicated electric system, a quantity called electric field is defined and used to account for the forces exerted on charges by one another. The electric-field strength vector E is defined in terms of a very simple and idealized model experiment. A point test body charged to a very small net positive charge q is brought into a region of space where an electric field exists. According to Coulomb's law, the force F on the test charge is proportional to q. E is defined as:

where it is understood that q is infinitesimally small so that it does not affect the electric field that is being measured. The unit of E is V m-l. Thus one could, in principle, determine whether an electric field existed a t a given point in space by placing a small test charge a t that point and measuring the force on it. If there were no force on it, the electric field would be zero a t that point. If there were a force on it, the direction of the force would be the direction of E a t

2.2

ELECTROMAGNETIC FIELDS

/

17

that point, and the magnitude of E (E) could be determined from the definition. Of course, this is not a practical way to detect or measure a n electric field strength, but this idealized thought experiment is valuable for understanding the basic nature of electric fields. From the definition of electric field strength, it follows that the force on a charge q placed in a n electric field is given by:

Thus, if E is known, the force on any charge placed in E can be determined easily.

2.2.2 Magnetic Fields

When electric charges are moving in a magnetic field, there is another force exerted on them i n addition to t h a t described by Equation 2.8. In order to account for this additional force, another force field is defined, analogous to the definition of the electric field described in the previous section. This second force field F, is associated with the magnetic-flux density vector B. B is defined in terms of the force exerted on a small test charge q. The magnitude of B is defined as:

where F, is the magnitude of the maximum force on q in any direction, and v is the magnitude of the velocity of q. The unit for B is tesla T, which is equivalent to Wb m-'. The magnetic field is more complicated than the electric field in that the direction of the force exerted on q by B is always perpendicular to both the velocity of the particle and the direction of B. This force is given by:

which is analogous to Equation 2.8. This force is sometimes referred to as the Lorentz force. The quantity in parentheses in Equation 2.10 is the vector cross product. The direction of the vector cross product is perpendicular to both v and B and is in the direction that a righthanded screw would travel if v were turned into B (see Section 2.1.3). When a moving charge q is placed in a space where both an electric field and a magnetic field exist, the total force exerted on the charge is given by the sum of Equations 2.8 and 2.10.

18

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2. BASIC CONCEPTS

2.2.3 Static Fields The basic concepts of electric and magnetic fields are explained below, first in terms of static fields, because the evaluation is simpler for static fields than for the more complicated time-varying fields.

2.2.3.1 Static-ElectricFields. Perhaps the simplest example of an electric field is that of a static point charge Q in space. Let q be a small test charge used to determine the field produced by Q. Then, by the definition of E in Equation 2.7 and the force on q from Equation 2.6, the magnitude of the electric field E due to Q is found to be:

A diagram of the electric field associated with a point charge is shown in Figure 2.5(a). The direction of the arrows shows the direction of the electric field, and the spacing between the field lines shows the intensity of the field. The field is most intense when the spacing of the field lines is the closest. Thus, near the charge, where the field lines are close together, the field is strong, and it decreases as the reciprocal of the square of the distance from the charge W2), as indicated by Equation 2.11. The electric field produced by an infinitely long uniform line of charge is shown in Figure 2.5(b). In this case, the field decreases as the reciprocal of the distance (r-l) from the line charge. Note that in every case, the direction of each

A

Line Charge ,

Point Charge

Fig. 2.5. (a) The electric field produced by a single point charge in space. (b) Electric field produced by a uniform line of charge (looking along the axis of the line charge).

2.2 ELECTROMAGNETIC FIELDS

1

19

electric field line is the direction of the force that would be exerted on a small test charge q placed a t that point in the field. For a negative point charge, the electric field lines would point toward the charge, since a positive test charge q would be attracted toward the negative point charge producing the field. The sources of electric fields are charges. Electric fields can be produced by charges picked up by a person walking across a deep pile rug, for example. The presence of this electric field is sometimes manifested in terms of an unpleasant shock when the person touches something that is grounded, such as a water faucet. The charge configurations that produce electric fields are often found in mechanical devices, such as electric generators, or in electrochemical devices, such as automobile batteries. Figure 2.6 shows a sketch of the electric field lines between a pair of parallel infinite plates. This field could be produced by connecting a voltage source across the plates, which would charge one plate with positive charge, and the other plate with negative charge. An important characteristic of electric fields is illustrated by the configuration shown in Figure 2.7(a), where a small conducting object is placed in the field between the parallel plates of Figure 2.6. The sharp corners of the object concentrate the electric field, as indicated by the crowding of the field lines around the corners. Figure 2.7(b) shows how the edges of finite plates also concentrate the field lines. It is generally true that any sharp object will tend to concentrate electric field lines. This explains why arcs often occur a t corners or sharp points in high-voltage devices. If sharp edges and corners are rounded, such arcs will often be prevented. Another important principle is that static-electric field lines must always be perpendicular to surfaces with high electrical conductivity. An approximate sketch of the electric field lines can often be made on the basis of

Fig. 2.6. Electric field lines between infinite parallel conducting plates.

20

.

1

2. BASIC CONCEPTS

Fig.2.7. (a)Electricfield lines when a small conducting object is placed between the plates (the electric field in the conductor is zero). (b) Electric field lines between parallel conducting plates of finite size.

this principle. For example, consider the field plot in Figures 2.7(a) and 2.7(b). This sketch can be made by noting that the originally evenly spaced field lines of Figure 2.6 must be modified so that they will be normal (perpendicular) to the surface of the metallic object placed between the plates and they must also be normal to the plates. This concept is often sufficient to understand qualitatively the electric field behavior for a given configuration. 2.2.3.2 Static-MagneticFields. Perhaps the simplest example of a static magnetic field is that produced by an infinitely long, straight conductor carrying a direct current (dc) element, a s shown in Figure 2.8. The field lines circle around the current, and the field dies away as the reciprocal of the distance from the current element. Figure 2.9 shows another example, the magnetic field produced by a simple circular loop of current. A simple qualitative rule for sketching static-magnetic field lines is that the field lines circle around the current element and are strongest near the current. The direction of the field lines with respect to the direction ofthe current is obtained

2.2 ELECTROMAGNETIC FIELDS

1

21

Fig. 2.8. The magnetic field produced by an infinitely long straight direct current element. The direction of the current is upward from the page toward the reader.

Current Loop

I

Fig. 2.9. The magnetic field produced by a circular current loop.

22

/

2. BASIC CONCEPTS

from the right-hand rule: the thumb is pointed in the direction of the current (positive charge) and the fingers will circle in the direction of the magnetic field lines.

2.2.4

Quasi-Static Fields

An important class of EM fields is quasi-static fields. These are fields that have the same spatial patterns as static fields, but vary with time. For example, if the charges that produce the electric fields in Figures 2.5 to 2.7 were to vary slowly with time, the field patterns would vary correspondingly with time, but a t any one instant would be similar to the static field patterns shown in the figures. Similar statements could be made for the static magnetic fields shown in Figures 2.8 and 2.9. Thus, when the frequency of the motion of source charges or currents is low enough, the fields produced by the sources can be considered quasi-static fields, and the field patterns will be the same as the static field patterns, but changing with time. This makes the analysis of quasi-static fields much easier than the analysis of fields that change more rapidly with time.

2.2.5 Interaction of Fields with Materials

The interaction of electric and magnetic fields with a material consists of two parts. First, the electric and magnetic fields exert forces on the charged particles in the material, thus altering the charge pattern that originally existed. Second, the altered charge patterns in the material produce electric and magnetic fields in addition to the fields that were originally applied. Materials are usually classified as being either magnetic or nonmagnetic. Magnetic materials have molecular or atomic dipoles that are strongly affected by applied fields. Nonmagnetic materials do not have significant magnetic dipolar interactions with applied fields. 2.2.5.1 Nonmagnetic Materials. In nonmagnetic materials, mainly the applied electric field has an effect on the charges in the material, and this occurs in three primary ways: 1. polarization of charges 2. orientation of permanent dipoles 3. drift of conduction charges

2.2 ELECTROMAGNETIC FIELDS

1

23

The polarization of bound charges is illustrated in Figure 2.10(a). Charges in a material that are so tightly held by restoring forces that they can move only very slightly are called bound charges. Without an applied electric field, the positive and negative bound charges in an atom or molecule are essentially superimposed upon each other and effectively cancel. When an electric field is applied, the forces on the positive and negative charges are in opposite directions, and the charges separate, resulting in an induced electric dipole moment. A dipole consists of a combination of a positive and a negative charge separated by a small distance. In this case, the dipole is said to be induced, because it is caused by the applied electric field. When the applied electric field is removed, the dipole disappears. When the charges are separated by the applied electric field, the charges no longer cancel, and in effect, new charge is created. This new charge is called polarization charge. Polarization charge creates new fields that did not exist previously. The orientation of permanent dipoles is illustrated in Figure 2.10(b). The arrangement of charges in some molecules produces permanent dipoles that exist whether or not an electric field is applied to the material. In the absence of an applied electric field, these permanent dipoles are randomly oriented because of thermal excitation. When

Force Up

ir

Force Down Fig. 2.10. (a) Polarization of bound charges. (b) Orientation of permanent dipoles.

24

1

2. BASICCONCEPTS

an electric field is applied, the resulting forces on the permanent dipoles tend to align the dipole with the applied field, as shown in Figure 2.10(b). The orientation of each dipole is slight, because the thermal excitation is relatively strong, but on the average, there is a net alignment of dipoles over the randomness that existed without the applied field. Like induced dipoles, this net alignment of permanent dipoles also produces new fields. The drift of conduction charges in a n applied electric field occurs because conduction charges are free enough that they can move significant distances in response to the forces of the applied fields. The movement of the conduction charges is called drift, because thermal excitation causes random motion of the conduction charges, and the forces due to the applied fields superimpose only a slight movement in the direction of the forces on this random movement. The drift of conduction charges amounts to a current, and this current produces new fields that did not exist previously.

2.2.5.2 Permittivity. The two effects, the creation of new charges by an applied field, and the creation of new fields by these new charges, are both taken into account (for induced dipoles and orientation of permanent dipoles) by a quantity calledpermittivity. Permittivity is a measure of how easily polarization in a material occurs. If a given applied electric field results in a great number of induced dipoles per unit volume, or a high net alignment of permanent dipoles per unit volume, the permittivity is high. The drift of conduction charges is accounted for by a quantity called conductivity. Conductivity is a measure of how much drift occurs for a given applied electric field. A large drift means a high conductivity. For sinusoidal steadystate applied fields, complexpermittivity is defined to account for both dipole charges and conduction-charge drift. Complex permittivity (F m-'1 is usually designated as: (2.12) E = E,(E' - j E") where (E' - j E") is called the relative permittivity, E' is called the real part and E" is called the imaginary part of the relativepermittiv= E' is also called the dielectric constant. E"is related ?ih4?1?#ctiue conductivity by:

m.

where a is the effective conductivity, and is the radian frequency (radians per second) of the applied fields. E, is called the permittivity of free space. E, is equal to 8.854 X 10-l2 F m-l and f is the frequency of the applied field.

2.2 ELECTROMAGNETIC FIELDS

1

25

2.2.5.3 Energy Absorption. Energy is transferred from electric fields to a material in the form of kinetic energy of the charged particles in the material. The time rate of change of the energy transferred to the material is the power P. The power transferred to a material is often called absorbed power, although the term is regarded by some as inappropriate. A typical manifestation of average (with respect to time) absorbed power is heat. The average absorbed power results from the "friction" associated with the movement of the induced dipoles, permanent dipoles, and drifting conduction charges. If there were no friction in the material, the average rate of energy absorption would be zero. Since the absorbed power is proportional to the product of the electric field in the material and E", E" is a measure of the lossyness of the material. In general, a larger E" means a more lossy material. Highly conducting metals are an exception to this rule, however, because for these metals E"is extremely large, but the electric field is very small, so that the product of the two and, therefore, the absorbed power, is small. In some tables a quantity called the loss tangent is listed instead of E". The loss tangent, often designated as tan 8, is defined as:

The loss tangent usually varies with frequency. For example, the loss tangent of distilled water is approximately 0.040 at 1MHz and is about 0.265 at 25 GHz. Sometimes the loss tangent is called the dissipation factor. Generally speaking, the "wetter" a material is, the more lossy it is, and the "drier" it is, the less lossy. For example, a wet paper placed in a microwave oven will get hot as long as it is wet, but when the paper dries out, it will no longer absorb energy and, hence, will no longer be heated by the EM fields of the oven. For steady-state sinusoidal fields,the time-averaged rate of energy absorption per unit volume (W m-3) at a point inside an absorber is given by:

where E is the root-mean-squared (rms) magnitude of the electric field vector at that point inside the material and u is the conductivity. If the peak value of a sinusoidal electric-field vector is used, a factor of 0.5 must be included on the right-hand side of Equation 2.16. (The rms and peak values are explained in Section 2.2.6 and unless otherwise noted, rms values are used.) To find the total power absorbed by an object, Equation 2.16 must be used to calculate P at

26

1

2. BASIC CONCEPTS

each point inside and summed (integrated) over the entire volume of the object. This is usually a very complicated calculation. Ebctric-Flux Density. A quantity called electric-flux density D or displacement flux density is defined as:

2.2.5.4

D

=

EE

(2.17)

where D has the property that the integral of D over any closed surface (that is, the total flux passing through the closed surface) is equal to the total free charge inside the closed surface. Free charge is defined to be charge not contained in the material and, therefore, it does not include polarization and conduction charge. This relationship is called Gauss's law. Figure 2.11 shows an example of this, where Q is the total free charge inside the surface. The total flux passing out through the closed surfaces is equal to the total free charge Q within, regardless of the permittivity. D is a convenient quantity because it is independent of the induced charges in material. Magnetic Materials. Magnetic materials have magnetic dipoles that tend to be oriented by applied magnetic fields. The resulting motion of the magnetic dipoles produces a current, which produces new electric and magnetic fields. Both the effect of the

2.2.5.5

Surface A Fig. 2.11. Example of Gauss' law applied to a charge inside a dielectricspherical shell.

2.2 ELECTROMAGNETIC FIELDS

/

27

applied fields on the material and the creation of new fields by the moving magnetic dipoles in the material are accounted for by a property of the material called permeability. Permeability (h m-l) is usually designated as: (2.18) CL = CL,(P' - jpn) where p1 - jpNis the relative permeability and is the permeability of free space (which is rigorously equal to 477 x h m-l) or approximately 1.257 x h m-l. Another field quantity is the magnetic-fild strength H (A m-'1 which is defined by:

H

=

-.Bc1

(2.19)

Magnetic-fieldstrength is a useful quantity because it is independent of magnetic currents in materials. The term magnetic field is applied both to B and H, which is frequently confusing. Whether to use B or H in a given situation is not always clear, but since they are related (see Equation 2.19), it does not really matter which one is specified. In situations pertinent to this Report, H is the quantity that is usually measured. Although small quantities of magnetic material have been found in some birds, bees and bacteria, most biological materials are nonmagnetic and permeability is usually not an important factor in bioelectromagnetic interactions. 2.2.6

Wave Propagation

The motion of source charges or currents produce radiating electric and magnetic fields. At low frequencies, these radiated fields are usually negligibly small. At high frequencies, the fields can be substantial. A convenient and commonly used description of radiation is wave propagation. The basic ideas of wave propagation are illustrated in Figures 2.12 and 2.13. Propagation of an EM sine wave is analogous to water waves rolling in on a beach. As shown in Figure 2.12, the distance from one crest to the next in meters (or some other appropriate unit of length) is defined as the wavelength, which is usually designated as A. The phase velocity is the velocity at which the wave is traveling. It is called the phase velocity because it represents the velocity of a point of constant phase. From Figure 2.12, the magnitude of the phase velocity is equal to the distance traveled Az divided by the time it took to travel the distance Az:

28

/

2.

BASIC CONCEPTS

Fig.2.12. A traveling wave at two instants of time, tl and t p

Fig. 2.13. The time variation of an electric field at a point in space.

A detector fixed a t one point in space would be subject to a function that oscillated with time as the wave passed by. This is similar to an individual standing on the beach and watching a wave go by. The height of the water above some reference plane would change with time similarly to the plot in Figure 2.13.The value of the crest of the wave is called the peak value or amplitude of the wave. The amplitude of the wave shown in Figure 2.13 is 10 V m-'. The period T of the oscillation is the time between adjacent corresponding points of the function (Figure 2.13). The frequency f is defined as the reciprocal of T, i.e.:

The unit of T is second; that o f f is Hz, which is equivalent to cycles per second. The frequency of a water wave could be obtained

2.2

ELECTROMAGNETIC FIELDS

/

29

by counting the number of crests (or troughs) that passed by a fixed point in 1s. The quantities defined above are related by the following important equation: In free space, v is often designated by c, the velocity of light. In a material such as a dielectric, the velocity of the wave is slower than that in free space. There are two commonly used idealizations of wave propagation, spherical waves and plane waves. These are described in Sections 2.2.6.4 and 2.2.6.5. In power relationships, as explained in Section 2.2.5.3, the rms value of a function is convenient to use. This term has traditionally been associated with the effective heating value of an electrical current or voltage. For a general periodic function At), the definition of the rms value F is:

where to is any value of t, and T is the period of the function. Equation 2.23 shows that the rms value is obtained by squaring the function f(t), to obtain f(t), integrating the square of the function over the period, dividing by the period, and taking the square root. Integrating over a period T is equivalent to calculating the area between the function, f(t), and the t axis. Dividing this area by T is equivalent to calculating the average, or mean of f(t) over one period. A function f(t) is shown in Figure 2.14(a) and f(t) is shown in Figure 2.14(b).' The rms value of f(t) is F and is calculated as follows (see Figure 2.14): the area between the f(t) curve and the t axis between to and f + T is 25 x 30 + 4 x 10 = 790. Hence the rms value F is:

2.2.6.1 Modulation. Modulation is the process of impressing information on an EM wave by varying some aspect of the wave. The information may be in the form of telegraphy, voice, data, video, etc. RF fields may also be pulse modulated, e.g., for radar applications. 'The function f(t) could represent any periodic function, eg., instantaneous electric field strength or instantaneous power density S of an EM field.

30

1

2. BASICCONCEPTS

f (t)

Is--

10 I--3o--t--(

t(ms)

-T-+

2- -

-

-

-

-b

Fig. 2.14. (a) f(t) versus t. (b) f(t) versus t. The darkened area between f (t) and the t axis over one period is that used for calculating the rms value of f(t).

Various modulation methods have been developed to optimize information performance for specific requirements. For each type of modulation, some property (amplitude, frequency, or phase) of a carrier wave (the RF field which is to be modulated) is changed in proportion to the instantaneous amplitude of the information-bearing waveform to be transmitted. RF carrier waves possess two properties that may be modulated, i.e., amplitude and phase angle. Angular modulation includes FM and phase modulation. With all modulation schemes, the deviation of the modulated property of the carrier (with respect to the value of that property in the unmodulated carrier) is made

2.2

ELECTROMAGNETIC F'IELDS

/

31

proportional to the instantaneous amplitude of the modulating signal. The rms value of sinusoid G is given by:

where g, is the peak value of the sinusoid. 2.2.6.2 Amplitude Modulation. Amplitude modulation (AM) results in a variation of the carrier amplitude in proportion to the amplitude of the modulating signal. The degree of modulation is called the modulation index m, and is usually expressed as a percentage. When a sinusoidal signal is used to AM a sinusoidal carrier, modulation sidebands are produced at frequencies displaced fkom the carrier frequency (above and below) by the frequency of the modulating signal. The modulation index for an AM carrier wave would be given by the expression:

where Em, is the instantaneous electric-field strength during modulation and E, is the instantaneous electric-field strength of the unmodulated carrier wave. In AM fields, the detected average field strength may be substantially less than the instantaneous peak value. The exact value of the average field strength of an AM RF field will be dependent on the nature of the modulating signal. For example, voice modulation, telegraphy and pulse modulation (such as used by radars) will all result in characteristic ratios of average to peak field strengths. 2.2.6.3 Frequency Modulation. Frequency modulation (FM) occurs when the instantaneous frequency deviation of the modulated carrier wave with respect t o the frequency of the unmodulatedcarrier wave is proportional to the amplitude of the modulating signal. The ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating signal is m, the modulation index. With FM RF fields, the instantaneous field strength is equal to the average field strength. Examples of FM RF sources include FM radio, some point-to-point microwave radio systems, earth SATCOM systems and cellular telephone systems.

Spherical Waves. A spherical wave is a model that represents approximately some actual EM waves that occur physically,

2.2.6.4

32

1

2. BASIC CONCEPTS

but of course a true spherical wave does not really exist. Spherical waves have several characteristic properties (see Figure 2.15):

1. The wave fronts are spheres and propagate radially outward from the source. E and H are both tangential to the spherical surfaces. 2. Each crest and each trough is a spherical surface and on every sphere's surface, both E and H are constant values. 3. E and H and the direction of propagation are all mutually perpendicular. k is a vector in the direction of propagation. 4. EH-I = p 5 ~ - %(called the wave impedance). For free space, EH-I = 377 ohms. 5. The magnitude of both E and H vary as r - l , where r is the distance from the source.

Fig. 2.15. A spherical wave. The wavefronts are spherical surfaces. The wave propagates radially outward in all directions. The magnetic field component, H, is directed into the page.

2.2 ELECTROMAGNETIC FIELDS

/

33

6. The magnitude of the velocity of propagation (phase velocity) The velocity is less and the wavelength is given by v = (p)-%. is shorter for a wave propagating in matter than for a wave propagating in free space.

Plane Waves. A plane wave is another model that represents approximately some actual EM waves, but true plane waves do not exist. Plane waves have characteristics similar to spherical waves: 2.2.6.5

1. The wave fronts are planes. 2. E and H and the direction of propagation k are all mutually perpendicular. 3. EH-I = p%-% (called the wave impedance). For free space, EH-I = 377 ohms. 4. E and H are constant in any plane perpendicular to the direction of propagation. 5. The magnitude of the velocity of propagation is given by v = ( p ~ ) - %The . velocity is less and the wavelength is shorter for a wave propagating in matter than for a wave propagating in free space. Figure 2.16 shows a plane wave. E and H could have any direction in the plane, so long as they are perpendicular to each other. Far away from its source, a spherical wave can be considered to be approximately a plane wave in a limited region of space because the curvature of the wavefronts is so small that they appear to be almost planar.

Peak

Trough

Peak

Trough

Fig. 2.16. A plane wave. H is directed out of the page.

34

1

2. BASIC CONCEPTS

2.2.7 Near Field

In regions close to a source, the field is called the near field. In the near-field region, E and H are not necessarily perpendicular, and in fact, the field cannot always be conveniently characterized. The field is often more nonpropagating in nature and is called a fringing field, reactive field or induction field. The near field often varies rapidly with space. The mathematical expressions for E and H in the near field generally contain the terms r-l, r-', r-3,. . ., where r is the distance from the source to the field point (point at which the field is being determined). Objects placed near a source may have a strong effecton the nature of the near fields. For example, placing a probe near a source to measure the field may change the nature of the field considerably. This will be seen to be an important factor in measurement accuracy and the interpretation of fieldstrength measurements made very near a source. 2.2.8 Far Field

When (21rrlA)>>I, where r is the distance from the source and X is the wavelength, the r-', r-3 and higher-order terms are negligible compared with the r-' term in the expressions for E and H, and the field is called the far field. The far field is approximately a spherical wave that can in turn be approximated in a limited region of space by a plane wave. Making measurements in the far field is usually easier than making measurements in the near field, and calculations for far-field energy absorption are much easier than for near-field absorption. 2.2.9 Interaction of Fields with Objects

When a propagating wave strikes an object, part of the wave is reflected or scattered by the object and part of it penetrates the object. The total electric and magnetic fields at any point outside the scatterer consists of the incident fields and the scattered fields. The simplest example of scattering is a plane-wave incident on a planar object. Planar Conductors. Figure 2.17 shows a plane wave normally incident on a planar conductor. When the incident wave enters the conductor, it produces currents that are sources of additional electric and magnetic fields that are called scattered fields (or reflected fields). If the conductor has infinite conductivity (a perfect

2.2.9.1

2.2 ELECTROMAGNETIC FIELDS

1

35

Fig. 2.17. Plane-wave normally incident on a planar conductor. The conductor produces a normally scattered wave.

conductor), the sum of the incident and the scattered fields must equal zero everywhere inside the conductor. The total electric and magnetic fields (sum of the incident and scattered fields) to the left of the perfect conductor is shown in Figure 2.18(a) as a function of position for two different times, tl and t,. At any instant of time, the variation of the field with position is sinusoidal. At any position, the variation of the field with time is sinusoidal. The envelope of the field variation with position, as the field varies through a full cycle in time, is shown in Figure 2.18(b). Note that at certain positions, the electric field is zero for all values of time. These positions are called nodes. The nodes for the electric field occur at multiples of one-half wavelength from the conductor, and E is zero at the surface of the conductor. The magnetic field is not zero at the surface of the conductor [see Figure 2.18(c)l, but the nodes are still spaced one-half wavelength apart. Remember that E and H are constant everywhere in a plane perpendicular to the direction of propagation. The patterns shown in Figure 2.18 thus represent the magnitudes of E and H in planes parallel to the conductor. At an E node, E is zero everywhere in that plane. Because of the characteristic envelope shown in Figure 2.18(a), the patterns are called standing waves.

36

1

2. BASICCONCEPTS

(a) Total electric field as a function of position a t two different times, t, and tz.

(b) Total electric field as a function of position for various times through a full cycle, and the envelope of the standing wave.

Envelope of the wave

(c)Total magnetic field as a function of position a t various times through a full cycle, and the envelope of the standing wave.

Fig. 2.18. Total fields, incident plus scattered, for a plane-wave incident on a planar conductor.

2.2 ELECTROMAGNETIC FIELDS

1

37

A standing wave is produced by the combination of a wave traveling to the right (incident wave), and a wave of equal magnitude traveling to the left (reflected wave). When the reflected wave has an amplitude that is less than the incident wave, an envelope similar to that in Figure 2.18(b) and (c) results, except the fields do not cancel to produce nodes. There are corresponding points a t which a minimum value of the field occurs, but they are not nodes because the field does not vanish there. This pattern is explained in Section 2.2.9.2. Figure 2.19 shows a plane-wave obliquely incident on a perfect planar conductor. In this case, the angle of reflection is equal to the angle of incidence. The angles are defined as the angles between the direction of propagation and the normal (perpendicular)to the planar conductor. The sum of the incident and the scattered waves for oblique incidence is also a standing wave, but in this case, the nodes do not occur a t half wavelength spacing. Their spacing depends on the angle of incidence. For large angles of incidence, the spacing between nodes is much larger than one-half wavelength. 2.2.9.2 Planar Dielectrics. When a plane wave is incident on a planar dielectric, the incident wave produces currents in the dielectric that produce additional fields,just as in the case ofthe conductor. Unlike the fields inside a conductor, however, the fields inside a dielectric do not necessarily add to zero. A plane-wave incident on a planar dielectric produces scattered fields outside the dielectric and a wave inside the dielectric called the transmitted (refracted) wave, as shown in Figure 2.20 for oblique incidence. If the dielectric

Fig. 2.19. Plane-wave obliquely incident on a planar conductor.

38

1

2. BASIC CONCEPTS

Fig. 2.20. Plane-wave obliquely incident on a planar dielectric.

is lossy (Section 2.2.5.3), the transmitted wave is attenuated a s it travels into the dielectric, becoming essentially zero a t some depth. The penetration depth is related to the imaginary part of the relative permittivity E". The larger the value of E", the smaller the penetration depth. 2.2.9.3 Standing-Wave Ratio. Figure 2.21 shows the top half of the envelope of the electric-field pattern produced by the sum of a n incident wave and a wave reflected by a planar dielectric. The electric field a t each position is a sinusoidal function of time. The figure shows the magnitude of the sinusoid as a function of position. Since the magnitude of the reflected wave is smaller than the magnitude of the incident wave, there are no nodes. There are, however, maxima and minima that occur a t specific values of z. The maximum values are spaced one-half wavelength apart, and the minimum values are also spaced one-half wavelength apart. A typical electric field probe that measures the magnitude of E (E) could be used to obtain a distribution like the one shown in Figure 2.21. An i m p r t a n t quantity called the standing-wave ratio (often designated by s) is defined as the ratio of the maximum value of the sinusoidal magnitude of a wave a t any position to the corresponding minimum value a t any position. For the wave pattern shown in Figure 2.21, the standing-wave ratio is:

2.2 ELECTROMAGNETIC FIELDS

1

39

Fig. 2.21. Top half of the envelope resulting from an incident and reflected electric field wave. This envelope is similar to the one in Figure 2.18(b) (the solid lines), except that Ed, = 0 in Figure 2.18(b) and here it is not zero. Only the top half of the envelope is shown here (above the axis).

where Em, is the maximum value of the sinusoidal electric field at any value of z and Eminis the minimum value of the sinusoidal electric field a t any value of z. The standing-wave ratio is a measure of the reflection at an interface. When there is no reflection, s = 1.When there is total reflection, s = because Em, = 0. In terms of the reflection coefficient, s is given by:

where p is the reflection coefficient, which is equal to the ratio of the magnitude of the reflected wave to the magnitude of the incident wave. 2.2.9.4 Nonplanar Objects. The scattering of electric and magnetic fields by nonplanar objects is, of course, more complicated than that by planar objects. The scattering depends on the size, shape

40

/

2. BASIC CONCEPTS

and electrical properties of the object and the frequency of the incident fields. If the object is very small compared with the wavelength of the incident field, very little scattering occurs. If the relative permittivity of the object is very close to unity, very little scattering occurs. When the size of the object is comparable to or larger than a wavelength, significant scattering generally occurs. More specific information about scattering and absorption by biological tissue is given in Section 2.3. 2.2.10 Poynting's Theorem (Power-Conservation Theorem)

Poynting's theorem, which is an important statement relating the rate of energy absorption in an object to the incident fields, is often misunderstood and misinterpreted. Avoiding complicated mathematical expressionsis a goal of this Report, but explaining Poynting's theorem is a situation where a mathematical statement is necessary for a good explanation. Enough explanation will be given, however, to allow understanding with only a minimum knowledge of mathematics. According to Poynting's theorem, if A is any closed mathematical surface and V is the volume inside A, then:

dl at ( W , + E E - E + ~ H . H ) ~ V + ~ E X H - ~ ~(2.28) =O V

A

where w, is the energy per unit volume possessed by charged particles at a given point in the volume, &E.Eis the energy per unit volume stored in the electric field at a given point in the volume, da is a differential surface element of A, and pH-H is the energy per unit volume stored in the magnetic field at a given point in the volume. A closed surface is any surface that completely encloses a volume. The volume integral corresponds to summing the terms in the integrand over all points inside the volume. Thus the integral over the volume corresponds to the total energy possessed by all charged particles within the volume plus the energy stored in the electric and magnetic fields. The integral on the left is the time rate of change of the total energy within the volume, which is total power. The term on the right is an integral over the closed mathematical surface enclosing the volume. For convenience, let

S = E x H

(2.29)

where S, which is called the Poynting vector, has the unit of W mP2 and is interpreted as the power density S. As explained in Section 2.1.3, the direction of the cross product of E and H is perpendicular

2.2 ELECTROMAGNETIC FIELDS

1

41

to both E and H, and the vector dot product of E x H with d a selects the.component of E x H that is parallel to da (see Figure 2.22). S-da is the power dP passing out through the differential surface element da, and the surface integral is the sum of the power passing through each surface element over the entire surface A, which is equal to the total power passing out through A. Thus Poynting's theorem is a statement of the conservation of energy: the time rate of change of the total energy inside the volume is equal to the total power passing out through A. The Poynting vector E x H is useful in understanding energy absorption, but it is important to realize that Poynting's theorem applies only to a closed surface and the volume enclosed by that surface. Misapplying Poynting's theorem to field relations at a given point, or to only part of a closed surface instead of to the entire closed surface can lead to serious misunderstanding of energy absorption characteristics.This will be illustrated after consideration of Poynting's theorem for time-averaged fields. Poynting's theorem for time-averagedpower and energy of steadystate sinusoidal electric and magnetic fields is of special interest. Equation 2.28 is valid at any instant of time for fields of general time variation. For sinusoidal fields, however, the average of the time rate of change of energy stored in the electric and magnetic

Fig. 2.22. The volume V is bounded by a closed surface A; da is the differential surface vector. S da is the projection of S on da, which corresponds to the power (dP) passing through the differential surface element da.

.

42

1

2. BASIC CONCEPTS

fields is zero. This is analogous to energy stored in a frictionless spring. Over one part of the cycle the spring is compressed so that it stores energy, but over the next part of the cycle, the spring extends and gives back the energy, so the average of the time rate of change of the energy stored is zero. The time average of the rate of change of energy possessed by charged particles is also zero if there is no "friction" (such as that due to collisions) involved in the motion of the particles. Friction, however, results in energy loss (usually transformed to heat) that cannot be returned, which corresponds to a nonzero time average change in energy. In terms of average power, Poynting's theorem is:

where the brackets designate the time average of the quantity. Equation 2.30 states that the sum of the average power possessed by charged particles in the volume and the total average power passing out through A is always equal to zero. Another way to look at this relationship is indicated in Equation 2.31: -

]<s>-da

=

f

dV.

(2.31)

V

A

The term on the left is the total average power passing in through A, which is equal to the total average power transferred to the charged particles in the volume. Thus, if interpreted properly with respect to a closed surface, the Poynting vector E x H can be a useful parameter in describing nonionizing radiation fields, but it must be used with considerable caution. The following examples illustrate this point. Consider the case of a plane-wave incident on an absorbing object, as illustrated in Figure 2.23. Using the impedance relationship for plane waves, EH-I = 377 ohms, the magnitude of the Poynting vector for a plane wave in free space is the familiar expression:

S

E2

= -=

377

377P

(2.32)

When the incident fields Ei Hi impinge on the absorber, electric and magnetic fields E, H, are scattered by the absorber. Poynting's theorem applied to this situation yields

Since

( E i + E , ) x ( H i + H , ) = E i x H i + E i ~ H , + E , Hi+E,xH, x (2.34)

2.2 ELECTROMAGNETIC FIELDS

1

43

Hi

from the left

A

Fig. 2.23. A plane wave irradiating an absorber. A is any closed surface. Scattered fields are produced by reflection of the incident plane-wave.

i t is clear t h a t t h e Poynting vector for the incident wave, Ei x Hi, even when integrated over A, would not yield the total power transferred from the incident wave to the absorber. Finding the total power transferred to the absorber from integration of the Poynting vector over A would require knowing the scattered fields and including them in the calculation according to Equation 2.33, but the scattered fields are not usually known because calculating or measuring them is generally difficult. It is true that the power transferred to the absorber would be proportional to the Poynting vector of the incident plane wave. For a given absorber and a given plane wave, for example, the power transferred to the absorber would be twice as much if the incident S (Poynting vector of the incident wave) were 2 mW ~ r n a- s~i t would be if the incident S were 1mW cmP2.The actual amount of power transferred to the absorber in each case, however, would depend on the characteristics of the absorber. Even though the incident S of a plane wave is commonly

44

1

2. BASIC CONCEPTS

used as an indication of the ability of the plane wave to cause power absorption in irradiated objects, it must be remembered that this is only a relative indication, not an absolute one. The situation shown in Figure 2.24 further illustrates difficulties sometimes encountered in applying Poynting's theorem. Suppose a plane wave is incident on a perfectly conducting plane, which will produce a reflected wave that combines with the incident wave to produce a standing wave (Section 2.2.9). Measurement of E and H in front of the conductor would show a pattern like that of Figure 2.18. Could S calculated from E x H of these fields at a given distance from the conductor be used to indicate the relative amount of power that would be absorbed by a small absorber placed at that point? To answer this question, first suppose that the absorber was placed at a distance of 3h14 from the conductor. As shown in Figure 2.18,H at that distance is zero. Hence, E x Hat that distance is zero, which might lead one to believe that no power would be absorbed by an object at that point. Even though E x H is zero there, it is certainly not necessarily true that an absorber placed at that point

Conducting Plane

Fig. 2.24. Absorber placed between an incident plane wave and a conducting planar surface. Scattered fields are produced by both the absorber and conducting surface.

2.2 ELECTROMAGNETIC FIELDS

1

45

would absorb no energy. The only way that E x H can be used to obtain information about energy absorbed by an object is to integrate E X H over a closed surface surrounding the object. This requires having knowledge of the incident and scattered electric and magnetic fields, including E and H reflected by the conductor and E and H scattered by the absorber. For the same reasons, it is also not correct to add the power density S of the incident and reflected waves to determine what S would be effective in causing energy absorption by the object. Poynting's theorem is a statement of conservation of power, but only the total power passing through a closed surface. It does not state that S is conserved from place to place. The concept of S becomes more useful at microwave frequencies, where energy can be transmitted in narrow beams. However, caution must still be used. The best procedure for assessing potential absorption of energy by an object in a field (at least for frequencies below a few hundred MHz) is to measure both E2 and H2, because energy absorption can more easily be predicted from them, and many radiofrequency protection guides are expressed in terms ofE2and@. Unfortunately, some guidelines are expressed and some meters are calibrated in terms of S, i.e., "equivalent plane-wave power density." What this usually means is that the meter actually measures either E or H or E2 or P,and then the "equivalent" plane-wave power density is calculated from Equation 2.32.This could cause misunderstanding andlor measurement errors. Suppose, for example, that a meter that measures P, and is calibrated in equivalent plane-wave power density, is placed a distance of 3,414 from a conducting plane in the presence of an incident plane wave. As indicated in Figure 2.18,the meter would read zero because H would be zero at that point. One might erroneously concIude that since the meter indicates zero S, a person located at that point would be safe in the sense that the person would absorb no energy. Furthermore, one meter measuring E2 and another measuring H2 would indicate different values of equivalent plane-wave power density, perhaps causing one to conclude erroneously that one meter or the other were faulty. The best procedure is to measure both,!? and@ separately. Further information can be found in Section 2.3 on dosimetry.

2.2.1 1 Antennas

Antennas are devices used to radiate or receive RF energy, generally for purposes of communication or broadcasting. The antenna for a radio station accepts power from the transmitter and radiates

46

1

2. BASICCONCEFTS

this power in various directions away from the antenna. Depending upon the application, antennas may possess radiation characteristics tailored to specific requirements. Knowledge of the radiation characteristics of an antenna is required for calculating the expected RF fields associated with the antenna. The most important property of an antenna for evaluating RF exposures is the gain; a measure of the ability to direct RF power in specific directions. The concept of gain may be understood by considering a point source which radiates RF energy outward uniformly in all directions. If the point source is radiating power P , then S (W m-') at a distance r meters, from the source is:

Such an imaginary antenna is called an isotropic source. Real antennas do not perform as isotropic sources, but rather concentrate the radiated power in such a way that the flux through a spherical surface about the antenna is not uniform. In some directions the radiated fields will be stronger while in other directions they will be weaker. This property of concentrating the radiated power is called gain (sometimes called directional gain or directivity) and is determined by the ratio of the field strength or S at a given point to the field strength or S that would exist at the same point from an isotropic radiator radiating the same power. In this case the gain is referenced to an isotropic radiator. The gain of some antennas, particularly linear antennas such as those used for FM and TV broadcasting, may be referenced to a A12 dipole. Thus, if a particular antenna creates a Sof 10mW a t a specificpoint, but an isotropic radiator would create only 1mW ern-' at the same point for the same input power, then the power gain of the antenna in that direction is said to be ten. While an antenna may possess power gain greater than unity in some directions, it will also exhibit power gain less than unity in other directions. The general approach to calculating the expected value of S from an antenna is to introduce the antenna gain G into Equation 2.35 as follows:

In this case, P is interpreted as the input power to the antenna in watts. The product PG is called the effective radiated power (ERP), or effective isotropic radiated power (EIRP) when referenced to an isotropic source. When the gain is referenced to a dipole, the gain

2.2 ELECTROMAGNETIC FIELDS

1

47

of the antenna must be multiplied by the gain of a dipole (1.64) in order to use Equation 2.36. Conventionally, antenna gains are specified in the far-field. This is appropriate since the normal purpose of an antenna is to produce a field at some distant point. However, for the evaluation of human exposure to RF fields, calculations are usually performed for areas in the near-field of many antennas, where fields will be strongest. In these cases, a knowledge of the near-field gain of the antenna is helpful in accurately estimating the field strengths. Near-field gain generally is less than the far-field gain. Figure 2.25 illustrates the computed gain of an antenna array consisting of 12 dipoles spaced one wavelength apart (Tell, 1978). The gain approaches an asymptotic value at large distances (far field) from the array. At close-in distances (near field) the array gain is seen to decrease in an oscillatory manner. The extent of near-field gain reduction is, in general, a functionof the size of the antenna in terms of wavelengths. Typically, antennas which exhibit relatively high far-field gains also exhibit significant reductions in gain at close distances. The matter of nearfield gain of an antenna plays an important role in calculating potential exposure near certain types of RF radiating systems. This will be discussed further in a later section of this Report. -

-

-

F = 100 MHz

-

I

1

I

1

1 1

1 1 1 1

10

I

1

1

1

1

1 1 1 1

1

I

1

100

1

1

1 1 1

1000

Distance From Array (meters) Fig. 2.25. Near-fieldgain reduction for a 12 bay dipole antenna array operating at 100 MHz with one wavelength (3 m) bay spacing and uniform in-phase power division to each bay.

48

1

2. BASICCONCEPTS

2.2.11.1 Near and Far Fields. Various methods exist for distinguishingbetween the far-fieldand the near-field region of an antenna radiation field. A definition frequently encountered in engineering texts specifies that the far field begins at a distance d away from the antenna equal to W 2A - l where D is the largest dimension of the radiating portion of the antenna and h is the wavelength of the RF wave. This definition is based on a maximum phase variation of 22.5 degrees between waves arriving at d from all points on the surface of the antenna. For RF exposure assessments, other expressions may be desirableand more practical. For many types of antenna systems the extent of the near field defined in terms of the distance beyond which S falls off as the inverse of d2,is substantially closer to the antenna than W 2A - l . For circular parabolic antennas, this distance may be closer to 0.35 D~ A - l (Hankin, 1974). In the most general sense, three zones may be defined for the radiation fields of an antenna: the near field, in which S varies dramatically with distance, although S maxima remains relatively constant; the intermediate-field, in which S may vary approximately inversely with distance; the far-field, in which S varies inversely with distance squared. It must be stressed that these are not absolute definitions for the physical extent of any of these zones. The polarization of the radiated RF field is determined by the orientation of the electric field vector E with respect to the surface of the earth. If E is parallel with the ground, the polarization is horizontal; if E is normal to the ground, the polarization is vertical. RF' waves may also be elliptically polarized in which case E rotates as it propagates. A special case of elliptical polarization is circular polarization: if the rotation of the oncoming E is clockwise, the polarization is said to be right-hand polarization, if counter-clockwise, left-hand. In a circularly polarized wave E describes a circle as it rotates. RF waves transmitted by FM radio broadcast antennas are commonly circularly or elliptically polarized. Such polarization is effective in providing better reception for a broad range of receiving antenna orientations (polarizations).In contrast,AM radio broadcast stations that operate in the 535 kHz to 1,605kHz range use vertically polarized monopole antennas. Antennas are classified as either linear or aperture. Linear antennas consist of metallic rods, tubing or wire conductors arranged in such a way as to radiate the desired RF field. Examples of linear antennas are dipoles, monopoles, Yagis, long-wires, etc., and collections of such types of antennas to form antenna arrays. For example, dipole elements may be arranged to form a dipole array that has higher gain than a single dipole. Aperture antennas are normally characterized by metallic surfaces formed into the shape of a horn or reflector which may be

2.2 ELECTROMAGNETIC FIELDS

1

49

round or rectangular. Parabolic reflector antennas, trapezoidal horn antennas and pyramidal gain horn antennas are common examples of aperture antennas. In the case of horns, the transmitter power is fed directly into the throat of the horn via a waveguide. Parabolic reflector antennas employ a small antenna generally called a feed or feed-horn, of either the linear or aperture type, usually located at or near the focal point of the reflector. The feed-horn illuminates the reflector which concentrates the radiating RF energy into a beam.

2.2.11.2 Radiation Patterns. The directive properties of antennas are commonly illustrated by a radiation pattern. The radiation pattern may be thought of as the relative electric field strength at a constant distance away from the antenna plotted as a function of angular direction about the antenna, or in some cases it may be a plot of the antenna gain. Typically, radiation patterns are measured in the azimuthal and elevation planes. By employing such data, accurate computations of the expected field strength may be made at any given spatial point in the vicinity of the antenna. Figure 2.26 illustrates the radiation pattern of a dipole antenna. Note the broad beamwidth of the bidirectional pattern. Beamwidth is usually defined as the angular deviation in the main beam (or lobe) of radiation for which the relative power density S is one-half (-3 dB) of the maximum value of S on the antenna's axis.' A half-wave dipole's 'Decibel, (dB), is a dimensionless quantity that is proportional to the logarithm of the ratio of two values of the same physical quantity.

END VIEW OF DIPOLE

$=0°

Fig. 2.26. Radiation patterns of the half-wave dipole.

50

/

2. BASICCONCEPTS

beamwidth is 78 degrees. Large aperture antennas may have beamwidths significantly less than one degree. If the maximum on-axis gain of the antenna is known, then the effectivegain of the antenna at off-axis locations may be determined from the radiation pattern. Sometimes such a pattern is called a gain pattern. It is important to keep in mind that radiation patterns of antennas are conventionally measured or computed for the farfield, i.e., as though one were an infinite distance away from the antenna. In reality, it may be of interest, or necessity, to have knowledge of the gain patterns in the near-field. Figure 2.27 illustrates one method of displayingthe radiation pattern of a parabolic reflector

Y-Axis Distance Divided by D

Fig. 2.27. Radiation pattern of a parabolic reflector antenna with a diameter D of 20 wavelengths (frequency = 3.95 GHz and D = 1.5 m). Each curve represents the radiation pattern as determined at a different distance from the antenna; at short distances, in the near field, the pattern becomes much broader than it is in the far field, but the gain at the peak of the pattern becomes less. The y-axis distance is that distance normal to a line in the direction of the main beam. A value of 0.5 on the horizontal axis is equivalent to a point at the edge of cylindrical projection of the reflector.

2.3 DOSIMETRY

/

51

antenna having a diameter equal to 20 times the wavelength. Radiation patterns at many different distances from the antenna are shown in Hankin (1986).

2.3 Dosiinetry Dosimetry is the calculation and measurement of the energy absorbed by irradiated objects. Dosimetry also includes the determination of the internal field distribution in irradiated objects. Some of the important concepts and information about dosimetry are summarized in this Section. 2.3.1 Electrical Properties of Tissue

The permeability p of tissue is essentially equal to that of free space. In other words, tissue is essentially nonmagnetic. The permittivity E of tissue is a strong function of frequency. Figure 2.28 shows a plot of the average value of E' and E" for the adult human body as a function of frequency. Calculations have shown that the whole-body averaged value of E' and E" is approximately equal to two-thirds that of muscle tissue. At frequenciesbelow about 1MHz,

1

0

lo1

0

1

I

1o2

I

1o3

1o4

1o5

Frequency (MHz)

Fig. 2.28. The frequency dependence of tissue with a permittivity equal to twothirds that of muscle tissue. (Calculations have shown that the average permittivity of the human body is approximately equal to two-thirds that of muscle tissue.)

52

1

2. BASICCONCEPTS

body tissue is anisotropic. That is, the conductivity in one direction is significantly different from the conductivity in another direction. The permittivity generally decreases with increasing frequency. This is a manifestation of the inability of the charges in the tissue to respond to the higher frequencies of the applied fields, which results in a lower value of E.

2.3.2

Plane-Wave Absorption as a Function of Frequency

The absorption of energy by an object irradiated by EM fields is a strong function of frequency. Although calculation of the absorbed energy is generally difficult, many calculations have been made and significant data, both calculated and measured, are available. Absorption characteristics are explained below, first for plane-wave models, which are the simplest, but least representative of the human body, and then for more realistic models.

2.3.2.1 Planar Models. Although planar models do not represent the human body well, important qualitative understanding of energy absorption characteristics has been obtained from analyses of planar models. When a plane-wave is incident on a planar dielectric object, the wave that is transmitted into the dielectric is attenuated as it travels and transfers energy to the dielectric, as explained in Section 2.2.9.2. For very lossy dielectrics, the wave attenuates rapidly. This characteristic is described by the penetration depth, which is defined as the depth a t which the magnitude of the E and H fields have decayed to e-I (36.8 percent) of their value at the surface of the dielectric. The penetration depth is also the depth at which the Poynting vector S has decayed to e-2 (13.5 percent) of its value at the surface. The penetration depth 6 for a plane-wave incident on a planar dielectric is given by: 67.52 6 = -[{(&I)'

f

+( E " ) ~-)~~' '1~- l 'meters, ~

(2.37)

where f is the frequency in MHz, E' is the real part of the complex permittivity E,, and E" is the imaginary part of the complex permittively. Figure 2.29 shows the penetration depth as a function of frequency for a planar dielectric with a permittivity equal to twothirds that of muscle tissue. At higher frequencies, the penetration depth is small, which means that most of the energy from the fields absorbed by the material occurs near the surface. For example, at 2.45 GHz, the penetration depth in tissue is about 2.0 cm. At 10 GHz, the penetration depth is about 0.4 cm. The results for planar

2.3 DOSIMETRY

1

53

100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) Fig. 2.29. Penetration depth versus frequency for a dielectric half-space with permittivity equal to two-thirds that of muscle.

models show a characteristic that turns out to be generally true for other objects as well. At lower frequencies the fields penetrate much deeper than at the higher frequencies. At higher microwave frequencies, any heating that occurs in a lossy material as the result of plane-wave irradiation will be primarily surface heating.

2.3.2.2 Other Models. Other models, including spheres, cylinders, prolate spheroids (shaped like an egg) and block models (cubical mathematical cells arranged in a shape like a human body) have been used to represent the human body in making calculations and measurements of the energy absorbed during plane-wave irradiation. The internal electric and magnetic fields are a function of the incident fields, the frequency, the permittivity of the object and the size and shape of the object. Some typical absorption results and characteristics are given below. 2.3.3 Polarization The orientation of the incident electric and magnetic fields with respect to the irradiated object has a very strong effect on the strength of the fields inside the object. This orientation is defined in terms of the polarization of the incident fields. For objects of revolution (circular symmetry about the long axis), the polarization is defined in terms of which vector of the incident

54

/

2. BASIC CONCEPTS

field, E, H or k, is parallel to the long axis of the body. The polarization is called E polarization, if E is parallel to the long axis, H if H is parallel to the long axis and K if k is parallel to the long axis of the body. This definition is illustrated in terms of prolate spheroids in Figure 2.30. For other objects, like the human body, which are not objects of revolution, six polarizations are defined, as illustrated in Figure 2.31 for ellipsoids (shaped like a flattened egg).The ellip-

E polarization

H polarization

K polarization

Fig.2.30. Illustration of the definition of polarization of the incident fields with respect to an irradiated object.

EHK polarization

KEH polarization

HEK polarization

KHE polarization

EKH polarization

HKE polarization

Fig.2.31. Definition of polarizationsfor objectsthat do not have circular symmetry about the long axis.

2.3 DOSIMETRY

1

55

soid has three semiaxes with lengths a, b and c, where a > b > c. The polarization is defined by which vector (E,H or k) is parallel to which axis (a, b, or c). For example, EHK polarization is defined as the orientation for which E lies along a, H lies along b and k lies along c. 2.3.4

SpecificAbsorption Rate Characteristics

The transfer of energy from electric and magnetic fields to charged particles in an absorber is described in terms of the specificabsorption rate (SAR). SAR is defined at a point in the absorber as the time rate of change of the energy transferred to the charged particles in an infinitesimal volume at that point divided by the mass of the infinitesimal volume. "Specific" refers to the normalization to mass, "absorption" refers to the absorption of energy and "rate" means the time rate of change of the energy absorption. From Equation 2.38, the local SAR is

where p, is the mass density of the object at that point and dWc/dt is the rate of change of the energy per unit volume of charged particles at that point. The whole-body average SAR is defined as the time rate of change of the total energy transferred to the absorber divided by the total mass of the object. From Poynting's theorem for the time-average sinusoidal steady-state case (Equation 2.391, the whole-body average SAR is given by

The term represents the power transferred to chargedparticles in an infinitesimal volume at a given point and M is the total mass of the absorber. In practice, the term "whole-body average SAR" is often shortened to just "average SAR." The local SAR is related to the internal electric field through Equation 2.40, i.e.,

Thus if E and the conductivity are known at a point inside the object, the SAR at that point can be found; conversely, if the SAR and conductivity at a point in the object are known, E at that point can

56

1

2. BASIC CONCEFTS

easily be found. The relation in Equation 2.38 illustrates why SAR is also called absorbed power density; P has traditionally been called absorbed power density (with the unit W m-3). SAR is generally accepted as the preferred quantity. SAR is an important quantity in dosimetry, both because it gives a measure of the time rate of energy absorption that can be manifested as heat, and because it gives a measure of the internal fields, which could affect the biological system in ways other than through heating. As explained above, the internal rms electric fields, and hence the SAR, are a strong function of the incident fields, the frequency and the size, shape and properties of the absorber. Since any biological effect would be a result of the internal fields, it is important to be able to determine internal fields or SAR in humans and experimental animals for given exposure conditions. Meaningful extrapolation of observed biological effects in irradiated animals to similar effects that might occur in irradiated humans could not be accomplished without determination of the internal fields or SARs in both. Since a great deal of information about SARs is available in the literature, only a few examples of SAR characteristics are discussed here. For more information see Durney et al. (1978; 1986). Also see the following special issues devoted to these subjects; Radio Science (1977; 1979), Proceedings of the IEEE (1980). The general dependence of whole-body average SAR on frequency in the far-field irradiation zone is illustrated in Figures 2.32 and 2.33 for models of an average-size adult human and a medium-size rat for the three standard polarizations (Durney et al., 1978). For the models of an adult human, a resonance occurs a t about 70 MHz for E polarization. For the rat the resonance occurs a t a frequency of about 600 MHz. As shown in the figures, the resonance frequency is related to the length of the body. In general, resonance occurs a t a frequency for which the length of the body is approximately onehalf of a free-space wavelength. A more accurate formula for the resonant frequency is given in Section 2.5. Below resonance, the SAR varies approximately as fL, and just beyond resonance it varies approximately as f (within limits). Figures 2.32 and 2.33 also indicate that below resonance the SAR is generally higher for E polarization, intermediate for K polarization, and lower for H polarization. These characteristics can be explained by two qualitative principles: 1. The SAR is higher when the incident electric field is more nearly parallel to the long axis of the body than when it is more perpendicular. 2. The SAR is higher when the cross section of the body perpendicular to the incident magnetic field is larger than when it is smaller.

'

2.3 DOSIMETRY

/

57

Fig. 2.32. Calculated whole-body average SAR versus frequency for models of an average individual for three standard polarizations. The incident power density S is 1 mW

The average SAR is higher for E polarization, because the incident electric field is more nearly parallel to the body than perpendicular to it, and the cross section of the body perpendicular to the incident magnetic field is relatively larger (Figure 2.30). For H polarization,

58

/

2. BASIC CONCEPTS

Fig. 2.33. Calculated whole-body average SAR versus frequency for models of a medium-sized rat for three standard polarizations. The incident power density S is 1 mW

however,the incident electric field is more perpendicularto the body than parallel to it, and the cross section of the body perpendicular to the incident magnetic field is relatively smaller, and both contribute to a lower average SAR. The average SAR for K polarization is

2.3 DOSIMETRY

/

59

intermediate between the other two because the incident electric field is more perpendicular to the body, contributing to a lower SAR, but the cross section perpendicular to the incident magnetic field is large, contributing to a larger SAR. When a person is standing on a perfectly conducting ground plane, for E polarization, the ground plane has the effect of making the individual appear electrically to be about twice as tall, which lowers the resonant frequency to approximately one-half of that in free space. For an individual on a ground plane, a plot of SAR versus frequency (for E polarization) would therefore be almost like that shown in Figure 2.32, but shifted to the left approximately 40 MHz. This decrease in resonant frequency generally occurs for objects on ground planes for E polarization. Another important qualitative characteristic is that when the incident electric field is mostly parallel to the body, the average SAR increases if the body is made longer and thinner. Some of these "rules of thumb" are summarized in Section 2.5.

2.3.5

Dosimetry Concepts as Applied to Radiofrequency Prottrction Guides

Many presentday radiofrequency protection guides (RFPG) use the dosimetric concepts described above as a basis for specifying the maximum incident RF field strengths for exposure. Based on research findings and consideration of the uncertainties associated with sensitivity of individuals in the population, eg., occupational versus general population exposure circumstances, a threshold SAR is defined below which it is believed that no adverse health effects will occur in people. Then, using the frequency dependency of the SAR function for people of different sizes, and a suitable safety factor, incident RF field strengths are determined that will limit the SAR to the defined value. The most common application of this approach limits RF field strengths most stringently in the so-called body resonance frequency range of approximately 30 to 300 or 400 MHz. In this frequency range the whole-body SAR is maximum for a given field strength. At frequencies lower than approximately 30 MHz, the field strength is permitted to rise, reflecting the reduced efficiency of coupling of the field to the body. But below approximately 1 to 3 MHz, other biological phenomena such as RF shocks and burns may become the dominant concerns for health and safety rather than whole-body SAR. Since shock and RF burn phenomena can occur at these frequencies for RF field strengths significantly less than that which would limit the SAR to the required value,

60

1

2. BASIC CONCEPTS

some RFPGs have a cap or ceiling value on the RF field strength for frequencies below 1to 3 MHz.

2.4

Concepts of Measurements

Three kinds of EM measurement techniques are of primary interest; measurement of the electric field, the magnetic field, and SAR. The basic concepts underlying these measurement techniques are discussed in this Section. 2.4.1 Electric Field Measurements

Devices for measuring the electric field usually consist of two main components; a small antenna or other pickup device that is sensitive to the electric field, and a detector that converts the signal to a form that can be registered on a read-out device such as a meter. The antenna is typically a short dipole, as illustrated in Figure 2.34. The dipole can be two short pieces of thin wire [Figure 2.34(a)], or two short strips of t h i n metal, a s on a printed circuit board [Figure 2.34(b)]. Sometimes the dipole is flared to look like a bow tie [Figure 2.34(c)] to improve the frequency bandwidth characteristics. The detector is usually a diode or a thermal sensor. A diode rectifies the signal, i-e.,converts the time varying signal to a steady-state dc voltage or current, so that it can be registered on a dc meter. A thermal sensor responds to heat produced by the absorption of energy from the electric field in some lossy material placed on the pickup. The heat produces a voltage or current that can be registered on a meter. An example of a thermal sensor is a thermocouple, which produces a voltage that is proportional to the temperature difference produced across a junction of two appropriate dissimilar materials. Although commercial instruments for measuring electric field are based on the simple concepts described here, they are very sophisti-

Fig. 2.34. Short dipole as used for measuring an electric field.

2.4 CONCEPTS OF MEASUREMENTS

/

61

cated in their design and fabrication. Some commercially available instruments are described elsewhere in this Report. Leads of some kind are required to transmit the voltage or current from the detector to the meter or other read-out device. These leads often cause problems, because they themselves can be sensitive to the presence of an electric field and may produce erroneous measurements because of unwanted electric field pickup. To reduce this problem, high-resistance leads are often used. The sensitivity of the antenna or pickup element is roughly proportional to its Iength compared with the wavelength of the electric field to be measured. There is, however, a difficult trade-off between sensitivity and field perturbation. At low frequencies, where the wavelength is very long, short elements are sometimes not sensitive enough. It is important, however, not to make the element too long, because a long element may perturb the field being measured. To avoid field perturbation, the element should be less than a tenth of a wavelength a t the frequency of interest. The dipole element is sensitive only to the component of the electric field that is parallel to the dipole. In principle, an electric field that is perpendicular to the dipole will not be sensed a t all. This can be understood in terms of the force that the electric field exerts on the charges in the dipole, for that is the basic mechanism by which the field is sensed. When E is parallel to the dipole, it produces forces on charges that tend to make them move along the dipole from end to end producing current. When the electric field is perpendicular to the dipole, however, it tries to force the charges out through the walls of the dipole, which produces essentially no useful current. Rather than making three orthogonal measurements with a single dipole, three orthogonal dipoles are often used. Electronic circuitry may be used to obtain the square of each component and add the squares of the three components to obtain E2,the square of the amplitude of the total E.

2.4.2 Magnetic Field Measurements

Devices for measuring magnetic fields also consist of two basic components, the pickup or antenna and the detector. For magnetic field measurement, the antenna is usually some kind of loop, as shown in Figure 2.35. The loop is sensitive only to the component of magnetic field perpendicular to the plane of the loop, as indicated. A time-varying magnetic field produces a voltage in the loop that is proportional to the area of the Ioop and the frequency of the magnetic field. Thus, at low frequencies, the loop must be large to

62

/

2. BASICCONCEPTS

Fig. 235. h o p used as an antenna for measuring magnetic fields.

be sensitive to weak fields. As with the electric-field probe, there is the same trade-off between making the probe large to improve the sensitivity, and making it small to minimize the perturbation of the field being measured. Diode detectors and thermal sensors are also commonly used with magnetic field probes. As in the case of the electric-field probe, leads can also cause unwanted pickup in magnetic-field instruments. An additional problem with loop sensors is that they may be sensitive to the electric field as well as to the magnetic field, but shielding techniques can be used to minimize electric-field pickup. Some commercially available magnetic-field instruments are described elsewhere in this Report. 2.4.3 Specific Absorption Rate Measurements

Specific absorption rate (SAR) measurements are usually made only in research laboratories because they are relatively difficult to make and require specialized equipment and conditions (Polk and Postow, 1986). There are two basic techniques for measuring SAR. One is to measure the electric field inside the body with implantable electric field probes and then calculate the SAR from Equation 2.40. This requires a knowledge of the conductivity of the material. This technique is suitable only for measuring SAR at specific points in an experimental animal. Even in models using tissue-equivalent synthetic material, it is often not practical to measure the internal electric field at more than a few points.

2.5 FREQUENTLY USED RELATIONSHIPS

1

63

The second basic technique for measuring SAR is to measure the temperature change caused by the heat produced by the absorbed energy, and then to calculate the SAR from the result. Local SARs can be measured by inserting temperature probes in experimental animals or models and calculating the SAR from the temperature rise a t a given point. SARs can be calculated from temperature rise only if the temperature rise is linear with time. This means that the irradiating fields must be intense enough that the temperature rise is not influenced significantly by heat transfer within and out of the body. This is sometimes difficult to accomplish. SAR distribution patterns in models may be measured by thermographic camera techniques (NCRP, 1981). In this technique, the model is made in two pieces, which are to be joined together a t the interface on which the SAR pattern is to be measured. The model is joined together with a thin silk screen placed between the two pieces a t the interface. The model is then irradiated, the two pieces are separated and a thermographic camera used to immediately scan the interface and record the temperature distribution. The purpose of the silk screen is to allow easy separation of the two pieces after irradiation, but allow conduction current to flow across the interface during irradiation. Whole-body average SARs can be measured in small animals and in small models by measuring the total heat absorbed with wholebody calorimeters and calculating the SARs from that. Whole-body average SARs have also been measured in saline-filled models by shaking them after irradiation to distribute the heat and then measuring the average temperature rise of the saline solution.

2.5 Generalizations and Frequently Used Relationships Generalizations concerning EM fields that have been discussed in previous sections can be summarized as follows: 1. "Wetter" materials (muscle, high-water content tissues) are generally more lossy than "drier" materials (fat, bone), and hence absorb more energy from EM fields. 2. The SAR is higher when the incident electric field is more parallel to the body than when it is more perpendicular to the body. 3. The SAR is greater when the cross section of the body perpendicular to the incident magnetic field is larger than when it is smaller.

64

1

2, BASICCONCEPTS

4. Sharp comers, points and edges concentrate electric fields.

Conducting wires and plates cause minimum perturbation to electric fields when placed perpendicular to the fields, and maximum perturbation when placed parallel to the fields. 5. A uniform incident field does not generally produce a uniform internal field. 6. Depth of penetration decreases as conductivity increases, and depth of penetration decreases as frequency increases. 7. Objects small compared with a wavelength cause little perturbation andfor scattering of EM fields. 8. Below resonance, the SAR varies approximately as fL. 9. For E polarization, SAR increases faster than fL just below resonance. Just beyond resonance, SAR decreases approximately as f 1 and then levels off. Variation of SAR with frequency is greatest near resonance. 10. Near resonance and below, SAR is greatest for E polarization, least for H polarization, and intermediate for K polarization. 11. For E polarization, the SAR increases as an object becomes longer and thinner, and decreases as an object gets shorter and fatter. Table 2.3 provides a summary of some of the more frequently used relationships of EM fields.

0

u =

=

27rf

O E ~ E ~

= EE

=

-

P

pH

-

E"/E'

D

=

tan S =

B 8.85 x lo-'' F m-'

=

1IT

=

377 ohms in free space

= 457 x lo-' h m-I

uE2

e0 =

f h = vf

EH-'

TABLE2.3-Frequently used relationships. u is the conductivity in siemens per meter e0 is the permittivity of free space in farads per meter E" is the imaginary part of complex relative permittivity o is the radian frequency in radians per second f is the frequency in hertz tan 6 is the loss tangent E' is the real part of the complex relative permittivity P is the absorbed power a t a point in watts per cubic meter a is the conductivity in siemens per meter a t that point E is the root-mean squared electric-field strength in volts per meter D is the electric-flux density vector in coulombs per square meter E is permittivity in farads per meter E is the electric-field strength vector in volts per meter B is the magnetic-flux density vector in tesla p is the permeability in henries per meter H is the magnetic-field strength vector in amperes per meter e0 is the permittivity of free space wo k the permeability of free space f is the frequency in hertz T is the period in seconds h i s the wavelength in meters v is the magnitude of the velocity of propagation in meters per second f is the frequency in hertz

cont

@or lane waves) E H - ~is the wave impedance in ohms E is the magnitude of the electric-field strength in volts per meter H is the magnitude of the magnetic-field strength in amperes per meter

v = OLE)-% v = 3 x 10' meters per second in free space

=

E2/377

S = E x H

S

I

67.52 [{(E')~+ ( E ' ' ) ~ } -~ ~ f

s = E,,IEmin

6 =

v is the magnitude of the velocity of propagation in meters per second p is the permeability in henries per meter E is the oermittivitv in farads oer meter S is Poynting's vector in watts per square meter E is the electric-field strength vector in root-mean squared volts per meter H is the magnetic-field strength vector in root-mean squared amperes per meter

(For plane waves) S is the magnitude of the Poynting vector in free space E is the magnitude of the electric-field strength in root-mean squared volt meter 377 is the wave impedance of free space in ohms s is the standing-wave ratio (unitless) E , is the maximum value of the magnitude of the electric-field strength anywhere along the wave E ~ is, the minimum value of the magnitude of the electric-field strength anywhere along the wave s is the standing-wave ratio (unitless) p is the reflection coefficient (ratio of the reflected to the incident electric-f strength) S is the penetration depth in meters E' is the real part of the permittivity E" is the imaginary part of the permittivity f is the frequency in megahertz

=

2.75 x -%

los 2 I2 + !f ( I 2 + d 2 ) [ 4 1

SAR = V E ~ ~ ; '

fo

= gp2-5

1

F = $[lfz(t)dt]*

G

SAR is the specific absorption rate in watts per kilogram u is the conductivity in siemens per meter E is the root-mean squared electric-field strength in volts per meter p, is the mass density in kilograms per cubic meter f, is the resonant frequency in hertz of the specific absorption rate for E polarization 1 is the average length of the absorbing object in meters d is the average diameter of the absorbing object in meters

F is the root-mean squared value of the periodic function At) T is the period of the function gpis the peak value of a sine wave G is the root-mean squared value of the sine wave

3. Procedures for Evaluation of Exposure In previous sections, basic concepts relating to the nature and quantification of EM fields were discussed. This Section is concerned with the evaluation of exposure. Subsections include the general objectivesof exposure evaluation, protection guide criteria, data necessary for exposure evaluation and data analysis. 3.1 General Objectives

Measurements of exposure are usually undertaken to relate particular exposure situations to protection guides adopted by government or nongovernment organizations to assure the safety of workers or the general public. An example of such a protection guide is the incorporation of exposure criteria in NCRP Report No. 86,Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields (NCRP, 1986). In the planning of measurement procedures, the requirements of the particular guide must be understood so that the exposure measurements can be directed specifically to the need to accumulate data appropriate to that guide. Criteria are usually related to whole-body exposure. The presence of small areas where field strengths may be found of substantially greater magnitude than the spatial average (usually called "hot spots") are often of little significance relative to exposure except as they contribute to the average for the whole-body SAR.An exception to that general case can be found in the instance where an evaluation is being made of the exposureof an individual at a fixed work station. If the elevated level of field strength is concentrated in the vicinity of body areas that have been identified as of particular sensitivity, perhaps the eyes or gonads, that condition may not be disregarded on the basis of the whole-body SAR.

3.2 Protection Guide Criteria Protection guides promulgated by standard-setting organizations may be expressed in terms of a single value for the frequencies

3.2 PROTECTION GUIDE CRITERIA

1

69

involved, or they may be frequency dependent. Accordingly, the selection of equipment and methodology to be employed for evaluation of compliance must take into consideration the characteristics of the particular protection guide. 3.2.1

Single-Value Protection Guide

If the protection guide specifies a single field level with no frequency dependence over the pertinent range of frequencies, average levels and peak levels may be specified as a percentage of the specified criterion. For frequencies below 300 MHz, where plane-wave conditions may not exist because of either proximity to the radiating source or the nearby presence of reradiating objects, E and H data are to be referenced separately to the protection criterion. Planewave field-strength data may be expressed in terms of electric or magnetic fields or, if preferred, in terms of S. If the particular protection guide is expressed only in terms of S and near-field conditions exist, E or H measurements may be converted to equivalent far-field S. The conversion is accomplished by employment of the impedance of free space (120 7~ = 377 ohms). S in W m-2 is calculated by applying the formula S = E2 x 377-l, for the electric field and S = P x 377 for the magnetic field. (E is expressed in V m-l and H i n A m-'. Dividing S i n W m-2 by ten changes the unit to mW ~ m - Some ~ . protection guides use the approximate figure of 400 ohms for the impedance of free space rather than the more accurate 377 ohms. 3.2.2

Frequency-DependentProtection Guide

If the protection guide is frequency dependent, and the measured levels approach the most restrictive level, measurement of the individual frequency components of E and H is advisable. For each contributor, the ratio of measured level to the corresponding recommended maximum, as set forth in the protection guide, is required. The sum of those ratios is then the ratio of total exposure to the recommended maximum of the guide. Where near-field conditions exist for any exposure source, the highest ratio for that source, whether electric or magnetic, is to be applied. Examples 1and 2 below illustrates two situations where application of a frequency-dependent protection guide is used to determine compliance a t a site where AM, FM and VHF TV broadcast stations contribute to the total exposure.

70

3. PROCEDURES FOR EVALUATION OF EXPOSURE

Example 1.Consider AM and FM radio station co-located AM limit = 100 mW : actual field FM limit = 1 mW cm-2 : actual field

= =

5 mW .5% 0.8 mW cm-2 .80%

lbtal ,855 site in compliance

Example 2. Consider AM and FM radio station and VHF' TV station co-located AM limit = 100 mW ~ r n :- actual ~ field = 5 mW cm-2 ,5% FM limit = VHF TV limit =

1 mW cm-2 : actual field = 0.8 mW cm-2 .80% 1 mW cm-2 : actual field = 0.3 mW cm-2 ~ 3 0 %

.

'Ibtal 115%site NOT in compliance

3.3 Data Necessary for Exposure Evaluation

In order that meaningful conclusions can be drawn from measurement data, sufficient measurements must be made to provide an adequate sample with no significant component of the total field omitted. Depending on the particular purpose of the survey, measurements may be needed not only at a sufficient number of locations in a horizontal plane, but also at several heights above ground in order to encompass the total volume wherein exposure is to be evaluated. Furthermore, appropriate instrumentation must be selected so that the data accumulated meets the requirements of the protection guide.

3.3.1 Frequency Spectrum Coverage Prior knowledge of the frequencies being used in the area of interest is essential to assure that the instruments used measure the full range of frequencies with reasonable accuracy. Of almost equal importance is the need to know whether or not strong frequency components may exist beyond the useful range of the instrument(s) used, but which could cause incorrect readings. Some probes exhibit resonance effects above their useful range. The presence of energy at, or near, the frequency of resonance may result in high readings not reflecting actual field-strength levels. 3.3.2

Variability with Time

Within the area being studied, one or more sources of EM energy may be intermittent. Examples of such situations are radar sources,

3.3 DATA NECESSARY FOR EXPOSURE EVALUATION

1

71

which, because of their rotating antennas, are likely to provide intermittent exposure, as are industrial processes involving the intermittent application of EM energy. In such instances, data must be collected over a sufficient period of time to permit averaging. Usually, the averaging time is specified in the protection guide, e.g., 6 min. The collection of such data may require the use of recordings for analysis. Instrumentation is available to permit collection and storage of data a t fixed time intervals. The instruments provide automatic analysis of those data, together with the capability of later extraction, either in a step-by-stepprocess for viewing the data in sequence, or by transferring the data directly to a printer. Where EM emitter operation is intermittent, or even where intermittency is not expected, assurance must be obtained that all significant sources of EM energy are both in operation and a t normal power output. Other instruments provide for time-averaged fields directly. 3.3.3 Near- Versus Far-Field Conditions

Determination of the suitability of instruments requires knowledge of whether near-field or far-field (plane-wave)conditions exist. If near-field conditions prevail in all or parts of the area of interest, H as well as E data must be known for complete knowledge of the exposure. At frequencies above 300 MHz,the assumption may be made that only E measurements are necessary. If the instrument used has been calibrated only in terms of equivalent far-field S, information must be available as to whether the instrument is responding to the electric or magnetic field. Furthermore, caution must be exercised even when the calibration is given in terms of E. A line of instruments widely used for the measurement of field strengths in the frequency range of 540 to 5,000kHz provides calibration in terms of E, but the loop antenna employed is sensitive only to magnetic fields. In this case, H can be determined from the relationship between E, H, and wave impedance (see Section 3.2.1). In a plane-wave field, and not in the presence of reradiating objects, the use of either E or H data is appropriate when the wavelength is sufficiently short to ensure that measurements of either field adequately describe the potential exposure from the unmeasured field component. At long wavelengths an undetected standing wave may exist so that at any particular point, the measurement of one field component may not be indicative of the other. 3.3.4 Probe Characteristics For the instrument to satisfy the requirements of any protection guide, there must be knowledge of the nature of the probe. If the

72

1

3. PROCEDURES FOR EVALUATION OF EXPOSURE

probe performs essentially as an isotropic antenna, the assumption may be made that each reading represents the total field. If the probe is linearly polarized, field strength is required a t each measuring point in three mutually orthogonal planes, e.g., vertical, north/ south and eastlwest. The total field strength is the square root of the sum of the squares of the three orthogonal measurements (Bowman, 1974).

3.3.5 Time and Spatial Averaging In general, exposure standards specify a maximum recommended level averaged over a fixed time-typically 0.1 h (6 min). Time averaging may be appropriate from a thermal standpoint due to the dynamics of the body's thermal regulation characteristics. If the duty cycle of the source is known, e.g., the pulse duration, repetition rate and rotation rate of a radar or the onloff cycle period of a heat sealer, the average E, H or S over any period of time can be calculated from the peak value. To be certain t h a t t h e peak value of the appropriate parameter is being measured, the time constant of the meter must be known. If the time to reach peak meter reading is less than the pulse width (or "on" time), the true peak value will be measured. For pulse durations less than the period required to achieve full reading, consultation with the meter manufacturer may be necessary to learn the rise and decay times applicable to the meter in use. In the absence of knowledge as to the time characteristic of the signal being measured, continuous recording of the meter output may be necessary to determine a true average. Spatial averaging of the EM fields is important from two viewpoints: generally, the whole-body average exposure is desired and, except for fixed work stations, an approximate spatial average is desired to gauge the level of exposure as people move through the area of interest. To achieve a n understanding of average exposure, the survey area must be divided into sufficiently small segments to yield a valid sample and a cubic volume of approximately 1m on a side should be explored a t each measuring location. Another approach is to perform a planar scan of space, approximately 1to 2 m on a side (Tell, 1986). Since, in some contexts, peak values are also desired, both the peak and the average for the volume planar scan should be noted. In any survey area, limited sectors may exhibit substantially higher field strengths than generally prevail. In the context of accessible areas occupied only occasionally, such "hot

73

1

3.3 DATA NECESSARY FOR EXPOSURE EVALUATION

spots" have little significance. Far more important is the prevailing average since "exposure" exists only when the location is occupied. If, for example, an RF exposure standard specifies a maximum S of 1 mW cm-2 as averaged over any 6 min period, the allowable exposure level may be expressed as St = 6 mW min cmP2

(3.1)

where S is in mW cm-2 and t is in minutes. Use of this concept is illustrated by Figure 3.1, where the area under the curve represents the time-averaged exposure level expressed in units of mW min cm-'. Any combination of exposure S and time is permitted as long as Equation 3.1 is satisfied. Two different combinations are illustrated in period one and period two of Figure 3.1. Thus, for an exposure lasting 3 min, the permitted S level would be 2 mW rather than only 1mW cm-'. However, during the remaining time of the 6 min period, no exposure would be allowed. For shorter periods, higher exposure levels are permissible in accord with the expression given in Equation 3.1. More generally, RF exposures vary in time either because of source operational characteristics or movement of the individual through the field, resulting in changing exposure. This is also represented in Figure 3.1, where the area under the curve represents the timeaveraged exposure level. In the more representative case given in Figure 3.1, assessment of time-averaged RF exposure levels becomes rapidly more difficult. Hence, the determination of compliance with an applicable RFPG is also more difficult, particularly in complex exposure situations. Tell (1986) reported on the use of recently developed data-logging instrumentation to determine real-time averages of RF exposure levels. The instrumentation described provides continual recording of exposure levels registered on a broadband field-strength meter (FSM) and the Period 1

Perlod 2

Period 1

Period 2

6

5 W

E 4 9

z

5 2 1 ~

1

2

3

4

5

6

1

2

Time (mn)

3

4

5

6

0

1

2

3

4

5

6

1

2

3

4

5

Time (min)

Fig. 3.1. Illustration of American National Standards Institute (ANSI) time averaging of RF exposure.

6

74

1

3. PROCEDURES FOR EVALUATION OF EXPOSURE

determination of the running average of the measured value over a 6 min period. A powerful advantage of such a technique is that, by being able to observe the current 6 min average of exposure level, personnel exposure may be managed actively in such a way as to permit higher momentary exposure levels while remaining within the time-average provisions of the exposure standard. Such instrumentation also may be used to retain a historical record of exposures for permanent recording at a later time. This is a useful feature, particularly for awkward exposure situations such as for workers who climb communications and broadcast antenna towers. 3.3.6 Effects of Secondary Sources

When an object consisting of conducting material is immersed in an alternating EM field, currents are induced in that object. These currents produce EM fields of their own. Since no new energy is contributed by that conducting object and the entire driving force is the energy that can be extracted from the primary field, such objects are known as "secondary sources," "reradiators" or "parasitic radiators." In the very near proximity of such secondary sources, field strengths substantially in excess of those encountered in the general area may be found. However, as noted below, such high field strengths often have little significance from the viewpoint of human exposure. Field strength in the near vicinity of a secondary source contains a large reactive component representing the storage rather than the radiation of EM energy. That reactive component, not contributing to the transfer of EM energy from the field to the body, decreases much more rapidly than the usual linear inverse relationship between field strength and distance from the radiating source. Typically, such large reactive fields are found close to metallic fence posts, or linear metallic devices used for support or decorative purposes. Measurements within a few centimeters of such secondary sources are of little significance; furthermore, the close coupling of the measurement probe to a metallic object affects the accuracy of the instrument. In order to avoid distortion of survey results, measurements generally should be made no closer than 20 cm from secondary sources. Although exact quantification of the extent of the reactive field cannot be made in the context of the typical measurement survey situation, 20 cm is a reasonable choice since the field is known to decrease rapidly, and the presence of a human body in that region would significantly alter the local fields. A possible exception to the

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foregoing can exist where the conducting objects are not poles or isolated linear conductors, but instead are conducting surfaces so configured as to provide a focusing effect. This condition could occur at the right angle junction of two conducting surfaces. If doubt exists a simple test to define the situation can be made. Observe the field strength as the probe is moved from the close proximity of the secondary source to a distance of several meters. If the field strength drops rapidly, then tends to taper off, a reactive field is indicated and the 20 cm convention should be employed. If, on the contrary, the field remains high, or even increases with distance, a focus effect exists and measurements at less than 20 cm may be justified, if an individual could have access to that space. 3.3.7

Uncertainty Factor

As part of any report of exposure measurements, a factor of uncertainty should be determined. That factor includes meter accuracy, the variation of probe sensitivity with orientation, and the likely disturbance of the EM field produced by the presence of the operator in the vicinity of the measuring device. The measurement report should include the manufacturer's specified uncertainty in the probe calibration. Meter accuracy and probe sensitivity to orientation should have been provided by the meter manufacturer. If data are not available from previous work, uncertainty of measured field strength caused by the presence of the operator should be determined experimentally. The meter and probe should be fixed in position on a nonconducting support. The meter variations should be observed (through binoculars, if necessary) with the operator distant from, and in normal proximity to, the meter and probe.

3.4

Data Analysis and Exposure Evaluation

The methodology to be employed in the analysis is dependent on whether information is required over a limited area, such as at a particular work station in an industrial plant, or for a general area where people are likely to be moving about. 3.4.1 Limited Area Survey

Limited area surveys generally involve a work station in an industrial plant or office. Particularly i n the industrial context, the

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application of the EM field is likely to be intermittent, but the on/ off cycles are also likely to be of a relatively uniform nature. The time-weighted average for a 6 min period may be obtained by converting the 6 min period into 360 s. The time-weighted average would be equal to the sum of the product of the mean-squared (ms) field strength times the duration of each exposure during the entire 360 s interval divided by 360. For the normal position of a person a t a work station, data should be provided not only for an average over the volume of the person in either a standing or sitting position, whichever is appropriate, but also for the separate body parts; head, neck, torso, arms, legs and feet. The location and level of maximum exposure should be noted. 3.4.2 Area Survey

If the purpose of a survey is to determine the exposure in a large area where movement of people through the area is likely to be random, a sampling technique is appropriate. Over a plot of the area to be surveyed, lay a grid with line spacing of approximately 1 m for areas to 20 m2 in extent, 1 to 2 m for areas to 100 m2 (with no fewer than 25 grid intersections) and 2 to 3 m grid spacing for areas greater than 100 m2 in extent (with no fewer than 20 grid intersections for each 100m2of area). Measurements should be taken a t intervals as close to the grid intersections as feasible. In the evaluation, average the exposure data for the entire volume of interest, but also identify and quantify the measured field strength, or power density, for all locations where measured data are 50 percent greater than the average. Where exposures vary with time, average over the appropriate time specified in the RFPG, applying any weighting factors which may be appropriate.

4. Instruments and Measurement Techniques 4.1 Introduction

The EM fields associated with familiar RF and microwave devices can occasionally be characterized in sufficient detail analytically, e.g., antenna fields in the far-field region, but in most cases of interest, measurements must be relied upon. Unlike many physical quantities that can be measured with an uncertainty of a few tenths of a percent or less, the uncertainty associated with the measurement of EM radiation fields is approximately + 2 dB ( + 58 percent, - 37 percent) in terms of equivalent S.3This uncertainty results from instrument limitations and errors associatedwith field perturbations caused by field interactions with the instrument and the operator. Instruments that are commonly used for the measurement of EM fields are either broadband (respondover a wide range of frequencies) or narrowband (respondover a narrow range of frequencies). A third category of instruments is the so-called quasi-narrowband, that respond over a relatively wide-frequency range, but are equipped with filters to limit the response to a selected frequency range. A broadband instrument requires no tuning and is characterized by a frequency-independentsensitivity over a wide range of frequencies, e g . , up to several decades. A device of this type may indicate the root-mean squared (rms) or mean squared (ms)sum of the intensities of all the field polarization components at all frequencies within the usable frequency range of the instrument, but does not provide 3Decibel (dB) is a dimensionless quantity that is proportional to the logarithm of the ratio of two values of the same physical quantity. In particular: dB = 10 logP,/Po = 20 log V,/Vo = 20 log Z,lZ0 where P, V and Z are power, voltage and current, respectively. The term dBm means decibels referenced to 1 mW of power and dBW means decibels referenced to 1 W of power. That is: dBm

=

10 log Pl(1 mW), dBW

=

10 log PI1 W

Thus, 10 dBm equals 10 mW, -20 dBm equals 0.01 mW, etc.

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frequency information. What is measured depends on the antenna and detector used. Such instruments need not measure rms values and need not measure all three components. An example of this type of device is the broadband isotropic survey (hazard) meter. The term narrowband, meaning a response to a narrow range of frequencies, generally implies frequency selectivity, i.e., tuning is required, and refers to the instantaneous or impulse bandwidth of the instrument and not to its overall usable frequency range, which in some cases, may be tuned over a considerably larger range than that of a broadband instrument. An instrument of this type indicates the strength of each frequency component of an EM field individually, and hence provides both amplitude and frequency information. Examples of narrowband instruments are field-strength meters (FSM) and spectrum analyzers used in conjunction with calibrated antennas. The term quasi-narrowband refers to an instrument or system that requires no tuning, but has frequency-dependent response. These systems are generally used for the measurement of fields significantly smaller than the minimum discernible level (MDL)of a broadband instrument, but they will only provide accurate results for a single-frequency or multiple sources that are close in frequency. The system does not provide detailed frequency data. The usable frequency range of a quasi-narrowband system, without changing components, is typically a few octaves. An example of a quasinarrowband system is a power meter coupled with a calibrated antenna. All instruments used for the measurement of EM fields consist of essentially the same basic components; an antenna or sensor to sample the field, a detector to convert the time varying (at the frequency of the EM wave) output of the antenna to a proportional steady-state or slowly varying signal, and electronic processing circuitry and a readout device to display the measured field components in the appropriate units.

4.1.1 Broadband Survey Meters

The basic components of a broadband survey meter are a sensor (antenna), a detector, and the appropriate electronic circuitry and readout device. The antenna, or sensor, generally used with a broadband device is either a dipole, which responds to the electric field, or a loop, which responds to the magnetic field (Sections 2.4.1 and 2.4.2). Some instruments are available that consist of combinations of dipoles and loops in a single probe. In order to achieve a uniform

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response over a wide range of frequencies, the physical size of the dipole must be small compared with the wavelength a t the highest frequency to be measured (Bassen and Smith, 1983) or the dipole must be resistive (Aslan, 1972; Hopfer and Adler, 1980; Kanda and Driver, 1988). Similarly, in order to achieve a response that is linear over a wide range of frequencies, the physical size of a loop must be small compared with the wavelength at the highest frequency to be measured (Greene, 1975). Physically small elements also ensure minimum field perturbation by the antenna itself and allow fields with a small radius of curvature (those exhibiting rapid spatial variation) to be measured accurately, e g . , leakage fields from a narrow slot or crack. A physically small dipole responds to an electric field in a manner such that the open-circuit voltage V, that appears at the terminals is equal to the product of the effective length I, of the dipole and the tangential component of E [see Figure 4.l(a)l. The effective length of a short dipole (less than one-tenth of a wavelength) is equal to one-half of the physical length I,. Thus,

V,

=

112 I, cos 8

(4.1)

where 8 is the angle between the axis of the dipole and E. Thus the open-circuit voltage is at a maximum when E is parallel to the dipole, and is equal (in principle) to zero when the two are perpendicular. In order to measure E of unknown polarization (referring to the direction of E) with a single dipole, measurements must be made at each of three mutually orthogonal orientations and the results summed in a prescribed manner. Instead of having a single dipole that must be

(a)

(b)

Fig. 4.1. (a)An electricallyshort dipole antenna,(b)a small closed-loopantenna.

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4. INSTRUMENTS AND MEASUREMENT TECHNIQUES

rotated, modern broadband survey instruments have three mutually orthogonal dipoles arranged as a single unit (see Figure 4.2). When the output of the dipoles is summed appropriately, the response of the device is independent of orientation and is called isotropic. Isotropic is a term used for the conceptually useful, but physically unrealizable antenna that radiates or receives uniformly in all directions. A small closed loop [see Figure 4.l(b)] responds to a time varying magnetic field in a manner such that the open-circuit voltage V, induced in the loop is proportional to the product of the frequency, the area of the loop and the component of H that is perpendicular to the plane of the loop (Jordan and Balmain, 1950). (The fact that the voltage induced in a loop is proportional to frequency complicates the design of magnetic-field instruments and limits the use of a given loop to a narrower range of frequencies compared with that for dipole instruments.) As with an electric dipole, the size of a loop must be small compared with the wavelength at the highest frequency to be measured in order to minimize field perturbation and coupling with the electric field, as well a s to maintain the

Fig. 4.2. Three mutually orthogonal dipole elements for isotropic response to an arbitrary electric field.

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81

angular pattern of an elementary dipole. Most modern broadband magnetic-field survey instruments utilize an arrangement of three mutually orthogonal loops to achieve an isotropic response (see Figure 4.3). The open-circuit voltage of a dipole, or the voltage induced in a loop, is time varying at the frequency of the EM wave being measured. In order to provide useful information,the output of the dipoles or loops must be processed in a manner such that the value of the measured parameter can be displayed on a readout device. To accomplish this, the time varying output voltage of each dipole or loop is converted to a proportional steady-state, or approximately steady-state voltage or current. This conversion in most modern broadband survey instruments is accomplished with either diodes or thermocouples. A diode is a device having nonlinear characteristics that produces a direct conversion (called rectification or detection) of the time varying voltage (current) to a proportional steady-state voltage (current). Although point-contact diodes are frequently used at microwave frequencies, modern diode-based broadband survey instruments use metal-barrier or Schottky diodes. At low levels of W, the diode operates in the input power, typically less than square-law region, and the output voltage (current) is proportional

Fig. 46. Three mutually orthogonal elements for isotropic response to RF magnetic fields.

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to the square of the input voltage (current) and also to the square of E or H, i.e.,E2or P.At higher input levels, the response deviates from square-law operation and, in order to provide an output proportional to E2 or lP over a wide dynamic range, the output of the diodes must be processed electronically. In the usual arrangement, the diode is physically located at the terminals of the dipole or loop along with lumped or distributed circuit elements that provide filtering (Figure 4.4). Three such units are arranged mutually orthogonal at one end of a hand-held probe. The output of each diode is brought separately to the opposite end of the probe, or perhaps a preamplifier, via a pair of high-resistance leads. The purpose of the high-resistance leads is to minimize interaction with the field being measured, thereby reducing the effects of pick-up and field perturbation. The diodeldipole (loop) assembly is generally enclosed in an electrically transparent sphere, e.g., polystyrene foam, in order to minimize errors associated with environmental factors, i.e., light, rapid thermal fluctuations, etc., to provide

/

INSULATING SUBSTRATE

DIODE ELEMENTS

TRANSMISSION LINE Fig. 4.4. An electrical dipole element with a diode detector at the centerjunction connected to resistive transmission lines for conveying the detected signal to an electronics package.

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83

protection from mechanical damage and to provide some minimum space between the sensor elements and outside conductors. The advantages of diode detectors are their availability, low cost, high sensitivity and the ability to withstand relatively high field strengths compared with thermocouple detectors (Tell, 1983). The disadvantages are temperature sensitivity and the dependence of the response or transfer function on the input signal level, which limits the useful dynamic range. Both of these disadvantages can be reduced or eliminated with appropriate electronic circuitry. Disadvantages in some instruments are the errors introduced when the input consists of multiple frequency components of the same relatively high amplitude, such that the diode operates in the linear region area, and its sensitivity to modulation. When two such signals of identical level are present, a positive error will result which can be as high as 3 dB or greater for field strength squared, i.e., the instrument may indicate a field that is two times the actual value (Bowman, 1974). The same characteristic results in errors when the time average value of certain pulse modulated signals is measured. In each case the magnitude of this error should be evaluated in order to assess the actual uncertainty in measurement. In addition to diodes, thermocouples are frequently used to convert the time varying RF current or voltage to the required proportional steady-state current or voltage. A thermocouple is a device that consists of a junction formed between two dissimilar metals. When the junction is heated, for example heating associated with an induced RF current in the elements of the probe, a proportional steady-state voltage called the thermoelectric potential is developed across the junction. At 300 "C the rate of change of the thermoelectric potential with temperature for typical thermocouple junctions is of the order of tens of microvolts per degree celsius. Since the junction temperature change is proportional to the square of the induced junction current, the thermocouple is truly a "square-law" device. To minimize errors associated with ambient temperature changes, thermocouples are usually arranged in series-connected pairs that are located in close proximity to and in thermal equilibrium with one another, and with the environment. The energy associated with the induced RF current is dissipated in one junction, but not in the other, which is called the reference or cold junction. The potential difference between the hot and coldjunctions is proportional to their temperature difference, which in turn is proportional to the energy dissipated in the hot junction, thus to the square of the induced RF current. One class of thermocouple-based broadband electric field instruments consists of three lossy, linear orthogonal arrays (which

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function as both antenna and detector)formed from series-connected thin-film (antimony and bismuth) thermocouples (Aslan, 1972).The geometry is such that alternate hot junctions, with relatively small cross-sectional areas and high resistance, and cold jundions with relatively large cross-sectional areas and low resistance, are formed along the array. Because of the high junction resistance, most of the energy associated with the induced RF current is dissipated in the hot junctions, resulting in a proportional temperature rise. The steady-state output voltage of each array is proportional to the total energy dissipated in the array and, thus, to the square of the tangential component of the electric field. The sum of voltages from the three orthogonal arrays is thus proportional to the square of the total E. The advantage of a thermocouple-based instrument is a true "square-law response." This means that the device will respond correctly to simultaneous multiple frequency fields and will correctly indicate the time-averaged value of pulse modulated fields (Tell, 1983).The disadvantages are limited sensitivity and dynamic range, susceptibility to burnout (typicallyat levels greater than three times the full scale rating for CW fields, which may occur below full scale rating for pulsed fields of high-peak power), temperature sensitive zero-setting and sensitivity, thermal drift and slow response on the most sensitive range (NCRP, 1981). The configuration of most commercial broadband survey instruments, both diode and thermocouple types, is a relatively long, nonperturbing, hand-held probe containing the sensor (dipoles or loops) and detectors at one end and a handle at the other. The output of each detector is brought to a preamplifier or a connector in the handle via nonperturbing high-resistance leads. The probe itself is connected with a flexible cable to the electronics package containing the signal processing circuitry and an appropriate readout device. Many units provide a means for changing the instrument response time and means for examining the output of each dipole or loop either separately or as the sum oftwo or all three sensors. In addition, many instruments provide peak-hold circuitry, an audible output with the intensity or frequency proportional to the meter reading, overload alarms, recorder outputs and other useful features. A variation of the above configuration incorporates an active monopole or monopoles. In this device, one or three (for an isotropic response) monopoles are mounted on a small (approximately 10 cm on edge) electro&cs package containing the detedors (diodes),signalprocessing circuitry readout and batteries (Bassen and Smith, 1983). The entire unit is generally fixed to one end of a nonperturbing dielectric support, or handle, to minimize errors associated with

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85

probeloperator or sourceloperator mutual coupling. An option for this device, as well as for others mentioned above, is an optical fiber telemetry system that transmits information from the electronics package to a remote readout, thereby eliminating errors associated with mutual coupling between the operator and the source or instrument (Bassen et al., 1977). In addition to short dipoles and monopoles, a form of parallel plate capacitor, called a displacement-current sensor (Guy, 1987), can be used to measure electric fields normal to its surface or normal to any larger conducting surface. Recently developed instruments designed primarily for measuring fields associated with visual-display terminals (VDT), are based on the displacement-current sensor concept. The sensors of these devices are a double-sided circuit board about 30 cm indiameter. On one side, the front, an annular ring electrically isolates a smaller circular area. The smaller area, approximately 10 cm in diameter, is the active area; the larger annular ring serves as a guard ring to eliminate errors associated with fringing fields at the edge of the sensor. The smaller area is connected through an integrator (an operational amplifier with capacitive feedback)to the backplane of the sensor which serves as a reference ground. When a time varying electric field is normally incident on the front surface of the device, the induced surface charge produces a current which flows to the backplane through the integrator circuit. This current, called a displacement current because it is proportional to the time derivative of the electric displacement (electric flux density) (Section 2.2.5.4), is thus proportional to the time derivative of the incident electric field strength. Integration of the displacement current results in a voltage a t the output of the operational amplifier that is directly proportional to the incident E and the cross-sectional area of the smaller circular area. The output of the operational amplifier drives a true rms-voltmeter, located within a shielded electronics package. The electronicspackage, which contains a microprocessor, digital readout and an optical-fiber telemetry system for remote monitoring, is mounted to the edge of the sensor. Displacementccurrent sensors are typically used at frequencies in the LF and VLF regions (see Table 1.1), eg., from a few to a few hundred kHz, but can also find application in the ELF range. Depending on the fashion in which the displacement current sensor is designed, such as its physical symmetry, and used, as when being directly held by an individual, the sensor can perturb the local electric fields. With some instruments, the perturbation effects simulate the presence of the conductive body of a person sitting in front of a VDT. Finally, although not currently available as a commercial instrument, several broadband isotropic combination instruments have

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been developed with which both E and H can be measured simultaneously (Babij and Bassen, 1980; 1986). These devices consist of a set of three mutually orthogonal dipoles and a set of three mutually orthogonal loops collocated within the same relatively small space. The use of electrically small elements (dipoles and loops) minimizes mutual coupling between similar elements and between the dipoles and the loops and also minimizes the response of the loops to the electric field. When these instruments are available commercially, they will be particularly useful in those situations where the fields are rapidly changing with time (or space) and the value of both E and H is desirable at each measurement point. 4.1.1.1 Desirable Characteristics of Broadband Survey Instruments. In order to characterize, by measurement, a poorly defined complex EM environment, eg., a leakage situation, a broadband survey instrument must meet certain physical and electrical specifications (ANSI, 1981; Babij and Bassen, 1986; Kanda and Driver, 1988; Tell, 1982). These specifications are outlined in the following subsections. 4.1.1.1.1 Isotropic response. The response of the instrument should be constant, regardless of the orientation of the probe in a given field. (This characteristic does not apply to instruments designed specifically for microwave oven leakage measurements. The response of an oven survey instrument is equivalent to that of two orthogonal dipoles, i.e., it is insensitive to the direction of polarization in a plane and its response should be constant with rotation about the longitudinal axis of the probe when the polarization of the field is in the plane of the probe elements.) An isotropic response is especially important under leakage conditions or when the polarization of the field is not known. Rotating a single-axisprobe through three orthogonal orientations and summing the squares of the three corresponding field components, is very time consuming. The ellipticity, or deviation from a true isotropic response, must be made known to the operator. This information is generally specified by the instrument manufacturer, and for many commercially available devices is of the order of + 0.5 dB ( 12, - 11percent) for S or field strength squared.

+

4.1.1.1.2 Frequency response. The frequency response generally means the upper and lower frequencies between which the error of response of an instrument is within a specified limit. Ideally, the response should be flat (constant) between these limits and fall off rapidly with decreasing frequency at the low end and with increasing frequency a t the high end. Since this is rarely the case for available

4.1 INTRODUCTION

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87

instruments, the usable frequency range and the error of response over the usable frequency range must be known. Manufacturers generally provide this information based on a calibration a t specific frequencies over the usable frequency range. Since these instruments are commonly used for the measurement of fields associated with industrial heating equipment, these calibration frequencies should include, as a minimum, 13.56, 27.12 and 40.68 MHz. These frequencies are referred to a s industrial, scientific and medical (ISM) frequencies. A typical response of a thermocouple type electric field probe is: 1to 12 GHz, ? 0.75 dB; 0.85 to 16 GHz, +0.75dB7 - 1dB; 0.3 to 26 GHz, + 0.75dB, - 3 dB (Narda, 1985). The usable frequency range for the above probe is specified to be 0.3 to 26 GHz (Narda, 1985). A typical response for a diode type electric field probe is: 3 to 500 MHz, k 0.5 dB; 1 MHz to 1 GHz, k 1 dB; 0.5 MHz to 6 GHz, ? 2 dB (Holaday, 1985). The frequency response of a commercially available electric field sensor of the active monopole type is: 10 kHz to 100 MHz, + 0.25 dB (IFI, 1984). As indicated above, the response of a small loop is frequency dependent and, therefore, the usable frequency range of commercially available magnetic-field probes is relatively narrow when compared with that of an electric-field probe. For example, the frequency response of a typical HF, thermocoupletype magnetic field probe is: 13 to 200 MHz, + 0.5 dB; 10 to 300 MHz, 2 dB maximum total deviation (Narda, 1985). The frequency response of a similar LF probe is 0.3 to 10 MHz, 20.5 dB (Narda, 1985). Similarly, the frequency response of a HF magnetic field probe of the diode type is: 10 to 200 MHz, -+ 1dB; 5 to 300 MHz, -+2dB and the frequency response of a similar LF magnetic field probe is: 0.3 to 10 MHz, r 2 dB (Holaday, 1985). The reported frequency response of a displacement-current sensorlfrequency-compensated loop type of instrument is: 15 to 250 kHz, 0.5 dB; 10 to 300 kHz, + 0.5, - 2.0 dB (Holaday, 1987).It should be noted that the frequency response characteristics quoted by manufacturers may not be supported by sufficient test data. 4.1.1.1.3 Absolute accuracy. The absolute accuracy of the instrument for ideal uniform plane-wave fields a t the calibration frequencies should be known. Because of errors and uncertainties associated with the calibrating fields and response errors inherent in the instrument itself, e-g.,deviations from isotropy, nonuniform response over the dynamic range of the instrument, etc., achievable absolute accuracies a r e limited to approximately -+ 1.0 dB and can approach 2 2 dB. Uncertainties of up t o + 3 dB a r e associated with complex measurements, such as implanted probes or measurements in the near field. Although t h e operator normally has to rely on

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specificationsprovided by the instrument manufacturer, evaluations of commercially available instruments are frequently made by government agencies such as the EPA, NIOSH and FDA (Mantiply, 1984; Nesmith and Ruggera, 1982), and reports of the results are available. It is certainly worthwhile to obtain such information before selecting a particular instrument. Out-of-band response. As important as the knowledge about instrument response a t frequencies within the specified usable range, is information about regions of enhanced response, or resonances, at frequencies outside of the specified range. Many magneticfield and some electric-field instruments exhibit a significantly enhanced response at frequencies above the specified usable frequency range, while some electric field instruments exhibit this phenomenon at frequencies below the specified usable frequency range. A knowledge of the out-of-band characteristics is important when measuring mixed frequency fields or sources rich in harmonics, since frequency components may exist in these resonance regions, thereby causing significant errors. Unfortunately, manufacturers rarely provide the out-of-band performance of their instruments, but such information is reported in instrument evaluations such as those cited above.

4.1.1.1.4

Dynamic range. For general use, a desirable dynamic range for a single probe, is + 10 dB of the appropriate RFPG. Since the range of current RFPGs varies with frequency and country of origin, a universally useful instrument would require a dynamic range of about 70 dB in order to meet the above criterion. Rather than offeringa single instrument with a large dynamic range, most instrument manufacturers offer a selection of probes, i.e., probes for low levels and probes for the measurement of high levels, each with a dynamic range of at least 30 dB. Usually, an instrument can be selected to provide the desired dynamic range with respect to almost any RFPG. In addition to matching the dynamic range of an instrument to the appropriate RFPG, there are situations where the fields may be significantly greater than 10 dB above the RFPG and the dynamic range of the instrument must be compatible with the highest fields expected. For example, based on survey data, the desirable dynamic range of an electric-field instrument and a magnetic-field instrument used for the measurement of leakage fields associated with RF dielectric heaters is 1,800 V2 m-2 to 5 x lo6 V2 mW2and 0.1 to 100 A2 m-', respectively (Cox et al., 1982). Similarly, the desirable dynamic range for an electric-field instrument and a magnetic-field instrument used for the measurement of leakage fields 4.1.1.1.5

4.1

INTRODUCTION

associated with RF induction heaters is 1 x and 1to 1 x lo4 A2m-2, respectively.

lo3 to 1 x

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89

10' V2 rn-'

Meter output units. The meter scale should be calibrated in units of the quantity that is measured (see Section 2.2.10). An electric field probe responds to E or E2, and a magnetic field probe responds to H or H2 regardless of the field region in which measurements are made, i.e., far field (plane-wave conditions) or near field. Although many instruments are calibrated to display S, i.e., W m-2 or mW cm-', none actually measure this quantity. Instead, the calibration is derived from the relationship between S and E or H and the free-space plane-wave impedance of 377 ohms, i.e., EH-I = 377 ohms. If a device such as this is used to make measurements under near-field conditions and the wave impedance is not equal to 377 ohms, or is complex (reactive), the meter does not indicate the actual S. In these cases, equivalent plane-wave S , generally expressed in mW cm-', is indicated, i.e., S equivalent to E2/3,770or 37.7 H-', where E2 and H2 are the squares of the actual fields being measured. Since virtually all current RFPGs are expressed in terms of E2and I??, (or E and H),and since the actual quantities measured are P or E and I?? or H, it is desirable to have the readout calibrated in the same units.

4.1.1.1.6

Response to the pammeter being measured. It is important that the instrument respond only to the field parameter being measured. A loop responds to the magnetic field, but if the physical size of the loop is not sufficiently small compared with the wavelength at the frequency being measured, there may be a significant response to the electric field (Greene, 1975).Similarly, a short dipole responds to the electric field, but if sufficient care is not given to the design of the probe, loops may be formed by the various filter elements at the dipole terminals, and there may be a response to the magnetic field. Normally the extent of these spurious responses is not specified by instrument manufacturers, and evaluations such as those cited above should be referred to for this information. 4.1.1.1.7

4.1.1.1.8 Electromagnetic interference. The shielding and filtering of the instrument should be adequate to eliminate spurious responses associated with EM interference (EMI). Such interference can arise from direct penetration of the electronics package by the EM field being measured or from pickup by the cable connecting the probe to the electronics package. Many of these effects become apparent when relatively intense or pulsed fields with high-peak powers are being measured. An indication of this effect is a meter response accompanying a movement of the electronics package and the

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interconnecting cable while the probe is held fixed. Also, cable flexure effects, associated with the high input impedance circuitry used in many devices, frequently result in spurious responses manifested as a momentary deflection of the readout meter. The latter transient effect, which may lead to high artifactual readings when only peakhold circuitry is relied upon, can be especially bothersome when low levels are being measured outdoors during windy conditions. 4.1.1.1.9 Probe burnout alarm. There should be a means for the operator to verify that a probe being used is operating correctly and does not have one or more defective elements. The alarm may be in the form of a "probe test" switch or some other means, eg., the inability to zero the meter if the probe is defective. Similarly, a means should be provided to alert the operator to overload conditions. Generally an audible or visual signal is used to provide this function. 4.1.1.1.10 Probe overloadlburnout rating. The steady-state burnout rating should be, as an absolute minimum, a factor of two times the full-scale rating of the probe. Practical eledric-field and magneticfield instruments used for the measurement of leakage associated with RF dielectric heaters should continuously withstand fields of at least 1 x lo7 V2 m-2 and 500 A2 mp2,respectively, and at least 2 x lo7 V2 mp2and 1 x lo4 A2 m-2 for instruments used for the measurement of leakage associated with induction heaters. 4.1.1.1.11 Peak hold. A peak-hold circuit provides a useful function when measurements are made under conditions of changing field amplitude with time, either from the source changing output or from moving the probe through a nonuniform field. In addition to allowing the operator to concentrate on the location of the probe, the peak-hold circuitry generally responds significantly faster than the meter needle and consequently, the peak reading of a transient that may not produce a meter deflection, i-e.,transients of less than approximately 1 s in duration can often be obtained. The response time of the peak-hold circuit and that of the meter movement should be made known to the operator. It is desirable to be able to disable the peak-hold feature for many applications. 4.1.1.1.12 Static-charge sensitivity. The instrument should be insensitive to false readings associated with static charge on various dielectric instrument components, e.g., the meter face or protective covering over the probe elements. Generally, coating a component with a conductive film eliminates or reduces the problem. Without proper treatment, static-charge sensitivity can be significant when performing a survey in windy conditions or when the operator

4.1 INTRODUCTION

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91

possesses a static charge build-up on certain types of clothing, such as nylon. 4.1.1.1.13 Battery opemtion. The instrument should be operated with either replaceable or rechargeable batteries. A "press to test" switch or some other means should be provided for determining the state of charge of the batteries. The instrument should be capable of at least 8 h of operation within the rated accuracy without replacing or recharging the batteries. 4.1.1.1.14 Response time. The overall response time, defined as the time for the instrument to reach 90 percent of the value of a step input, should be less than 1s and should be made known to the operator. A fast response time is necessary for conducting rapid surveys over regions with complicated spatial variations in the amplitude of the fields or when intermittent fields or rapid time varying fields are to be measured.

Stability. The stability ofthe device should be such that it does not require frequent rezeroing. This can be a significant problem if instruments are used where large temperature variations may be encountered, e.g., outdoors or near heating equipment. Although temperature drift may seem like a minor point, the instrument must be zeroed with the probe isolated from the field by shielding with metallic foil, switching off all nearby sources (which may not be possible), or removing the instrument to a remote site. Each of these methods is time consuming and can create uncertainty in the measurement results. Some instruments are available that feature a full-time automatic-zero circuit that requires no zero adjustment by the operator.

4.1.1.1.15

Spatial resolution of the instrument. It is important that the instrument present a small electrical cross section to the field. This implies that the dimensions of the probe elements should be less than one-tenth of a wavelength at the highest frequency measured. This will ensure that the spatial distribution of standing wave fields can be adequately resolved. In addition, small elements (and high impedance lines from these elements) will reduce the probability that the probe itself will perturb the field and will assure that the measured field will be representative of the field in the absence of the probe. Finally, a small electrical cross section also ensures that the instrument not only responds correctly under planewave conditions, but also under conditions of high-field curvature, i.e., near-field leakage conditions. 4.1.1.1.16

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Multiple signal addition. The instrument should correctly measure the total mean squared (ms) value of a11 simultaneously occurring frequency components throughout the specified usable frequency range of the instrument. As indicated above, diodes operating in the square-law region and thermocouples will achieve this, but positive errors as large as 3 dB for field strength squared may occur when two componentsof the same amplitude, but different in frequency, are measured with a diode type instrument operating in the linear region. 4.1.1.1.17

Modulation response. The instrument should indicate the time averaged value of a modulated field, independent of the modulation characteristics. This goal is readily achieved with thermocouple type instruments, but significant errors may be introduced by diode based instruments operating in the linear region. 4.1.1.1.18

Readability. The instruments' readout should be sufficiently large to be easily read a t arms length. Experience has shown that analog type readouts are preferable to digital readouts for performing area surveys of fields that may vary by location andlor time.

4.1.1.1.19

Ease of adjustment and use. The instrument should have a minimum number of controls, each labeled as to its function. Complicated operating procedures should be eliminated so that the technician can make accurate measurements with only the information provided in the instruction book.

4.1.1.1.20

4.1.1.1.21 Portability. Broadband instruments are frequently used in physically awkward conditions, eg., while climbing a radio tower. The instrument should be portable and lightweight, with a carrying handle located such that the instrument can be carried in one hand for long periods of time without undue fatigue. A carrying sling and a means for clamping the probe to the instrument are desirable features.

Durability. The instrument should be sufficiently rugged to withstand the shock and vibration associated with shipping. A transit case should be provided. 4.1.1.1.22

Recorder output. It is useful to have a recorder output that provides a voltage proportional to the meter reading which can be used to drive a recorder, digital voltmeter or data logger (see Section 4.1.1.2). 4.1.1.1.23

Response to other environmental factors. The response of the instrument should not be affected by exposure to ionizing

4.1.1.1.24

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93

radiation, light or corona. If the accuracy of the instrument is affected by temperature, humidity or pressure, the degree of the effect and the range over which it occurs should be made available to the operator.

Peripheral Equipment for Broadband Survey Instruments. Most commercial broadband survey instruments feature an analog output suitable for driving external recording equipment, e.g., a strip-chart recorder. This feature is useful for recording field strengths a t a point, as a function of time. An extension of this, albeit not simply realized, would be to use a recorder for spatial averaging by calibrating the scan rate with the linear speed at which the operator progresses along a measurement path. Recently, a device called a data logger has become available that can be used in conjunction with a series ofbroadband instruments for both spatial and time averaging (Tell, 1986). The data logger is a small, portable microprocessor-based instrument that allows automatic recording of data during prescribed time periods. The data are retained in a memory for subsequent evaluation. Features of the device are a programmable measurement period, variable from 1s to 4 h; a record of the minimum, maximum and average values for each sample period are provided; the computation and continuous display of a sliding 6 min time-averaged value (6 min is the averaging time of many RFPGs) of the field level; and retention of the maximum sliding 6 min average. As indicated, time averaging is accomplished electronically. Spatial averaging can also be achieved by determining the time needed to perform a linear scan over a specified distance and programming the data-logger measurement period to correspond to that time interval. When traversing the area in the specified time period (note that this requires that the probe be moved at a constant speed), the average value output of the data logger thus corresponds to the spatial average of the fields along the traverse. The minimum level and the maximum level which is useful for locating field maxima are also provided. 4.1.1.2

4.1.2

Narrowband Systems

In contrast to the broadband type of instrument described above, a narrowband instrument has selectable bandwidths that range from a few Hz to several hundred kHz or more. The unit itself, however, may be operated over a range of frequencies extending from a few Hz to tens or even hundreds of GHz. Whereas the broadband survey

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instrument responds uniformly and simultaneously to multiple fields of any polarization and any frequency, within the usable frequency range of the instrument, and provides a reading equal (in principle) to the ms sum of all such inputs, the narrowband system must be tuned from frequency to frequency and the level of the field a t each frequency measured separately. In addition, three orthogonal measurements a t each frequency generally must be made if the polarization of the field being measured is unknown. The corresponding ms values of the results must be added to the ms values obtained at every other frequency in order to establish the ms value of the total field. That is:

where E,, Eyi and E,, are the x, y and z polarization components of the electric-field strength for all frequencies from i = 1to n. Although the effort is considerable,the use of a narrowband system provides frequency as well as amplitude information. This may be important when identifying a source, or when comparing an exposure level associated with multiple sources at widely separated frequencies, with a frequency-dependent exposure guide. As is the case of broadband devices, a narrowband system also consists of an antenna to sample the appropriate field component, an electronics package such as a spectrum analyzer or a FSM that contains a detector and a readout to display both amplitude and frequency information, and interconnecting cables. Antenna Types. The antennas used with narrowband systems are either linear or aperture antennas. Examples of linear antennas are loops, dipoles, monopoles (rod antennas, see Section 2.2.11), log-periodic, conical log-spiral, biconical and discone antennas. Examples of aperture antennas are pyramidal standardgain horns and parabolic reflectors, which are used almost exclusively a t microwave frequencies. The voltage appearing at the terminals of an impedance-matched antenna immersed in an EM field is proportional to E. An exception is the loop antenna, in which case the voltage is proportional to H. When the antenna polarization and the polarization of the field are aligned, the constant of proportionality is called the antenna factor A , which is expressed in units of V m-' V-' or, for the loop antenna, A m-' V-'. A , which is usually frequency dependent, is obtained by measurement or calculation and is provided by the manufacturer. When the polarization of the antenna and the field are not in alignment, the voltage a t the terminals of an impedance-matchedantenna 4.1.2.1

4.1 INTRODUCTlON

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95

is equal to the product of the Af and the tangential component of the electric field. For the case of a loop antenna, the voltage is equal to the product of the magnetic field component perpendicular to the plane of the loop and the Af. Thus, accurate measurements of E with an antenna such as a dipole, requires alignment of the antenna axis with the direction of E. In the case of directive antennas, such as log-periodic and aperture antennas, the response will also depend on the direction of propagation of the plane waves (the angle of incidence). For an arbitrary field where the polarization is not known, three separate measurements, one for each orthogonal orientation of the antenna must be made. The magnitude of the electric field is equal to the square root of the sum of the squares of the three individual components (see Figure 4.2). A, is the parameter that is usually associated with a linear antenna, while effective area A, is the parameter usually associated with an aperture antenna. (Effective area is also sometimes used with linear antennas.) The effectivearea, which at a given frequency is a function of the direction angles between the antenna axis and the propagation direction of the incident field, is defined as the ratio of the power at the terminals of an impedance-matched antenna to S of the incident field. That is, under free-space uniform plane-wave conditions, and with the polarization of the field and the antenna aligned, the power received PR that is delivered to a conjugately matched load is equal to the product of S and A,, i.e.,

PR = SA,

(4.3)

The directional gain G relative to that of an isotropic antenna, a dimensionless measured parameter, is usually supplied with an aperture antenna rather than the effective area. The gain G, which is a function of frequency and of the direction angles between the antenna axis and the direction of propagation, is related to A, by the following expression:

where A is the wavelength at the frequency of interest. In general, A, is about one-half the physical area for many antennas. In addition, G and Af are related by:

where Z, is the terminating impedance specified for the antenna, usually 50 ohms. When the gain or Af is known, the appropriate field

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4. INSTRUMENTS AND MEASUREMENT TECHNIQUES

parameter can be determined from a measurement of the voltage or power a t the terminals of the impedance-matched antenna. As indicated above, a number of different types of antennas exist that are suitable for the measurement of EM fields. The appropriate antenna type for a given situation is generally determined by the frequency range of interest, but is also influenced by such source related factors as location of the source or sources, whether the source is stationary or rotating, whether the expected field levels may be high or low and the proximity of large metallic surfaces at the measurement site. The antennas commonly used at lower frequencies (10 kHz to 30 or 40 MHz) are active or passive rod antennas (monopoles above a ground plane) and shielded loop antennas. Loop antennas respond to the magnetic field, but A, is frequently provided to convert the measured output to equivalent E. Implicit here is the assumption that measurements are made under uniform plane-wave conditions where the fields are orthogonal and simply related by the impedance of free space. Unless this is known to be the case, loop antennas should be used strictly for the measurement ofH. Rod antennas, both passive and active, are generally 1.1m in height, and are mounted on a ground plane. The active rod antenna contains electronic circuitry shielded from the field, which shapes the frequency response to provide a constant A, (usually unity) over the entire usable frequency band. The advantages of the active rod antenna are E can be read directly from the FSM or spectrum analyzer; and swept frequency measurements can be made readily. The disadvantage of the active rod antenna is that it can only be used for the measurement of relatively low-level fields. Electric field strengths greater than a few V m-I, correspondingto S of a few p W cm-2 will generally overdrive the electronics circuitry causing large errors and an output rich in harmonics. Under these conditions, not only will the indicated value of the field strength be in error (lower than the actual value), but artifactual responses will appear at harmonics of the source frequency. Another problem is the uncertainty produced by the inadequacy of the ground plane a t low frequencies. The passive rod antenna can be used to measure significantly higher field strengths than can its active counterpart. The passive rod antenna generally contains impedance matching circuitry comprised of passive linear components and, therefore, has a linear transfer function. A typical passive rod antenna may contain up to eight separate impedance matching networks, each providing an impedance match over a specific frequency band. The appropriate network for a given frequency band is selected by means of a switch located at the base of the antenna or, in some cases, is controlled

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97

automatically by the range switch of a FSM. Finally, in contrast to the uniform frequency response of the active rod antenna, At for the passive rod antenna is frequency dependent and may range from a value of several hundred to several thousand depending on the frequency. Rod antennas are used to measure the vertical component of an electric field and should be oriented in a vertical position. Since the response of the rod antenna is omnidirectional in the plane perpendicular to the antenna axis, the antenna cannot be rotated to determine the azimuthal direction of a source. In addition, care must be taken to place the antenna away from large metallic surfaces or objects and away from the FSM or spectrum analyzer in order to avoid errors associated with mutual coupling between these objects and the antenna. Errors may also be introduced when fields of high curvature are measured, such as those in close proximity to small leakage sources, or at the base of AM broadcast antennas. In such cases, the difference in phase of the field along a rod antenna produces an indicated level that is usually less than the actual value of E. Shielded loop antennas are available for the measurement of H at frequenciesbetween a few Hz and a few tens of MHz. The diameter of a typical loop antenna that covers the frequency band of 10 Hz to 50 kHz is approximately 0.6 m; that of a similar device that covers the frequency band of 10 kHz to 30 MHz is approximately 0.4 m. The response of the loop antenna, and hence Af, is a function of frequency. The response of a loop antenna is omnidirectional in the plane of the loop, and measurements must be made in each of three orthogonal antenna orientations to determine the resultant magnetic field strength. Finally, although the effect of mutual coupling between a shielded loop antenna and nearby metallic objects is less severe than for other antenna types, measurements should be made with the antenna no closer than one loop diameter from such objects. Half-wave dipole antennas may be used for measurements of E at frequencies up to about 1 GHz. At frequencies above 1 GHz, the errors associated with coupling between the small antenna elements and the highly conductive transmission line become a limiting factor. Because of the large physical length of the antenna at low frequencies, the practical LF limit of a resonant half-wave dipole is about 30 MHz for measurements made outdoors and approximately 100 MHz for measurements made indoors. The response of the halfwave dipole is omnidirectional in the plane perpendicular to the antenna axis, thus measurements must be made at each of three orthogonal antenna orientations to obtain the total field. In addition, the impedance of the antenna, and hence A , is affected by mutual coupling between the antenna and nearby conducting objects,

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including the ground. If the separation distance between the antenna and nearby conducting objects, including any componentof the transmission line that is parallel to the antenna, is greater than one freespace wavelength, coupling errors will be minimal. A disadvantage of the resonant half-wave dipole is that the length of the antenna must be adjusted at each frequency that a measurement is made. Although mechanical tuning of an antenna is not a problem when a single source or even a few sources are involved, it becomes time consuming when a large number of sources are involved. For these cases, broadband dipoles and biconical antennas, neither of which require tuning or adjustment, are available for the 20 to 200 MHz frequency band. In addition to ease of use, broadband (or fat) dipoles tend to be shorter in length at the lower frequencies than resonant half-wave dipoles, and should, therefore, be the choice for indoor measurements. The responses ofbroadbanddipoles and the biconical antennas are omnidirectionalin the plane perpendicular to the antenna axis, hence measurements at each of three orthogonal antenna orientations must be made in order to determine the total value of E of an arbitrary field. A practical problem associated with half-wave dipoles is the size of the antenna relative to areas in which it is desired to measure the strength of RF fields. Often it is not possible to map the spatial variation of fields in certain environments because of the large antenna size required. In addition to tuned dipoles, planar log-periodic antennas and conical log-spiral antennas are commercially available, and each covers the frequency range of approximately 100to 1,000MHz. These antennas are broadband in the sense that tuning is not required, but like the broadband dipole and the biconical antenna, the gain and A, vary with frequency. In addition, the response of the logperiodic and log-spiral antenna is unidirectional, unlike the omnidirectional (in a plane) response of the antennas discussed above. Since the gain is maximum in only one specific direction, i.e., along the antenna axis, and lower elsewhere, the antenna must be pointed in the direction of the source or incoming wave in order to make valid measurements. However, since the 3 dB beamwidth, i.e., the angle between the points at which the gain is one-half of the maximum value, of typical log-spiral and planar log-periodic antennas varies from about 50 to 130 degrees in the two orthogonal planes that contain the antenna axis, small misalignment errors will not result in large measurement errors. Significant errors may occur, however, when signals arrive from opposite directions, eg., in a reflection situation. Planar log-periodic antennas are linearly polarized and can be used to measure linearly polarized fields as well as elliptically

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99

polarized fields. In the latter case, two orthogonal measurements must be made, each with the polarization axis of the antenna in the plane of polarization of the incident wave. The response of conical log-spiral antennas is generally uniform in a plane perpendicular to the antenna axis and can be used to measure circularly polarized or elliptically polarized fields, as well as fields that are linearly polarized. In each case only one measurement is necessary. Whereas the direction of polarization of a linear polarized field can be determined with a planar log-periodic antenna, it cannot be determined with a conical log-spiral antenna. The response of the latter is similar to that of two orthogonal dipoles and is insensitive to the direction of the electric field in the plane of polarization of the antenna. At frequencies above 1 GHz, conical log-spiral and planar logperiodic antennas are also available, as are standard pyramidal gain horns and parabolic reflector antennas. Conical log-spiral antennas with a usable frequency range of 1to 10 or 12 GHz and planar logperiodic antennas with a usable frequency range of 1to 18 GHz are readily available. The physical sizes of these antennas are considerably smaller than their lower frequency counterparts,but the electrical characteristics are similar. Standard pyramidal gain horn antennas are relatively narrowband when compared with planar log-periodic or conical log-spiral antennas. The usable frequency bandwidth of a standard gain horn antenna is limited by the dominant mode bandwidth of the waveguide that terminates the horn and is usually less than one octave. Several antennas are needed to cover the fkquency range of 1to 18 GHz. Compared with the various broadband antennas discussed, the gain of a standard gain horn is high, typically 15 dB to 18 dB, and the 3 dB beamwidth is small, typically 25 to 35 degrees, depending on the frequency. Both of those parameters are functions of frequency. A variation of the standard gain horn is the ridged-guide antenna, characterized by a Iower gain and a larger usable bandwidth. Ridgedguide antennas with a usable bandwidth of 1to 18 GHz are commercially available. Both the standard gain horn and the ridged-guide antenna are linearly polarized, hence are similar to the planar logperiodic antenna in terms of polarization response. Broadband and narrowband parabolic reflector antennas are also available. A typical broadband parabolic antenna, with a usable frequency range of 2 to 18 GHz, consists of a 45 cm diameter circular parabolic reflector with a cavity-backed planar spiral-antenna located at the focal point. The gain and beamwidth at 2 GHz is 4 dB and 18 degrees and at 18 GHz is 30 dB and 2 degrees, respectively. The antenna is circularly polarized and, hence, is similar to the

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conical log-spiral antenna.4 Because of the relatively narrow bearnwidth of the parabolic reflector antenna at the upper end of the usable frequencyrange, measurement errors associated with misalignment with the source may be significant. In addition, the parabolic reflector antenna is large compared with standard gain horns, ridged-guide antennas, or log-spiral antennas and, therefore, along with its support structure, may considerably distort the field being measured. This type of antenna is generally used only under plane-wave conditions when a high gain is needed to measure very low levels. A problem associated with RF measurements, particularly in highly reflective environments, is spatial resolution of the fields. Small isotropic broadband electric- and magnetic-field survey meters are often effective in such applications. However, if determination of the frequency components of the composite field is necessary, eg., when assessing exposure in terms of a frequency-dependent RFPG, the use of broadband instruments requires turning off various sources so that measurement of the individual field components may be accomplished. Often such control of the RF emitting systems is not possible or desirable. Thus, there is a strong need for small antennas for such applications (EPA, 1982). This problem has been approached from at least two separate directions. Kanda et a1. (1979) describes the development of a resistively loaded passive dipole antenna system designed to exhibit a relatively flat frequency response over a large frequency span. This resistively loaded probe has been further developed by Kanda and Driver (1987).An orthogonal arrangement of three of these antennas has been used in environmental RF measurements (Tell, 1983).This antenna, while useful at frequencies above approximately 200 MHz, suffers from coaxial cable pickup and field perturbation at lower frequencies. Another approach to the problem was the development of small, active dipole antennas which are electro-optically isolated from their environment (Larsen et al., 1976). In this type of antenna, the electronics for converting the RF induced voltages on the antenna elements to an amplitude modulated optical signal are self-contained within the elements themselves. The RF current amplitude modulates a light source, which is then demodulated with a separate 4When using circularly polarized antennas, one must be certain that the gain provided corresponds to the polarization being measured. Although many of these antennas are said to be circularly polarized, the value of G or A, provided by the manufacturer usually corresponds to a linear polarized field. In some cases, the gain does correspond to circular polarization. For the latter case, the antenna can be used to measure linearly polarized fields if 3 dB is subtracted from the given gain.

4.1 INTRODUCTION

101

package of electronics at the other end of a long optical fiber. Ultimately the light signal is used to reconstruct the original RF spectrum of signals incident on the antenna elements. The reconstructed signals are then available for conventionalmeasurement techniques. This technique has been used for electromagnetic-pulseE measurements (Baum et al., 1978), and more recently has been extended to measurement of relatively weaker RF fields (EPA, 1985; 1987).The small size of the antenna allows high resolution of the spatial variation of the fields and, more importantly, removes a major problem associated with measurements of lower frequency fields where cable pick-up can lead to erroneous values of field strength. The small size provides a relatively flat frequency response over a reasonably wide range of frequencies. A recently developed fiber-optic, spherical dipole antenna has a flat response & 1dB over the frequency range of 10 kHz to about 800 MHz (EPA, 1987). With the exception of the active monopole, the resistively loaded passive dipole and the fiber-optic spherical dipole antennas, Af and the gain of each of the above antennas is frequency dependent.Therefore, the frequency of the source, for a single source situation, must be known in order to determine the correct value forA,for calculating the field strength. When the frequency of the source is not known, or sources are of multiple frequencies, a tunable device that can discriminate between frequencies, while measuring the power or voltage level a t each frequency, is required for measuring the antenna output. Instruments that perform this function are spectrum analyzers and field-strength (intensity) meters or receivers. 4.1.2.2 Spectrum Analyzers. A spectrum analyzer is an instrument that graphically displays voltage or power as a function of frequency on a cathode-ray tube (CRT). There are two basic types of spectrum analyzers; swept-tuned and real-time. As the name implies, a swept-tuned, or serial analyzer, is electrically swept (tuned) a t a selectable rate over a selectable frequency range sampling the frequency components within that range sequentially in time. Since the frequency band is sampled sequentially in time, periodic and random signals may be displayed on a swept-tuned spectrum analyzer, but not single transients. Real-time spectrum analyzers simultaneously display the amplitude of all signals within the frequency range of the analyzer, and hence are capable of displaying not only periodic and random signals, but many transient signals as well. There are two basic types of real-time spectrum analyzers: multichannel or bank-of-filters analyzers and Fourier transform analyzers or fast Fourier transform (FFT) analyzers. The multichannel

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4. INSTRUMENTS AND MEASUREMENT TECHNIQUES

analyzer consists of a set of narrow-bandpass filters, each tuned to a different frequency (see Figure 4.5). Ideally, the filters should have rectangular transfer functions with the center frequency of adjacent fdters separated by the filter bandwidth. An electronic switch sequentially samples the filters at a rate fast enough to display the instantaneous response of each filter to the input signal. The combined output of each filter displayed at the center frequency of that filter must be extremely narrow, and a large number of filters are required to cover a wide frequency range. This type of analyzer is essentially limited to frequencies of a few tens of kHz. In essence, all spectrum analyzers convert from the time domain to the frequency domain. The FFT analyzer uses an algorithm to perform this conversion. The FF"r analyzer operates as follows: the input signal is fed to a low pass filter, the analog output of which is sampled at a predetermined rate; the analog samples are digitized and these data "words" are stored in a digital memory at a predetermined location; the discrete Fourier transform of the digital

,,,

,INPUT SIGNAL

k

FILTER +DETECTOR 2 2 FILTER -DETECTOR 3 3

-

ELECTRONIC SCAN SWITCH

t

l

-

-

SCAN GENERATOR

-

C RT

I

fl

f2

A AMPLITUDE FREQUENCY RESOLUTION BANDWIDTH

bFREQUENCY RANGE 4 f,

b

FREQUENCY

f2

Fig. 4.5. A multichannel analyzer consisting of a set of narrow bandpass filters, each tuned to a different frequency.

4.1 INTRODUCTION

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103

data is computed in accordance with the FFT algorithm, and the real and imaginary coefficients of the frequency components are generated; the coefficients are squared and displayed as a function of frequency to yield the power spectra of the input signal. The analysis frequency range of an FFT analyzer, which is limited by the sampling rate, is of the order of 100s of kHz for modern instruments. For higher frequencies, swept-tuned spectrum analyzers must be used. There are two types of swept-tunedspectrum analyzers; sweptfilter or tuned radio frequency analyzers, and superheterodyne analyzers. The swept-filter spectrum analyzer (see Figure 4.6) consists of a tunable narrow-bandpass filter, which is swept across the frequency range of interest. The output of the filter is plotted against the frequency sweeping waveform to produce a display of amplitude versus frequency. The simplicity of the swept-filter analyzer is obtained via a trade-off in performance. For example, the bandwidth of a tunable filter is generally not constant, but is dependent on the center frequency to which it is tuned. Therefore, the resolution is frequency dependent and the frequency sweep range is limited, usually to less than a decade. In addition, swept-filter analyzers lack sensitivity and require extremely long sweep times when high resolution is required. In spite of these shortcomings, swept-filter spectrum analyzers are used, particularly at high frequencies. However, the most commonly used type of spectrum analyzer is the superheterodyne type. In the superheterodyne spectrum analyzer, the spectrum is essentially swept through a fixed bandpass filter (in contrast to a sweptfilter analyzer where the filter is swept through the spectrum). In a TUNABLE BANDPASS FILTER

lNPUT SIGNAL

Fig. 4.6. A swept-filter spectrum analyzer consisting of a tunable narrow-bandpass filter which is swept across the frequency range of interest.

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simple superheterodyne spectrum analyzer (see Figure 4.7), the input signal is fed to a low pass (or tunable wide-bandpass) filter. The output of the filter is fed to a nonlinear device called a mixer. The output of a voltage controlled oscillator, called the local oscillator, which is electronically swept in frequency by the same voltage ramp that drives the horizontal deflection plates of the CRT, is also fed to the mixer. The output of the mixer consists of the two signals (and their harmonics) plus their sum and difference frequencies. Following the mixer is a fixed frequency bandpass filter, called the IF amplifier, which is tuned to a frequency called the IF. Only when the frequency difference between the input signal and the local oscillator is equal to the intermediate frequency, is there an output from the IF amplifier. The output of the IF amplifier is fed to a detector, followed by a filter and then to the vertical deflection plates of a CRT. Thus, the display consists of a graphical representation of the amplitude of all input signals within the sweep range, as a function of frequency. Sophisticated modern superheterodyne spectrum analyzers provide a great deal of flexibility in use. For example, the bandwidth of the IF amplifier is generally adjustable in discrete steps from a few Hz to a few MHz, the sweep width and sweep time are adjustable, linear as well as logarithmic amplitude display modes are provided as is a means for the absolute measurement of power or voltage, and variable time constant video filters are provided to average internal noise and enhance sensitivity. Many modern instruments can be interfaced with, and controlled by, a computer in which the appropriate antenna factors are stored, thereby providing a means for displaying the field strengths directly, as well as providing a hard copy output of the scanned spectrum. The characteristics of typical instruments of the superheterodyne type are: usable frequency range, 20 Hz to 40 GHz with three RF section plug-in units and an external mixer for frequencies above 18 GHz; frequency response, better than t 2 dB for frequencies below INPUT

SIGNAL

VIDEO FILTER

LPF

(f*)

&' LOCAL OSCILLATOR

Fpfl GENERATOR

Fig. 4.7. Block diagram of superheterodyne spectrum analyzer.

-

4.1 INTRODUCTION

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14 GHz; sensitivity at 100 Hz bandwidth of - 125 dBm; resolution bandwidth switchable from 10Hz to 10kHz in a 1to 3 to 10 sequence for the low frequency (20Hz to 300 kHz) RF section and switchable from 100 Hz to 300 kHz in the same sequence for the microwave (10MHz to 40 GHz) RF section; and display range of 70 dB. The characteristics of a more sophisticated instrument are: usable frequency range, 100 Hz to 300 GHz with an external mixer for frequencies above 22 GHz; frequency response, better than +2 dB for frequencies below 20 GHz; sensitivity a t 10 Hz bandwidth of - 119 dBm a t 18 GHz; resolution bandwidth switchable from 10 Hz to 30 MHz in a 1 to 3 to 10 sequence; and display range 90 dB (Hewlett Packard, 1985). The spectrum analyzer and calibrated antenna are, under certain conditions, an ideal arrangement for rapidly characterizing the EM environment when both frequency and amplitude information are required. There are, however, inherent limitations and sources of error. For example, the spectrum analyzer is usually a laboratory instrument rather than a field instrument because it is fragile, large, heavy and requires line power (although recently several smaller, truly portable analyzers have been introduced). In addition, the shielding of most spectrum analyzers is inadequate when the unit is immersed in fields exceeding a few V m-' (corresponding to S of the order of 1 to 10 p W cm-') and significant measurement errors associated with RF interference (RFI) and other electromagnetic interference (EMI) can occur. Also, many instruments commonly used are subject to signal identification problems associated with image, multiple and spurious responses. An image response can occur when two input signals are present, one equal to the sum of and the other the difference between the local oscillator and the input frequency (IF). In this case, both signals will appear at the same position on the CRT even though the frequencies are different. Multiple responses occur when the input signal mixes with harmonics of the local oscillator. Since several harmonics of the local oscillator are used to obtain tuning over a wide range of frequencies, a single input signal can produce several responses, each appearing as a different frequency. This is often encountered when low-level fields a t microwave frequencies are measured with an antenna that also responds to frequencies in the broadcast band, e.g., a conical or planar log-periodic antenna. Under these conditions it is very common to see the spectrum of the entire FM broadcast band reproduced on the display even though the spectrum analyzer is tuned to a much higher frequency. Finally, spurious responses associated with nonlinear behavior of the spectrum analyzer may occur when one or more high-level signals are being measured. In an extreme case,

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not only will the input signals appear on the display, but also their sum, difference and harmonics. The above implies that under certain circumstances,the data displayed may be ambiguous. It is, therefore, important to be certain that the signal displayed is the signal being measured. Generally, a means for identifying the signal is provided, e.g., a switch that moves only the actual signal a specific number of divisions to the right or left on the display. Although this removes some of the ambiguity, it greatly prolongs the measurement time. A more positive approach is the use of a tracking preselector. A tracking preselector, which is generally incorporated into the design of the more sophisticated spectrum analyzers, is a tunable bandpass filter that automatically tracks the appropriate tuning curve of the spectrum analyzer. The use of a tracking preselector effectively eliminates the undesirable responses discussed above at the expense of added complexity, increased equipment size and weight (when an external preselector is required, as is the case for many of the commonly used instruments), loss in sensitivity and some degradation in the overall flatness of the frequency response. Another limitation of the spectrum analyzer is the lack of specialized detector functions. The spectrum analyzer is a peak detector, i.e., it responds to the instantaneous amplitude of the signal, not the time-averaged or rms value. However, for many types of signals, eg., CW, FM, pulsed RF, etc., the rms value can be determined from the peak value. In the case of a pulse amplitude modulated (AM) carrier, e.g., a radar pulse, the utility of the spectrum analyzer is clearly demonstrated because it can be used to determine the peak and rms value of the signal, as well as the pulse repetition frequency f, and pulse width. The modulation characteristics of many other types of signals can also be determined. When the modulation characteristics are complex or when the amplitude of the signal is varying randomly, some form of data processing is required to obtain the time-averaged or rms value. This may consist of photographic techniques if the CRT is of the variable persistence type, or digital signal processing if the spectrum analyzer is controlled by a microprocessor (Tell, 1982) (see Section 4.1.2.4). 4.1.2.3 Field-Strength Meter. A field-strength meter (FSM) or field-intensity meter, is a manually tuned or microprocessor controlled narrowband tunable voltmeter or receiver. In many respects the FSM is similar to a superheterodyne spectrum analyzer (see Figure 4.8). For example, both instruments utilize superheterodyne techniques to achieve a wide usable frequency range. Harmonic mixing is generally used in a spectrum analyzer, i.e., the input signal is mixed with the fundamental local oscillator frequency and its

I F INPUT

GENERATDR

Fig. 4.8. Block diagram of a manually tuned narrowband FSM.

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harmonics, while only fundamental mixing is used in a FSM. (A significantly greater usable frequency range can be achieved with harmonic mixing with a sacrifice of sensitivity in those ranges where it is used.) Consequently, several different FSMs may be required to cover the frequency range of a spectrum analyzer. The display of a FSM consists of one or more meters. One meter is generally calibrated in units of microvolts (pV), decibels above one microvolt (dB pV), and dBm. When corrected for the setting of the input attenuator, the displayed voltage is the input voltage across the input impedance (50 ohms) of the FSM. The other meter, when two meters are present, is calibrated in kHz, MHz or GHz. The indication on this meter is the frequency to which the FSM is tuned. An alternate method of displaying frequency is a linear scale, calibrated in kHz, MHz, etc., and a pointer mechanically coupled to the frequency tuning control. Unlike the spectrum analyzer, FSMs are manually tuned, but many also may be swept electronicallyat a slow rate. These frequency displays generally provide adequate resolution and accuracy. When a higher degree of accuracy is required, a frequency counter can be used to measure the local oscillator frequency. Subtracting the IF from the local oscillator frequency (which can be done automatically with modern counters) yields the signal frequency. The amplitude response of most spectrum analyzers is calibrated at a single frequency, eg., 30 MHz, by connecting the output of a built-in calibration generator to the input of the spectrum analyzer. If extreme amplitude accuracy is required at other frequencies, i.e., better than the overall flatness limitations of the frequency response of the instrument, some form of substitution measurement must be used a t the frequency of interest. In contrast, FSMs are calibrated with built-in tracking reference generators or with impulse generators that effectively produce a continuum of energy throughout the entire usable frequency band. Although the overall frequency response of a FSM may be no flatter than that of a spectrum analyzer, the response a t any given frequency over the entire usable frequency range of a FSM can readily be determined. The ease of achieving accurate amplitude measurements may be, for certain measurements, an advantage of the FSM. FSMs are designed to operate in relatively high fields (tens of V m-') and considerable effort goes into shielding the components within the unit. This provides a high shielding effectiveness (the ratio of the voltage indicated in a given field with an antenna connected to the input, to that indicated with the input terminated with a resistive load), typically 90 to 100 dB. It also adds to the unit's weight. In addition to shielding, many FSMs are battery operated

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to eliminate conducted interference associated with the power cord and to provide flexibility with respect to measurement location. A significant difference between a spectrum analyzer and a FSM is that the spectrum analyzer is a peak detector, whereas the FSM may contain several detector options. Average-value detection, peak detection, quasi-peak detection and a peak hold function with selectable holding times are often included in modern FSMs (Figure 4.8). True rms detection is also available, generally as a separate unit. In addition, AM and FM detectors connected to an internal loudspeaker or an audio output jack are provided in order to listen to the demodulated signal. This facilitates signal identification, especially those signals associated with telecommunications equipment. Similarities between the FSM and spectrum analyzer are; both instruments offer selectable bandwidths, FSMs contain a tracking preselector to eliminate spurious responses as do some spectrum analyzers, and both have high sensitivity and a wide dynamic range. The characteristics of a typical FSM are: usable frequency range, 30 to 1,000 MHz in eight bands (additional units are available that cover the frequency range of 20 Hz to 18 GHz); flatness of response, 2 dB typical; sensitivity, - 124dBm for frequencies below 100 MHz and - 117 dBm for frequencies below 1,000MHz; bandwidth, switchable from 0.01 to 1MHz in a 1to 10 to 100 sequence; display dynamic range, 60 dB; instrument dynamic range, 140 dB (Eaton, 1985).The characteristics of a similar FSM with a wider usable frequency range are: usable frequency range, 0.01 to 1,000 MHz in fifteen bands; flatness of response, 2 1dB typical; sensitivity, - 131 dBm for frequencies below 2.4 MHz, - 109 dBm for frequencies below 100 MHz and - 103 dBm for frequenciesbelow 1,000 MHz; bandwidth, switchable from 500 Hz to 1 MHz in a 5 to 10 to 50 sequence; display dynamic range, 60 dB; and instrument dynamic range, 160 dB. There are also a variety of microprocessor controlled FSMs available to provide automated data collection over the frequency range of 20 Hz to 18 GHz. There are also simpler special purpose units available that cover only a specific narrow range of frequencies, e.g., the AM broadcast band (535 to 1,605 kHz). As indicated above, there is a great deal of overlap in the characteristics of a FSM and those of a spectrum analyzer. Because of the similar characteristics, either one can be used for most narrowband measurement applications. In many cases, only the ease of measurement or the measurement time will dictate which instrument should be used. For example, since a spectrum analyzer can very rapidly scan and display simultaneously all signals within a broad frequency range, it is ideal for taking a "quick look" at the spectrum and for intermittent signal situations such as land mobile radio, particularly

*

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cellular radio where a large number of closely spaced channels are used. Such intermittent signals may be missed with a FSM because the unit was tuned to a slightly different frequency during the time of transmission. With a spectrum analyzer, the frequency band of interest is scanned in real time (compared with the time duration of typical intermittent transmissions) and all signals are displayed simultaneously, greatly reducing the time required for the measurement. In addition, many spectrum analyzers have a storage feature whereby all signals can be retained on the display until erased. In other cases, e.g., where the fields are of the order of several V m-l, andlor where line power is not available, the well-shielded, battery operated FSM is the clear choice. Compared with the broadband survey instruments described in Section 4.1.1, the limitations of narrowband systems are: the spectrum analyzer and many FSMs are laboratory type instruments (particularly the spectrum analyzer) rather than field instruments in terms of size and complexity; considerable expertise is required to make meaningful measurements; the time required to make a measurement is considerable if many sources operating at different frequencies are present or if measurements are made at many locations around a single source; the antennas used are generally linear, requiring three separate orthogonal measurements at each point; and the antennas are large resulting in errors associated with field perturbation, field curvature and coupling with nearby conductive objects. 4,1.2.4 Automated Measurement Systems. An automated measurement system is an assembly of equipment which permits collection of RF field-strength data with some form of digital control of the various elements of the system. Several approachesto automated measurements have been developed. Most automated systems have been applied to the measurement of band occupancy, i.e., the determination of the presence of radio signals at specified frequencies(Hagn et al., 1971; Matheson, 1977; McMahon, 1973). Such studies are primarily designed to aid in efficient spectrum management. Tell et al. (1976a) described an automated system used for collecting environmental field-strength data through the use of a computerautomated spectrum analyzer and associated antenna systems. Figure 4.9 provides a block diagram of a typical automated measurement system. Such a system consists of a remote, digitally controlled FSM or spectrum analyzer, appropriately calibrated receiving antennas, antenna switching hardware and a digital computer to effect the actual control. Some form of mass storage device, such as a magnetic tape drive or rotating disk memory system, is usually

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MUTUALLY ORTHOGONAL DIPOLES

RF SWITCH ASSEMBLY

4 ANALOG SWITCHING VOLTAGE

HIGH PASS FILTER

I -

ANALOG AMPLITUDE DATA SPECTRUM ANALYZER

4 -

t

) DATA ACQUISITION

SCAN TRIGGER SIGNAL

PROCESSED FIELD INTENSITY DATA

Fig. 4.9. Block diagramof a typical automatedRF measurementsystem consisting of calibrated receiving antennas and a computer controlled spectrum analyzer.

used to store applications software and the data resulting from the measurement process. Figure 4.9 shows the use of an orthogonal antenna system, individual elements of which are automatically selected for connection to the input of the spectrum analyzer. Several special features of automated field-strength measurement systems are worthy of discussion. Since antenna systems composed of several individual elements, as in the case of three mutually orthogonal dipoles, may be used in routine measurements, any one element will provide field-strength data associated with only the element's particular polarization direction. In general, it is necessary to perform the vector addition of these field components; such addition is readily accomplished in the computer controller used in this system. The resultant E is obtained by squaring each of the individual field-strength components, summing and extracting the square root of this sum. Note in the case of a spectrum analyzer, the output signal can normally be obtained directly in units of power, which is

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proportional to the square of the input voltage from the antenna and thus the square of the field strength. Thus, it is not necessary to square each datum obtained from the analyzer, resulting in a reduction in processing time. Another valuable feature of computer based systems is the ability to perform signal averaging. Thus, even in the case of signals whose amplitudes vary in time, it is easy to perform continual computations on the incoming signal values to obtain the time average of the amplitude. In a similar fashion the computer may be programmed to check each signal value to determine whether it is greater or less than the previous sample value. Consequently, peak retention software routines may be implemented which permit the maximum value of signals at each frequency to be retained in memory and used for further analysis after some suitable observation period. Integration of measured amplitude spectra may be used to find the resultant exposure intensity of a complete band of individual signals. Figure 4.10 illustrates a typical measured spectrum of signals obtained in New York City (Tell, 1983) using an automated measurement system. Note that the ordinate, which corresponds to electric field strength, is labeled in units of dB f l m-'. These values are the time averaged amplitudes of the signals obtained over a sample period of several minutes. Although E of the individual FM radio stations is given directly, an integration of this spectrum by the computer, in effect, converts each measured field-strength value to the equivalent plane-wave S, sums S of all signals, and prints the value of S for that particular band in units of p W cm-'. Probably the single most powerful advantage of automated RF measurement systems is the ability to store A, data in the computer memory. This allows automatic correction of the incoming data from the spectrum analyzer to directly yield field strength. It is useful to store A, as a function of frequency. This allows computation of an appropriate Af over any selected part of the spectrum for which the system is employed. In fact, mathmatical expressions for many different antennas may be stored and accessed by appropriate input from the operator. Upon completion of a spectral measurement, the processed data are typically written onto a tape or flexible disk file for subsequent computerized access andlor re-evaluation. It is clear that automated systems permit a rapid and accurate indication of the field intensities of large numbers of signals during environmental evaluations. Figure 4.11 is a photograph of an automated system installed in a four-wheel drive utility vehicle. This system is designed for environmental measurement and analysis over a wide frequency range. Shown in the photo, mounted on top of the vehicle, is a

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RES BW= 30 kHz POLARIZATIONS - 3 Average

Frequency (MHz) Fig. 4.10. Graphical output of the measured spectrum of FM broadcast signals obtained with an automated measurement system in New York City. The vertical axis represents the resultant E in units of dB pV m-'. These values of field strength are averaged over a short observation period by the computer. The horizontal line represents a threshold value for integrating S equivalents of all of the signals; signals above the threshold are included in the summation process, the result of which is printed at the side of the graph.

steerable dish antenna for measurements in the 1to 12 GHz band. This antenna features a selection of individual horizontal and vertical polarization components. A small gasoline powered generator or dc to ac inverters are used to provide ac power for the instrumentation. There are several types of signal processing which are of particular value in the measurement of radar signals. Some of these processing techniques lend themselves to the application of computer automated RF measurement systems. Radar sources represent a considerable problem in environmental measurements; this problem is principally related to: (1)the scanning or moving beam which typically illuminates a given measurement area for only a fraction of a second, and

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Fig. 4.11. An automated RF measurement system installed in a four-wheel drive utility vehicle. The system is designed for environmental measurement and analysis over a large frequency range. Thetop photograph shows a steerable parabolic dish antenna which features selection of individual horizontal and vertical polarization components. The bottom photograph shows the layout of equipment inside the vehicle.

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(2) the task of determining the average field strength of the pulsed emission. Since the spectrum analyzer, when used with a sufficiently wide IF bandwidth, will properly respond to the instantaneous peak signal level of the radar, the average signal level must usually be determined by measuring the duty cycle of the radar emission. Duty cycle 6 is given by Equation 4.6:

where t is the pulse width and fp is the pulse-repetition frequency (Hz). From a knowledge of the duty cycle and the peak-incident value of S (S,), the average S (SAV)is given by Equation 4.7: This means both f, and the pulse width t must be measured and this is commonly accomplished by observing the radar signal in the time domain. Tell and Nelson (1974)reported on measurements of both f, and t of air traffic control radars using a manually operated spectrum analyzer. Computer enhanced spectrum analysis techniques permit more rapid and accurate assessments of these parameters and, consequently, the duty cycle which is used to obtain the average field value from the peak field value. One area deserving special mention is the long-term averaged field strength of a rotating radar due to the movement of the radar beam over the area of interest. It is desirable to measure the apparent rotational duty cycle of the radar antenna, and this is a relatively simple matter for an automated measurement system. In essence, the technique consists of monitoring, in time, the variation of the peak field strength over one or more complete rotations of the antenna. The computer is used to find the average of the fieldstrength values and to relate this average value to the one measured as the main radar beam passes by the measurement site. The antenna rotational duty cycle is used to correct the measured field strengths and arrive at a true average value, averaged over the transmitter duty cycle and also the rotational duty cycle of the antenna. An example measurement of the peak signal arriving from a rotating radar is illustrated in Figure 4.12. Here, one complete antenna rotation is apparent from the presence of two main beam peaks of the signal observed on the left and the right of the display. The distance between the two peaks corresponds to 360 degrees of antenna rotation. The ultimate accuracy of any field-strength measurement system depends on the accuracy of the antenna factors used in the measurement process. When employing antennas mounted on vehicles, it is preferable to calibrate the receiving antennas in place as they will

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RES BW = 3 MHz Radar Measurements

Time (seconds) Fig. 4.12. An example measurement of the antenna rotational duty cycle of a common air-traffic control radar using an automated measurement system. The two tall peaks represent main beam alignment with the receiving location from one full rotation.

be used and thus fully characterize any influence of the vehicle on the antenna patterns. Tell et al. (1976a) describes a technique for measuring the radiation pattern of several antennas, including orthogonal dipole arrays, used with the measurement van shown in Figure 4.13. For this case, the entire vehicle was rotated on a large turntable and pattern variations were measured using the equipment in the vehicle. From an analysis of these data it was concluded that a worst case error of 2.5 dB was associated with the measurement system. This uncertainty is primarily related to the absolute

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Fig. 4.13. A large mobile EM field measurement van illustrating an orthogonal, calibrated measurement antenna system for absolute field-strength measurements in the VHF range.

accuracy of the A, and pattern variation of individual antenna elements. 4.1.3 Quasi-Narrowband Systems

Quasi-narrowband in this Report refers to a system that responds over a limited band of frequencies without tuning, is frequency dependent, and provides no frequency information. Such instruments can be used over a wide range of frequencies. Because the system is frequency dependent and does not provide frequency information, the frequency of the source must be known in order to make a meaningful measurement. An example of a quasi-narrowband system is a calibrated antenna coupled to a broadband power meter in which the frequency response characteristics of the antenna defines the useable band of frequencies. Although such a system can be assembled to operate a t almost any frequency, it is used most often at microwave frequencies. A variety of power meters, both analog and digital, are available commercially, including battery operated units. A power meter is similar to a broadband survey instrument without its antenna elements, i.e., it consists of a broadband detector coupled by cable to

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an electronicspackage. The detectors are enclosed in a small (approximately 3 to 5 cm in cross section, 10 to 15 cm long) metal package with a coaxial connector or a waveguide transducer at one end and a multiwire cable at the other. The detectors themselves are usually metal-barrier (Schottky) diodes, thermocouples or thermistors. A thermistor is a semiconductor device that has a large negative temperature coefficient of resistance. Induced RF currents result in a change of resistance which can be measured with a bridge circuit. The large temperature coefficient leads to high sensitivity, but may also lead to instabilities associated with fluctuations in ambient temperature (NCRP, 1981). For this reason, thermistor, as well as diode and thermocouple type power meters, must be temperature compensated. The metal-barrier diodes used in modern sensors/detectors are operated in the square law region over their full dynamic range. Hence, the output is a measure of true or rms power regardless of the input waveform. The characteristics of a typical diode type sensor are: usable frequency range of 10 MHz to 18 GHz, sensitivity of - 70 dBm, power range of - 70 to - 20 dBm, dynamic range of 50 dB, drift (after 24 h warm up) of less than 20 pW, and response time of 0.1 to 10 s depending on the range to which the power meter is set. The characteristics of a typical thermocouple type sensor are: usable frequency range of 50 MHz to 26.5 GHz, sensitivity of -30 dBm, power range of -30 to +20 dBm, dynamic range of 50 dB, drift (after 24 h warm up) of 10 nW, and response time of 0.1 to 10 s depending on range. The characteristics of a typical thermistor type sensor are: usable frequency range of 10 MHz to 18 GHz, sensitivity of - 30 dBm, power range of -30 to + 10 dBm, dynamic range of 40 dB, and response time of 0.035 s (Hewlett Packard, 1985). Cables of varying length (up to approximately 60 m) are available for connecting the detector mount to the electronics package. The electronics package contains the readout, generally calibrated in units of nW, pW, mW and dBm, and may be digital or analog. Many of the newer instruments are autoranging (no range switch is needed) and display the proper units, i.e., pW, nW, etc., eliminating one possible source of error. In addition, many units have an internal calibrator, i.e., a fixed frequency oscillator that provides a reference amplitude (usually 1mW), that can be used to calibrate the detector head with any length cable in place. The power meter can be used to accurately measure the power at the terminals of a calibrated antenna. If A, or the effective aperture area of the antenna is known, the value of the field can be determined. This implies that the quasi-narrowband system can be used only for single frequency sources or multiple sources operating at frequencies

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within a relatively narrow frequency range, i.e., a frequency range where At remains relatively constant. Examples of such situations are terrestrial microwave radio operating in a single frequency band and SATCOM earth stations. In these situations, levels well below existing RFPGs can be measured easily. When the above conditions apply, the advantage of the quasi-narrowband system over a narrowband system, for measurements of low-level microwave fields are: speed of measurement, simplicity, portability, and in many cases higher sensitivity and stability. With respect to the latter, the detector head of the power meter may be coupled directly to the antenna, eliminating the need for a coaxial cable, in which the signal attenuation can be appreciable (of the order of 1 dB m-'1. Quasi-narrowband instruments are especially useful for the measurement of low-level broadband signals, i.e., where the bandwidth of the signal exceeds the resolution bandwidth of typical narrowband systems. When a narrowband system is used for this situation, the apparent level indicated on the display is lower than the actual signal level by the ratio of the instrument bandwidth to the signal bandwidth. This is because spectral density (W per MHz), not total power, is the quantity that is being measured. In this case, the total power of the signal is equal to the integral of the spectral density over the bandwidth of the signal, or approximately equal to the measured level times the ratio of the signal to instrument bandwidth. In many practical situations, the apparent reduction in signal amplitude is sufficient to cause the signal to be lost in the noise of the system. When this happens, increasing the bandwidth of the instrument will not help since the input noise also increases and the signal to noise ratio remains about the same. To illustrate the advantage of the quasi-narrowband system for the measurement of broadband signals, consider a signal that is distributed uniformly over a frequency band of 30 MHz, e.g., a signal associated with phase-shift-keyeddigital microwave radio. This type of signal has the appearance of white noise when displayed on a spectrum analyzer. If a spectrum analyzer with a 0.03 MHz bandwidth is used to measure this signal, the indicated power level, which actually corresponds to the spectral density of the signal, will be approximately 30 dB below the total power of the signal. If the bandwidth of the spectrum analyzer is changed to 0.3 MHz, the signal level will increase by 10 dB and the difference will be 20 dB, but the input noise will be increased by 10 dB and the signal to noise ratio will be about the same. In many cases the signal levels are sufficiently low to preclude measurements with a narrowband system unless low-noise amplifiers are used. When a power meter is used, however, total power is measured directly since the band-

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width of the power meter exceeds that of the signal. This, coupled with the fact that there is no cable loss C, since the detector mount is usually connected directly to the antenna, yields a significantly higher sensitivity for the quasi-narrowband system compared with that of a narrowband system. (Seethe point-to-point microwave radio sample problem in Appendix B.2 for further information.) In many cases, the dynamic range of narrowband and quasinarrowband systems can be increased with low-noise amplifiers for small signals or attenuators for large signals. Low-noise amplifiers are especially useful to make up the C1when narrowband instruments are used to measure the spectral density of broadband signals. Care should be taken when using low-noise amplifiers to ensure that the input signal is not overdriving the amplifier. This can usually be checked with a calibrated attenuator before the amplifier. If the system is linear, a 3 or 10 dB increase in attenuation should result in the same 3 or 10 dB decrease in the indicated signal level on the display. Calibrated attenuators can be used to extend the upper limit of the useful dynamic range of a narrowband system, when the spectrum analyzer or FSM is shielded from the field and the coaxial cable connecting the antenna to the receiver has a sufficient shielding effectiveness to preclude pickup. If this is not the case, large errors associated with RFI may be introduced into the measurement. Whenever relatively strong fields, i-e., above a few V m-', are measured with a spectrum analyzer or FSM,it is good practice occasionally to replace the antenna with a 50 ohm termination resistor and observe the display in order to assess the influence of RFI and EM1 on the system. 4 6 General Measurement Techniques and Pitfalls 4.2.1

Characterizing the Source

The correct choice of an instrument for a particular situation is determined primarily by the following: The source frequency or frequencies Whether or not the frequencies are known a priori If multiple sources are involved, whether or not the frequency distribution of the exposure levels must be known, eg., in order to compare with a frequency-dependent RFPG Whether the measurements are to be made in the near field, in close proximity to a leakage source or under plane-wave conditions, eg., an antenna field

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Whether it is sufficient to ascertain that an exposure level does not exceed a particular RFPG, or whether it is necessary to determine the magnitude of the fields even if the levels are well below the RFPG The anticipated level of the fields; and for the case of pulsed fields, the ratio of the peak to average values Therefore, in order to choose the appropriate instrument, the source (or sources) must first be characterized in as much detail as possible. For the general case of leakage fields associated with industrial heating, cooking, drying equipment, etc., an attempt should be made to obtain the following information: the frequency and maximum output power of the source in question as well as for any nearby sources;the power at which each device normally operates; the operating duty cycle, i.e., the ratio of the length of time that the device operates to perform its intended function (on-time) to the total on plus off-time during each operating cycle; and the normal position of the operator relative to the source as well as the locations where people are likely to spend considerable time, i.e., the points where measurements are to be made; and the location of nearby reflecting surfaces that could produce regions of field intensification. For the case of leakage fields (not antenna fields) associated with telecommunications equipment and radar, e.g., exposure levels inside of transmitter rooms, the following information should be obtained: the output power and frequency of all transmitters in the area of concern; the type of modulation; for pulsed systems such as radar, f, and the pulse width, i.e., the duty cycle; the locations where people may spend considerable time and locations occupied by personnel servicing or maintaining the equipment; and the location of nearby reflecting surfaces that could produce regions of field intensification. For the case of antenna fields associated with the same equipment, the above information plus the following antenna characteristics should be obtained: antenna gain width of the beam polarization of the beam scanning rate (if applicable) antenna azimuth and elevation or the direction of maximum gain For most leakage situations the frequency of the source and nearby sources can be determined readily. In many cases the frequency as well as the maximum output power will appear on the

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manufacturer's label posted somewhere on the equipment. This is especially true of telecommunications equipment and industrial heating equipment certified as ISM equipment. In the case of the latter, Part 18 of the Federal Communications Commission (FCC) rules and regulations requires that the owner of such equipment retain a certification manual, supplied by the manufacturer, which contains all pertinent operating parameters such as the maximum output power, operating frequency or range of frequencies. Many industrial heating devices, such as heat sealers, RF cooking and drying equipment operate at a specific ISM frequency, e.g., 13.56, 27.12, 40.68, 915 or 2,450 MHz. Induction heating equipment, in many cases, may operate anywhere within a relatively wide range of frequencies (typically one or two octaves). The exact operating frequency depends on the size of the induction coil and the properties of the material being heated. The actual operating frequency of these devices is rarely known and must generally be measured. A spectrum analyzer, FSM, or a frequency counter and an appropriate antenna can be used to measure the frequency. Less sophisticated instruments, e.g., grid dip meters, can also be used to measure the operating frequency. In addition to the frequency, an attempt should be made to obtain the harmonic content of the emission from the manufacturer's data. Serious measurement errors may be introduced if a significant amount of energy associated with harmonics lies outside of the usable frequency range of the survey instrument. The error may be positive or negative depending on the instrument's out-of-band response.

4.2.2

Leakage Fields

One of the most frequently encountered exposure situations is a single contributing source, for example, industrial heating equipment or a telecommunications transmitter, that operates at a fixed known frequency. The points at which measurements are to be made are generally in the near field or the reactive near-field region. Since the emission is associated with unintentional leakage, estimates of the field levels cannot be determined a priori. In this situation, the broadband isotropic survey instrument, calibrated at the source frequency or with an accurately known frequency response, is the preferred instrument. The choice of probe, i.e., the field parameter to be measured, depends on the frequency of the source. Generally a t frequencies above about 300 MHz, the energy absorbed from an EM field by an object the size of an adult human is determined primarily by the electric field. Hence, at this frequency and above, it is sufficient to measure E2.At frequencies below 300 MHz, both

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the electric and magnetic fields influence the energy absorption and both quantities should be measured separately and independently. Under conditions known to be plane-wave, either the electric- or the magnetic-field component can be measured. The other component can be determined from Equation 4.8: Also, under plane-wave conditions, E and H can be related to the equivalent plane-wave power density S by Equation 4.9: where S, E and H are expressed in the units of W m-2, V m-' and A m-', respectively. These expressions are also useful for determining the values of E2and I% when near-field measurements are made with a broadband instrument that has a readout calibrated in units of S. Before proceeding with the survey, care should be taken to document locations normally occupied by equipment operators, service personnel or others in the immediate area relative to the location of the source(s). These are the areas that must be surveyed to establish an exposure from the measured field information. It is also important that the operator, survey equipment and surrounding personnel remain stationary while the measurements are being made to minimize field perturbations caused by reflection. A large sampling of data must be taken to spatially resolve areas of field intensification caused by reflection and multipath interference. Since the spacing between peaks of a standing wave are separated by one-half wavelength, sufficient measurements should be made to ensure that regions of field intensification are identified (ANSI, 1981). This can be facilitated by using a survey instrument that contains a visual or audible alarm that can be adjusted to trigger at any desired percentage of the full scale reading, or an audible output that increases in intensity or rate of ticking with increasing field level. These provisions allow one to survey large areas in relatively short periods of time. The areas of field intensification should be noted and can be examined in detail later. For the case of standing waves, eg., where waves may be reflected from the ground or surrounding objects, measurement of only one field component may, for practical purposes, be sufficient. Such a measurement, however, requires that a clearly defined peak in the measured field parameter, be it electric or magnetic, is observed. In this case, it can usually be assumed that the maximum in the unmeasured field parameter will not exceed the plane-wave equivalent value derived from the measured field parameter. Thus, in

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practice, the exposure assessment can be based on the limiting value for the measured parameter. It is important that the measured peak in the standing wave be determined within the volume of space to which an individual may have access. For example, the area should be surveyed to a height above ground at least as high as a person. If, in the search for the standing wave maximum, a clearly defined peak cannot be determined within the area of interest, then measurement of both electric and magnetic fields should be performed. When proceeding with the survey, one should approach the source using the high-power probe with the instrument set to the least sensitive range and fast response. (One should also be aware of the probe burnout limitations to avoid probe failure.) The source then should be approached with the instrument range switch adjusted accordingly. The regions normally occupied by personnel should be examined in detail as should the various anatomical locations of equipment operators or service personnel, i.e., personnel that are required to spend long periods of time at a specific location relative to the source. In this case, measurements should be made approximately 10 cm from the body in the region of the eyes, neck, chest, stomach, gonads, knees and ankles. The location and magnitude of the field maxima should also be identified. In addition, measurements should be made in close proximity to the source (taking care not to damage the probe), i.e., high levels around the cabinet or enclosure, the die of a heat sealer or the coil of an induction heater, feed wires and transmission lines, primary power cables, panel meters, etc., since in many cases the source of leakage can be identified and the emission reduced by shielding or filtering (Ruggera and Schaubert, 1982). Once the electric field has been characterized in sufficient detail, the entire procedure should be repeated using a magnetic field probe if the operating frequency is below 300 MHz. In addition to quantifying the EM fields, the temporal characteristics of the person's exposure must be determined. This may be based on the operating duty cycle of the device under test, or the exposure time patterns associated with movement from point to point in the field, or a combination of both. This must be done to determine the actual time-weighted exposure to ascertain compliance with the appropriate RFPG, most of which are time-averaged.Also, judgment must be used in interpreting the results of the exposure to obtain a realistic exposure value. For example, levels measured in close proximity to an induction coil or a few centimeters from a metal surface, which may greatly exceed the appropriate RFPG, are not representative of an exposure if personnel cannot or normally do not place themselves at the point of measurement. Similarly, the same consideration should be given to the interpretation of high fields

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that are localized to a volume of space small compared with the size of an adult human. When measurements are made a t a location in the near field of several sources, each of which operates at different, but known frequencies, the procedure described above can also be followed, using the broadband survey instrument. If the sources are located in close proximity to one another, then, even though the fields may decay rapidly with distance, several nearby sources may contribute significantly to the total field at any point, particularly when reflecting surfaces are located in the same area. Therefore, it is important that the frequency response of the survey instrument be flat over the entire range of frequencies of the source, including those harmonics where appreciable energy may be expected, unless each source can be operated and measured individually. It would be helpful, also, to be able to control the operation of devices that are operated intermittently in order to separately time average the contribution of each emitter. The sum of all such averages would represent the exposure at that point, provided that the averaging time used is that of the appropriate RFPG. The alternative would be to use a sufficiently long measurement time at each point to ensure that the proper time average is obtained. When the sources cannot be individually controlled, and the measured level exceeds the minimum value of the RFPG over the frequency range of interest, the contribution at each frequency must be determined to ascertain whether or not one is in compliance. In this case, narrowband instrumentation should be used to determine the spectral distribution of the emitted energy. Compliance with a specific RFPG is ascertained when the following inequalities are satisfied:

where f l and E: are the measured ms values of H and E, respectively, at frequency i, and and EESiare the corresponding values of the RFPG at frequency i. When measurements are made sufficiently far from a number of leakage sources operating a t different, but known frequencies, so that plane-wave conditions apply, the same general procedures apply. In many practical situations however, the levels are generally below the MDL of a broadband survey instrument. In such a case, reporting that the exposure level is below the value of the MDL should be sufficient, provided the MDL is well below the RFPG. If the levels must be determined in detail, narrowband instruments must be used.

eYi

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Antenna Fields

4. INSTRUMENTS AND MEASUREMENT TECHNIQUES

When the fields are associated with one well-characterized source (in terms of frequency, modulation and duty cycle for pulsed systems), the same procedures outlined above may be followed. If the levels are to be determined close to the radiating elements of an antenna, broadband survey instruments can be used. In many cases, however, the locations of interest may be sufficiently far from the antenna that the levels will be below the MDL of the survey instrument. In that case, narrowband or quasi-narrowband instruments and an appropriate antenna must be used. Many commonly used antennas, including some aperture antennas (see Section 4.1.2.1), are linearly polarized, and the squares of the fields corresponding to each of three mutually orthogonal antenna orientations must be summed to determine the total field a t a given point. This may not be necessary if the polarization of the fields is known a priori, e.g., AM broadcast. When a directional antenna such as a pyramidal standard gain horn or a log-periodic antenna is used, it should be rotated in both azimuth and elevation until a maximum is obtained. After the measurement is made for that particular orientation, the antenna should be rotated 90 degrees about its axis (if it is linearly polarized), and the measurement repeated. This is to ensure that both the vertical and horizontal components of the field are measured. If the antenna is circularly polarized and the fields linearly polarized or circularly polarized in the same direction as the antenna, this last step is not needed (see Footnote 3).Since most directional antennas have high front-to-back gain ratios, the antenna should be rotated plus or minus approximately 180 degrees from the direction of t h e maximum signal to determine whether reflections are important. When measurements are to be made in an area where many antennas operating a t widely different frequencies are located, eg., an antenna farm, measurements should first be made with a broadband survey instrument at all points of interest. If the highest levels measured are below the minimum RFPG value over the frequency range of the emitters, this would generally be sufficient to show compliance. If, however, the measured values a t some points exceed this value, then narrowband instruments must be used a t each of these points to determine the spectral distribution of the total field. At remote distances, where the fields are below the MDL of the survey instrument, narrowband instruments must be used if a knowledge of the level is important.

4.2 GENERAL MEASUREMENT TECHNIQUES AND PITFALLS

4.2.4

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127

Precautions for Ensuring Measurement Accuracy

There are several precautions and sources of error that one should be aware of while performing surveys. First, it is important that one become familiar with the instructions and cautionary statements found in the operating manual supplied with the survey instrument. For example, some manufacturers recommend a preferred probe orientation that should be consistently used to ensure maximum accuracy and repeatability. Statements to the effect that measurements should not be made with the probe in close proximity to high voltage transformers or in contact with the cabinet of a VDT, eg., near the flyback transformer, should be heeded. Second, when making measurements, certain procedures should be followed to minimize errors. For example, when the polarization of the field being measured is known, all cables associated with the survey instrument should be held perpendicular to the direction of the electric field in order to minimize pickup. At frequencies below about 1 MHz, it appears that the body of the operator becomes part of the probelantenna of some broadband instruments (Kucia, 1972) and, if possible, the probe and electronics should be held or supported with a dielectric structure such as wood or preferably polystyrene foam. This is particularly relevant when measuring the electric fields associated with induction heaters or AM broadcast antennas. When measurements are made of relatively high fields or where pulsed fields exist with high peak to average power ratios, it is usually a good idea to hold the probe fixed and note the meter reading while rotating the readout and moving the interconnecting cable. Any significant change usually indicates pickup and interference problems. If a device that requires zeroing is being used, especially on the most sensitive range, it should frequently be checked for zero drift. This should be done with the probe removed from the field, preferably by switching offthe source(s).It is possible that the survey instrument cannot be zeroed a t the work site, because sources cannot be turned off or completely identified. Zeroing must then be done at a remote location and substantial errors may be introduced because of zero drift. In this situation, zero driR should be checked frequently at the remote location during the survey. An alternate method of zeroing an electric-field type instrument is wrapping the probe and handle with a metallic foil such as copper or aluminum to shield the sensor from the fields. A large metal can be effectively used a t long wavelengths (lower frequencies) to shield the probe elements. However, at short wavelengths such as microwave frequencies, such procedures must be used with caution since the metal can and probe act as a coaxial cavity, actually enhancing the fields in the vicinity

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of the probe elements. Foil lined plastic bags may also be used, but care must be exercised to eliminate static-charge effects caused by the friction of sliding the bag over the probe. The use of foil normally will not work with LF magnetic-field probes. Recently developed shielding fabrics may be effective for wrapping or covering the probe in the field. Finally, when a survey is being conducted, all unnecessary personnel should be removed from the area to minimize errors caused by reflection and field perturbation.

4.2.5

Precautions for Protection of the Operator

In addition to instrument considerations and measurement techniques, one should be aware of potential hazards to the operator in the area being surveyed. Obviously, one should keep in mind the RFPG and avoid exposing oneself to time-averaged values that exceed the RFPG. Protective clothing fabricated from conductive material may prove useful in preventing excessive exposure to survey personnel that require access to high strength RF field areas such as broadcast towers (FCC, 1985). Today, protective clothing has been developed that overcomes limitations exhibited by earlier developments (Amato, 1994). One should also be aware of other potential hazards that may be associated with very high-power RF generating equipment, such as high voltage, x rays and the possibility of receiving an RF burn if one inadvertently touches or comes close to exposed conductors of RF current, such as induction coils and antenna or induction coil feed-wires. One should not disable or interfere with cabinet interlocks which could allow access to potentially dangerous levels of RF, x rays and high voltage. Similarly, one should observe the usual safety precautions associated with moving or rotating machinery. Also, operators with implanted pacemakers, metal implants, intrauterine devices, etc., should carefully consider possible RF-induced effects prior to conducting a survey. Finally, jewelry such as metal bracelets and decorative metal belts should not be worn because of the potential for RF burns associate with induced RF currents. Use caution when first arriving a t the survey location to minimize the opportunity that high strength fields might damage the instrumentation prior to conducting a survey. For example, when driving into a large shortwave broadcasting facility antenna area, avoid parking directly beneath transmission lines which may have extremely strong associated fields. When approaching a high-power radar facility, use care to avoid accidental exposure of thermocouple

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probes to excessive peak-power fields which could damage the probe elements, even with the instrument turned off. 4.3 Special Measurements The first consideration in any exposure evaluation for RF fields is the determination of the field strengths themselves. This is because most RFPGs specify limiting values for the field strengths as surrogates for assessing the SAR that may result from the exposure. In conditions in which essentially whole-body exposure may occur, certainly the field strengths need to be determined, but exposure circumstances exist in which individuals are so close to some sources that significant RF currents may flow between the source and the body through the phenomenon of capacitive coupling. When individuals may come into physical contact with the source, such as when touching an antenna or a reradiating object, the contact currents which result will generally be greatest. When current flows in various anatomical regions of the body, the local SAR will increase; the SAR at a specific point is related directly to the local current density in the tissues and the conductivity of the tissue. Hence, two issues arise relative to exposure: the question of local SAR and whether an RF burn may occur due to high point-contact surface-current density. RF field measurements are not good indicators of compliance with RFPGs for circumstances of contact with energized objects. In this case, a direct measurement of the contact current provides more relevant information about the potential for exceeding the fundamental SAR limits of a RFPG. Body currents and contact currents may be measured in several ways. For body current measurements, generally taken to represent the case of exposure of the body to RF fields, but without any direct contact with objects other than the ground upon which one may be standing, the most common technique is the measurement of the current flowing between the feet and ground. This can be accomplished by constructing a low-profileplatform which consists of two parallel conductiveplates. The individual stands on the platform and the body current which flows to ground is measured by determining the RF potential difference (voltage drop) across a low resistance resistor connected between the two plates. Current flow will be through the bottom plate to the ground. By relating the voltage drop to current using Ohm's law, the body current is determined (Tell et al., 1979). As an alternative to the voltage drop measurement from which the body current is deduced, a current transformer may be used to

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determine directly the current flowing between the two plates of the platform. In this case, the two plates should be electrically shorted together and the current transformer placed around the shorting conductor. Again, the detection device, which is used in conjunction with the current transformer, may be of the broadband type such a s a broadband power sensor and power meter combination, or a tuned receiver. Another alternative is to place a thermocouple RF meter directly in series with the two conducive plates. In some cases, this may be the preferred method since the meter movement is direct reading and does not require a power supply or associated electronics. Two factors which may reduce the attractiveness of the thermocouple meter are its physical size and sensitivity to burnout. Measurement of contact currents may be accomplished by inserting a current measuring device between the hand and the object to be tested. This may take the form of a thermocouple type meter (Tell, 1991) or a metallic probe, one end of which is held by the individual and the other end of which is touched to the object under test. A clamp-on type RF current transformer, a s discussed above, may then be used to determine the contact current (Tell, 1990).In this case, the current measured is exactly that flowing into the hand of the individual due to contact with the object.

5. Recommended Areas for Further Research and Technical or Engineering Development Although the current instrumentation for characterization of EM fields is considerably further advanced than it was just a few years ago, there is still room for development andlor improvement (EPA, 1982). Several such areas are outlined below: 1. Meter Output Units-Some instruments are not calibrated in terms of the field parameter which they directly measure. For example, some isotropic, broadband survey instruments are calibrated to read in units of plane-wave equivalent power density S even though they actually respond to the square of E or H. A narrowband instrument commonly used for the measurement of AM broadcast signals, which is a standard in the broadcast industry, is calibrated in terms of E while i t actually responds to the magnetic field. Many loop antennas are supplied with antenna factors that yield E instead of H to which they respond. Although a loop antenna can be used to determine E under plane-wave conditions, many measurements are made in the near field where the free-space impedance is not equal to 377 ohms. Using a loop antenna to obtain E under such conditions may lead to large measurement errors. It is clear that it is desirable to have instruments which measure those parameters of the EM field that are stated in exposure standards and RF'PGs. 2. Achievable Accuracy-At present, a range of accuracy of 2 to 4 dB is possible, 2 dB being associated with microwave ovens and 4 dB being associated with more difficult field measurements. In typical measurement situations, accuracy is usually limited by confounding factors and measurement technique rather than by the accuracy of the instruments. 3. Out-of-Band Response-Typical commercially available RF hazard meters are frequently used in environmental surveys where the situation is not well controlled. Fields from unwanted

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sources, combined with poor and generally unknown out-ofband instrument response, can produce highly erroneous results to the unaware operator. At present, many instruments exhibit a n enhanced or resonant type of response above or below the useful frequency range specified by the manufacturer. In many cases, the enhancement is greater than 10 dB. There is a need for making this information available to the operator, and even more importantly, the instrument's responses should be diminished a t a specified rate, eg., 3 or 6 dB per octave, above and below the upper and lower 3 dB frequency points to eliminate the problem altogether. 4. Instrument Specifications-Published instrument specifications are generally incomplete. For example, the applicability of frequency response data provided by the manufacturer a t frequencies where the instrument was not calibrated is often unclear and, as above, there is usually no indication of out-ofband performance or susceptibility to EMI. In addition, published instrument specifications are often unreliable, e.g., the response of individual instruments does not always agree with the calibration provided by the manufacturer. 5. Antennas for Narrowband Measurements-There is a definite need for suitably calibrated isotropic antennas for use throughout the spectrum (particularly for frequencies between 10 kHz and 1 GHz). The size of such devices should be such that they can be used for the spatial characterization of EM environments. In addition, it would be useful if the response of the antenna were flat with frequency to facilitate the characterization of the multifrequency environments. At present, no such devices are commercially available. 6. Reporting Requirements-There is a definite need for industry standards; for minimal uniform reporting of performance characteristics for existing instrumentation, for minimal testing procedures for evaluating instrument performance, and for performance characteristics for future instruments.

APPENDIX A

Hazard Evaluation Procedures for Common Sources A.l Amplitude-Modulation Radio Broadcast

Purpose: Domestic radio broadcasting.

Operational Characteristics: AM radio stations rely principally upon ground-wave propagation to provide broadcast programming within specific geographical regions. Ground-wave coverage extends outward up to approximately 160 km depending on station power, antenna characteristics and propagation conditions. AM stations typically employ vertically polarized antenna towers called monopoles. The monopole works in conjunction with the earth to radiate the signal uniformly in all compass directions making the antenna's radiation pattern omnidirectional. Many AM stations employ a n array of several towers which are spaced, oriented and phased in such a way as to create nulls and enhance levels in the radiated fields a t prescribed azimuthal directions. This property is used when it is necessary to reduce the potential for interference with another AM station operating a t the same frequency in a distant city or to improve service in a particular area.

Frequency Range: AM radio broadcasting in the United States is authorized to operate within the 535 to 1,705 kHz frequency range.

Power Levels: In the United States AM radio stations operate with antenna input power levels of 250 W to 50 kW.

Estimated Number in Use: In 1993 there were in excess of 4,950 AM radio stations authorized within the United States.

Authorization for Use: AM radio broadcasting is regulated in the United States by the FCC.

Typical Exposure Levels: The strengths of the electric and magnetic fields associated with AM radio broadcasting are dependent on the distance from the transmitting tower. Table A.l illustrates the required distance from an AM tower such that the resulting fields will be equal to a specified value (Gailey and Tell, 1985). The data in Table A.l is based on TABLEA.1-Distances at which fields from A M radio stations will fall below given field-strength values. This Table applies to any A M frepuency and antenna heights ranging for 0.1 to 1.0 wavelength (from Gailey and Te11,1985)." Electric-field strength Transmitter power (kW) V rn-'

50.00

25.00

10.00 5.00

2.50

1.00

0.50

0.25

0.10

Meters

49 38 30 23 67 10.0 202 171 113 87 18 13 27 22 25.00 104 35 57 45 80 11 8 18 14 50.00 23 62 49 35 28 11 8 6 18 13 75.00 47 37 27 23 12 10 7 5 87.00 42 17 33 25 21 9 7 5 100.00 38 11 30 23 19 15 5 4 8 6 11 150.00 29 24 18 15 3 5 4 7 194.00 10 25 21 15 12 5 4 3 7 200.00 25 9 20 15 12 4 3 <2 7 5 275.00 21 17 12 10 5 4 3 <2 7 300.00 9 20 16 11 6 4 3 <2 <2 400.00 16 7 9 13 3 <2 <2 3 6 5 500.00 14 11 8 <2 <2 3 <2 632.00 12 5 4 9 7 <2 <2 3 <2 5 11 4 750.00 8 6 1,000.00 9 7 5 4 3 <2 <2 <2 <2 "The distances shown in this Table are those a t which E will fall below the specified values in the left hand column. The distances indicated will also ensure that H does not exceed the far-field equivalent value of E obtained from the relation EH-' = 377.

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numerical calculations of fields from monopole antennas with heights ranging from 0.1 to 1.0 wavelength and frequencies ranging from 600 to 1,600kHz. Limitations inherent to the numerical computation technique for very close-in near-field calculations are believed to overpredict the distance at which various field strengths may exist. Additional near-field measurements should help refine the accuracy at these distances (Tell, 1989). Surface fields on AM towers can reach very high values (>1,000 V m-'1. Commonly, AM towers are climbed with the tower energized by the transmitter. Because the actual electric and magnetic fields the climber may come in contact with are not well defined, caution is warranted for such activities. High RF voltages exist on the tower and severe RF burns may occur if contact is made with the tower by a grounded individual.

InstrumentationlMeasurement Techniques: Within distances equal to the tower height, the ratio of E and H will generally not be equal to 377 ohms (Tell et al., 1979). In such cases, measurement of both the electric and magnetic fields is required to assess the RF exposure. E may be measured with broadband electric FSMs or tunable, narrowband instruments. In the case of tunable FSMs, the use of separate, external attenuators may be appropriate to permit measurement of high fields that might be beyond the normal range of field strengths that the meter is designed to measure. An external attenuator may also protect the input circuitry of the meter. Either small loop antennas or vertically polarized electric-field rod antennas may be used in conjunction with the tunable meter for measurement of H and E, respectively. In either case the operator must know the value of the A, to interpret the FSM readings properly. Care must be exercised in the use of broadband measurement instruments with separate probes connected via cables to the electronic readout package. This is because RF currents are induced on the connecting cables at the low frequencies used in AM radio broadcasting (Aslan, 1985).The induced currents may lead to erroneous readings. Various precautions are advised before interpreting such field-strength measurements. Often a sweeping of the probe from near ground level to above the head will help identify erratic indications on the meter which suggest that erroneous values may be indicated. Meters wherein the sensing element and the electronic circuitry is self-contained, i.e., the active monopole type of instrument (see Section 4.1.1.1), perform well in LF environments.

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Special Considerations: AM tower tuning circuits have been identified as a source of locally intense magnetic fields (Tell, 1986). These magnetic fields decrease rapidly with distance from the tuning circuits, but should be carefully considered when evaluating exposure very near the base of AM towers. Often, it may be possible to locate the tuning circuits in such a way as to greatly reduce the potential for high-level exposures. As an example, the circuits may, in some cases, be moved from a location near the protective fencing around the tower to a more central point, increasing the distance between the circuits and normally accessible areas. Contact currents associated with the climbing of energized towers have been investigated by Tell (1991).

A.2

Frequency-Modulation Radio Broadcast

Purpose: Domestic commercial and noncommercial radio broadcasting.

Operational Characteristics: FM broadcast radio stations commonly employ transmitting antennas which are omnidirectional in the azimuth plane, i.e., the signal is propagated uniformly in all directions of the compass to create a circular audience-coverage area. The antennas used in FM radio broadcasting typically exhibit gain in the elevation plane, i.e., the radiation pattern may resemble a doughnut shape with a maximum in field strength horizontal to the antenna and lower intensities at steep elevation and depression angles. These antennas, however, can exhibit significant sidelobes. A common occurrence is the presence of a so-called grating lobe, or a lobe of radiation pointed nearly directly vertically downward. With some antennas, these downward pointed grating lobes may be as intense as points within the main beam and often lead to relatively high RF field strengths near the base of the antenna tower. FM stations usually employ "circular polarization" with equal ERP in both the horizontal and vertical polarization planes. FM antennas are commonly side-mounted on towers and are typically composed of between 1and 12 radiating elements configured into an antenna array. Common design practice calls for a n element spacing of one wavelength but some antennas use shorter element spacing to prevent grating lobes. Most commonly, transmitter power is uniformly divided among the elements of the antenna and usually, the excitation of each element is

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approximately in electrical phase with the other elements. Antennas are generally located a t high elevations to maximize signal coverage of a geographical area, but may be found on towers as low as 10 m in height above ground.

Frequency Range: FM broadcasting in the United States is assigned to the 88 to 108 MHz frequency band by the FCC.

Power Levels: Authorized ERP ranges from 10 W to 100 kW. (Some stations authorized prior to adoption of the present standards in 1964operate with ERP in excess of 100 kW.) FCC regulations specify the ERP in the horizontal plane rather than the sum of ERP for both polarization planes. Thus, for a 100 kW FM station using circular polarization, the total ERP is 200 kW and this value should be used to perform estimates of the RF field strength in the vicinity of the antenna. The ERP is referenced to a half-wave dipole.

Estimated Number in Use: In 1993, there were in excess of 6,450 FM radio stations authorized in the United States.

Authorization for Use: FCC.

Typical Exposure Levels: Equivalent S near the base of FM antenna towers may reach values as high as several mW cm-'. Areas subject to such levels are typically within 30 m of the tower base. Calculations of S should be accomplished by referring to a radiation pattern for the subject antenna. However, calculations may be performed by assuming that the main beam of the antenna, i.e., full stated ERP, is pointed toward the area of interest. For these calculations the sum of both the horizontal and vertical polarization ERP should be used. An example of the calculation is as follows: S(pW cm-') = (W-+ Wt )(1.64)(100)(4~?)-~

(A.1)

where S is the power density (pW ~ r n - ~plane-wave )) equivalent, W, Wt is ERP in horizontal and vertical polarization planes in watts, 1.64 is the power gain of half-wave dipole (corrects for ERP

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referred to a dipole), and r is the distance from calculation point to center of radiation of antenna array (physical center) in meters. This formula assumes that there are no reflections from objects or fmm the ground. Reflections can increase the field strengths and thereby increase the equivalent plane-wave power density. For example, a ground reflection could potentially add in phase with the direct wave from the antenna to result in a doubling of the field strength at some point. Since S is proportional to the square ofE, the resulting planewave equivalent power density could be four times the value computed from the above formula. Often, for conservative estimates (i-e., estimates of fields which may over predict the actual field strengths), the factor of four in ( 4 1 d - l is omitted. A practical value of 2.56, to account for ground reflection, has been recommended for use in computing S associated with FM stations (Gailey and Tell, 1985). The factor 2.56 was derived from the observation that, under most situations, the electric field reflection coefficient is approximately 1.6. The square of this value (2.56) is used for S calculations. For exposures in the very near vicinity of the antenna structure, such as on the supporting tower, exposures may be high and can exceed the plane-wave equivalent S of several hundred mW ~ m - ~ . Even relatively low-power FM stations produce intense RF fields within 1 to 2 m of the radiating antenna elements. Extra caution should be exercised for individuals who must work on towers supporting FM broadcast antenna systems.

Instrumentation/MeasurementTechniques: Isotropic E- and H-field survey meters may be used to determine the exposure levels. If more than one source is present at the transmission site, eg., several FM radio stations collocated on the same tower or in the immediate vicinity, individual stations will have to be selectively switched off for a period sufficient to measure the fields of the desired station in the areas of interest. If this cannot be arranged in such cases, narrowband measurement techniques will be required. Generally, measurements of the electric field alone will be sufficient to determine compliance with a RFPG. This may be accomplished by ensuring that in the case of reflections (from the ground), a peak in the measured E is found. In this case, H peak is usually found to be at a different height above ground, but will not exceed the measured maximum E in terms of plane-wave equivalent S. Because of the wavelengths involved with FM broadcasting, about 3 m, standing wave patterns usually exhibit peaks in the electric field within distances (height above ground) normally accessible by common broadband-field survey instruments.

A.3 VHF AND UHF TELEVISION BROADCAST

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139

Narrowband measurements are commonly performed by use of a tuned half-wave dipole or a broadband (fat) dipole antenna connected to either a spectrum analyzer or FSM. In this case, three orthogonal measurements of E should be made by sequentially orienting the dipole. It is usual practice to support the dipole on a nonconductive tripod at a height such that the dipole, when vertically oriented, does not touch the ground. Since the length of a broadband dipole may be considerably shorter than that of a tuned dipole, the former may offer an advantage. Ideally, the lower end of the dipole, when oriented vertically, should be a t least one wavelength above ground. Special Considerations: Because RF exposures near FM antenna elements can be intense, care must be exercised when performing measurements on antenna towers to avoid over-exposure of survey personnel. In environments where multiple FM frequencies are present, care must be exercised in interpreting measurement data obtained with broadband field probes that use diode detectors. Diode detectors tend to act as peak detectors and may give readings higher than the true values (see Section 4.1.1.). Large antennas, such as tuned half-wave dipoles, are not easily used for spatial resolution of localized regions of elevated field strength. Measurement data should be reviewed from the standpoint of anomalies, which may be a function of the proximity of the dipole to nearby metallic surfaces.

A.3 Very-High-Frequency a n d Ultra-High-Frequency Television Broadcast Purpose: Domestic, commercial, and noncommercial television (TV)broadcasting. Operational Characteristics:

TV broadcast stations commonly employ transmitting antennas which, like those used for FM radio broadcasting, are omnidirectional in the azimuth plane. TV transmitting antennas achieve their gain by concentrating the radiated power in a doughnut shaped lobe, with the maximum field strength occurring in a plane horizontal to the vertical center of the antenna. At steep elevation and depression angles from the antenna, the radiated signal is of lesser intensity

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for the same radial distance. Similar to FM broadcast antennas, TV antennas also exhibit sidelobes of radiation which create maxima and minima in the ground level field strength. This effect occurs from the base of the antenna tower out to a distance determined by the number of elements or sections in the antenna and its height above ground. However, in contrast to typical FM antennas, most TV antennas tend not to exhibit pronounced grating lobes, i.e., the relatively strong lobes of radiated signal pointed steeply downward toward the ground. Because of this, typical ground level RF field strengths near TV antenna towers are not usually as intense as near FM antenna installations of the same height and average ERP. TV transmission consists of two separate signals; one carrying the video information for producing a picture on a TV receiver and a separate audio signal 4.5 MHz higher in frequency than the video signal for conveying the audio portion of the transmission.

Frequency Range:

TV broadcasting in the United States is assigned to the following frequency bands by the FCC: Channels 2 to 6 7 to 13 14 to 83

Frequency mnge (MHz) 54 to 88 174 to 216 470 to 890

It should be noted that channels 70 to 83 (806to 890 MHz) are no longer available for TV broadcast in the United States.

Power Levels: Authorized ERP ranges from 10 W to 100 kW for VHF channels 2 to 6;316 kW for VHF channels 7 to 13;and 5,000kW (5MW) for UHF channels 14 to 69.The ERP for W stations is specified as the maximum ERP at some particular elevation angle. Some stations employ a certain amount of beam tilt purposely to direct the signal slightly below the horizontal plane. Also, some W stations use circular polarization, and the total ERP, horizontal plus vertical polarization components, for such a station may be as much as twice the stated ERP. In the case of circularly polarized W stations, the total ERP must be used in the calculation of the expected RF field levels. ERP for W stations are specified in relation to the peak video power, i.e., the instantaneous peak power of the AM video signal of the W station. Because the video signal is AM and contains synchronization pulses necessary for proper TV receiver operation,

A.3 VHF AND UHF TELEVISION BROADCAST

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the average power of a TV video signal is approximately 40 percent of its peak video-carrier power. Thus, a 100 kW ERP TV signal is actually equivalent to about 40 kW of an FM signal, such as from an FM radio station. In addition to the video signal, a TV station transmits an FM audio signal. Typical FM sound signals of TV stations vary in ERP, but lie in the range of 10 to 20 percent of the peak video effective radiated power. For example, a 100 kW TV station would have an approximate average ERP of 50 to 60 kW (40 kW average video and 10 to 20 kW average audio).

Estimated Number: In 1993, there were approximately 3,850 VHF and UHF TV stations authorized in the United States.

Typical Exposure Levels: Equivalent S near TV broadcast antenna towers depends on the antenna height above ground and the TV station's ERP, but normally will not exceed 1 mW ~ m - To ~ .calculate the expected S associated with a TV station, the following formula may be used:

S(fl cm-') = (0.4 ERP,

+ ~ ~ ~ ~ ) ( 1 . 6 4 ) ( 1 0 0 ) ( 4(A.2) &)-~

where S is the plane-wave equivalent power density (pW cm-'1 ERP, is the effective radiated power in watts of the peak video signal a t the required depression angle (angle between a horizontal to the antenna and a line from the physical center of the antenna to the point of interest on the ground),ERPAis the effective radiated power of the audio signal at the required depression angle, 1.64 is the power gain of a half-wave dipole (corrects for ERP referred to a dipole), and r is the distance from calculation point to the center of radiation of antenna (physical center) (m). If the TV station uses circular polarization, the sum total ERP of the horizontal and vertical polarization components must be used for both the video and audio ERP. For example, for a 316 kW circularly polarized TV station, the ERP, value would be 632 kW. As with the FM broadcast example, no account has been taken of ground reflections. An additional factor may be used in the numerator of the above expression to account for such reflections and this factor may take on values between one and four, depending on the degree of reflection at a given point. A realistic value for many exposure situations is 2.56 (Gailey and Tell, 1985).

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APPENDIXA

Instrumentation/MeasurementTechniques: See the example for FM broadcast.

Special Considerations: In the case of performing narrowband measurements of W signals with a spectrum analyzer, it is offen more convenient to measure t h e instantaneous peak of t h e video signal t h a n to attempt to measure the average field strength. This is because of the rapidly changing character of the video programming material which modulates the video signal. While the average value of power changes somewhat, according to programming, the peak value of the synchronization pulses remains constant. Upon determination of the peak-ofthe-video-signal equivalent S, the average value can be reasonably assessed by taking 40 percent of the peak value (Gailey and Tell, 1985).

A.4 Terrestrial Microwave Radio (Pointcto-Point Radio Relay)

Purpose: An economic alternative to wire and cable for the transmission of information, both voice and data. Commonly used by the telecommunications industry (common carriers), public utilities, broadcasters, military, private industry, independent companies, schools, hospitals, etc.

Operational Characteristics: Systems may be unidirectional or bidirectional. Frequently a single antenna may be used to transmit and receive simultaneously. In most systems, however, a single transmit antenna and one or more receive antennas will be used. Both vertical and horizontal polarization are used, frequently both are used simultaneously with the same antenna. The most commonly used antennas are horn reflectors and circular parabolic reflectors (dishes). Occasionallypassive reflectors are used to change the direction of the beam. The antenna characteristics are: high gain (typically greater than 40 dB) and narrow beamwidth (typically 2.5 degrees or less). The antennas may be mounted on towers, rooftops, sides of buildings or anywhere else that a clear unobstructed line-of-sight path to the next antenna can be obtained. Repeater stations are typically spaced distances of

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up to 50 km or more depending on frequency and topography. FM is commonly used for analog radio and quadrature amplitude modulation (QAM) and phase shift keying (PSK) are commonly used for digital radio.

Frequency Range: There are several frequency bands between 1and 40 GHz dedicated to fixed microwave radio. Three common carrier bands that are used extensively are: 3.7to 4.2,5.925to 6.425and 10.7to 11.7 GHz. Some microwave stations operate in all three bands with a single antenna.

Power Levels: From a few hundred mW to 20 W per channel. Generally, systems operating in the 4 and 6 GHz common carrier bands operate at levels of 2 to 5 Wand 10 to 12.5 W per channel, respectively. A fully loaded system may contain twelve 4 GHz channels and eight 6 GHz channels on a single antenna. Systems operating in the 11 GHz common carrier band generally operate a t power levels of 5 W per channel or less.

Estimated Number in Use: The number of authorized terrestrial microwave stations is in the tens of thousands.

Authorization for Use: FCC assigned communications bands for fixed radio.

Typical Exposure Levels: The equivalent S for off-axis points are of the order of a few pW cm-' or less. Levels measured in proximity to the base of typical microwave antenna towers are of the order of a few tens of nW cm-' or less. The near-field maximum S, S,, on axis, is approximately given by Equation A.3, where where P is the antenna input power, q is the antennas aperture efficiency(typically in the range of 0.4 to 0.7)and A is the antenna's physical aperture area. This value (S,) ranges from a few tens or hundreds of pW ~ m for - most ~ systems, up to several mW cm-' for fully loaded high-capacity systems (Petersen, 1979).

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Instrumentation/MeasurementTechniques: Isotropic E-field survey meters may be used to ascertain compliance with a RFPG or to document levels in the beam of the antenna. If the actual levels must be known at points off axis, narrowband or quasi-narrowband instruments should be used. Quasi-narrowband instruments and narrowband antennas facilitate measurement of digital PSK and QAM systems. Care must be taken to ensure that all polarizations are measured. Measurements are usually made with the test antenna mounted at head height on a wooden tripod. At frequencies above a few GHz, semi-rigid or precision flexible microwave coaxial cable can be used, if necessary, to connect the antenna to the power meter or spectrum analyzer. Although the attenuation of cable such as RG 142 B/U is comparable with that of semi-rigid or precision flexible cable, the attenuation stability with flexure of this and other common coaxial cables preclude accurate measurements a t t h e higher microwave frequencies (see Appendix B.2 for an illustrated example).

A.5 Satellite Communication-Earth

Stations

Purpose: SATCOM is often an economical alternative to wire and cable, including undersea cable, for relaying voice, W and data. SATCOM systems are commonly used by the telecommunication industry (common carriers), public utilities, broadcasters, military and private industry. Operational Characteristics: Earth station antennas are generally of the circular parabolicreflector type at least 100 wavelengths in diameter. The physical dimensions range from a few meters in diameter to a few tens of meters depending on the operating frequency and system performance requirements. Many antennas currently in operation are approximately 10 or 11m in diameter. A single antenna is commonly used to simultaneously transmit and receive. Vertical polarization, horizontal polarization or both may be used. The antenna characteristics are: high gain (typically of the order of 60 dB or more) and narrow beamwidth (typically a few tenths of a degree). The antennas may be mounted almost anywhere that a clear line-of-sitepath exists between antenna and satellite. Since interference is frequently a problem, antennas are usually located in radio-quiet areas, either

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naturally quiet because of local topography or as a result of distance from interfering sources or man-made quiet as a result of fences or surrounding buildings. Earth station antennas are often found in parking lots, on rooftops or a t ground level between buildings. Most SATCOM antennas are essentially fixed in orientation (azimuth and elevation) for long periods of time (months to years); others are reoriented frequently, particularly those used for telemetry and satellite control. FM, single-sideband AM, PSK, frequency shift keying (FSK) and QAM are commonly used, as is frequency division multiple access, where data is sent in bursts of a few milliseconds so that several operators may access the same satellite transponder. Frequency Range: Several frequency bands between 100 MHz and 275 GHz are dedicated for fixed and mobile satellite service. The most commonly used frequency bands are: 5.85 to 6.65 GHz (C-band), 14.0 to 14.5 GHz (Ku-band) and to a much lesser extent 27 to 31 GHz (Ka-band). Power Levels:

Power levels are from a few tens of watts to more than a kilowatt per channel. The radiated power of any given earth station antenna a t any specified time depends on the number of channels in use and climatic conditions. In some cases the output power of an earth station antenna may be relatively constant over long periods of time. In other cases, transmission on some channels may last a few minutes or a few tens of minutes while other channels on the same antenna may be active for hours. Still others may transmit continuously. It is important that the person conducting measurements of an earth station antenna coordinate with the station engineer so that the measured results can be compared with or normalized to the transmitted power. Changes in output power to compensate for additional path loss caused by climatic conditions,eg., rain or snow, are usually less severe than those associated with the addition or deletion of radio channels and are typically of the order of 3 dB or less. In a few cases, e.g., during heavy rain or snow or when the reflector is covered with snow or ice, the increase in output power may be considerably greater than 3 dB.

Estimated Number in Use: The number of authorized SATCOM earth station transmitting antennas of all types is of the order of thousands.

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Authorization for Use: FCC authorizes all domestic telecommunications systems in the United States. The National Telecommunications and Information Administration (NTIA) authorizes all government systems in the United States. Typical Exposure Levels: The equivalent S a t off-axis points may be as large as a few mW cm-2 close to the edge of the reflector and at points close to the antenna near the edge of the geometrical projection of the reflector. At points in the near field a few degrees off axis, S, of a few tens of pW cm-2 may be expected. At points in the near field on-axis S, is approximately where P is the antenna input power, .rl is the antenna aperture efficiency (approximately0.6 for most modern reflector or dish antennas) and A is the physical aperture area. S at off-axis points in the near field is difficult to determine analytically, but estimates can be obtained from the normalized graphical results of various models that have been developed for carrying out such estimates (e.g., Lewis and Newell, 1985). At points on-axis in the far field where the distance r is given by Equation A.5 with d defined as the antenna diameter and A is the wavelength, S can be determined from Equation A.6.

where P is the antenna input power and G,is the gain of the antenna in the direction 6. At points off-axisin the far field, G, can be estimated from a suitable specified peak-sidelobe-envelope for earth station antennas, eg., G, = 32 - 25 log 8 dB, for 1" < 19I 48"

G,

=

(A.7)

10 dB, for 48" C 8 < 180"

Instrumentation/MeasurementTechniques: Isotropic E-field survey meters may be used to ascertain compliance with an RFPG at points close to the antenna or beam edge. If the actual levels must be known at points off-axis, the sensitivity of broadband instruments is inadequate and narrowband or

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quasi-narrowband instruments must be used. Quasi-narrowband instruments facilitate measurement of digital systems, eg., FSK and PSK. Since the levels off-axis may be of the order of a few pW cm-2 or less, it is tempting to use a large diameter (of the order of 1 m) parabolic reflector antenna to increase measurement sensitivity. If this is done, care should be taken to ensure that the entire SATCOM antenna falls within the acceptance angle of the measurement antenna (3dB beamwidth), otherwise the measured values will be lower than the actual values. Generally, when narrowband or quasi-narrowband measurements are made close to a SATCOM antenna, a measurement antenna such as a conical log-spiral antenna with a typical beamwidth of 60 degrees or a standard gain horn antenna with a typical beamwidth of 30 degrees would be an appropriate choice. The advantage of the conical log-spiral antenna is that both polarizations are measured simultaneously. Measurements are usually made with the test antenna mounted at head height on a wooden tripod. For quasi-narrowband measurements the power sensor is normally connected directly to the antenna terminals. For narrowband measurements, semi-rigid or precision flexible coaxial cable should be used to connect the antenna to the spectrum analyzer or FSM. Although the attenuation of cable such as RG 142 BIU is comparable with that of semi-rigid or precision cable, the attenuation instability with flexure of this and other common coaxial cables can lead to large errors. In some cases, a low-noise amplifier may be needed to overcome the loss of the cable. Finally, because of transmitter power changes associated with communication traffic and climatic conditions, it is extremely important to coordinate the measurements with the SATCOM station-engineer and record the time of each measurement. This will allow normalization of the measured levels with a nominal transmitter output power.

A.6 Hand-Held Portable Radios (Including Cordless and Hand-Held Cellular Telephones) Purpose: These devices are used for communications by police, broadcast news-gathering personnel, security and business personnel, the general public and amateur radio operators. Conventional hand-held mobile radios, cellular radio telephones and cordless phones, as well as citizens band (CB) transceivers, are used by private citizens in professional and nonprofessional applications.

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Operational Characteristics: Hand-held units are small devices with a short single "rod" antenna or monopole (a flexible or rigid antenna, usually less than 30 cm in length). CB units require longer antennas to operate efficiently. Cordless phones use lower power and smaller antennas.

Frequency Range: Communications bands are between 27 to 50 MHz, 150to 170 MHz, 450 to 512 MHz and 800 to 900 MHz. CB radios operate a t about 27 MHz, cordless phones operate between 46 to 60 MHz and some at about 915 MHz, and cellular radios operate in the 800 to 900 MHz band.

Power Levels: Less than 10 W for transceivers. Less than 1W for cordless phones. Typically 0.6 W or less for hand-held and up to about 3.5 W for transportable cellular phones.

Estimated Number in Use: Millions of hand-held transceivers are in use by police, security and business persons. Cordless and hand-held cellular phones are also in use by a rapidly-growing number of private citizens.

Authorization for Use: Except for cellular telephones, hand-held transceivers require a station or operator (amateur) license.

Typical Exposure Levels: Local E can be very high, over 200 V m-' a few cm from the tip of the antenna. However, the near-field exposure results in a relatively low SAR (peak SARs less than 1 W kg-', except at the higher microwave frequencies)(Balzanoet al., 1977)(see Cautionary Notes).

InstmentationlMeasurement Techniques: Isotropic E- and H-field survey meters may be used if they are specified to operate in the frequency range of the transmitter. If measurements are made at distances closer than 30 cm, an erroneous interpretation (false alarm) may be reached when a localized high field strength is observed for a unit operating a t a frequency below

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400 MHz, even though the degree of hazard is insignificant in terms of localized SAR (see the Additional Information).

Additional Information: ANSI (1982), ANSMEEE (1992b) and NCRP (1986) specifically address the lower-power, hand-held devices and other partial body exposures from near-field sources. These sources do not usually possess sufficient radiated power to deliver an SAR that is above the criteria set forth in the exclusion clauses of the above recommendations. These criteria set a value for a localized (peak) SAR of 8 W kg-I in any 1g of body tissue. Because of their long wavelength, 5 to 10 W hand-held radios generating high localized fields at 27 MHz and 150 MHz normally cannot induce high SAR levels anywhere in a human body. Only if a long-wavelength RF source exposes a significant part of the human body to high field strengths can the SAR conditions of these exclusion clauses be violated. Higher-frequency hand-held radios can induce high peak (local) SARs if operated with approximately 10 W or more and if the antenna is placed very close to the head (see Cautionary Notes).

Cautionary Notes: There is some concern that operators of higher frequency (microwave) units (400 MHz or above), which transmit powers of 5 to 10 W or more, or who talk for extended periods of time (over 6 min) may be exposed to localized SAR levels that exceed the limits of prevailing exposure standards. This concern applies only if the tip or shaft of the antenna is kept within 1or 2 cm of the head during this time, and RF transmission occurs for about 50 percent or more of the time. In this case, the limits of the exclusion clause above may be exceeded, depending on the RF power transmitted and the position of the antenna with respect to the head. Therefore, operators of high powered microwave units should be advised to keep the antenna away from their head and eyes.

A 7 Mobile Radios (Vehicle Mounted-Including Citizens

Band and Cellular Radios) Purpose: These devices are used for communications by police, security, business personnel and the general public. They are also used by amateur radio operators. CB units and cellular radio telephones are

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used by private citizens in both professional and nonprofessional applications.

Operational Characteristics: Vehicular units use a monopole or "rod antenna, usually about 15 to 150 cm long, depending on the operating frequency.

Frequency Range: Communication bands from 27 to 50 MHz, 150 to 170 MHz, 450 to 512 MHz and 800 to 900MHz. CB radios operate a t approximately 27 MHz, cellular radios between 800 and 900 MHz. Power Levels: From 3 to more than 100 W are delivered to the antenna. Mobile cellular radios are limited to an ERP of 7 W, which limits the transmitter power to approximately 3.5 W.

Estimated Number in Use: In the CB, millions of 4 W units are in use by private citizens and truck drivers. Millions of cellular radio units and other mobile radios are in use in other applications.

Authorization for Use: Except for cellular telephones, these units require a station or operator (amateur) license in accordance with FCC rules. Qpical Exposure Levels: At distances of 1 m from the antenna, exposure levels are below 1 mW cmV2except for the highest power units (see Cautionary Notes). The near-fields decay quickly with increasing distance (Ruggera, 1979),and exposure of vehicle occupants results in a relatively low SAR (less than 1 W kg-' at any localized point in the body). For example, even though the local fields near a cellular radio antenna exceed commonly used RFPGs, a t normal transmitter power levels (3.5W) the peak exclusion (8 W kg-') for a person standing 9 cm from the antenna is only exceeded for transmitter powers greater than 35 W (Guy and Chou, 1986). Instrumentation/MeasurementTechniques: IsotropicE- and H-survey instruments may be used. The transmitter's frequency must lie within the specified operating range of the survey instrument.

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Additional Information: At frequencies of 400 MHz or above, resonant pickup may induce RF currents in the steering wheel or other circular or straight metallic objects in the vehicle, causing local high E or H to exist near these objects. Similarly vertical metallic elements such a s window partitions may re-radiate resulting in high local fields. High strength RF fields can also lead to the potential of RFI with automotive electronic systems (Lambdin, 1979;NBS, 1979).

Cautionary Notes: As described above, for high-power mobile-radio transmitters, local E can be high (over 200 V m-') within a meter of the transmitting antenna. It is unlikely, however, that during transmission the antenna on a vehicle would ever be near enough to a person (except for a parked vehicle), or that persons outside would remain stationary within a meter of the transmitting antenna long enough to exceed typical RFPGs. Special units used by law enforcement and security personnel may have transmitters that generate up to 100 W and some of these units may have their transmitting antenna concealed inside the front or back window of the vehicle. In this case occupants could be subjected to significant RF exposure due to the presence of high field strengths when long-term transmissions occur (e.g.,over 6 min in duration).

A.8 Diathermy Equipment (Microwave and Shortwave) Purpose: Medical heat therapy. Operational Characteristics: These units operate a t specific RF or microwave frequencies. They are intended to produce localized heating of an area of the body to a maximum temperature of about 42 "C. An applicator (special antenna) is placed close to or surrounds the part of the body selected for treatment. The applicator couples RF or microwave energy to that part of the body.

Frequency Range: Shortwave diathermy units operate a t fixed frequencies of 13.56 or 27.12 MHz (see Additional Information). Microwave diathermy devices operate a t a fixed frequency of 2,450MHz.

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Power Levels: Maximum output powers are 50 to 200 W, with microwave diathermy units generating the higher power levels. Estimated Number in Use: Many thousands of units are in use in medical and athletic facilities. Authorization for Use: FCC assigned ISM bands. Qpical Exposure Levels: Microwave units expose the operator to less than 10 mW cm-2 at distances of more than 60 cm (measured from the applicator). Shortwave units can create high operator exposures (greater than 10 mW ~ m - at~ distances ) of up to 1m from the applicator (Ruggera, 1980) (see Cautionary Notes).

Instrumentation/Measurement Techniques: Isotropic E (and H for shortwave units) survey instruments should be used. A careful, wide-area scan should be performed for shortwave units, since high fields can exist at locations other than close to the applicator (see Cautionary Notes). For microwave units, surveys within a 60 cm radius of the applicator are sufficient. Additional Information: The RF or microwave frequency a t which the unit operates can be determined by inspecting the operator manual, or inspecting the unit itself for FCC labeling which normally states the frequency (in MHz) of operation. EM1 is oRen generated by these devices, creating an indirect hazard to patients on monitoring equipment located near a diathermy machine. This is because stored data in electronic (especially critical-function monitoring) medical devices and control software can be destroyed in computerized systems, rendering the affected device inoperative until manually reset. Cautionary Notes: For shortwave (27 MHz)diathermy, very-highE exists along twinlead or unshielded (noncoaxial) cables that run from the operator's console to the applicator. These fields can expose the operator (at the console), and nonprescribed areas of the patient's body to high

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levels of RF radiation. In addition, contact with these cables by certain objects (a foam pillow for example) have caused the objects to burn and emit smoke which also may be hazardous.

A.9 Electrosurgical and Electrocautery Units Purpose: To cut andlor cauterize tissue (stop blood flow) for general and specialized surgical procedures.

Operational Characteristics: Electrosurgical and electrocautery units (ESUs) consist of a RF generator, two insulated, but unshielded cables, a grounding, or dispersive metallic plate coated with conductive saline gel and a "knife" or hand-held probe similar in function to a scalpel.

Frequency Range: CW or amplitude modulated RF from 0.5to 2.4 MHz. Strong harmonics and spurious frequencies produce a broad spectrum with frequency components up to 100 MHz. The modulation frequency is approximately 10 to 30 kHz.

Power Levels: No intentional radiated RF power is generated.

Estimated Number in Use: Thousands are in use, with one or more units in every operating room, and many in physician's offices.

Authorization for Use: These devices do not intentionally radiate RF, so no FCC authorization is assigned. However, see the Additional Information Section concerning local EM1 caused by these devices.

'I'ypical Exposure Levels:

E of several hundred V m-' and H of over 0.1 A m-' are present a t a distance of 30 cm from the "knife."

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Instrumentation/MeasurementTechniques: Caution must be exercised in the proper use and selection of measurement instruments or totally erroneous data will be obtained. RF isotropic survey meters must NOT be used in the frequency range of about 300 kHz to 100 MHz unless so specified in that range. Even then, caution must be used to prevent measurement errors associated with, among other sources, the operator becoming part of the probeantenna system (see Section 4.2.4). Also, the electronic readout of the meter must be well shielded from EMI. Totally self-contained RF survey instruments (active monopoles) are commercially available that are designed so that thereadout (display)and the associated electronics are enclosed in a metal box. The antennas for sensing RF fields are monopole (single wire) antennas extending through the walls of the box. These survey instruments are the most resistant to EM1 (see Section 4.1.1).

Additional Information: High field strengths also are present along the unshielded cables of ESUs. The fields decrease as a function of increasing distance and, a t 1 m E is about 50 V m-'. No studies or measurements of SAR have been performed with these units. These medical devices primarily generate fields with very long wavelengths, fields which do not couple well to the body. Also, they are used intermittently, so the time-averaged SAR in an exposed person is reduced. However, since the "knife" is held by the surgeon or other operator, and the unshielded cable is sometimes draped over the surgeon's shoulder, high currents may be induced in the body of the operator. No studies of induced currents have been performed. In terms of indirect hazards to a patient, the EM1 generated by these units is so severe that most medical electronic instrumentation in use in an operating room is temporarily "disabled whenever the ESU is used. This could result in more permanent problems in computerized medical devices where data and programs stored in semiconductor memories may be temporarily destroyed, rendering critical monitoring and other devices "inoperative" until the affected device is manually reset.

Cautionary Notes: Erroneous field-strength readings will be obtained unless the operator uses proper instrumentation that covers the entire frequency band of the ESU emissions, and does not pick up spurious interference by the cable between the probe and the readout meter.

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A.10 Hyperthermia Equipment Purpose: Medical treatment of cancer. Operational Characteristics: These units operate over a wide range of RF and microwave frequencies. They are intended to produce localized or regional (large area) heating of tumor tissue to a temperature of 42 "C or higher, while sparing normal tissue from this tumoricidal temperature. Normally, hyperthermia is used in conjunction with ionizing radiation and drug therapy. In some cases, an applicator (antenna) is placed in contact with the skin, or in other cases, microwave heating is delivered with an insertable needle (interstitial applicator). For regional heating, an array of surface applicators or a single large coil surrounds the patient's body. Both the surface and regional heating applicators are specifically designed to couple RF energy into the tumor. Interstitial applicators deliver microwave heating at the tip or along the shaft of the inserted applicator. Implanted thermometers monitor the temperature of the tumor during the time of the treatment, normally 20 to 50 min. Frequency Range: Some units operate a t a fixed ISM frequency, i.e., 13.56, 27.12, 433,915 or 2,450 MHz. Other units can be operated at any frequency from 1to 1,000 MHz. Power Levels: The power levels range from less than 25 W for interstitial applicators in small tumors to 2,000 W for regional heating systems. Estimated Number in Use: Several hundred hyperthermia devices are in use in the United States. In addition, there are many one-of-a-kind devices and medical diathermy devices being used by individual practitioners. Authorization for Use: Some units operate in the ISM bands. Units operating a t other frequencies are usually placed in a "screen room" to prevent RF energy from radiating and interfering with communications and navigational equipment.

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Typical Exposure Levels: Small surface applicators and interstitial applicators for localized heating usually require 50 to 150 W and 10 to 25 W of power, respectively. These produce S less than 10 mW cm-2 when measured at a distance of about 10 cm from the applicator. High-power regional heating units (using over 500 W) are usually placed in screened rooms constructed of metallic walls, floor and ceiling. These units can create high operator exposure levels (greater than 10mW at distances of up to 1 m from the applicator. In addition, in a screened room, "hot spots" caused by reflection can appear in other parts of the room, far removed from the regional heating unit's array of multiple applicators or single coil (Bassen and Coakley, 1981; Bassen, 1985a; 198523).

Instrumentation/MeasurementTechniques: Isotropic E and H survey instruments should be selected according to the frequency being used a t the time of the survey. A careful area scan should be performed. Probe burnout can occur when an unsuspected "hot spot" is encountered, unless the probe is removed from this localized area immediately.

Additional Information: EM1 is often generated by RF hyperthermia devices, creating an indirect hazard to patients being treated nearby. This is due to the fact that stored data and control software (especiallycritical-function monitoring) in computerized systems can be destroyed rendering the device inoperative until it is manually reset.

Cautionary Notes: As stated above, screened rooms containing high-power regional heating hyperthermia systems can create distant "hot spots." These hot spots can create resonant-coupling effects in distant metallic objects that are in the room. This coupling or "capture" of RF energy can cause the plastic insulation on metallic cables to melt, when the cable contacts an object that is electrically L'grounded"to the metallic floor or walls of the screened room, or when the cable touches the floor directly. Also, the melting of plastic floor tiles andlor the rubber wheels of metal carts has been observed during treatment sessions, at points where the carts legs touch the floor, if the cart is in a part of the room containing a "hot spot." Burnout of some probes can occur when an unsuspected "hot spot" is encountered, unless the probe is removed immediately from the area. Medical personnel

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should be waned of the existence of these high-exposure zones, both near the patient and at the remote hot spots, and be told to not occupy these locations for periods of time longer than 1or 2 min so that the time-averaged exposure does not exceed the appropriate RFPG. Since the location of the hot spots can change, depending on the treatment setup, frequency and the position of conducting objects and people in the room, the equipment operator should scan the room containing a high-power, regional heating system, during each treatment.

A.ll Magnetic Resonance Imaging Systems Purpose: Medical diagnostic imaging.

,

Operational Characteristics: These systems use a combination of a very strong static and pulsed magnetic field and pulsed RF' fields to produce three-dimensional views of the internal structures of the body. This technique provides better images of certain soft tissue structures than can be obtained with x-ray systems, and does so without exposing the patient to ionizing radiation.

Frequency Range: 1to 100MHz pulsed RF, static magnetic fields and pulsed magnetic fields with a rate of change of 0.1 to 10 T s-l.

Power Levels: Average RF powers of up to 100 W (10 kW of instantaneous peak RF power).

Estimated Number in Use: Several hundred.

Authorization for Use: FCC's limits for radiated power in navigation and communications bands must be maintained outside of the hospital. To meet this requirement, shielded rooms are utilized to house magnetic resonance imaging (MRI) devices. (Note: The shielded room also reduces

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the potential for interference to MRI systems from external RF sources.)

Typical Exposure Levels: Patient exposure is described in terms of SAR. In some systems, with very high pulsed RF power and high duty cycle, these levels may approach or exceed 0.4 W kg-' whole-body average SAR for periods exceeding 6 min. However, this is a medical system where risk versus benefit to the patient must be considered. Personnel operating the equipment are exposed to much lower levels of RF radiation (Athey and Ruggera, 1982; Tenforde and Budinger, 1986). Static magnetic fields of 0.1 to 2 T and pulsed magnetic fields with a rate of change of 0.1 to 10 T s-I exist at the patient position. Athey and Ruggera (1982), Budinger (1981) and Tenforde and Budinger (1986) provide useful background on MRI systems and associated fields.

Instrurnentation/MeasurementTechniques: RF isotropic survey instruments (E and HI plus static magnetic field meters (gauss meters) with high field strength measurement capabilities should be used to perform wide area surveys in the vicinity of MRI machines.

Cautionary Notes: Care should be exercised while an operator is in the presence of very strong magnetic fields so that magnetic objects do not fly into the MRI device. Also, peak RF field-strength levels increase rapidly as the patient treatment "tunnel" is approached and care should be taken to avoid burnout of the probe sensors. Individuals wearing cardiac pacemakers should be excluded from areas near the MRI device where the static magnetic field level exceeds 1mT. At higher field levels, inadvertent closure of the pacemaker's reed switch can occur, leading to an asynchronous pacing mode that can be competitive with the heart's endogenous pacing rate. Care must be used in observing the instrumentation to determine that, for example, mechanical, analog meter movements are not affected by the strong static magnetic fields.

A.12 Radar Purpose: Radio navigation, weather detection, vehicle speed measurement. Operational Characteristics: Radar is used in a variety of applications. Conventional radar relies on the transmission of pulses of microwave energy and the

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subsequent detection of a return signal, i.e., pulses which have been reflected by some object (target). By measuring the amount of time between the transmitted pulse and the return pulse, the distance to the reflective target can be determined. By using a rotating, directional antenna, the direction or the elevation angle of the target may also be determined. The pulse repetition frequency fp and pulse widths are selected on the basis of the desired range of the radar and other intended applications. Typical fp falls in the range of a few hundred to a few thousand pulses per second. Pulse widths are normally in the range of 0.5 to 10 ps. CW, or doppler radar, is used for weather surveillance and for measuring the speed of vehicles, such as in highway law enforcement. CW radars use the Doppler effect to determine the speed of the target by measuring the change of frequency of the reflected signal, caused by the moving target, relative to the transmitted signal. Frequency Range:

Radars normally use frequencies greater than 1 GHz, but in some applications may operate as low as 300 MHz. Greater spatial resolution is achieved with higher frequencies. An extremely small number of military over-the-horizon radars operate in the HF spectrum of 3 to 30 MHz. Police traffic radar in the United States operates at 10.525 and 24.15 GHz and approximately 35 GHz. Power Levels: Radar transmitters operate a t a wide range of power levels, depending on the specific application. Aircraft weather-radars and small marine radars may use peak transmitter powers in the range of 40 to 80 kW. Radar powers are normally specified in terms of peak-pulse-power during the transmitted pulse, not the average or rms power which is normally associated with RF hazard assessments. Thus, while an aircraft weather-radar may operate at a peak power of 80 kW, the rms value is probably closer to 80 W. The average power is determined by the duty cycle of the transmitter, which is given by: 8 = fpt

(A.8)

where 6 is the duty cycle, fp is the pulse repetition frequency and t is the pulse width. The duty cycle is, thus, the ratio of the time the pulse is on, to the total time from the beginning of a pulse to the beginning of the next pulse, i.e.,the period. Typical pulsed-radar duty cycles are in the range of to lob3.Large air-route surveillance radars, used for air-traffic control, operate a t transmitter peak

.

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powers as high as several megawatts. Such powers coupled with high gain antennas can result in ERPs as high as 10 to 15 GW or higher. CW traffic radar units operate at very low average power levels, typically 100 mW or less, most commonly in the 15 to 25 mW range.

Estimated Number in Use: The total number of radars in operation, although unknown, is large. Every major airport has some form of surveillance radar for air-traffic control. At other locations (normally not at airports), longer-range radars are employed in the national air-traffic system. Similar size radars are found at military air bases. Thousands of small boats, as well as all large ships, possess some form of marine radar. Virtually all commercial aircraft are equipped with a weathersurveillance radar in the nose of the plane. Thousands of small, vehicular speed radar units are in operation. Authorization for Use: Civilian radars are licensed by the FCC while government operated systems, such as those used by the Federal Aviation Administration, are authorized by the Interdepartment Radio Advisory Committee within NTIA of the Department of Commerce. mica1 Exposure Levels: The primary area of concern with RF exposure from radars is the occupational setting. This is because of the highly directive nature of the transmitting antennas which tend to greatly reduce radiation levels at locations outside of the main beam. In general, radars are designed to radiate toward the horizon with minimal exposure of objects in the foreground (which causes ground clutter and reduces effectiveness). Relatively low average powers also reduce rms exposure levels. Most radar antennas are some form of aperture antenna, usually having a parabolic shape with cross-sectional shapes that are either circular or rectangular, and are fed with some form of horn radiator positioned in front of the main reflector. For aperture antennas, the near-field S is a function of the reflector efficiency and the cross-sectional area. A commonly used equation (Hankin, 1974), for the circular parabolic reflector antenna, for computing the maximum expected near-field on-axis S is S = 4gA-' (A.9) where S is the power density on axis, .II is the reflector efficiency (typically in the range of 0.55 to 0.75), P is the average power delivered to the antenna and A is the cross-sectional area of the reflector.

A.12 RADAR

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It is clear for a given power level, that antennas with small areas produce higher S levels. Thus radars with small antennas should be given as much, if not more, consideration than larger antenna systems of comparable power. Figure A.l, taken from Tell et al. (1976a1, illustrates measured S values for several aircraft radar units. In specifying exposure levels for radars, it is important to note whether the value is for the antenna fixed in a specific direction or when rotating. Often, because the measurement of exposure levels is complicated by moving beams, the antenna is stopped to facilitate the measurement process. In such cases, the indicated exposure levels are not applicable to routinely encountered exposure at a given distance from the antenna. It is necessary to correct such "fixed beam" values by multiplying by the antenna's rotational duty cycle. The rotational duty cycle is normally defined as the ratio of halfpower beam width, of the radiation pattern, to the angular extent of one complete cycle of the antenna scan. For antennas rotating in a complete circle, this would be 360 degrees; for sector scan antennas, this would be less than 360 degrees.

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Distance from Radome (m) Fig. A.1. Measured data for several aircraft radar systems including the WP-103 on a BAC-111 aircraft with radome; WP-103 on a BAC-111 without radome; WP103 on a BAC-111; AVQ-20 on a Convair 520; AVQ-2OC on a DC-9; AVQ-50 on a Convair 520. The measurements were made in the main beam with antenna rotation stopped and then an average exposure calculated with the antenna rotating.

'

The area of maximum hazard potential with radars is between the feed-horn and the reflector surface. Maintenance activities on radar antennas should include special caution for workers who may need to get directly in front of the aperture or between the feed-horn and reflector. On large radar systems the transmitter should be shut down to prevent excessive exposure. Also, caution must be used to avoid being struck by moving parts of the antenna system.

Instrumentation/MeasurementTechniques: Both narrowband and broadband instrumentation is suitable for radar exposure measurements. Commonly, portable survey meters will sufficefor assessing hazards near aradar. Because radar systems are generally pulsed, it is useful to employ an average responding detector, such as a thermocouple type element, in a survey instrument. However, a word of caution is appropriate when using such instruments in areas subject to high-peak field strengths. The manufacturer's specification on peak S limitations should be studied in relation to the known duty cycle of the radar transmitter. It is possible to subject the probe to average S which are well below the "CW burnout" level of the instrument, while at the same time, because of the small duty cycle of some radars, the instrument's peak S burnout limit is exceeded. If a thermocouple exceeds its peak power dissipation limit, it will become an open circuit, preventing the instrument from responding to the field. In some cases, because of an upset in the balance of the circuitry of the instrument, a negative reading may result. Instruments with diode detectors are essentially burnout proof, but generally do not provide correct average fieldstrength measurements for pulsed radars because in many cases, they are driven beyond the square law region of operation (see Section 4.1.1). When surveying rotating radar antennas, the rotation should, if possible, be stopped to facilitate measurements along the axis of the radar antenna. If the antenna is not stopped, the relatively long time constant of the instrument will generally preclude accurate measurement of the maximum value of S when the radar antenna is momentarily pointed in the direction of the operator. The indicated reading will be less than the true value. Monheiser (1985)developed a system for evaluating the time response of R F hazard survey probes by simulating actual rotating radar antenna beams. The use of broadband survey meters in surveying rotating radar antennas has been addressed by Aslan (1980).In the described technique, the survey instrument response time to a step input is characterized. Then, making measurements with the max-hold feature of

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the instrument, the peak S readings, obtained during maximum exposure when the rotating antenna sweeps past the measurement point, are corrected for the fact that they are lower than the stoppedbeam S. The technique relies on a knowledge of the radar antenna's beam width, the antenna's rotation time and a careful determination of the time constant of the instrument for various illumination periods. In certain situations, the use of peak signal holding spectrum analyzers can be helpful in determining exposure levels. In this case, a measurement of the instantaneous peak field strength is made as the moving radar beam passes by and then, through a measurement of the radar's duty cycle, the peak readings may be corrected to equivalent average field-strength values.

Special Considerations: Because radars typically possess rather high-peak powers, instantaneous peak field strengths can also be high. High-power radars may cause RFI in electronic devices, including field measuring instrumentation within 1 to 2 km. Special precautions should be taken not to exceed the peak S ratings of thermocouple measurement probes in high-peak power-pulsed radar fields.

A.13 Marine Radar

Purpose!: These systems are used on large and small vessels for navigation and for harbor surveillance.

Operational Characteristics: These systems produce pulsed microwave transmissions that are reflected by objects such as docks, buoys and other vessels. The antenna sweeps over a 360 degrees rotational angle (full scan), or less than 360 degrees (sector scan),in a horizontal plane and parallel to the surface of the water. The units use a rotating parabolic reflector antenna (circularor rectangular) or a rotating phased-array antenna consistingof many individual waveguide radiators aligned alongside one another, to produce a narrow, far-field beam. The duty cycle (the ratio of time on to time of0 of the RF transmitter is typically 0.00075, so the average radiated power is less than 0.1 percent of the peak radiated power. S a t a fixed point, is further reduced by the rotational

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duty cycle of the antenna. The microwave beam is narrow, especially in the horizontal plane.

Frequency Range: 9,330 to 9,500 MHz.

Power Levels: These units transmit up to 20 kW peak pulse power, with a typical unit generating 10 kW.

Estimated Number in Use: Thousands of units are installed on pleasure craft and small commercial craft.

Authorization for Use:

FCC authorized bands for marine radar. Typical Exposure Levels: Approximately 1mW cm-2 average at distances of more than onehalf the length of the antenna (typically about 1 m) (Peak et al., 1975).It is not normally possible to get closer to the rotating antenna (see Additional Information).

InstrumentationJMeasurernent Techniques: A microwave isotropic survey instrument E of the thermocouple type is the preferred instrument. Diode-based units may not respond correctly due to the extremely high instantaneous S that occurs during each pulse (lasting less than 1ms) (see Section 4.1.1). Generally, measurements are made in the main beam or at the point of interest with the antenna stationary. These data are then corrected for the rotational duty cycle to determine potential time-averaged exposure.

Additional Information: It is not normally possible to get closer to an exposed rotating antenna than 1 m, since this is approximately one-half its length. An instrument placed closer than this distance would be struck by the rotating antenna. The microwave beam is sufficiently narrow in the vertical plane, so that a person directly below or above the antenna will be exposed to levels of RF that are a small fraction of

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power density in the main beam. In addition, since the antenna is rotating, S at any fixed point, averaged over many seconds (several rotational cycles) is reduced by the rotational duty cycle, i.e., the ratio of the beamwidth to the scan width. For example, the duty cycle for an antenna with a 2degrees beamwidth and a full rotational scan would be 21360 degrees or 0.006.

Cautionary Notes: Persons should not be exposed at very close range to a malfunctioning, nonrotating antenna, since the increased exposure levels would be many times more than the exposure levels experienced during a rotating-antenna situation. Also, an isotropic survey meter using diode detectors may not work properly, due to the high-peak powers occurring over a fraction of a microsecond. Care should be exercised when working near moving antennas to avoid physical injury to measurement personnel and instruments.

A.14 Police and Sports Radar

Purpose: Determination of the speed of moving automobiles, in-flight baseballs, etc.

Operational Characteristics: These are low-power doppler radar systems that detect objects moving through or along a narrow beam path originating from a small, hand-held transmitter or radar "gun."

Frequency Range:

10.525 GHz, 24.15 GHz and approximately 35 GHz. Power Levels: The radiated power is less than 100 mW, typically less than 35 mW.

Estimated Number in Use: Many thousand in police vehicles. Sports units are used by athletic departments and in amusement parks.

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Authorization for Use: FCC assigned ISM bands.

Typical Exposure Levels: Less than 1mW cm-2 at distances greater than 30 cm from the transmitter. Table A.2 shows measured and calculated S for an older style traffic radar unit (Hankin, 1976),see also NBS (1981). Instrumentation/MeasurementTechniques: Isotropic microwave survey instruments E should be scanned slowly throughout the interrogated area.

Cautionary Notes: Persons should not position any part of their bodies within approximately 30 cm of the radiating surface (aperture) of the antenna, along the "beam path" (in front of the antenna) for periods of time, e.g., exceeding 6 min. This may, but not necessarily, result in "farfield" microwave exposures to small areas of the body that approach applicable RFPGs.

A.15 Microwave Industrial HeatingIDrying Equipment

Purpose: These units are used for the thermal processing and drying of manufactured goods such as rubber, wood or paper. Operational Characteristics: These devices are almost always totally-enclosed systems with metal shielding and leak-proof doors or metal tunnels that allow the insertion or removal of the object to be processed. TABLEA.2-Predicted and measured S for MR7 t m f i mdnr analytical values (from Hankin, 1976). Distance value (meters)

0.089 0.66 0.91 1.32 2.67 28.3

Point source Value model (W m-2)

4

2.1 1 2.44 x lo-' 2.17 x

Circular aperture Value model (W m-')

Measured Value (W m - 9

1.25 X 10' 4.08 2.15 1.02 2.49 x lo-' 2.22 x l o - 3

4 2.19 2.5 X lo-' 6.92 X lo-' 3.47 x lo-' 2.76 x

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Frequency Range: 915 or 2,450 MHz.

Power Levels: Several thousand watts or more are commonly used.

Estimated Number in Use: Several thousand in configurations customized for the particular product to be processed.

Authorization for Use: FCC assigned ISM bands. Typical Exposure Levels: In general, S is less than 1m W cm-2 at or beyond the surface of these easy-to-shield enclosed systems. However, due to the high power levels involved, higher exposure fields may exist, especially on older units where the seals and shielding may have degraded.

Instrumentation/Measurement Techniques: Isotropic E survey instruments should be used. After performing a rapid area scan of the surroundings and finding no high-level fields, the probe of the survey instrument should slowly be scanned along any cracks or seams in the metallic enclosure. The tunnel opening should be approached gradually from a distance of about 1 m, to avoid possible instrument burnout.

Additional Information: These are specialized, custom-made systems, produced by only a few companies.

Cautionary Notes: As mentioned above, due to the high power levels involved, highexposwe levels may exist. Therefore, the operator should carefully

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scan all areas of the surroundings,approaching the microwave device from a distance (see Section 4.2.2).

A.16 Radiofkequency Induction Heaters Purpose:

RF induction heaters are used for efficient heating or melting of materials such as metals, semiconductorsand some dielectrics. Some major applications of these heaters in metal processing are deep and surface hardening, tempering, annealing, stress relieving, brazing, soldering, glass-to-metal sealing, fatigue testing and melting. Other applications are electron-tube flashing, semiconductor processing (zone refining), crystal growing, epoxy curing, cap sealing, heating and drawing optical fibers and plasma torching. Operational Characteristics: The products to be processed by an induction heater are placed inside or near a coil applicator which has a high RF current and which generates a dominant magnetic field. Many induction heaters are fully automated and require no hands-on operation. Some require limited hands-on operation and some are portable or free-standing with the operator using an RF applicator attached to a flexible cable. The latter requires almost constant operator involvement (Centaur, 1982) and, consequently, results in higher operator exposure for greater periods of time.

Frequency Range: The frequencies typically used for induction heating range from about 60 Hz to 8 MHz, depending on the application.

Authorization for Use: There is no limit on fields in the ISM bands. Fields emitted outside of ISM bands are limited by FCC regulations (Part 18) to minimize interference with other FCC authorized communications services.

Power Levels: About 300 W to 5,000 kW. Number of Exposed Workers: Greater than 100,000.

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Typical Exposure Levels: Exposures to t h e hands have been reported to range from 2.3 x lo5 to 4.0 x 10' A2 m-2 (Lovsund and Mild, 1978) and from 3.8 x lo5 to over 10' V2 m-2 (Centaur, 1982). In areas commonly occupied by operators, exposures have been reported to range from 1.13 x 10' to 1.13 x lo4 V2 m-2 (Centaur, 1982) and from 0.01 to 300 A2 m-2 (Conover et al., 1986).

Survey Instrument Specifications: A broadband isotropic RF survey meter (with electric- and magnetic-field probes) should be used for induction heater surveys at frequencies above a few MHz. At frequencies below a few MHz,the preferred instrument for the E measurement is the self-contained active monopole device. The appropriate isotropic magnetic field probe should also be used. Special attention should be given to the following Cautionary Notes when selecting an instrument.

Cautionary Notes: Spurious responses of survey instruments can cause sizable measurement errors. For example, the high-resistance leads often used to connect the field sensor assembly and the meter electronics can have significant field pickup (resulting in an error response) below about 300 kHz. This is why the self-contained monopole device is appropriate for measuring the electric field. This type of device also eliminates the problem of scalar potential effects. The operator should also be aware of possible measurement errors associated with capacitive coupling between combinations of the following: the RF source, components of the field survey instrument, the operator and earth ground (Kucia, 1972).

Dynamic Range: For the magnetic field strength, the instrument should be capable of accurately measuring mean-squared (ms) fields from about 1 to 1 x lo4A2m-'. For E, it should be capable of accurately measuring ms fields from about 1 x 10' to 1 x 10' V2 m-'.

Probe Overload/Burnout Rating: The electric field probe should continuously withstand at least 2 x lo7 V2 rn-' ms and the magnetic field strength should continuously withstand at least 2 x lo4 A2 m-2 ms. These ratings also include the high-resistance leads in the probe handles.

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Temperature Sensitivity: Possible measurement errors due to responses of the survey instrument to high temperatures should be given careful attention. This is important because many products processed by induction heating reach 1,000 to 2,000 OF and some reach even higher temperatures. The actual ambient air temperature and the duration that the survey instrument is a t the measurement location should be considered. Response to Other Radiation: Possible error responses associated with exposure to high intensities of infrared, ultraviolet and visible light radiation emitted by products at high temperatures, should be considered. Survey Procedures: General survey procedures are given in Section 4 (Instruments and Measurement Techniques). Additional specific information for RF induction heater survey procedures are given below. 1. Frequency Measurement: An appropriate frequency counter and antenna or grid-dip meter should be used to determine the fundamental frequency. The fundamental frequency of an induction heater may vary considerably with the material being processed. Since the response of the field survey probe and the RFPG can vary significantly with changes in the fundamental frequency, it is important to measure the fundamental frequency to interpret properly the measurements relative to a given RFPG. 2. Duty Factor Determination: The duty factor is the time the operator is actually exposed during the averaging time specified by the RFPG (eg., during any 6 min operation period) divided by the averaging time. Many induction heaters operate continuously for periods up to several hours. However, induction heater operators often are not in close proximity to the heater (where the exposures exist) for long periods of time. For example, if an averaging time of 6 min is specified in an RFPG, and an operator was only near the heater for 2 min, and the induction heater was on continuously, the duty factor would be 0.333 and the measured levels should be multiplied by the duty factor to determine exposure. 3. Measurement of Operator Exposure: a. Measure both E2 and P at least at the following operator positions: eyes, neck, chest, waist, gonads, knees and ankles; and search for the maximum field strength. b. Measure with the surface of the survey probe close to, but never touching, the operator. A separation distance of at least 10 cm is acceptable.

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c. Measure operator exposures while the induction heater operates with different applicator coils, materials being processed, power output and frequency to determine maximum "worstcase" field strengths. 4. Data Analysis and Interpretation: The exposure measurements must be corrected for duty factor and frequency response (of the survey instrument) before comparing them with occupational exposure standards or RFPGs. It should be noted that field measurements made in close proximity to an operator will represent perturbed fields. Such fields are not useful for relating accurately to exposure limits in RFPGs (which are typically based on uniform field exposures). Nonetheless, pertured field measurements are usually made due to practical consideration related to operation of induction heaters.

A17 Radiofrequency Dielectric Heaters (Heat Sealers)

Purpose: RF dielectric heaters are used to heat, dry, emboss, melt, seal or cure materials which are poor electrical conductors. Some examples of industries using these heaters are as follows: automotive, food, furniture, glass, paper, plastics, rubber, textile and wood products. Operational Characteristics: The products are usually processed by placing the dielectric material between two parallel metallic plates or a similar configuration, and applying a high RF voltage between the plates. Frequency Range: Frequencies used for dielectric heating typically range from 3 to 100 MHz, depending on the application. RF heaters operating within the ISM bands a t 13.56, 27.12 and 40.68 MHz can emit unlimited fields in these bands according to FCC Rules and Regulations (Part 18). Authorization for Use: The fields emitted from RF dielectric heaters operating outside these ISM bands are limited by Part 18 of the FCC Rules and Regulations to minimize interference with other electronic devices.

Nominal Output Power:

Number of Exposed Workers: Approximately 50,000.

Typical Exposure Levels: Survey data indicate that operators are exposed to electric-field strengths ranging from 0.001 to 1,500 V m-' and magnetic-field strengths from 0.001 to 6.5 A m-l (Cox et al., 1982). Field survey measurements on 82 RF heaters document that 38 percent of the heaters exceeded 61 V m-I or 0.16 A m-l (Stuchly et al., 1980). Exposure measurements on another 82 RF heaters indicate that 96 percent of the heaters surveyed exceeded 50 V m-' and 80 percent exceeded 0.15 A m-l (Conover et al., 1980).

Survey Instrument Specifications: A broadband isotropic RF (1 to 100 MHz)survey meter (with E and H probes) should be used. Special attention should be given to the following cautionary notes when selecting an instrument.

Cautionary Notes: Extreme caution should be taken by the operator to avoid serious

RF electrical shocks and burns, and survey instrument burnout. These can occur if the operator touches the RF sealer "top plate," which can be at high RF potential (thousands of volts above ground potential). Also, the high fields near the RF sealer will destroy most survey instruments if the probe is placed within a few inches of the high RF voltage areas.

Spurious Responses: Spurious responses of survey instruments can cause sizable measurement errors. Survey meters used for RF dielectric heater exposure measurements are particularly susceptible to spurious error responses due to the very high field strengths existing near these heaters. Normally, the specifications for such spurious responses are not given by instrument manufacturers. However, performance evaluations have been made on RF survey meters for spurious responses and other parameters (Nesmith and Ruggera, 1982).

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173

Dynamic Range: For E, the instrument should be capable of accurately measuring fields from about 1.8 x lo3 to 5 x lo6 V2 m-'. For H, it should be capable of accurately measuring fields from about 1 x lo-' to 1 x lo2 A2 mP2.

Probe Overload/Burnout Rating: The E probe should continuously withstand a t least 1 x 10' V2 m-2 and the H probe should continuously withstand at least 500 A2mP2. These ratings minimize the possibility that probes will burn out because of high RF fields commonly found around RF heaters. 'l'hese ratings also apply to the high-resistance leads in the probe handle.

Survey Procedures: General survey procedures are given in Section 4 (Instruments and Measurement Techniques). 1. Frequency Measurement: The proper frequency counter and antenna, grid dip meter, etc., should be used to determine the fundamental frequency. Care should be taken that the location of the antenna used with a counter is sufficiently removed from the heater to preclude burnout of the input circuitry of the counter. 2. Measurement of Operator Exposure: a. Measure both E2 and ?P at least at the following operator positions: eyes, neck, chest, waist, gonads, knees and ankles; and search for the maximum field strength. Measurements at only a few isolated locations can easily fail to detect higher field strengths at other positions because the levels are variable and unpredictable. It should be noted that field measurements made in close proximity to an operator will represent perturbed fields. Such fields are not useful for relating accurately to exposure limits in RFPGs (which are typically based on uniform field exposures). Nonetheless, pertured field measurements are usually made due to practical considerationsrelated to operation of induction heaters. b. Measure with surface of probe close to but never touching the operator. A minimum separation distance of about 10 cm is recommended. c. Keep the probe handle parallel to the floor (horizontal)and to the front edge of the RF heater applicator press (wherepresent) to minimize false meter readings caused by the interactions of the field with the high-resistance Ieads inside the probe

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handle and with the cable connecting the probe and readout meter electronics. d. Measure operator exposures while the sealer operates with different dies, materials being processed, power output, plate current, etc., to determine maximum "worst case" fieldstrength levels. 3. Data Analysis and Interpretation: The exposure measurements taken must be corrected for duty factor and RF frequency response (of the survey meter) before comparing them with exposure standards or RFPGs. 4. Worker Exposure Reduction Techniques: Effective techniques to reduce worker exposures from RF dielectric heaters, especially the installation of shielding, can be successfully applied to numerous types of RF dielectric heaters.

A.18 Anti-Theft Devices

Purpose: These devices interrogate persons and objects that pass through a "portal" area containing one or two transmittreceive "pillars" or posts. These systems detect the presence of special "tags" that are attached to merchandise or books. Only tags that are less than a few feet from the transmitting pillar are detected.

Operational Characteristics: These systems produce weak, pulsed or continuous microwave and1 or LF fields that are produced by the "pillar." The tags contain a diode and antenna that re-radiate a modified signal back to the system.

Frequency Range: VLF, LF and HF bands. Some systems generate fields in only one of the above bands while other systems generate both a LF and a microwave field simultaneously.

Power Levels: Only a few milliwatts of microwave power are radiated. Magnetic fields associated with the LF systems may be in the 1 A m-' range. Estimated Number in Use: Thousands of units are installed in retail stores and libraries.

A.18

ANTI-THEFTDEVICES

I

175

Authorization for Use: The microwave emissions are in the ISM band and FCC approval is issued for the LF (10 kHz) devices, under Part 15 of the FCC Regulations (Restricted Radiation Devices).

Typical Exposure Levels: Less than 0.1 mW ernA2(microwave) at distances of more than 5 cm from the pillar. LF field-strength data have not been reported in the literature, but are presently thought to be less than 100 V m-l. Based on the lack of induced SAR that can occur in the human body a t these low frequencies and the lack of sufficient energy to induce RF currents in nearby metallic objects that could cause shocks or burns, these devices are presumed to be "harmless" (see Cautionary Notes and Additional Information).

InstrumentationlMeasurementTechniques: A microwave isotropic survey meter (El is used to scan the area near each pillar. A potential problem may exist if the low frequency fields that are emitted by some of these devices, along with the microwave fields, cause electrical interference with the survey meter, and thus produce erroneous measurement data. The LF devices should be surveyed using the self-containedactive monopole instrument. Additional Information: The microwave anti-theft devices that have been tested in the past are not a radiation hazard, since persons are exposed only to the very low levels that exist at distances of more than a few inches from the transmitting pillars (see Cautionary Notes). For information on proper instrumentation to use for surveying the "microwave plus 10 kHz" anti-theft systems, the operator should check with the instrument manufacturer to see if the RF instrument to be used is susceptible to interference from LF, 10 kHz "out-of-band" fields (see Cautionary Notes).

Cautionary Notes: Persons should not lean on the pillars of microwaves systems for extended periods of time (more than 6 min). This could possibly produce local SARs in small regions of the body that might approach the RFPG. However, no study has been performed to verify this concept. Also, there have been older prototype systems, and there

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may be new or different systems that will appear on the market, which do emit strong microwave fields. There are also non-RFmicrowave devices used to detect magnets placed in objects (such as library books). Some systems use only VLF RF energy (10 kHz) to detect "tagged" items, and must be surveyed with LF instrumentation (see Section 4.1.1). Exposures to fields at frequencies of 10 kHz are not covered by some standards and exposure guides including those of NCRP (1986). Exposures at these frequencies are, however, covered by ANSYIEEE (1992b)and ACGIH (1993). The ACGIH RFPG is based on consideration of new technical publications citing the possibility of shocks or burns from broadly-distributed LF fields which can induce RF currents in large metal objects such as metallic fences. These objects could induce RF shocks or burns in persons touching the metallic object. The total power radiated from anti-theft devices at 10 kHz is generally low, and is localized so that this should preclude the induction of RF shocks or burns in persons touching any large metal object surrounding the anti-theft device. Finally, the operator should exercise caution in attempting to make accurate measurements by first determining the operating frequency of the anti-theft device being evaluated. If the device emits LF (10 kHz) fields, and an improperly-designed RF survey meter is used, erroneously-high readings can result in a "false alarm" situation (see Additional Information). A.19 Microwave Door Openers

Purpose: Automatic detection of persons approaching a door. Operational Characteristics: These systems are low-power doppler radar systems that detect objects moving in or through a narrow beam path that originates from a small, box-like transmitter. Frequency Range: 10.5 GHz. Power Levels: The radiated power is less than 10 mW.

Estimated Number in Use: Many thousands. These are commonly used in retail stores, businesses and industrial environments.

A.20 MICROWAVE INTRUSION ALARMS

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Authorization for Use: FCC assigned ISM bands. Typical Exposure Levels: Less than 1 mW cmP2at distances greater than 5 cm from the transmitter. Instrumentation/Measurement Techniques: An isotropic microwave (E) survey instrument should be used to slowly scan the area of interest.

Cautionary Notes: If an operator places the measurement probe in direct contact with the surface (aperture) of this device, S indications may exceed 1mW ~ m - This ~ . situation may create a "false alarm" on the part of an inexperienced operator who does not realize that the field strengths drop off rapidly with increasing distances, resulting in very low exposure levels only inches from the device. Measurements should be made at those points normally occupied in order to obtain realistic exposure values. A.20 Microwave Intrusion Alarms

Purpose: Personnel detection for theft protection or security monitoring. Operational Characteristics: These systems are low-power doppler radar systems that detect objects moving in or through a narrow beam path that originates from a small, box-like transmitter. Frequency Range: 10.5 GHz. Power Levels: The radiated power is less than 10 mW. Estimated Number in Use: Several thousand. These are commonly used in retail stores, industrial environments and homes.

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Authorization for Use: FCC assigned ISM bands. Typical Exposure Levels: Less than 0.5 mW at distances greater than 5 cm from the transmitter (Bassen, 1980).

Instrumentation/Measurement Techniques: An isotropic microwaveE survey instrument should be used slowly to scan the area of interest.

Additional Information: Many devices that are used for intrusion detection utilize ultrasonic or infrared radiation and not microwave radiation. The operator must identify the type of radiation being emitted, by examining the device label.

Cautionary Notes: If an operator places the measurement probe in direct contact with the surface (aperture) of this device, S indications may exceed 1mW cm-'. This situation may create a "false alarm" on the part of an inexperienced operator who does not realize that the field strength drops off rapidly with increasing distance, resulting in low exposure levels only inches from the device. Measurements should be made in areas normally occupied to provide realistic exposure values.

A.21 Microwave Ovens

Purpose: To cook, heat or thaw foods rapidly.

Operational Characteristics: These devices consist of a sealed metal container with a rectangular inner cavity in which the object to be heated is placed. Some industrial units have conveyorized metallic "tunnels" for the input and output of a continuous flow of foods.

A.21 MICROWAVEOVENS

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Frequency Range: 915 or 2,450MHz.

Power Levels: Less than 600 W to several thousand watts.

Estimated Number in Use: Many tens of millions in homes, factories, offices, canteens and restaurants; many thousands in food manufacturing plants.

Authorization for Use: FCC assigned ISM bands.

Typical Exposure Levels: Typically less than 0.1 mW cm-2 a t operator locations.

Instrumentation/MeasurementTechniques: A special purpose microwave oven survey meter is used to scan the door seams and other seams. These instruments contain a special 5 cm spacer cone which is placed in contact with the metal surface of the oven. For measurements that are not being made for official tests of compliance with the FDA performance standard (21 CFR 1030.10),isotropic E instruments may be used at a greater distance, so that the probe tip (typically 10 cm diameter) is not in contact with the oven's surface. Caution should be used when selecting an oven survey instrument. Evaluation of commercially available oven survey instruments has been carried out by the FDA (1977)and only instruments judged acceptable should be used. There are many inexpensive devices on the market with effective advertising claims for oven survey use, but their actual characteristics, accuracy and quality are uncertain. Also, the larger industrial "tunnel" ovens (that may be conveyorized), should be surveyed according to the methods described in Appendix A.15 (Microwave Industrial HeatingIDrying Equipment).

Additional Information: FDA under 21 CFR 1030.10 (Public Law 90-602)regulates RF emissions from microwave ovens. The microwave oven performance standard allows a 5 mW cmP2maximum leakage level, at any point along the oven surface, when measured a t a distance of 5 cm.

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Cautionary Notes: Conveyorized, industrial microwave food processing devices may not be covered by the FDA performance standard, and may emit higher leakage levels. These devices should be surveyed according to the methods described in Appendix A.15 (Microwave Industrial HeatingtDrying Equipment).

A.22 Visual-Display Terminals

Purpose: To display computer, word processor and other electronicallygenerated data.

Operational Characteristics: Alphanumeric characters and graphics are generated by electronically scanning and intensity modulating an electron beam@)that is incident on the phosphor-coated inner surface of the glass faceplate of a CRT. Upon collision with the phosphor, a portion of the kinetic energy of the electrons is converted to radiant energy, i.e., light. The display is similar in operation and appearance to a standard TV monitor or receiver.

Frequency Range: Visual-display terminals (VDT) are not intentionally designed to radiate EM energy other than light. However, spurious emissions predominantly associated with the sweep circuitry, digital clocks, etc., have been measured over a range of frequencies extending from 15 or 20 kHz to well beyond several hundred MHz (Petersen et al., 1980). The predominant emissions, both electric and magnetic, are pulsed fields, associated with the flyback transformer and deflection system. Harmonic analysis of these fields indicates that the energy content above 200 kHz is negligible (Petersen et al., 1980;Wolbarsht, 1980).

Power Levels: No RF power is radiated intentionally. The total input power to typical terminals is approximately 100 W or less.

A.22 VISUAL-DISPLAYTERMINALS

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Estimated Number in Use: Millions are in use, in industry, schools and homes.

Authorization for Use: These devices do not intentionally radiate RF energy, therefore no authorization is needed. However, compliance with Part 15 of the FCC Rules and Regulations, which define emission limits for computing equipment, is required. These rules and regulations are based on potential EM1 with telecommunication and navigation equipment.

Typical Exposure Levels: At the operator location electric fields of the order of 50 V m-I rms or less may be expected (FDA, 1981;Guy, 1984).Magnetic fields, of the order of 0.5 A m-' rms or less may be expected at the same location. The predominant fields are usually associated with the horizontal sweep circuitry (e.g., flyback transformer and deflection coils). Although the fields are nonlinear (pulsed) and, therefore, rich in harmonics, the energy associated with harmonics above 200 kHz is negligible. The ratio of the peak to rms values of both the electric and the magnetic field is typically between five and ten to one. Instrumentation/Measurement Techniques:

Caution must be exercised to use proper instrumentation and measurement techniques, or erroneous data will be obtained. Electric field measurements should be made with a displacement-current sensor type of instrument (Guy, 1987).Under laboratory conditions, a metal foil-covered phantom containing one or more displacementcurrent sensors (Harvey, 1984) may be used. Dipole type broadband probes should not be used. E measurements made at distances greater than 30 cm from the terminal can be made with the selfcontained active monopole type of instrument provided that the sensitivity is adjusted such that the lower end of the meter scale is used. This will ensure that the diodes are not driven beyond their design range of operation by the pulsed fields. Instruments possessing true rms response have been developed that are especially suitable for VDT measurements. Measurements of magnetic fields may be accomplished with either single loop or orthogonal loop type sensors (see Section 4.1.1). Magnetic field measurements should be made with a frequency-compensated loop sensor designed and calibrated for use down to 10 kHz (Guy, 1987). Under laboratory conditions, body current measurements may prove useful (Guy, 1987).

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Measurements of electric and magnetic fields and body currents associated with the operation of VDTs should only be made by personnel with specialized experience and knowledge in making these measurements. No other equipment or measurement techniques should be used to evaluate VDT operator exposures. Measurements can be made to satisfy either one of two criteria: exposure measurements to determine operator exposure or performance measurements to compare the emissions of different terminals. Measurements for assessing operator electric field exposure should be made with the operator in place. A displacement-current sensor is used and should be supported with a dielectric support. The sensor should be placed parallel to and in contact with the surface of the operator at the point at which the fields are to be evaluated, e.g., at the chest, abdomen and lap. Since the electric field distribution is determined primarily by the position of the operator, the sensor will not perturb the local fields and the value measured will be representative of the component of the field terminating on the body. If exposure measurements are to be made at the sides or back of the terminal, the same procedure should be followed, i.e., with a person located at the point of interest. The measurements in both cases should be repeated with the arms and hands at various positions, eg., in contact with the keyboard a t the operator's sides. Also, if the operator normally uses a conductive glare shield, measurements should be made with the shield in place and repeated with it removed. Measurements for assessing operator exposure to the magnetic field may be made at the position of the operator with the operator absent. In this case, the compensated loop need not be supported with a dielectric support since the operator will not appreciably affect the magnetic field distribution. The loop must, however, be rotated for a maximum meter deflection. Measurements should also be made a t a prescribed distance from the sides and back of the terminal, eg., 30 cm or the normal location of a nearby worker. If a combination displacement-current sensor/compensated loop type of instrument is used, the ratio of the peak to rms values of the electric field can be determined by viewing the auxiliary output with an oscilloscope. Similarly, the rate of change of the magnetic field (dB/dt) can be determined with the instrument operating in the magnetic field mode by viewing the auxiliary output on an oscilloscope. Performance types of measurements are useful for comparing the emissions of different terminals. This type of measurement is normally made with the operator absent. However, i t is extremely important to follow consistent procedures, e.g., a prescribed

A.22 VISUALDISPLAY TERMINALS

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measurement distance, measurement location (sides, back, etc.), height above the floor. As above, the compensated loop must be rotated for a maximum meter indication a t each measurement point. The electric field sensor should be supported with a dielectric support and also must be rotated for a maximum meter indication unless a n isotropic self-contained active monopole type of device is used. Body current measured in the laboratory also may be of use. One method is to measure the voltage a m s s a small resistor (10 to 100 ohms) connected between a person seated a t the terminal (using EKG type gel pads connected to the skin of a n arm or leg) and ground. A true rms voltmeter should be used. Alternatively, a digital microampere meter may be connected between the operator's body and ground. Body current measurements are generally used for comparison purposes since the magnitude of the current is greatly influenced by the location of the operator, the reference ground chosen, the length of the connecting leads, the location of the operator's hands, etc. Also, one should be aware of the fact that body current is usually associated with capacitive coupling between the operator and the terminal. Body current is not, to any great extent, a measure of internal eddy-currents associated with the time rate of change of the magnetic field.

Additional Information: Relatively high E, i.e., hundreds of V m-l or more, associated with the sweep circuitry in some VDTs may be found close to the surface of the device near the flyback transformer. These fields are so low in frequency, and decrease so quickly with increasing distance, that virtually no coupling to the body is possible, even a t close distances, and no measurable SAR results. This low degree of coupling from VDTs also eliminates the potential for RF shocks or bums in persons touching surrounding metal objects. The use of isotropic survey instruments of the dipole type, will indicate erroneously high E due to scalar potential field interactions with probes which are designed for vector fields only. These probes SHOULD NOT be used for VDT measurements. (Note: the instruction manual for these instruments should be read and thoroughly understood. I n many cases the instrument manufacturer warns against using particular instruments for VDT measurements.) The only type of instrument that should be used for electric field measurement is the displacement sensor type or the active monopole type. High static-electric fields also exist around the CRT of many VDTs, a s well as home TV receivers. These fields are associated with the "surprise reaction" that a person might have if a harmless spark

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discharge occasionally occurs when walking across a rug in a dry room and then touching a metal object (doorknob). In conducting a field survey, in most cases, it will be observed that the magnetic field possesses a principal polarization, i.e., the field will be predominantly vertically polarized for the horizontal deflection fields and horizontally polarized for the vertical deflection system fields. Knowledge of this property may be used in designing a practical and simplified measurement procedure for VDT magnetic fields. For example, since in many cases only one polarization component is significant, measurements with a single axis responding instrument oriented in the appropriate polarization plane may be entirely sufficient for determining the fields.

Cautionary Notes: Occasionally, a n operator will encounter a VDT with a conductive cabinet or a shielded flyback transformer, in which case the electric fields will be unusually low.

A.23 LORANIOMEGA Navigational Stations Purpose: To provide signals useful for worldwide navigation of ships and aircraft.

Operational Characteristics: LORAN, or more precisely, LORAN-C, stations operate from fixed locations, transmitting a pulsed signal with a nominal pulse-repetition-frequency of 10 Hz, using large, vertically polarized antennas. OMEGA stations transmit continuous-wave transmissions which are intermittent in nature, lasting between 0.9 and 1.2 s. Groups, or chains, of stations operate together to allow ships or aircraft equipped with special LORAN or OMEGA receivers to determine their location. The OMEGA navigation system has a potential for providing a position fixing accuracy of 4 to 7 km for most ships and aircraft; LORAN provides a n accuracy of approximately 500 m. Though LORAN can achieve a greater degree of accuracy, OMEGA has more worldwide coverage. An earlier LORAN system known as LORAN-A, which operated in the 1,800 KHz frequency range, is no longer in use in the United States.

A.23 LORANIOMEGA NAVIGATIONAL STATIONS

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Frequency Range: LORAN signals are transmitted a t 100kHz, while OMEGA signals occur in the frequency range of 10 to 14 kHz.

Power Levels: LORAN stations vary in power from 250 kW to 3 MW radiated peak power. OMEGA stations operate with typically 10 kW of radiated power.

Estimated Number in Use: LORAN, approximately 55 worldwide; OMEGA, 8 worldwide.

Authorization for Use: Stations are authorized by FCC and operated by the United States Coast Guard. Typical Exposure Levels:

E up to several kV m-' may be found in the workplace environment near the antenna feed point. Electric fields found away from the transmitting facility, outside the antenna grounds, may be in the range of 10 to 30 V m-' rms (McEnroe, 1980). Instrumentation/MeasurementTechniques: Caution must be exercised in the use of proper instrumentation. The instrument must specify that it is capable of measuring fields as low in frequency as 10 kHz. The most effective survey devices are the self-contained active monopole type supported with a dielectric handle. Particular attention should be given to the instrument's response to pulsed fields like those from the LORAN system. The field-strength calibration should be specified in rms or peak fieldstrength units.

APPENDIX B

Radiofrequency Exposure Determination: Examples B.l Radiofrequency Dielectric Heater Exposure Survey: A Sample Problem An exposure survey was requested by Notebooks, Inc. on an RF dielectric heater used to seal the vinyl covering on notebooks that they manufacture. Mr. John Smith of Notebooks, Inc. requested the survey in order to determine the exposure levels (both E and HI to the operator. A telephone conversationwith Mr. Smith revealed that this heater was manufactured by Seal-Rite Corporation and it had a manufacturer's serial number of 123456. Subsequent telephone conversations with representatives of Seal-Rite revealed that the sealer had a nominal operating frequency of 27.12MHz and a nominal output power of 7.5 kW. Appropriate field-strength probes and a frequency meter were selected. Initially, frequency measurements were made on the heater. A monopole antenna was attached to the frequency meter and the meter was allowed to warm up for 5 min. Starting about 3 m away, the heater was approached with the meter in hand and the heater actually sealing the vinyl covering on a notebook. At about 1.5 m from the heater the frequency reading stabilized (remainedthe same) for about 1 s. The frequency reading was 27.12 MHz which agreed with the nominal frequency given on the metal specifications plate attached to the heater. The heater operator was then requested to continue his normal work cycle for processing the notebooks. It had been explained to the operator that data to determine his average RF exposure over a 6 min period was to be collected. A stopwatch

B.l

DIELECTRIC HEATER EXPOSURE SURVEY

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was used to determine the total time that the RF energy was "on" during the 6 min (360 s) time period. It was helpful to watch a fluorescent bulb (attached to the top press) which indicated that the RF energy was "on" when it was ignited. However, voltage/cur~ent meters or timers on the heater could be used to determine the total RF "on" time. The total RF "on" time for this heater was 36 s and, therefore, the duty factor was 0.10. This information was recorded on the attached data form (see Table B.1). Next, field-strength measurements were made to determine operator exposure (see Table B.2). The meter was zeroed (with the RF heater "off'), the battery condition was checked, the fast (1s) response mode and the maximum hold mode (causing maximum readings to be displayed) were selected and the meter was allowed to warm up. The operator's location was approached from about 3 m away with the meter set on the least sensitive range. It was determined that the range of the probe was sufficient for monitoring the range ofE exposures to the operator. Therefore, E measurements were made at each anatomical location (specified on the attached data form) with the operator in his normal working position. The operator, meter electronics, cable, RF heater operator and adjacent personnel and metal structures were stationary during the few seconds required to make each field-strength measurement. All other RF heaters and other RF/microwave sources within the plant were identified and shut off during these measurements. The surface of the field-strength probe was about 10 cm from the operator's body. The probe handle was parallel to the floor (horizontal) and to the front edge of the RF heater press. All readings were recorded on the survey form in the uncorrected reading column for E. After TABLEB.I-Radiofrequency dielectric heater exposure survey form. Employer: Notebooks, Inc. Date: May 7,1985 Contact: John Smith Telephone: 513-345-1234 RF Source: Manufacturer: Seal-Rite Corp. M o d e l b e : KF75 Mfr. S/N: 123456 Co. SIN: SR-10 Power Output: (labeled) 7.5 kW Age: 6 y old Frequency: Measured: 27.12 MHz 'Ittgged: 27.12 MHz RF Shielding: No Application: RF sealing of the vinyl covering on notebooks Operator(s) position and location: one operator situated in front of sealer Duty Factor: Total RF "ON" Time 36 s. Calculated Duty Factor: 36 d360 s = 0.1 Frequency Meter Manufacturer: Continental SIN: 25103 Mdl: Max 100 Survey Equipment: E probe (Frequency Response Correction = 1.10) H probe (Frequency Response Correction = 1.00).

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APPENDIX B

TABLE~ . s S u i v e yresults.

Location

Electric-field strength D.F.&Freq. Uncon: Corrected Readinf Reading' (V2m'2) (V2m-')

Magnetic-fieldstrength D.F.&Fq. Uncon: Corrected Reading' Readinga (A2m-') (A%i2)

Eye 20,000 Neck 15,000 Chest 14,000 Waist 18,000 Gonad 17,000 Knee 19,000 Ankle 16,000 1,600 1,760 6.0 0.6 0.6 "Meansquared field-strength readings. bMean squared field-strength readings corrected for a duty factor of 0.10. 'Mean squared field-strength readings corrected for duty factor and frequency response of each respective field strength probe;E probe frequency responsecorrection was 1.10 and H probe frequency response correction was 1.00.

completing E measurements, H measurements were made using the same procedures described previously. After the survey measurements were completed and the data recorded in the respective uncorrected reading columns, the data were analyzed. The uncorrected ms field-strength readings were multiplied by the duty factor of 0.10to obtain the duty-fador corrected readings. The duty factor corrected readings for E were then multiplied by the frequency response correction (1.10)for E probe. The duty factor corrected readings for ms H were then multiplied by the frequency response correction (1.00)for Hprobe. These duty-factor and frequency-response corrected readings can be directly compared to RF occupational exposure standards or RFPGs which specify 6 min exposure averaging. If the ms field-strength readings are higher than desired, control technology techniques such as shielding may be useful to achieve lower operator exposures. CornmentslConclusions: Control technology techniques such a s shielding may be useful t o achieve lower operator exposures if desired.

B.2 Point-to-Point Microwave Radio: A Sample Problem The characteristics of a point-to-point microwave-radio repeater station are a s follows: Antenna height: 70 m Horn reflector, circular aperture Antenna type: Aperture diameter: 2.9 m

B.2 POINT-TO-POINT MICROWAVE RADIO

Midband gain: Waveguide loss: Number of channels: Polarization: Transmitter output power: M~dulation:~

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39.5 dB at 4 GHz, 42.9 dB at 6 GHz 2.78 dB 8 at 4 GHz, 6 at 6 GHz 4V, 4H at 4 GHz; 4V, 2H at 6 GHz 33.0 dBmIchanne1 at 4 GHz, 37.0 dBdchanne1 at 6 GHz Analog FM at 4 GHz, Digital (PSK) at 6 GHz

Measurements are to be made on the tower platform directly in h n t of the antenna aperture and also at grade level at a point 274 m from the base of the tower, directly under the main beam. To ensure that the operator is not exposed to levels exceeding the RFPG while measurements are being made directly in front of the antenna, the maximum expected levels should first be calculated. These levels can be approximated by averaging four times the total antenna input power PA over the physical area of the aperture A. That is,

The total transmitter power PTis equal to the combined power of the eight 4 GHz transmitters plus the combined power of the six 6 GHz transmitters. The transmitter power is given in dBm and must be converted to watts. Thus, the total power, in watts, is:

The actual power delivered to the antenna PA is the transmitter power reduced by the waveguide loss or,

From Equation B.1, S,, along the antenna axis directly in front of the antenna is:

6The modulation of each digital channel is such that the signal is spread over a 30 MHz bandwidth and has the appearance of "white noise" when observed on the display of a spectrum analyzer.

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APPENDIXB

This value of 1.5mW cm-2is well within the range of most broadband survey instruments and, therefore, this type of instrument would be a convenient choice for making these measurements. Before making measurements under the antenna axis (at the point 274 m from the base of the tower), the expected level a t this point also should first be calculated. This will provide the information necessary for selecting appropriate instrumentation and and alert the operator to equipment problems that may result in gross measurement errors. In order to calculate this value, the antenna gain in the direction of the point in question must be found. This angle, the depression angle 8, can be found from: where h is the height of the antenna and d is the horizontal distance. Substituting 70 m for h and 274 m for d into Equation B.5 yields a value of 14.3 degrees for 8. Assume Figure B.l represents the 4 GHz vertical directivity pattern for this particular antenna. The gain is approximately 45 dB below the midband gain or, 39.5 - 45 = - 5.5 dB at the depression angle of 14.3 degrees. Assume the gain at 6 GHz is also down by the same amount, then the corresponding gain in the direction of the point in question will be 42.9 - 45 or - 2.1 dB. S,,a t frequency f and angle 8 can be found from the expression: where PA is the total transmitted power, r is the radial distance and Go,,is the gain for a given frequency in the direction of the radial. The net S (S,) is given by summing S for the individual frequencies as sh9wn in Equation B.7.

substituting t h e values for PA4 and PA, (8.4 x 1O3mW and 15.9 x 103mW, respectively), Go, and Ge6( - 5.5 dB and - 2.1 dB, respectively) and (h2 + d2)% = 2.83 x lo4 cm2 for r into Equation B.8 yields:

B.2 POINT-TO-POINT MICROWAVE RADIO

ELEVATION

- DEGREES DOWN FROM MAIN

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LOBE

Fig. B.1. Vertical directivity envelope for circular aperture horn reflector antenna.

Assume that the measurements are to be made with a spectrum analyzer and a conical log-spiral antenna that have the following parameters at 4 GHz: preselector loss PI 6 dB, Af 39 dB, and C114 dB. In order to determine S from a measurement of the power a t the test antenna terminals, the effective area, A,, of the conical logspiral antenna must be determined. It can be shown that A, and A, are related by: where Re is the antenna terminating impedance (50 a).The corresponding effective area at 4 GHz is equal to approximately 9.5 cm2. The total S at 4 GHz, as estimated above, is 2.3 x lo-' mW cm-2 and S per channel will be approximately one-eighth of that value

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APPENDIXB

or 3.0 x lo-' mW ~ m - Thus ~ . the corresponding power per channel delivered to t h e antenna terminals will be approximately 3.0 x lo-' mW cm2 x 9.5 cm2 or approximately -65 dBm. Levels of the order of -80to -90 dBm can be readily resolved with a modern spectrum analyzer using a relatively narrow bandwidth to reduce the input noise. When optimizing the signal-to-noiseratio by changing the bandwidth, care should always be taken to ensure that the level of the signal does not change when the bandwidth is changed. If the signal level decreases linearly with a decrease in bandwidth, it means that the bandwidth of the signal is larger than the bandwidth of the spectrum analyzer and the spectral density in dBm per MHz must be measured and integrated over the bandwidth of the signal in order to obtain the total signal power. The net S is obtained from the power level measured at each specific frequency P as follows: (1)add the C,, PIand any other losses to P to obtain the level at the antenna terminals; (2) convert from dBm to W, (3) divide by the antenna aperture area to obtain S for that specific frequency; (4) repeat for each frequency; and (5) sum S obtained above to obtain the total S. When the A, is given (rather than the gain), it is convenient to use the following equation: where A , C,, PIand any other losses between the test antenna terminals and the receiver input terminals are expressed in dB and P is expressed in dBm. As an example, suppose that the following levels and corresponding frequencieswere measured ( a m , GHz): ( - 89,3.71), (- 91,3.77), (-88,3.83),(-93,3.89),(-96,3.95),(-92,4.01),(-90,4.07),and (-97, 4.13). S a t 3.71 GHz would be

If the above exercise is repeated for the other seven frequencies and the results added, the total S at 4 GHz is equal to approximately 6.5 x lo-' mW ~ m - ~ . At 6 GHz the signals are broadband and the measurement problem is more complicated. For example, assume that C1, Pl and A, at 6 GHz a r e 17 dB, 6 dB and 43 dB, respectively. At 6 GHz, S was calculated earlier (9.7 x lo-" mW cm-2 or approximately 1.6 X mW cm-'per channel). From EquationB.11, the effective area of an antenna with Af of 43 dB is approximately 3.8 cm2. The

B.2 POINT-TO-POINT MICROWAVE RADIO

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corresponding power per channel a t the antenna terminals will be approximately 1.6 x lov7x 3.8 mW or - 62.2 dBm. The power per channel at the input of the spectrum analyzer will be lower by an amount equal to the loss of the cable and preselector or - 85.2dBm. Since the signal associated with the 6 GHz transmitters is broadband (30MHz bandwidth), the signal level displayed will be reduced by the ratio of the impulse bandwidth of the spectrum analyzer to the signal bandwidth. For example, if the impulse bandwidth is 300 kHz, the displayed level will be reduced by 20 dB or will be equal to - 105.2dBm. This extremely low level combined with the "white noise" characteristic appearance of a PSK modulated signal makes it extremely difficult, if a t all possible, to measure with a typical spectrum analyzer. An alternate measurement method would be to use a broadband power meter with the detector head connected directly to the antenna output. This configuration eliminates the RF cable and PI and the effective loss associated with the relatively narrow bandwidth of the spectrum analyzer. Since modern power meters can readily measure levels of -70dBm, there will be more than enough sensitivity. Although the conical log-spiral antenna can be used with the power meter, energy associated with the 4 GHz signals, as well as signals associated with land-mobile radio transmitter and radio and TV broadcast stations, may result in significant measurement errors. Also, Af is different at 4 GHz and 6 GHz, and therefore the contribution of each frequency must be known in order to determine the corresponding S. IfAf at the two frequencies is reasonably close, the smallest value can be used to determine the effective aperture area. Use of this value will introduce an error but it will be on the high side, yielding a conservative estimate. Normally, a power meter used to simultaneously measure signals in two frequency bands will produce inaccurate results unless some form of filtering is used. One method of resolving this problem is to use the conical log-spiral antenna and spectrum analyzer for 4 GHz and a narrowband antenna, such as a pyramidal-horn antenna and power meter for the 6 GHz band. For example, if a horn antenna terminated with WR 137 waveguide is used, it will be below cutoff for frequencies lower than approximately 4.3 GHz and the only energy reaching the detector head of the power meter will be that associated with the 6 GHz signals. The gain of a typical horn antenna is approximately 15 dB and the corresponding effective aperture area at 6 GHz is approximately 63 cm2. Thus, the level at the antenna terminals will be of the order of -50 dBm per channel. The total S a t 6 GHz is the power measured with the antenna polarized vertically

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plus the power measured with the antenna polarized horizontally, divided by the aperture area of the antenna.

B.3 Amplitude-Modulation Radio: A Sample Problem In granting a zoning variance to an AM broadcast station to permit construction of an antenna at the desired site, the local Zoning Board of Appeals has specified that the magnitude of E and H may not exceed the far field equivalent of 20 mW cm-2 at any location beyond the property boundaries. A survey to prove compliance with that limit is required before a use permit will be granted. Characteristics of the system are as follows: Frequency: Power: Antenna: Size of site:

800 kHz 10 kW Single, quarterwave tower (93 m) 300 m2 with tower at center of site Maximum permissible field strengths are determined as follows:

S in (W m-') = 10 times S in (mW cmP2) Impedance of free space = 1 2 0 ohms ~ = 377 ohms E2 = 200 x 377 V 2m-2 E2 = 75,400 V2rn-' E = 275 V m-' P = (200 W m-') (377 P = 0.53 A2 m-2 H = 0.73 A m-' Reference to Table A.l provides assurance that compliance with the zoning requirement will be achieved because a 10 kW system will produce a 10 V m-' field at 114 m and the station is allowed 275 V m-' at the property edge no less than 150 m from the tower location. But the building permit requires that the survey be made to prove compliance. Scanning the top line of Table A.1 shows that transmitter power somewhat less than 25 kW perhaps about 20 kW would be necessary to establish 10 V m-l at 150 m. Field strength varies as the square root of the power so the approximate maximum E expected is As an approximation,H may be assumed to be related to E through the relation EH-I = 377. At 150 m this gives:

B.3 AMPLITUDE-MODULATIONRADIO: A SAMPLE PROBLEM

H = 0.019A m-l.

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195

(B.18)

Since the AM radio station is sufficiently far from any other RF source that its field strength would be expected to be at least one order of magnitude above the sum of other signals in the area, the use of a broadband meter would be appropriate. Accordingly, a diode based instrument with both electric and magnetic probes is selected. This meter is suitable for use down to a frequency of 500 kHz and the most sensitive setting for the E will provide readings of V2rn-' in about the midrange since the full scale reading for that range is 100 V2 m-2. The H readings would be expected to be at the low end of the scale range, but with a full scale reading of 0.1 A2rn-' and a permitted H of 0.53A2m-', the low readings would provide ample assurance that the criterion is being met. If, contrary to expectations, any H exceeds 0.1 A2 m-2, use of the adapter furnished with the meter permits readings of field strengths as high as 1.0 A2 mT2. As noted by the manufacturer, caution needs to be exercised when using the instrument at frequencies below 3 MHz to avoid pick-up on the leads from probe to meter. The probe should be held at arm's length with the handle in a near horizontal position pointed toward the broadcast antenna. If at any time, a measurement is particularly critical, the meter and probe should be placed on a wooden tripod or other support such as a step ladder. The probe should be horizontal and directed toward the broadcast antenna. Excess probe lead should be coiled with the plane of the coil horizontal. The meter should be read from as far away as possible, avoiding any effect of the observer's body. Use of a test stand is cumbersome and not recommended for the usual survey where multiple measurements are necessary. To proceed with measurements, start at one corner ofthe property, connect the more sensitive electric field probe to the meter, set the range switch to the least sensitive range and turn the meter on. Allow a few minutes of warm up. Make sure the battery indicator is blinking, indicating sufficient voltage. Momentarily actuate "Peak Hold" switch to clear memory. Depending on accessibility, take the measurements either just inside, or just outside, the property boundary, keeping the probe a t least 1 m away from metallic objects such as fences. If a tall metallic fence surrounds the property, measurements are best made within the property. Take ten measurements, evenly spaced, along each property edge. At each location, after f i s t momentarily depressing the "Peak Hold" switch, move the probe slowly throughout an area covering a 1 m cube while observing the meter reading. (In this example, more

196

/

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likely than not, the range switch will have to be put in the most sensitive position and left there.) The lower side of the cube should be about 1m above ground. Mentally average the readings observed. Also actuate the "Peak Hold7' switch and observe that reading. Record the average and peak readings as well as the date and time of day. If a data logger is available, the averaging can be accomplished with greater accuracy. Proceed to the next location and repeat the process, being certain to start by depressing the "Peak Hold" switch to clear the memory. After completion of the E measurements, replace the E probe with the H probe, set the range switch to the appropriate position and repeat the foregoing procedure, recording H. An alternative to use of the broadband isotropic type of meter for measuring H would be to employ a meter commonly used for AM field-strength surveys. Such meters, although calibrated in E units, actually respond to H. Using the following relationship,

H (A m-')

=

E (V m-') (377)-'.

(B.19)

Another alternative for measuring the strength of the electric field is the use of the self-contained active monopole type of broadband electric field sensor. This type of instrument is usually equipped with a series of antenna elements of different lengths to permit adjustment of the instrument sensitivity. When using the Potomac instrument, or an active monopole type of instrument with a single antenna element (some versions are isotropic, i.e., have three orthogonal elements) it should be noted that measurements of only one polarization component may be performed a t any given time. A complete characterization of the field will require three mutually orthogonal measurements of the field and a computation of the rootsum-squared value to obtain the total magnitude of the field. On the log sheet for the measurements, note the parameters of the broadcast station and readings of either base current or final transmitter stage voltage and current made both before and after the radiation survey.

B.4 Multiple Frequency-ModulationRadio: A Sample Problem A major broadcast site includes a tower with six high-power FM radio stations and a separate AM radio tower located approximately 100 m away. It is desired to determine the RF field levels in the

B.4 MULTIPLE FREQUENCY-MODULATION RADIO

I

197

immediate vicinity of the base of the FM tower which is not fenced and where passers-by may become exposed to RF fields. Previous analyses of the stations at this site indicate that the composite RF levels near the FM tower base are in the range of 1 to 2 mW cm-'. The associated field strength of the nearby AM radio station is only 10 to 20 V m-l near the FM tower. In addition to measuring the resultant RF field levels, the broadcasters want to know the relative contributions of each of their stations to the total. An RF survey approach is defined as follows: A broadband survey instrument will first be used to conduct an area survey in the vicinity of the FM tower to identify relative "hot spots," i.e., places where the individual fields add in such a way as to cause the resultant field to be substantially greater than the field in the surrounding area. These hot spots are normally the result of ground reflections and standing waves. At some points, these waves tend to combine such that they add together in phase to create stronger resultant fields. Once the hot spots are located, detailed narrow-band measurements of all the incident fields will be performed with a spectrum analyzer and a calibrated half-wave dipole antenna. Through this process, the individual contributions of each station can be determined. Although the AM tower is expected to produce field strengths less than 20 V m-' in the area of interest, the survey team is concerned about the possibility that the presence of the AM field may interfere with the use of the broadband survey instrument. This was suspected when using both diode and thermocouple type detector broadband instruments near the AM station and observing that, in one case (diode instrument), the probe reading seemed to be more dependent on its location with respect to the ground surface, producing progressively higher readings with height above ground. In the other case (thermocouple instrument), the probe reading a t times was negative and erratic. With this in mind, a broadband survey was performed, nevertheless, in the area near the FM tower to identify relative hot spots. The survey indicated three spots where the broadband instruments seemed to show composite exposures above 1 mW cm-', this being the level of concern to the local planning bureau. At two of these spots, however, t h e readings on the instruments seemed to be extremely sensitive to probe position andlor somewhat erratic, making it difficult to relocate the exact spot of maximum field in repeated measurements. Following the area survey, a series of narrowband measurements were performed at the three spots identified with the broadband equipment. In this case, an adjustable half-wave dipole antenna was

supported by a nonmetallic tripod so that its center was positioned very near to the point of maximum total field as read by the broadband probe. The antenna was connected to a length of coaxial cable and, in turn, to an attenuator rated at 20 W to protect the input of the spectrum analyzer. Finally, the attenuator was connected to the input of the spectrum analyzer and each FM signal transmitted from the tower was observed on the CRT display. The amplitude of each of the six signals was carefully measured by observing its peak spectral height on the CRT display and recording the indicated power expressed in dBm (dB with respect to a mW). This measurement was performed for each of six stations and for each of three mutually orthogonal orientations of the dipole antenna. The three powers measured for each signal were summed, after converting dBm units to actual power, to obtain a total power fi-om the three polarization planes of the wave. Next, the power was re-expressed in units of dBm and, subsequently, dBpV (dB with respect to 1pV in a 50 ohm system). Then, the attenuation of the external attenuator and the connecting cable were added to arrive at the actual signal level (dBpV) delivered from the antenna. Finally, the Af appropriate to each frequency examined was added to the signal level in dBpV to arrive a t the measured E in units of dBpV m-' (dB with respect to 1 pV per m). This value was then re-expressed in terms of planewave equivalent S in pW cmP2.Subsequently, S for each station was added together to obtain the total RF exposure S. These results for each of the locations (corresponding to hot spots) were then compared with the broadband measurements and to the 1,000 p W cm-2 limit of concern to the planning b ~ r e a u (See . ~ at the end of this sample problem for technical details of the processing of data collected during the measurements.) Two additional aspects of this measurement project were then treated. By prior arrangement with the engineer of the AM radio station, the AM station was switched off for a limited period of time which allowed for a re-evaluation of the broadband measurements. During the power-off period, broadband measurements were per-

'Conversion of dBm to power: power in mW

=

(7:)

log-' -

Conversion of power in mW to dBm: dBm = 10 log,, P(mW) Conversion of dBm to dBpV: dBpV = dBm

+

107

Adding cable loss (C,) and attenuator values: dBpV = measured (indicated) value + C, (dB) + attenuator value (dB) Using A,: S (dBpV m-I) = meter reading (dBpV)

+ A( (dB m-'1.

B.4 MULTIPLE FREQUENCY-MODULATION RADIO

1

199

formed at precisely the same points where the resultant field readings had been determined earlier to be maximum. During the same time period, a broadband magnetic field probe (which used thermocouples as detectors) was used to evaluate the space above and below the point where an electric field maximum had been determined. ~ssociatedpoints of maximum magnetic field were identified and the magnitudes of the magnetic fields at these points were compared with the corresponding electric field magnitudes determined via the narrowband measurement technique. A thermocouple detector was chosen because of its ability to sum accurately multiple signals and to indicate the total, resultant field level. The results showed that when the appropriate calibration factor was applied to the probe reading, the plane-wave equivalent S obtained on the basis of both electric and magnetic fields were the same. However, the positions in space above the reflective ground were displaced from one another as would be expected in a standing wave situation. The final aspect of this evaluation was to determine the extent of RF fields above the 1 and 2 mW levels. This permitted the recorded absolute maxima to be placed in perspective with what one might realistically expect public exposure to be in the area near the tower.

References ACGIH (1993). American Conference of Governmental Industrial Hygienists. Threshold Limit Values and Biological Exposure Indices (American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio). AMATO, J.A. (1994). "Use protective clothing for safety in RF fields," Mobile Radio Techno]., April, 40-44. ANSI (1981).American National Standards Institute. Recommended Practice for the Measurement ofHazardous RF and Microwave Electromagnetic Fields, ANSI C95.5-1981 (American National Standards Institute, New York). ANSI (1982). American National Standards Institute. Safety Levels with Respect to Human Exposure to Radiofrequency Electromagnetic Fields, ANSI C95.1-1982 (American National Standards Institute, New York). ANSUIEEE (1992a). American National Standards Instituteflnstitute of Electrical and Electronics Engineers. ZEEE Recommended Practice for the Measurement of Potentially Hazardous Electromagnetic Fields-RF and Microwave, ANSUIEEE C95.3-1992 (American National Standards Institute, New York). ANSUIEEE (1992b). American National Standards Instituteflnstitute of Electrical and Electronics Engineers. Standard Safety Levels with Respect to Human Exposure to Radiofrequency Electromagnetic Fields, 3 khz to 300 Ghz, ANSI (295.1-1991 (American National Standards Institute, New York). ASLAN, E. (1972). "Broadband isotropic electromagnetic radiation monitor," page 972 in Proceedings of International ZEEE-GroupAntennas and Propagation Symposium (Institute of Electrical and Electronics Engineers, New York). ASLAN, E. (1980). Personal communication on use of Narda broadband radiation monitor for determining power density caused by rotating radars without stopping the beam. ASLAN, E. (1985). "Non-ionizing radiation-Measurement methods and artifacts," page 645 in Proceedingsof the 39thAnnual Broadcast Engineering Conference,National Association of Broadcasters (National Association of Broadcasters, Washington). ATHEY, R. and RUGGERA, P.S. (1982). "Magnetic fields associated with a nuclear magnetic resonance medical imaging system," Magn. Reson. Imag. 1, 149. BABIJ, T.M. and BASSEN, H.I. (1980). "Isotropic instruments for measurements of electric and magnetic fields in the 10-100 MHz range," Bioelectromagnetics 1, 248 (Abs).

REFERENCES

/

201

BABLJ, T.M. and BASSEN, H.I. (1986). Broadband Isotropic Probe System for Simultaneous Measurement of Complex E - and H-Fields, U.S. Patent 4,588,993 (United States Patent OfXce, Washington). BALZANO, Q., GARAY, 0. and STEEL, R.F. (1977). "Energy deposition in biological tissues near portable radio transmitters a t VHF and UHF," pages 25 to 39 in ZEEE Vehicular Technology Society Conference Record (Institute of Electrical and Electronics Engineers, New York). BASSEN, H.I. (1980). An Evaluation of Microwave Emissions from Sensormatic Electronic Security Systems, FDA Publication 80-8106 (National Technical Information Service, Springfield, Virginia). BASSEN, H.I. (1985a). "Computerized medical devices: Usage trends, problems, and safety technology," pages 180 to 185 in ZEEE Engineering in Medicine and Biology Conference Proceedings, 1985(A), Volume 1(Institute of Electrical and Electronics Engineers, New York). BASSEN, H.I. (1985b). "Quality assurance of RF and ultrasound cancer hyperthermia systems," pages 346 to 151in IEEE Engineering in Medicine and Biology ConferenceProceedings, 1985(B), Volume 1(Institute of Electrical and Electronics Engineers, New York). BASSEN, H.I. and COAKLEY, R.F., JR. (1981). "United States radiation safety and regulatory considerations for radiofrequency hyperthermia systems," J. Microwave Power 16, 215-226. BASSEN, H.I. and SMITH, G.S. (1983). "Electric field probes-A review," IEEE Trans. Antennas and Propagation AP-31,710-718. BASSEN, H.I., HERMAN, W. and HOSS, R. (1977). "EM probe with fiberoptic telemetry system," Microwave J. 20, 35-47. BAUM, C.E., BREEN, E.L., GILES, C., O'NEIL, J.P. and SOWER, C.D. (1978). "Sensors for electromagnetic pulse measurements both inside and away from nuclear source regions," pages 22 to 37 in IEEE Transactions on Electromagnetic Compatibility, Volume EMC-20 (Institute of Electrical and Electronics Engineers, New York). BOWMAN, R.R. (1974). "Some recent developments in the characterization and measurement of hazardous electromagnetic fields," pages 217 to 227 in Biological Effects and Health Hazards of MicrowaveRadiation, Proceedings of a n International Symposium (Polish Medical Publishers, Warsaw). BRODEUR, P. (1989a). "Annals of radiation. The hazards of electromagnetic fields. I. Power lines," The New Yorker, June 12,1989,51-88. BRODEUR, P. (1989b). "Annals of radiation. The hazards of electromagnetic fields. 11. Something is happening," The New Yorker, June 19, 1989, 47-73. BRODEUR, P. (1989~). "Annals of radiation. The hazards of electromagnetic fields. 111. Video display terminals," The New Yorker, June 26, 1989, 39-68. BUDINGER, T. (1981). "Nuclear magnetic resonance (NMR)in vivo studies: Known thresholds for health effects," J. Comput. Assist. Tomogr. 5, 800-811. CENTAUR (1982). "Study of radiofrequency and microwave radiation," pages 219 to 221 in The Final Report, prepared for the Occupational Safety

202

/

REFERENCES

and Health Administration by Centaur Associates, Inc. (Occupational Safety and Health Administration, Washington). CONOVER, D.L., MURRAY, W.E., FOLEY, E.D., LARY, J.M. and PARR, W.H. (1980). "Measurement of electric- and magnetic-field strengths from industrial radio-frequency (6-38 MHz) plastic sealers," pages 17 to 20 in Proceedings of the IEEE 68 (Institute of Electrical and Electronics Engineers, New York). CONOVER, D.L., MURRAY, W.E., LARY, J.M. and JOHNSON, P.H. (1986). "Magnetic field measurements near RF induction heaters," Bioelectromagnetics 7, 83-90. COX, C., MURRAY, W.E., JR. and FOLEY, E.P., JR. (1982). "Occupational exposures to radiofrequency radiation (18-31 MHz) from RF dielectric heat sealers," Am. Ind. Hyg. Assoc. J. 43, 149-153. DURNEY, C.H., JOHNSON, C.C., BARBER, P.W., MASSOUDI, H., ISKANDER, M.F., LORDS, J.L., RYSER, D.K., ALLEN, S.J. and MITCHELL, J.C. (1978).Radiofrequency Radiation Dosimetry Handbook, Report SAM-TR-78-22, Second ed. (National Technical Information Service, Springfield, Virginia). DURNEY, C.H., MASSOUDI, H. and ISKANDER, M.F. (1986). Radiofrequency Radiation Dosimetry Handbook, Report SAM-TR-85-73, Fourth ed. (National Technical Information Service, Springfield, Virginia). EATON (1985). EMIIEMS Test Instrumentation and Systems (Carnel Laboratories, Canoga Park, California). EPA (1982). Environmental Protection Agency. Radiofreguency Measurements Workshop, EPA 52012-82-010(National Technical Information Service, Springfield, Virginia). EPA (1984). Environmental Protection Agency. Biological Effects of Radiofrequency Radiation, EPA 60018-83-026F (National Technical Information Service, Springfield, Virginia). EPA (1985). Environmental Protection Agency. A n Investigation of Microwave and Radiofrequency Radiation Levels in Vernon Township (National Technical Information Service, Springfield, Virginia). EPA (1987). Environmental Protection Agency. A n Investigation of Radiofrequency Radiation Levels on Healy Heights, Portland, Oregon (National Technical Information Service, Springfield, Virginia). FCC (1985). Federal Communications Commission. Evaluating Compliance with FCC-Specified Guidelines for Human Exposure to Radwfiequency Radiation, FCC OST Bulletin 56 (Federal Communications Commission, Washington). FDA (1977). Food and Drug Administration. Procedures for Testing Microwave Ovens, FDA 77-8037 (National Technical Information Service, Springfield, Virginia). FDA (1981). Food and Drug Administration. A n Evaluation of Radiation Emission from VideoDisplay Terminals, HHS Report FDA 81-8153, NTIS No. PB 272839lAS (National Technical Information Service, Springfield, Virginia). GAILEY, P.C. and TELL, R.A. (1985). A n Engineering Assessment of the Potential Impact of Federal Radiation Protection Guidance on the AM,

FM, and TV Broadcast Services, U.S. Environmental Protection Agency Report No. EPA 5201685-011, NTIS No. PB 85-245868 (National Technical Information Service, Springfield, Virginia). GREENE, F.M. (1975). Development of Magnetic Near-Field Probes, National Bureau of Standards No. NIOSH 75-127 (National Technical Information Service, Springfield, Virginia). GUY, A.W. (1984).Health Hazards Assessment of Radio Frequency Electromagnetic Fields Emitted by Video Display Terminals, Report prepared for International Business Machines, Ofice of the Director of Health and Safety (IBM Corporate Headquarters, Armonk, New York). GUY, A.W. (1987). "Health hazards assessment of radiofrequency electromagnetic fields emitted by video display terminals," pages 69 to 80 in Work with Display Units 86, Knave, B. and Wideback, P-G., Eds. (Elsevier Science Publishers B.V., Amsterdam). GUY, A.W. and CHOU, C.K. (1986). "Specific absorption rates of energy in man models exposed to cellular UHF mobile-antenna fields," IEEE Trans. Microwave Theory Techn. 34, 671-680. HAGN, G.H., FRALICK, S.C., SHALTER, H.N. and BARKER, G.E. (1971). A Spectrum MeasurementlMonitoring Capability for the Federal Government, Final report on contract OEP-SE-70-102 prepared for Office of Telecommunications Policy by Stanford Research Institute, SRI project 8410 (National Technical Information Service, Springfield, Virginia). HANKIN, N.N. (1974). "An evaluation of satellite communications (SATCOM) systems as sources of environmental microwave radiation," U.S. Environmental Protection Agency Report No. 52012-74-008, NTIS No. PB 257138 (National Technical Information Service, Springfield, Virginia). HANKIN, N.N. (1976).Radiation Characteristics of Traffic Radar Systems, U.S. Environmental Protection Agency Technical Note ORPIEAD 76-1, NTIS No. PB 257077 (National Technical Information Service, Springfield, Virginia). HANKIN, N.N. (1986). The Radiofrequency Radiation Environment: Environmental Exposure Levels and RF Radiation Emitting Sources, U.S. Environmental Protection Agency Report No. 52011-85-014 (National Technical Information Services, Springfield, Virginia). HARVEY, S.M. (1984)."Electric field exposure ofpersons using video display units," Bioelectromagnetics 5, 1-12. HEWLElT PACKARD (1985). Test and Measurement Catalog (Hewlett Packard, Santa Clara, California). HOLADAY (1985). Broadband R F Instrumentation (Holaday Industries, Eden Prairie, Minnesota). HOLADAY (1987). VDTIVLF Radiation Survey Meter (Holaday Industries, Eden Prairie, Minnesota). HOPFER, S. and ADLER, Z. (1980). "An ultra broad-band (200 kHz-26 GHz) high sensitivity p ~ ~ ~ bIEEE e , " Trans. Instrum. Meas. 29, 445-451. IEEE (1980). Institute of Electrical and Electronics Engineers. "Special issue on biological effects and medical applications of electromagnetic

energy," pages 1to 192 in Proceedings of theIEEE 68 (Institute of Electrical and Electronics Engineers, New York). IF1 (1984). Instruments for Industry. E-Field Sensor (Instruments for Industry, Ronkonkoma, New York). IRPA (1988). International Radiation Protection Association. "Guidelines on limits of exposure to radiofrequency electromagnetic fields in the frequency range from 100 kHz to 300 GHz," Health Phys. 54, 115-123. IRPA (1990). International Radiation Protection Association. "Interim guidelines on limits of exposure to 50160 Hz electric and magnetic fields," Health Phys. 58,113-122. ITU (1982). International Telecommunications Union. Radio Regulations, page RR-11 (International Telecommunications Union, Geneva). JORDAN, E.C. and BALMAIN, K.G. (1950). Electromagnetic Waves and Radiating Systems (Prentice Hall, Inc., New York). KANDA, M. and DRIVER, L.D. (1987). "An isotropic electric-field probe with tapered resistive dipoles for broad band use, 100 kHz to 18 GHz," IEEE Trans. Microwave Theory Techn. 35,126130. KANDA, M. and DRIVER, L.D. (1988). "Optically-linked electric and magnetic field sensor for poynting vector measurements in the near fields of radiating sources," IEEE Trans. Eledromagn. Compat. 30,495-503. KANDA, M., RIES, F.X. and BELSHER, D.R. (1979). A Broadband, Isotropic, Real-time, Electric-field Sensor (BIRES) Using Resistively Loaded Dipoles, National Bureau of Standards Publication 79-1622 (National Technical Information Service, Springfield, Virginia). KUCIA, H.R. (1972). "Accuracy limitation in measurements of HF field intensities for protection against radiation hazards," IEEE Trans. Instrum. Meas. 24, 412. KULIKOVSKAYA, Y.L. (1970). Protection from the Effect of Radio Waves, Report JPRS-52622, translated from Russian (Joint Publications Research Service, Arlington, Virginia). LAMBDIN, D. (1979). A n Investigation of Energy Density in the Vicinity of Vehicles with Mobile Communications Equipment and Near a Hand-Held Walkie Talkie, U.S. Environmental Protection Agency ORPIEAP 79-2 (National Technical Information Service, Springfield, Virginia). LARSEN, E.B., ANDREWS, J.R. and BALDWIN, E.E. (1976). Sensitive Isotropic Antenna with Fiber-optic Link to a Conventional Receiver, National Bureau of Standards Publication 75-819 (National Technical Information Service, Springfield, Virginia). LEVIN, J.J. (1971). The Invisible Resource, Use and Regulation of the Radio Spectrum (The Johns Hopkins Press, Baltimore, Maryland). LEWIS, R.L. and NEWELL, A.C. (1985). A n E f f i k n t and Accurate Method for Calculating and Representing PowerDensity in the Near-Zone of Microwave Antennas. National Bureau of Standards Report No. 85-3036 (National Technical Information Service, Springfield, Virginia). LOVSUND, P. and MILD, K.H. (1978). Low Frequency Electromagnetic Fields Near Some Induction Heaters, Investigation Report 1978: 38 (National Board of Occupational Health and Safety, Sweden).

REFERENCES

/

205

MANTIPLY, E.D. (1984). A n Automated TEM Cell Calibration System, U.S. Environmental Protection Agency Publication No. 52011-84-024 (National Technical Information Sewice, Springfield, Virginia). MARHA, K., MUSIL, J. and TUHA, H. (1971). Electromagnetic Fields and the Life Environment (San Francisco Press, Inc., San Francisco). MASTRANTONIO, M. and RUSSO, A. (1989). "Protezione da esposizioni a campi elettromagnetici a radiofrequenze e mimonde: metadologia strumenti di misura," Associazione Italiana di Protezione control le Radiozioni (Universita di Chieti, Italy). MATHESON, R.J. (1977). "A radio spectrum measurement system for frequency management data," IEEE Trans. Electromagn. Compat. 19,225. MCENROE, W.E. (1980).Electromagnetic Radiation Survey of United States Coast Guard OMEGA, LORAN-C (navigation), and Communications Stations, U.S. Coast Guard Technical Report (U.S. Coast Guard Headquarters, Washington). MCMAHON, J.H. (1973). "Capability of the FCC mobile monitoring van," in the Proceedings of the IEEE Vehicular Technology Conference Record (Institute of Electrical and Electronics Engineers, New York). MININ, B.A. (1974). Microwave and Human Safety, Reports JPRS-65506-1 and JPRS-65506-2, translated from Russian (Joint Publications Research Service, Arlington, Virginia). MONHEISER, P.A. (1985).Development of a System to Measure the Response T i m of Microwave Survey Instruments to Rotating Radur Antenna Patterns, U.S. Environmental Protection Agency Report No. 52016-85-019 (National Technical Information Service, Springfield, Virginia). NARDA (1985). Broadband Radiution Monitoring Systems (Narda Miuowave Corporation, Hauppauge, New York). NBS (1979). National Bureau of Standards. Electronagnetic Interference (EMI) radiation Measurements for Automobiles, National Bureau of Standards Technical Note 1014 (National Technical Information Service, Springfield, Virginia). NBS (1981). National Bureau of Standards. Field Strength Measurements of Speed Measuring Radur Units, Electromagnetic Fields Division, contract Report NBSIR 81-2225,performed for the National Highway Tr&c Safety Administration (police radar) (National Technical Information Sewice, Springfield, Virginia). NBS (1986). National Bureau of Standards. The International System of Limits, NBS Special Publication 330, Goldman, D.T. and Bell, R.J., Eds. (National Technical Information Service, Springfield, Virginia). NCRP (1981). National Council on Radiation Protection and Measurements. Radiofrequency Electromagnetic Fields-Properties Quantities and Units, Biophysical Interaction, and Measurements, NCRP Report No. 67 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). NCRP (1985). National Council on Radiation Protection and Measurements. SI Units in Radiation Protection and Measurements, NCRP Report No. 82 (National Council on Radiation Protection and Measurements, Bethesda, Maryland).

206

1

REFERENCES

NCRP (1986). National Council on Radiation Protection and Measurements. Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields, NCRP Report No. 86 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). NESMITH, B. and RUGGERA, P.S. (1982). Performance Evaluation of RF Electric and Magnetic Field Measuring Instruments, Health and Human Sewices Publication FDA-82-8185 (National Technical Information Service, Springfield, Virginia). PEAK, D.W., CONOVER, D.L., HERMAN, W.A. and SHUPING, R.E. (1975).Measurement ofpower Density from Marine Radar, Food and Drug Administration Report No. 76-8004 (National Technical Information Service, Springfield, Virginia). PETERSEN, R.C. (1979). "Levels of electromagnetic energy in the immediate vicinity of representative microwave radio relay towers," pages 31.5.1 to 31.5.5 in Proceedings of theInternationa1 Conference on Communication, Catalog No. 79C41435-7(Institute of Electrical and Electronics Engineers, New York). PETERSEN, R.C., WEISS, M.M. and MINNECI, G. (1980). "Nonionizing electromagnetic radiation associated with video-display terminals," Society of Photo-Optical Instrum. Eng. 229, 179-185. POLK, C. and POSTOW, E. (1986).Handbook ofBiologica1Effects ofEbctromagnetic Fields (CRC Press, Boca Raton, Florida). RADIO SCIENCE (1977). 12,6(S), 1-293. RADIO SCIENCE (1979). 14, 6(S), 1-352. RUGGERA, P.S. (1979). Measurements ofElectromagnetic Fields i n the Close Proximity of CB Antennas, Food and Drug Administration Report NO.798080 (National Technical Information Sewice, Springfield, Virginia). RUGGERA, P.S. (1980). Measurements of Emissions Levels During Microwave and Shortwave Diathermy Treatments, Food and Drug Administration Report No. 80-8019 (National Technical Information Service, Springfield, Virginia). RUGGERA, P.S. and SCHAUBERT, D.H. (1982). Concepts and Approaches for Minimizing Excessive Exposure to Electromagnetic Radiation from RF Sealers, Food and Drug Administration Report No. 82-8192 (National Technical Information Sewice, Springfield, Virginia). STUCHLY,M.A., REPACHOLI, M.H., LECUYER, D. and MANN, R. (1980). "Radiation survey of dielectric (RF) heaters in Canada," J. Microwave Power 15,113-121. TELL, R.A. (1978). Near-Field Radiation Properties of Simple Linear Antennas with Application to Radiofrequency Hazards Broadcasting, U.S. Environmental Protection Agency Technical Report ORPIEAD (National Technical Information Service, Springfield, Virginia). TELL, R.A. (1982). Radiofrequency Measurements Workshop Summary, U.S. Environmental Protection Agency Report No. 52012-82-00(National Technical Information Sewice, Springfield, Virginia). TELL, R.A. (1983). "Instrumentation for measurement of electromagnetic fields: equipment, calibrations, and selected applications," in TheProceedings ofa NATOAdvanced Studylnstitute Course onAdvances inBiologica1

Effects and Dosimetry of Nonionizing Radiation: Radiofrequency and Microwave Energies, Gandolfo, M., Michaelson, S.M. and Kindi, A., Eds., NATO Advanced Study Institute Series, Series A: Life Sciences 49 (Plenum Press, New York). TELL, R.A. (1986). "Real-time averaging for determining human RF exposure," pages 388 to 394 in Proceedings of the 40th Annual Broadcast Engineering Conference (National Association of Broadcasters, Washington). TELL, R.A. (1989). Electric and Magnetic Fields and Contact Currents Near AM Standard Broadcast Radio Stations, Federal Communications Commission Technical Report No. 89131-1912 (National Technical Information Service, Springfield, Virginia). TELL, R.A. (1990). "RF hot spot fields: The problem of determining compliance with the ANSI radiofrequency protection guide," pages 419 to 431 in 1990NAB Engineering ConferenceProceedings,44th Annual Broadcast Engineering Conference (National Association of Broadcasters, Atlanta, Georgia). TELL, R.A. (1991). Induced Body Currents and Hot AM Tower Climbing: Assessing Human Exposure in Relation to the ANSI Radwfrequency Protection Guide, Federal Communications Commission Technical Report No. 89131-1912 (National Technical Information Service, Springfield, Virginia). TELL, R.A. and NELSON, J.C. (1974). RF Pulse Spectml Measurements in the Vicinity of Several Air Traffic Control Radars, U.S. Environmental Protection Agency Report No. 52011-74-005 (National Technical Information Service, Springfield, Virginia). TELL, R.A., HANKIN, N.N., NELSON, J.C., ATHEY, T.W. and JANES, D.E., JR. (1976a). "An automated measurement system for determining environmental radiofrequency field intensities 11," pages 203 to 213 in Proceedings of an NBS 75th Symposium on Measurements for the Safe Use of Radiation, Fivozinsky, S.P., Ed., National Bureau of Standards Publication S P 456 (National Technical Information Service, Springfield, Virginia). TELL, R.A., HANKIN, N.N. and JANES, D.E., JR. (1976b). "Aircraft radar measurements in the near field," page 239 in OperationalHealth Physics, Proceedings of the Ninth Midyear Topical Symposium of theHealth Physics Society, Carson, P.L., Hendee, W.R. and Hunt, D.C., Eds. (Central Rocky Mountain Chapter, Health Physics Society, Boulder, Colorado). TELL, R.A., MANTIPLY, E.D., DURNEY, C.H. and MASSOUDI, H. (1979). "Electric and magnetic field intensities and associated induced currents in man in close proximity to a 50 kW AM standard broadcast station," page 360 in Progmm Abstracts, United States National Committee on International Union of Radio Science and Bioelectromagnetics Symposium (Bioelectromagnetics Society, Frederick, Maryland). TENFORDE, T.S. and BUDINGER, T.F. (1986). "Biological effects and physical safety aspects of NMR imaging and in vivo spectroscopy," page 493 in NMR in Medicine: Instrumentation and Clinical Applications Medical

208

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REFERENCES

Monograph No. 14 (American Association of Physicists in Medicine, New York). WHO (1993). World Health Organization. Electromagnetic Fields (300 Hz to300 GHz), Environmental Health Criteria No. 137 World Health Organization, Geneva). WOLBARSHT, M.L.(1980). "Electromagneticemissions for visual display units: A non-hazard," Society for Photo-Optical Instrumentation Engineers 229, 187.

The NCRP The National Council on Radiation Protection and Measurements is a nonprofit corporation chartered by Congress in 1964 to: 1. Collect, analyze, develop and disseminate in the public interest information and recommendations about (a)protection against radiation and (b) radiation measurements, quantities and units, particularly those concerned with radiation protection. 2. Provide a means by which organizations concerned with the scientific and related aspects of radiation protection and ofradiation quantities, units and measurements may cooperate for effective utilization of their combined resources, and to stimulate the work of such organizations. 3. Develop basic concepts about radiation quantities, units and measurements, about the application of these concepts, and about radiation protection. 4. Cooperate with the International Commission on Radiological Protection, the International Commission on Radiation Units and Measurements, and other national and international organizations, governmental and private, concerned with radiation quantities, units and measurements and with radiation protection. The Council is the successor to the unincorporated association of scientists known as the National Committee on Radiation Protection and Measurements and was formed to carry on the work begun by the Committee. The Council is made up of the members and the participants who serve on the scientific committees of the Council. The Council members who are selected solely on the basis of their scientific expertise are drawn from public and private universities, medical centers, national and private laboratories and industry. The scientific committees, composed of experts having detailed knowledge and competence in the particular area of the committee's interest, draft proposed recommendations. These are then submitted to the full membership of the Council for careful review and approval before being published.

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The following comprise the current officers and membership of the Council: Officers

CHARLESB. MEINHOLD S.JAMES ADELSTEIN W. ROGERNEY CARLD. HOBELMAN JAMES F. BERG

President Vice President Secretary and Treasurer Assistant Secretary Assistant Treasurer Members

Honorary Members LAURISTON S. TAYLOR,Honorary President WARRENK. SINCLAIR, President Emeritus R.J. MICHAELFRY ROBERT0.G~RSON JOHN W. HEALY PAULC. HODGES GEORGEV. LEROY WILFRIDB. MANN A. ALANMOGHISSI KARLZ. MORGAN ROBERTJ. NELSEN WESLEYL. NYBORG CHESTERR. RICHMOND HARALDH. ROSSI WILLIAML. RUSSELL

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Currently, the following subgroups are actively engaged in formulating recommendations: Basic Radiation Protection Criteria SC 1-3Collective Dose SC 1-4Extrapolation of Risk from Non-human Experimental Systems to Man SC 1-5Uncertainty in Risk Estimates Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV X-Ray Protection in Dental Offices Operational Radiation Safety SC 46-8Radiation Protection Design Guidelines for Particle Accelerator Facilities SC 46-9ALARA a t Nuclear Plants SC 46-10Assessment of Occupational Doses from Internal Emitters SC 46-11Radiation Protection During Special Medical Procedures SC 46-12Determination of the Effective Dose Equivalent (and Effective Dose) to Workers for External Exposure to Low-LET Radiation Dosimetry and Metabolism of Radionuclides SC 57-2Respiratory Tract Model SC 57-9Lung Cancer Risk SC 57-10Liver Cancer Risk SC 57-14Placental Transfer SC 57-15Uranium SC 57-16Uncertainties in the Application of Metabolic Models Radiation Exposure Control in a Nuclear Emergency SC 63-1Public Knowledge Radionuclides in the Environment SC 64-6Screening Models SC 64-17Uncertainty in Environmental Transport in the Absence of Site Specific Data SC 64-18Plutonium SC 64-19Historical Dose Evaluation Quality Assurance and Accuracy in Radiation Protection Measurements Biological Effects and Exposure Criteria for Ultrasound Efficacy of Radiographic Procedures Radiation Protection in Mammography Guidance on Radiation Received in Space Activities Guidance on Occupational and Public Exposure Resulting from Diagnostic Nuclear Medicine Procedures Radionuclide Contamination SC 84-1Contaminated Soil SC 84-2Decontamination and Decommissioning of Facilities Risk of Lung Cancer from Radon Hot Particles in the Eye, Ear or Lung Radioactive and Mixed Waste SC 87-1Waste Avoidance and Volume Reduction SC 87-2Waste Classification Based on Risk SC 87-3Performance Assessment

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SC 88

Fluence as the Basis for a Radiation Protection System for Astronauts SC 89 Nonionizing Electromagnetic Fields SC 89-1Biological Effects of Magnetic Fields SC 89-3Extremely Low-Frequency Electric and Magnetic Fields SC 90 Precautions in the Management of Patients Who have Received Therapeutic Amounts of Radionuclides SC 91 Radiation Protection in Medicine Ad Hoc Committee on the Embryo Fetus and Nursing Child Ad Hoc Committee on Council Involvement in Public Decision Making

In recognition of its responsibility to facilitate and stimulate cooperation among organizations concerned with the scientific and related aspects of radiation protection and measurement, the Council has created a category of NCRP Collaborating Organizations. Organizations or groups of organizations that are national or international in scope and are concerned with scientific problems involving radiation quantities, units, measurements and effects, or radiation protection may be admitted to collaborating status by the Council. Collaborating Organizations provide a means by which the NCRP can gain input into its activities from a wider segment of society. At the same time, the relationships with the Collaborating Organizations facilitate wider dissemination of information about the Council's activities, interests and concerns. Collaborating Organizations have the opportunity to comment on draft reports (at the time that these are submitted to the members of the Council). This is intended to capitalize on the fact that Collaborating Organizations are in an excellent position to both contribute to the identification of what needs to be treated in NCRP reports and to identify problems that might result from proposed recommendations. The present Collaborating Organizations with which the NCRP maintains liaison are as follows: American Academy of Dermatology American Association of Physicists in Medicine American College of Medical Physics American College of Nuclear Physicians American College of Occupational and Environmental Medicine American College of Radiology American Dental Association American Industrial Hygiene Association American Institute of Ultrasound in Medicine American Insurance Services Group American Medical Association American Nuclear Society American Pediatric Medical Association American Public Health Association American Radium Society

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American Roentgen Ray Society American Society of Radiologic Technologists American Society for Therapeutic Radiology and Oncology Association of University Radiologists Bioelectromagnetics Society College of American Pathologists Conference of Radiation Control Program Directors Electric Power Research Institute Federal Communications Commission Federal Emergency Management Agency Genetics Society of America Health Physics Society Institute of Nuclear Power Operations International Brotherhood of Electrical Workers National Aeronautics and Space Administration National Electrical Manufacturers Association National Institute of Standards and Technology Nuclear Management and Resources Council Oil, Chemical and Atomic Workers Radiation Research Society Radiological Society of North America Society of Nuclear Medicine United States Air Force United States Army United States Department of Energy United States Department of Housing and Urban Development United States Department of Labor United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission United States Public Health Services Utility Workers Union of America

The NCRP has found its relationships with these organizations to be extremely valuable to continued progress in its program. Another aspect of the cooperative efforts of the NCRP relates to the Special Liaison relationships established with various governmental organizations that have an interest in radiation protection and measurements. This liaison relationship provides: (1)an opportunity for participating organizations to designate an individual to provide liaison between the organization and the NCRP; (2)that the individual designated will receive copies of draft NCRP reports (at the time that these are submitted to the members of the Council) with an invitation to comment, but not vote; and (3) that new NCRP efforts might be discussed with liaison individuals as appropriate, so that they might have an opportunity to make suggestions on new studies and related matters. The following organizations participate in the Special Liaison Program: Australian Radiation Laboratory Commissariat a 1'Energie Atomique (France)

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Commission of the European Communities Defense Nuclear Agency International Commission on Non-Ionizing Radiation Protection Japan Radiation Council National Radiological Protection Board (United Kingdom) National Research Council (Canada) Office of Science and Technology Policy Office of Technology Assessment Ultrasonics Institute (Australia) United States Air Force United States Coast Guard United States Department of Health and Human Services United States Department of Transportation United States Nuclear Regulatory Commission

The NCRP values highly the participation of these organizations in the Special Liaison Program. The Council also benefits significantly from the relationships established pursuant to the Corporate Sponsor's Program. The program facilitates the interchange of information and ideas and corporate sponsors provide valuable fiscal support for the Council's program. This developing program currently includes the following Corporate Sponsors: Amersham Corporation Commonwealth Edison Consumers Power Company Duke Power Company Eastman Kodak Company EG&G Rocky Flats Landauer, Inc. 3M Public Service Electric and Gas Company Southern California Edison Company Westinghouse Electric Corporation

The Council's activities are made possible by the voluntary contribution of time and effort by its members and participants and the generous support of the following organizations: Agfa Corporation Alfred P. Sloan Foundation Alliance of American Insurers American Academy of Dermatology American Academy of Oral and Maxillofacial Radiology American Association of Physicists in Medicine American Cancer Society American College of Medical Physics American College of Nuclear Physicians American College of Occupational and Environmental Medicine

American College of Radiology American College of Radiology Foundation American Dental Association American Healthcare Radiology Administrators American Industrial Hygiene Association American Insurance Services Group American Medical Association American Nuclear Society American Osteopathic College of Radiology American Pediatric Medical Association American Public Health Association American Radium Society American Roentgen Ray Society American society of ~ a d i o l o ~ i ~ ~ e c h n o l o g i s t 8 American Society for Therapeutic Radiology and Oncology American veterinary ~ e d i c a Association l American Veterinary Radiology Society Association of University Radiologists Battelle Memorial Institute Canberra Industries, Inc. Chem Nuclear Systems Center for Devices and Radiological Health College of American Pathologists Committee on Interagency Radiation Research and Policy Coordination Commonwealth of Pennsylvania Defense Nuclear Agency Edison Electric Institute Edward Mallinckrodt, Jr. Foundation EG&G Idaho, Inc. Electric Power Research Institute Federal Emergency Management Agency Florida Institute of Phosphate Research Fuji Medical Systems, U.S.A., Inc. Genetics Society of America Health Effects Research Foundation (Japan) Health Physics Society Institute of Nuclear Power Operations James Picker Foundation Martin Marietta Corporation National Aeronautics and Space Administration National Association of Photographic Manufacturers National Cancer Institute National Electrical Manufacturers Association National Institute of Standards and Technology Nuclear Management and Resources Council Picker International Radiation Research Society Radiological Society of North America Richard Lounsbery Foundation Sandia National Laboratory Society of Nuclear Medicine Society of Pediatric Radiology

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United States Department of Energy United States Department of Labor United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission Vietoreen, Inc.

Initial funds for publication of NCRP reports were provided by a grant from the James Picker Foundation. The NCRP seeks to promulgate information and recommendations based on leading scientificjudgement on matters of radiation protection and measurement and to foster cooperation among organizations concerned with these matters. These efforts are intended to serve the public interest and the Council welcomes comments and suggestions on its reports or activities from those interested in its work.

NCRP Publications NCRP publications are distributed by the NCRP Publications Office.Information on prices and how to order may be obtained by directing an inquiry to: NCRP Publications 7910 Woodmont Avenue Suite 800 Bethesda, MD 20814-3095 The currently available publications are listed below. NCRP Reports No. Title Control and Removal of Radioactive Contamination i n Laboratories (1951) Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides i n Air and in Waterfor Occupational Exposure (1959)[IncludesAddendum 1 issued in August 19631 Measurement of Neutron Flux and Spectm for Physical and Biological Applications (1960) Measurement of Absorbed Dose ofNeutrons, and ofMixtures of Neutrons and Gamma Rays (1961) Stopping Powers for Use with Cavity Chambers (1961) Safe Handling of Radioactive Materials (1964) Radiation Protection i n Educational Institutions (1966) Dental X-Ray Protection (1970) Radiation Protection in Veterinary Medicine (1970) Precautions i n the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides (1970) Protection Against Neutron Radiation (1971) Protection Against Radiation from Bmchytherapy Sources (1972) Specification of Gamma-Ray Brachytherapy Sources (1974) Radiological Factors Affecting Decision-Making in a Nuclear Attack (1974) Krypton-85 in the Atmosphere-Accumulation, Biological Significance, and Control Technology (1975)

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Alpha-Emitting Particles in Lungs (1975) Tritium Measurement Techniques (1976) Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV (1976) Environmental Radiation Measurements (1976) Radiation Protection Design Guidelines for 0.1 -100 MeV Particle Accelerator Facilities (1977) Cesium-137 from the Environment to Man: Metabolism and Dose (1977) Medical Radiation Exposure of Pregnant and Potentially Pregnant Women (1977) Protection of the Thyroid Gland i n the Event of Releases of Radioiodine (1977) Instrumentation and Monitoring Methods for Radiation Protection (1978) A Handbook of Radioactivity Measurements Procedures, 2nd ed. (1985) Operational Radiation Safety Program (1978) Physical, Chemical, and Biological Properties of Radiocerium Relevant to Radiation Protection Guidelines (1978) Radiation Safety Training Criteria for Industrial Radiography (1978) Tritium in t h Environment (1979) Tritium and Other Radionuclide Labeled Organic Compounds Incorporated i n Genetic Material (1979) Influence of Dose and Its Distribution in Time on DoseResponse Relationships for Low-LET Radiations (1980) Management of Persons Accidentally Contaminated with Radionuclides (1980) Radiofrequency Electromagnetic Fields-Properties, Quantities and Units, Biophysical Interaction, and Measurements (1981) Radiation Protection in Pediatric Radiology (1981) Dosimetry of X-Ray and Gamma-Ray Beams for Radiation Therapy i n the Energy Range 10 keV to 50 MeV (1981) Nuclear Medicine-Factors Influencing the Choice and Use of Radionuclides in Diagnosis and Therapy (1982) Operational Radiation Safety-Training (1983) Radiation Protection and Measurement for Low-Voltage Neutron Generators (1983) Protection in Nuclear Medicine and Ultrasound Diagnostic Procedures in Children (1983)

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Biological Effects of Ultrasound: Mechanisms and Clinical Implications (1983) Iodine-129: Evaluation of Releases from Nuclear Power Generation (1983) Radiological Assessment: Predicting the Transport, Bioaccumulation, a n d Uptake by M a n of Radionuclides Released to the Environment (1984) Exposures from the Uranium Series with Emphasis on Radon and Its Daughters (1984) Evaluation of Occupational and Environmental Exposures to Radon and Radon Daughters in the United States (1984) Neutron Contamination from Medical Electron Accelerators (1984) Induction of Thyroid Cancer by Ionizing Radiation (1985) Carbon-14 in the Environment (1985) SI Units in Radiation Protection and Measurements (1985) The Experimental Basis for Absorbed-Dose Calculations in Medical Uses of Radionuclides (1985) General Concepts for the Dosimetry of Internally Deposited Radionuclides (1985) Mammography-A User's Guide (1986) Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields (1986) Use of Bioassay Procedures for Assessment of Internal Radionuclide Deposition (1987) Radiation Alarms and Access Control Systems (1986) Genetic Effects from Znternally Deposited Radionuclides (1987) Neptunium: Radiation Protection Guidelines (1988) Public Radiation Exposure from Nuclear Power Generation in the United States (1987) Ionizing Radiation Exposure of the Population of the United States (1987) Exposure of the Population in the United States and Canada from Natural Background Radiation (1987) Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources (1987) Comparative Carcinogenicity of Ionizing Radiation and Chemicals (1989) Measurement of Radon and Radon Daughters in Air (1988) Guidance on Radiation Received i n Space Activities (1989) Quality Assurance for Diagnostic Imaging (1988) Exposure of the U.S. Population from Diagnostic Medical Radiation (1989)

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Exposure of the U.S. Population from Occupational Radiation (1989) 102 Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV (Equipment Design, Performance and Use) (1989) 103 Control of Radon in Houses (1989) 104 The Relative Biological Effectiveness ofRudiations ofDifferent Quality (1990) 105 Radiation Protection for Medical and Allied Health Personnel (1989) 106 Limit for Exposure to "Hot Particles" on the Skin (1989) 107 Implementation of the Principle of As Low As Reasonably Achievable (ALARA) for Medical and Dental Personnel (1990) 108 Conceptual Basis for Calculations of Absorbed-Dose Distributions (1991) 109 Effects of Ionizing Radiation on Aquatic Organisms (1991) 110 Some Aspects of Strontium Rudiobiology (1991) 111 Developing Radiation Emergency Plans for Academic, Medical or Industrial Facilities (1991) 112 Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination (1991) 113 Exposure Criteria for Medical Diagnostic Ultrasound:I. Criteria Based on Thermal Mechanisms (1992) 114 Maintaining Radiation Protection Records (1992) 115 Risk Estimates for Radiation Protection (1993) 116 Limitation of Exposure to Ionizing Radiation (1993) 117 Research Needs for Radiation Protection (1993) 118 Radiation Protection i n the Mineral Extraction Industry (1993) 119 A Practical Guide to the Determination of Human Exposure to Radiofrequency Fields (1993) Binders for NCRP reports are available. Two sizes make it possible to collect into small binders the "old series" of reports (NCRP Reports Nos. 8-30) and into large binders the more recent publications (NCRP Reports Nos. 32-119). Each binder will accommodate from five to seven reports. The binders carry the identification "NCRP Reports" and come with label holders which permit the user to attach labels showing the reports contained in each binder. The following bound sets of NCRP reports are also available: Volume I. Volume 11.

NCRP Reports Nos. 8,22 NCRP Reports Nos. 23, 25, 27, 30

NCRP PUBLICATIONS

Volume 111. Volume IV. Volume V. Volume VI. Volume VII. Volume VIII. Volume IX. Volume X. Volume XI. Volume XII. Volume XIII. Volume XIV. Volume XV. Volume XVI. Volume XVII. Volume XVIII. Volume XIX. Volume XX. Volume XXI. Volume XXII. Volume XXIII.

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NCRP Reports Nos. 32,35,36,37 NCRP Reports Nos. 38,40,41 NCRP Reports Nos. 42,44,46 NCRP Reports Nos. 47,49,50,51 NCRP Reports Nos. 52,53,54,55,57 NCRP Report No. 58 NCRP Reports Nos. 59,60,61,62,63 NCRP Reports Nos. 64,65,66,67 NCRP Reports Nos. 68,69,70,71,72 NCRP Reports Nos. 73,74,75,76 NCRP Reports Nos. 77,78,79,80 NCRP Reports Nos. 81,82,83,84,85 NCRP Reports Nos. 86,87,88,89 NCRP Reports Nos. 90,91,92,93 NCRP Reports Nos. 94,95,96,97 NCRP Reports Nos. 98,99,100 NCRP Reports Nos. 101,102,103,104 NCRP Reports Nos. 105,106,107,108 NCRP Reports Nos. 109,110,111 NCRP Reports Nos. 112,113,114 NCRP Reports Nos. 115,116,117,118

(Titles of the individual reports contained in each volume are given above.)

No. 1 2 3

4

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NCRP Commentaries Title Krypton-85 in the Atmosphere-With Specific Refemnee to the Public Health Significance o f the Proposed Controlled Release at Three Mile Island (1980) Preliminary Evaluation of Criteria for the Disposal of Transuranic Contaminated Waste (1982) Screening Techniques for Determining Compliance with Environmental Standards-Releases of Radionuclides to the Atmosphere (1986),Revised (1989) Guidelines for the Release of Waste Water from Nuclear Facilities with Special Reference to the Public Health Significance of the Proposed Release of Treated Waste Waters at Three Mile Island (1987) Review of the Publication, Living Without Landfills (1989) Radon Exposure o f the U.S. Population-Status o f the Problem (1991)

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Misadministration of Radioactive Material in MedicineScientific Background (1991) Uncertainty in NCRP Screening Models Relating to Atmospheric Transport, Deposition and Uptake by Humans (1993) Proceedings of the Annual Meeting No.

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Perceptions of Risk, Proceedings of the Fifteenth Annual Meeting held on March 14-15, 1979 (including Taylor Lecture No. 3) (1980) Critical Issues in Setting Radiation Dose Limits, Proceedings of the Seventeenth Annual Meeting held on April 89, 1981 (including Taylor Lecture No. 5 ) (1982) Radiation Protection and New Medical Diagnostic Approaches, Proceedings of the Eighteenth Annual Meeting held on April 6-7, 1982 (including Taylor Lecture No. 6) (1983) Environmental Radioactivity, Proceedings of the Nineteenth Annual Meeting held on April 6-7,1983 (including Taylor Lecture No. 7) (1983) Some Issues Important in Developing Basic Radiation Protection Recommendations, Proceedings of the Twentieth Annual Meeting held on April 4-5,1984 (includingTaylor Lecture No. 8) (1985) Radioactive Waste, Proceedings of the Twenty-first Annual Meeting held on April 3-4,1985 (includingTaylor Lecture No. 9) (1986) Nonionizing Electromagnetic Radiations and Ultrasound, Proceedings of the Twenty-second Annual Meeting held on April 2-3, 1986 (including Taylor Lecture No. 10) (1988) New Dosimetry at Hiroshima and Nagasaki and Its Implications for Risk Estimates, Proceedings o f the Twenty-third Annual Meeting held on April 8-9,1987 (includingTaylor Lecture No. 11)(1988) Radon, Proceedings of the Twenty-fourth Annual Meeting held on March 30-31, 1988 (including Taylor Lecture No. 12) (1989) Radiution Protection Today-The NCRP at Sixty Years,Proceedings of the Twenty-fifth Annual Meeting held on April 5-6, 1989 (including Taylor Lecture No. 13) (1990)

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Health and Ecological Implications of Radioactively Contaminated Environments, Proceedings of the Twentysixth Annual Meeting held on April 4-5, 1990 (including Taylor Lecture No. 14) (1991) Genes, Cancer and Radiation Protection, Proceedings of the Twenty-seventh Annual Meeting held on April 3-4,1991 (including Taylor Lecture No. 15) (1992) Radiation Protection in Medicine, Proceedings of the Twenty-eighth Annual Meeting held on April 1-2,1992 (including Taylor Lecture No.16) (1993)

No. 1 2 3

Lauriston S. Taylor Lectures Title The Squares of the Natural Numbers in Radiation Protection by Herbert M. Parker (1977) Why be Qwlntitative about Radiation Risk Estimates? by Sir Edward Pochin (1978) Radiation Pmtection-Concepts and Tmde 0fi by Hyrner L. Friedell (1979) [Available also in Perceptions of Risk, see above] From "QuantiCy ofRadiationY'and "Dose" to "Exposure" and "Absorbed Dose9'-An Historical Review by Harold 0. Wyckoff (1980) How Well Can We Assess Genetic Risk? Not Very by James F. Crow (1981) [Available also in Critical Issues in Setting Radiation Dose Limits, see abovel Ethics, Trade-offs and Medical Radiation by Eugene L. Saenger (1982) [Available also in Radiation Protection and New Medical Diagnostic Approaches, see abovel The Human Environment-Past, Present and Future by Merril Eisenbud (1983) [Available also in Environmental Radioactivity, see abovel Limitation and Assessment i n Radiation Protection by Harald H. Rossi (1984) [Available also in Some Issues Important in Developing Basic Radiation Protection Recommendations, see abovel Truth (and Beauty) in Radiation Measurement by John H. Harley (1985) [Available also in Radioactive Waste, see abovel Biological Effects of Non-ionizing Radiations: Cellular Properties and Interactions by Herman P. Schwan (1987) [Available also in Nonionizing Electromagnetic Radiations and Ultrasound, see abovel

224

NCRP PUBLICATIONS

How to be Qwlntitative about Radiation Risk Estimates by Seymour Jablon (1988) [Available also in New Dosimetry at Hiroshima and Nagasaki and its Implications for Risk Estimates, see abovel How Safe is Safe Enough? by Bo Lindell(1988) [Available also in Radon, see abovel Radiobiology and Radiation Protection: The Past Century and Prospects for the Future by Arthur C. Upton (1989) [Available also in Radiation Protection Today, see above] Radiation Protection and the Internal Emitter Saga by J. Newel1 Stannard (1990) [Available also in Health and Ecological Implications of Radioactively Contaminated Environments, see above] When is a Dose Not a Dose? by Victor P. Bond (1992) [Available also in Genes, Cancer and Radiation Protection, see above] Dose and Risk in Diagnostic Radiology: How Big? How Little? by Edward W. Webster (1992)[Available also in Radiation Protection i n Medicine, see abovel Science, Radiation Protection and the NCRP by Warren K. Sinclair (1993) Symposium Proceedings The Control of Exposure of the Public to Ionizing Radiation in the EventofAccident orAttack, Proceedings of a Symposium held April 27-29, 1981 (1982)

No. 1 2

NCRP Statements Title "Blood Counts, Statement of the National Committee on Radiation Protection," Radiology 63, 428 (1954) "Statements on Maximum Permissible Dose from Television Receivers and Maximum Permissible Dose to the Skin of the Whole Body," Am. J. Roentgenol., Radium Ther. and Nucl. Med. 84, 152 (1960) and Radiology 75, 122 (1960) X-Ray Protection Standards for Home Television Receivers, Interim Statement of the National Council on Radiation Protection and Measurements (1968) Specification of Units ofNatum1 Uraniumand Natural Thorium, Statement of the National Council on Radiation Protection and Measurements, (1973)

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NCRP Statement on Dose Limit for Neutrons (1980) Control of Air Emissions of Radionuclides (1984) The Probability That a Particular M a l i g m c y May Have Been Caused by a Specified Irradiation (1992)

Other Documents The following documents of the NCRP were published outside of the NCRP Report, Commentary and Statement series: Somatic Radiation Dose for the General Population, Report of the Ad Hoc Committee of the National Council on Radiation Protection and Measurements, 6 May 1959, Science, February 19, 1960, Vol. 131, No. 3399, pages 482-486 Dose Effect Modifying Factors I n Radiation Protection, Report of Subcommittee M-4 (Relative Biological Effectiveness) of the National Council on Radiation Protection and Measurements, Report BNL 50073 (T-471) (1967) Brookhaven National Laboratory (National Technical Information Service Springfield, Virginia) The following documents are now superseded andlor out of print:

No.

NCRP Reports Title X-Ray Protection (1931) [Superseded by NCRP Report No. 31 Radium Pmtection (1934) [Superseded by NCRP Report No. 41 X-Ray Protection (1936) [Superseded by NCRP Report No. 61 Radium Pmtection (1938) [Supersededby NCRP Report No. 131 Safe Handling of Radioactive Luminous Compound (1941) [Out of Print] Medical X-Ray Protection Up to Two Million Volts (1949) [Superseded by NCRP Report No. 181 Safe Handling of Radioactive Isotopes (1949) [Superseded by NCRP Report No. 301 Recommendations for WasteDisposal of Phosphorus32 and Iodine-131 for Medical Users (1951) [Out of Print] Radiological Monitoring Methods and Instruments (1952) [Superseded by NCRP Report No. 571 Maximum Permissible Amounts of Radioisotopes i n the Human Body and Maximum Permissible Concentrations in Air and Water (1953) [Superseded by NCRP Report No. 221 Recommendations for the Disposal of Carbon-14 Wastes (1953) [Superseded by NCRP Report No. 811

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Protection Against Radiations from Radium, Cobalt-60 and Cesium-137 (1954) [Superseded by NCRP Report No. 241 Protection Against Betatron-Synchrotron Radiations Up to 100 Million Electron Volts (1954) [Superseded by NCRP Report No. 511 Safe Handling of Cadavers Containing Radioactive Isotopes (1953) [Superseded by NCRP Report No. 211 Radioactive- Waste Disposal i n the Ocean (1954) [Out of Print] Permissible Dose from External Sources of Ionizing Radiation (1954) including Maximum Permissible Exposures to Man, Addendum to National Bureau of Standards Handbook 59 (1958) [Superseded by NCRP Report No. 391 X-Ray Protection (1955) [Superseded by NCRP Report No. 261 Regulation of Radiation Exposure by Legislative Means (1955) [Out of Print] Protection Against Neutron Radiation Up to30 Million Electron Volts (1957) [Superseded by NCRP Report No. 381 Safe Handling of Bodies Containing Radioactive Isotopes (1958) [Superseded by NCRP Report No. 371 Protection Against Radiations from Sealed Gamma Sources (1960) [Superseded by NCRP Reports No. 33,34 and 401 Medical X-Ray Protection Up to Three Million Volts (1961) [Superseded by NCRP Reports No. 33,34,35 and 361 A Manual of Radioactivity Procedures (1961) [Superseded by NCRP Report No. 581 Exposure to Radiation in a n Emergency (1962) [Superseded by NCRP Report No. 421 Shielding for High-Energy Electron Accelerator Installations (1964) [Superseded by NCRP Report No. 511 Medical X-Ray and Gamma-Ray Protection for Energies up to 10 MeV-Equipment Design and Use (1968) [Superseded by NCRP Report No. 1021 Medical X-Ray and Gamma-Ray Protection for Energies Up to 10 MeV-Structural Shielding Design and Evaluation Handbook (1970) [Superseded by NCRP Report No. 491 Basic Radiation Protection Criteria (1971) [Superseded by NCRP Report No. 911 Review of the Current State of Radiation Protection Philosophy (1975) [Superseded by NCRP Report No. 91.1 Natural Background Radiation in the United States (1975) [Superseded by NCRP Report No. 941 Radiation Protection for Medical and Allied Health Personnel (1976) [Superseded by NCRP Report No. 1051

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Review ofNCRPRadiation Dose Limit forEmbryo and Fetus in Occupationully-Exposed Women (1977) [Out of Printl Radiation Exposure from ConsumerProducts and MisceUaneous S o u m (1977) [Superseded by NCRP Report No. 951 A Handbook of Radioactivity Measurements Procedures, 1st ed. (1978) [Superseded by NCRP Report No. 58, 2nd 4 . 1 Mammography (1980) [Out of Printl Recommendations on Limits fir Exposure to Ionizing Radiation (1987) [Superseded by NCRP Report No. 1161

NCRP Proceedings Title Quantitative Risk in Standards Setting, Proceedings of the Sixteenth Annual Meeting held on April 2-3, 1980 [Out of Print]

Index Absolute accuracy 87 broadband survey instruments 87 Active antenna 96 Active rod antenna 96 advantages of 96 Amplitude modulation (AM)31 AM radio field-strength values 134 Antenna 8,45,94-100,163,191 active 96 broadband 99 broadband parabolic 99 directional gain (G) 95 circular aperture horn reffector 191 circularly polarized 100 conical log-spiral 100 effective area (A,)95 factor (A,) 94 half-wave dipole 97 log-periodic 98 log-spiral 98 narrowband parabolic reflector 99 omnidirectional 98 passive rod 96 planar log-periodic 98 pyramidal gain horn 99 rotating parabolic reflector 163 rotational duty cycle 115 shielded loop 97 types 94 Antenna characteristics 121 antenna azimuth and elevation 121 gain 121 polarization of the beam 121 m i n i n g rate 121 width of the beam 121 Antenna factor (A,) 94 Antenna fields 126 Antenna rotational duty cycle 116 measurement of 116 Anti-theft devices 174 Area survey 75-76 Automated measurement system 110 block diagram 110 ~ u t o r n a t e measurement d~~ system 112 advantage of 112

Broadband electric-field instruments 83 thermocouple-based 83 Broadband instruments 77,93 peripheral equipment 93 Broadband survey instruments 78,80, 81,84,86,88.89, 90 electromagnetic interference 89 desirable characteristics of 86 diode-based 81,84 dynamic range 88 frequency response 86 isotropic response 86 meter output 89 modem 80 modulation response 92 multiple signal addition 92 out-of-band response 88 peak hold for 90 recorder output 92 response 92 response time 91 spatial resolution 91 stability 91 static-charge sensitivity 90 thermocouple based 84 Circularly polarized antenna 100 Complex permittivity 24 Conductivity 24 Conical log-spiral antenna 100 Data logger 93 broadband instruments 93 Diathermy equipment 151 operational characteristics 151 Decibel 77 definition 77 Derived SI units 13 Desirable characteristics of broadband survey instruments 86-92 absolute accuracy 87 battery operation 91 durability 92 dynamic range 88 ease of adjustment and use 92

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INDEX

electromagnetic interference 89 frequency response 86 isotropic response 86 meter output 89 modulation response 92 multiple signal addition 92 out-of-band response 88 peak hold 90 portability 92 probe burnout alarm 90 probe overload/burnout rating 90 readability 92 recorder output 92 response 89,92 response time 91 spatial resolution 91 stability 91 static-charge sensitivity 90 Dielectric constant 8 Dielectric heating 4 Digital systems 147 measurement of 147 Diode detectors 83 advantages of 83 disadvantages of 83 Diodes 81 point-contact 81 Schottky 81 Dipole moment 23 Directional antenna i26 pyramidal standard gain horn 126 log-periodic 126 Displacement current sensor 85 Displacement flux density 26 Dissipation factor 25 Dosimetry 51 Durability 92 broadband survey instruments 92 Duty cycle (6) 115 Duty factor determination 170 Dynamic range 88 broadband survey instruments 88

Electric-field measurements 60 Electric fields 8,16 Electric-field strength (E) 8 Electric-flux density (displacement) (D) 9,26 Electric polarization 9 Electromagnetic interference 89 broadband survey instruments 89 Electrosurgical and electrocautery units 153 operational characteristics 153 Ellipticity 86 Emission 9 Energy density (surface) 9 Energy density (volume) 9 Evaluation of exposure 68 procedures for 68 Exposure 9 Exposure assessment 124 Exposure evaluation 70 necessary data 70 Exposure limits 5 former Soviet Union 5 Far field 9,34,47,48 Far-field conditions 71 Field point 9 Field-strength meters 78,106,109 usable frequency range 109 response 109 Fourier transform analyzers 101 Frequency 9 Frequency measurement 170 Frequency modulation (FM) 31 Frequency-modulation (FM) radio broadcast 136 operational characteristics 136 Frequency response 86 broadband survey instruments 86 Grating lobe 136

Ease of adjustment and use 92 broadband survey instruments 92 Effective radiated power 46 E polarization 54 Electrical properties of tissue 51 Electric dipole 8 Electric-field instrument 88 desirable dynamic range 88 Electric-field intensity 8

Hand-held portable radios 147,148 operational characteristics 148 Half-wave dipole antenna 97,98 disadvantage of 98 Hazard evaluation procedures 133 Hazards 1 radiofrequency 1 H polarization 54

INDEX Hyperthermia equipment 155 operational characteristics 155

IF amplifier 104 Impedance, wave 9 Isotropic response 86 broadband survey instruments 86

K polarization 54 Leakage fields 121,122 LORANIOMEGA navigational stations 184 operational characteristics 184 Lorentz force 17 Loss tangent 25 Lossy 9,63 Magnetic field 9,17 Magnetic-field instrument 80,88 design of 80 desirable dynamic range 88 Magnetic-field measurements 61 Magnetic-field probes 87 frequency range of 87 frequency response of 87 Magnetic-field strength (H) 10,27 Magnetic-flux density (B)10.17 Magnetic resonance imaging systems 157 operational characteristics 157 Marine radar 163 operational characteristics 163 Measurement of 61,62 magnetic fields 61 specific absorption rate (SARI 62 Measurement accuracy 127 precautions for ensuring 127 Measurement of operator exposure 170 Measurement techniques 77, 135, 138,142, 144,146,148,150, 152, 154,156,158,162, 164,166,167, 175,177,178,179,181,185 AM radio 135 anti-theR devices 175 diathermy equipment 152 electrosurgical and electrocautery units 154

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231

FM radio 138 hand-held portable radios 148 hyperthermia equipment 156 LORANIOMEGA navigational stations 185 magnetic resonance imaging systems 158 marine radar 164 microwave door openers 177 microwave industrial heating1 drying equipment 167 microwave intrusion alarms 178 microwave ovens 179 microwave radio 144 mobile radios 150 police radar 166 radar 162 satellite communication 146 sports radar 166 television 142 visual-display terminals 181 Meter output 89 broadband survey instruments 89 Microwave door openers 176 operational characteristics 176 Microwave industrial heatingtdrying equipment 166 operational characteristics 166 Microwave intrusion alarms 177 operational characteristics 177 Microwave ovens 178 operational characteristics 178 Microwave radio 142 operational characteristics 142 Mobile radios 149-150 operational characteristics 150 Modulation 29 Modulation response 92 broadband survey instruments 92 Multiple signal addition 92 broadband survey instruments 92 Mutual coupling 85 Narrowband instruments 78 field-strength meters 78 spectrum analyzers 78 Narrowband survey instruments 110 limitations of 110 Narrowband systems 93 Near field 10,34,47,48 Near-field conditions 71 Nodes 35

232

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INDEX

Nonmagnetic materials 22 Nonplanar objects 39 Out-of-band response 88 Oven survey instrument 86 response of 86 Passive rod antenna 96 Peak hold 90 broadband survey instruments 90 Permeability 10 Permittivity (010,24,51 frequency dependence 51 Permittivity of free space 16 Phase shifl keying (PSK) 143 Phase velocity 10,27 Planar dielectrics 37 Planar log-periodic antenna 98 Planar models 52 Planar wave 10,33 Polarization 53 Polarization charge 23 Polarization of EM wave 10 Police radar 165 operational characteristics 165 Portability 92 broadband survey instruments 92 Power density 10 Power meter 118 Poynting's Theorem 40 Poynting vector (S) 10,40 Probe burnout alarm 90 broadband survey instruments 90 Probe overload/burnout rating 90 broadband survey instruments 90 Propagation coefficient 10 Protection guide 68-69 criteria 68 single-value 69 frequency dependent 69 Protection of the operator 128 precautions for 128 Pulse-repetition frequency Vp) 115 Pyramidal gain horn antenna 99 Quadrature amplitude modulation (&AM)143 Quasi-narrowband instruments 77, 119 advantage of 119

Quasi-narrowband system 78, 117 frequency range 78 Quasi-static fields 22

Radar 158 operational characteristics 158 Radar exposure measurements 162 Radian frequency 11 Radiation patterns 49 Radiofrequency 1-3, 6.59 band designations 3 hazards 1 protection guides 59 safety standards 6 spectrum 2 Radiofrequency dielectric heaters 171 operational characteristics 171 Radiofrequency induction heaters 168 operational characteristics 168 Readability 92 broadband survey instruments 92 Recorder output 92 broadband survey instruments 92 Reflection coefficient 11 Relative permittivity 11 Response to environmental factors 92 broadband survey instruments 92 Response to the parameter being measured 89 broadband survey instruments 89 Response time 91 broadband survey instruments 91

Safety standards 6 Satellite communicationearth stations 144 operational characteristics 144 Scalar 12 Scattered fields 34,43 Secondary sources 74 effects of 74 Shielded loop antenna 97 Short dipole 79 effective length of 79 Sidelobes 136 Spatial averaging 72 Spatial resolution 91 broadband survey instruments 91 Special measurements 129 Specific absorption rate (SAR) 11

INDEX Specific absorption rate characteristics 55 Specific absorption rate measurements 62 Spectrum analyzers 78,101, 103, 105, 108,115 amplitude response 108 measurement e m r 105 real-time 101 superheterodyne 103 swept-tuned 101, 103 Spherical waves 11,31 Sports radar 165 operational characteristics 165 Square-law response 84 Stability 91 broadband survey instruments 91 Standing-wave ratio 11,38 Static-charge sensitivity 90 broadband survey instruments 90 Static-electric fields 18 Static-magnetic fields 20 Survey procedures 170 Swept-tuned spectrum analyzer 103 Television broadcast 139 operational characteristics 139

1

233

Temperature compensated power meter 118 Thermocouple-based instruments 84 advantage of 84 disadvantage of 84 Thermocouples 83 Thermocouple-type magnetic fieldprobe 87 frequency response of 87 Thermocouple-type sensor 118 Time averaging 72 Traveling wave 28 Vector 11.12 Vector cross product 14,15 Vector dot product 14, 15 Velocity of propagation 11 Visual-display terminals (VDT)85, 180 measuring fields associated with 85 operational characteristics 180 Wave impedance 11 Wavelength 11 Wave propagation 27 Whole-body average SAR 55

Research Needs for Radiation Protection: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report ; No. 117)

Environmental radiation measurements: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 50)

Operational Radiation Safety Training: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report, No. 134)

Radiation Protection in the Mineral Extraction Industry: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp, No)

Maintaining Radiation Protection Records: Recommendations of the National Council on Radiation Protection and Measurements : Issued November 30, 1992 (NCRP report)

Limit for Exposure to ''Hot Particles'' on the Skin: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report, No 106) (N C R P Report)

Control of Radon in Houses: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report ; No. 103)

Tritium measurement techniques: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 47)

Alpha-emitting particles in lungs: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 46)

Specification of gamma-ray brachytherapy sources: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report)

Tritium in the environment: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report 62)

Protection of the thyroid gland in the event of releases of radioiodine: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 55)

Exposure of the U.S. Population from Diagnostic Medical Radiation: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report)

Physical, chemical, and biological properties of radiocerium relevant to radiation protection guidelines: Recommendations of the National Council on Radiation ... and Measurements (NCRP report ; no. 60)

Biological Effects and Exposure Limits for ''Hot Particles'': Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report, No. 130)

Limitation of Exposure to Ionizing Radiation: Recommendations of the National Council on Radiation Protection and Measurements : Issued March 31, 199 (N C R P Report)

Operational Radiation Safety Program: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Radiation safety training criteria for industrial radiography: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 61)

Managing Potentially Radioactive Scrap Metal: Recommendations of the National Council on Radiation Protection and Measurements

Radiation Protection Guidance for Activities in Low-Earth Orbit: Recommendations of the Nattional Council on Radiation Protection and Measurements (Ncrp Report, No. 132)

Exposure of the U.S. Population from Occupational Radiation: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Management of Terrorist Events Involving Radioactive Material: Recommendations of the National Council on Radiation Protection and Measurements (Ncrp Report, No. 138)

Carbon-14 in the Environment: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Influence of dose and its distribution in time on dose-response relationships for low-let radiations: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report)

Tritium and other radionuclide labeled organic compounds incorporated in genetic material: Recommendations of the National Council on Radiation Protection and Measurements (NCRP report ; no. 63)

Radiological factors affecting decision-making in a nuclear attack: Recommendations of the National Council on Radiation Protection and Measurements

Uncertainties in Fatal Cancer Risk Estimates Used in Radiation Protection: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Effects of Ionizing Radiation on Aquatic Organisms: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Si Units in Radiation Protection and Measurements: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

Radiological Protection for Medical Exposure to Ionizing Radiation: Safety Guide