NCRP report no 119 a practical guide to the determination of human exposure to radiofrequency fields

electric-field strength: 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

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

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 i n 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-Publication Data

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

Preface

This Report is the third in a series ofNational Council on Radiation Protection and Measurements (NCRP) reports concerning radiofre- quency electromagnetic (RFEM) radiation which constitutes an extension of NCRP interest in the subject of nonionizing radiation The first report, NCRP Report No 67, Radiofrequency Ebctromag- netic 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 Effects and Exposure Criteria for Radiofrequency Electro- magnetic 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 Practi- cal Guidance on the Evaluation of Human Exposures to Radiofre- quency Radiation (formerly Scientific Committee 78) Serving on the Committee were:

Richard A Tell, Chairman

Richard Tell Associates, Inc

Las Vegas, Nevada

Members

Howard I Bassen David L Conover

Center for Devices and National Institute for

Radiological Health Occupational Safety and Health Rockville, Maryland 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

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 1 Introduction

1.1 The Study of Radiofrequency Hazards

1.2 The Radiofrequency Spectrum

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

iii

1

1

2

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) 2.2.1 1 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 Radio- frequency 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

CONTENTS 1 vii

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

4.2.3 Antenna Fields 126 4.2.4 Precautions for Ensuring Measurement

Accuracy 127

4.2.5 Precautions for Protection of the Operator 128

4.3 Special Measurements 129 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

CONTENTS / ix

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 poten- tial 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 work- place and the environment Also, developments in this new technical area have been stimulated by both the formulation of safety guide- lines 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 poten- tial 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 Determi- nation of Human Exposure to Radiofrequency Fields, has been pre- pared 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 expo- sure, 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

2 / 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 stud- ies 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 specific radiator 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 Multi- ple 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

Figure 1.1 provides 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

mm Waves - a- V~sible Light

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 approxi- mately 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 tele- communications, satellite communications (SATCOM), mobile (cel- lular) 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 dia- thermy equipment, radar and VLF radio-navigational aids In addi- tion, there are large numbers of nonintentional radiators, such as

RF heaters, dryers and plasma etchers The overall types and number

TABLE 1.1-Radiofmquency band designations

Frequencgr (MHz) Band Description

SELF ELF

VF VLF

LF

MF

HF VHF UHF SHF EHF SEHF

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-high frequency

4 1 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 a n exhaustible, invisible resource (Levin, 1971) Cer- tainly 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 tech- nology Human exposure to emissions from RF sources is thus gener- ally 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

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 fric- tional forces, results in the production of heat This phenomenon of dielectric heating is the principle of microwave ovens used for cook- ing It is this same potential for heat generation in humans, albeit usually at significantly lower intensity irradiation than in a micro- wave 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

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 rec- ommended exposure criteria (NCRP, 1986); the Environmental Pro- tection Agency (EPA, 1984) has reviewed the available literature

to support development of federal regulations; and the C95-1 commit- tee of the American National Standards Institute (ANSI) in develop- ing its RF protection guide (RFPG), has examined the biological- effects 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 Associa- tion (IRPA) has reviewed the literature (IRPA, 1988; 1990; WHO, 1993) and developed exposure criteria for both ELF and RF fields

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

6 / 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 Specific attention to these standards, from the viewpoint of measure- ment, 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 biologi- cal effects associated with exposure to RF energy These new insights have provided the framework for reevaluation of some existing expo- sure 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

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 Per- sonnel responsible for implementing andlor evaluating actual or potential hazards existing near RF emitting devices or the environ- mental impact of broadcast and communications systems, e.g., point- to-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 manag- ers and nontechnical individuals involved with health and safety

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 measure- ment data obtained in the workplace or environment Various pitfalls that can lead to erroneous conclusions from RF mea- surements 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 mea- surement data with exposure limits

3 Section 4 addresses measurement instrumentation and tech- niques 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 instru- ments 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 pro- vides a description of methods for performing practical mea- surements 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 sum- marize the most important of these concepts for the specific applica- tions that are described in this Report Where practical, concepts are explained without the use of complicated mathematical expres- sions so that they may be understood without an extensive back- ground 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 a n explanation of terms than precise defini- tions The definitions of many of these terms can be found in docu- ments 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-field strength: 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 a t 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.)

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 mate- rial 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 dissi- pation 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,

this Report

10 / 2 BASIC CONCEFTS

magnetic-field strength: A vector field, usually designated H, that

is equal to the magnetic-flux density 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 polar- ization 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 (W m-') The surface integral of S represents the instanta- neous 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)

radian frequency: The angular rate a t which a quantity is oscillat- ing 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 a t 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 Veloc- ity 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 Confer- ence on Weights and Measures held in Paris in 1970 SI is the

12 / 2 BASIC CONCEPTS

TABLE 2.1-The SI basic units

Quantity Common symbol Unit Symbol

abbreviation for Systeme International d'Unites (International Sys- tem of Units) (NBS, 1986; NCRP, 1985)

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 illus-

trated 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 under- standing 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

14 / 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:

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:

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:

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 oppo- site 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

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 experi- ment 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 associ-

ated 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 direc-

tion, 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 right- handed 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 / 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-Electric Fields 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 direc- tion 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 W 2 ) ,

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

Line Charge

A ,

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)

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 mechani- cal 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 config- uration 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 perpendicu- lar 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) Electric field 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-Magnetic Fields 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 sketch- ing 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

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

Current

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

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 analy- sis 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

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 direc- tions, 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

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 perma- nent 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 orienta- tion of permanent dipoles) by a quantity calledpermittivity Permit- tivity 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 Conductiv- ity is a measure of how much drift occurs for a given applied electric field A large drift means a high conductivity For sinusoidal steady- state applied fields, complex permittivity is defined to account for both dipole charges and conduction-charge drift Complex permittivity (F m-'1 is usually designated as:

E = E,(E' - j E") (2.12) 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-

= m E' is also called the dielectric constant E" is related

?ih4?1?#ctiue conductivity by:

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

2.2.5.3 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 1 MHz 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

2.2.5.4 Ebctric-Flux Density A quantity called electric-flux den- sity D or displacement flux density is defined as:

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 relation- ship 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

2.2.5.5 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

Surface A

Fig 2.11 Example of Gauss' law applied to a charge inside a dielectric spherical shell

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:

where p1 - jpN is 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:

B

c1

Magnetic-field strength 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 non- magnetic 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 sub- stantial A convenient and commonly used description of radiation

is wave propagation The basic ideas of wave propagation are illus- trated 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 repre- sents 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 a s 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 corres- ponding points of t h e function (Figure 2.13) The frequency f i s 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

by counting the number of crests (or troughs) that passed by a fixed point in 1 s

The quantities defined above are related by the following impor- tant 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 infor- mation 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

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)

30 1 2 BASICCONCEPTS

Various modulation methods have been developed to optimize infor- mation performance for specific requirements For each type of modu- lation, 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

proportional to the instantaneous amplitude of the modulating sig- nal 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 per- centage 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 modu-

lation 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 unmodulated carrier 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

2.2.6.4 Spherical Waves A spherical wave is a model that repre- sents approximately some actual EM waves that occur physically,