RF Safety section of Chapter 36, 1992 ARRL Handbook
Copyright 1992 American Radio Relay
League, Inc. All rights reserved.
The "Bio-Effects" Committee mentioned in this
article was an advisory committee established by the ARRL Board of Directors.
It was a forerunner of the more recent R.F. Safety Committee.
RF Radiation Safety
Amateur Radio is basically a safe activity, in recent years there has been
considerable discussion and concern about the possible hazards of electromagnetic
radiation (EMR), including both RF energy and power frequency (50-60 Hz)
electromagnetic fields. Extensive research on this topic is under way in
many countries. This section was prepared by members of the ARRL Committee
on the Biological Effects of RF Energy ("Bio Effects" Committee) and coordinated
by Wayne Overbeck, N6NB. It summarizes what is now known and offers safety
precautions based on the research to date.
All life on
earth has adapted to survive in an environment of weak, natural low-frequency
electromagnetic fields (in addition to the earth's static geomagnetic field).
Natural low-frequency EM fields come from two main sources: the sun, and
thunderstorm activity. But in the last 100 years, manmade fields at much
higher intensities and with a very different spectral distribution have
altered this natural EM background in ways that are not yet fully understood.
Much more research is needed to assess the biological effects of EMR.
Both RF and
60-Hz fields are classified as nonionizing radiation because the frequency
is too low for there to be enough photon energy to ionize atoms. Still,
at sufficiently high power densities, EMR poses certain health hazards.
It has been known since the early days of radio that RF energy can cause
injuries by heating body tissue. In extreme cases, RF-induced heating can
cause blindness, sterility and other serious health problems. These heat-related
health hazards may be called thermal effects. But now there is mounting
evidence that even at energy levels too low to cause body heating, EMR
has observable biological effects, some of which may be harmful. These
are athermal effects.
to the ongoing research, much else has been done to address this issue.
For example, the American National Standards Institute, among others, has
recommended voluntary guidelines to limit human exposure to RF energy.
And the ARRL has established the Bio Effects Committee, a committee of
concerned medical doctors and scientists, serving voluntarily to monitor
scientific research in this field and to recommend safe practices for radio
Thermal Effects of RF Energy
Body tissues that are subjected to very high levels
of RF energy may suffer serious heat damage. These effects depend upon
the frequency of the energy, the power density of the RF field that strikes
the body, and even on factors such as the polarization of the wave.
At frequencies near the body's natural resonant
frequency, RF energy is absorbed more efficiently, and maximum heating
occurs. In adults, this frequency usually is about 35 MHz if the person
is grounded, and about 70 MHz if the person's body is insulated from ground.
Also, body parts may be resonant; the adult head, for example, is resonant
around 400 MHz, while a baby's smaller head resonates near 700 MHz. Body
size thus determines the frequency at which most RF energy is absorbed.
As the frequency is increased above resonance, less RF heating generally
occurs. However, additional longitudinal resonances occur at about 1 GHz
near the body surface.
Nevertheless, thermal effects of RF energy should
not be a major concern for most radio amateurs because of the relatively
low RF power we normally use and the intermittent nature of most amateur
transmissions. Amateurs spend more time listening than transmitting, and
many amateur transmissions such as CW and SSB use low-duty-cycle modes.
(With FM or RTTY, though, the RF is present continuously at its maximum
level during each transmission.) In any event, it is rare for radio amateurs
to be subjected to RF fields strong enough to produce thermal effects unless
they are fairly close to an energized antenna or unshielded power amplifier.
Specific suggestions for avoiding excessive exposure are offered later.
Athermal Effects of EMR
Nonthermal effects of EMR, on the other hand, may
be of greater concern to most amateurs because they involve lower-level
energy fields. In recent years, there have been many studies of the health
effects of EMR, including a number that suggest there may be health hazards
of EMR even at levels too low to cause significant heating of body tissue.
The research has been of two basic types: epidemiological research, and
laboratory research into biological mechanisms by which EMR may affect
animals or humans.
Epidemiologists look at the health patterns of
large groups of people using statistical methods. A series of epidemiological
studies has shown that persons likely to have been exposed to higher levels
of EMR than the general population (such as persons living near power lines
or employed in electrical and related occupations) have higher than normal
rates of certain types of cancers. For example, several studies have found
a higher incidence of leukemia and lymphatic cancer in children living
near certain types of power transmission and distribution lines and near
transformer substations than in children not living in such areas. These
studies have found a risk ratio of about 2, meaning the chance of contracting
the disease is doubled. (The bibliography at the end of this chapter lists
some of these studies. See Wertheimer and Leeper, 1979, 1982; Savitz et
Parental exposures may also increase the cancer
risk of their offspring. Fathers in electronic occupations who are also
exposed to electronic solvents have children with an increased risk of
brain cancer (Johnson and Spitz, 1989), and children of mothers who slept
under electric blankets while pregnant have a 2.5 risk ratio for brain
cancer (Savitz et al, 1990).
Adults whose occupations expose them to strong
60-Hz fields (for example, telephone line splicers and electricians) have
been found to have about four times the normal rate of brain cancer and
male breast cancer (Matanoski et al, 1989). Another study found that microwave
workers with 20 years of exposure had about 10 times the normal rate of
brain cancer if they were also exposed to soldering fumes or electronic
solvents (Thomas et al, 1987). Typically, these chemical factors alone
have risk ratios around 2.
Dr. Samuel Milham, a Washington state epidemiologist,
conducted a large study of the mortality rates of radio amateurs, and found
that they had statistically significant excess mortality from one type
of leukemia and lymphatic cancer. Milham suggested that this could result
from the tendency of hams to work in electrical occupations or from their
However, epidemiological research by itself is
rarely conclusive. Epidemiology only identifies health patterns in groups--it
does not ordinarily determine their cause. And there are often confounding
factors: Most of us are exposed to many different environmental hazards
that may affect our health in various ways. Moreover, not all studies of
persons likely to be exposed to high levels of EMR have yielded the same
There has also been considerable laboratory research
about the biological effects of EMR in recent years. For example, it has
been shown that even fairly low levels of EMR can alter the human body's
circadian rhythms, affect the manner in which cancer- fighting T lymphocytes
function in the immune system, and alter the nature of the electrical and
chemical signals communicated through the cell membrane and between cells,
among other things. (For a summary of some of this research, see Adey,
Much of this research has focused on low-frequency
magnetic fields, or on RF fields that are keyed, pulsed or modulated at
a low audio frequency (often below 100 Hz). Several studies suggested that
humans and animals can adapt to the presence of a steady RF carrier more
readily than to an intermittent, keyed or modulated energy source. There
is some evidence that while EMR may not directly cause cancer, it may sometimes
combine with chemical agents to promote its growth or inhibit the work
of the body's immune system.
None of the research to date conclusively proves
that low-level EMR causes adverse health effects. Although there has been
much debate about the meaning and significance of this research, many medical
authorities now urge "prudent avoidance" of unnecessary exposure to moderate
or high-level electromagnetic energy until more is known about this subject.
Safe Exposure Levels
How much EM energy is safe? Scientists have devoted
a great deal of effort to deciding upon safe RF-exposure limits. This is
a very complex problem, involving difficult public health and economic
considerations. The recommended safe levels have been revised downward
several times in recent years--and not all scientific bodies agree on this
question even today. In early 1991, a new American National Standards Institute
(ANSI) guideline for recommended EM exposure limits is on the verge of
being approved (see bibliography). If the new standard is approved by a
committee of the Institute of Electrical and Electronic Engineers (IEEE),
it will replace a 1982 ANSI guideline that permitted somewhat higher exposure
levels. ANSI- recommended exposure limits before 1982 were higher still.
This new ANSI guideline recommends frequency-dependent
and time- dependent maximum permissible exposure levels. Unlike earlier
versions of the standard, the 1991 draft recommends different RF exposure
limits in controlled environments (that is, where energy levels can be
accurately determined and everyone on the premises is aware of the presence
of EM fields) and in uncontrolled environments (where energy levels are
not known or where some persons present may not be aware of the EM fields).
Fig. 20 is a graph depicting the new ANSI standard.
It is necessarily a complex graph because the standards differ not only
for controlled and uncontrolled environments but also for electric fields
(E fields) and magnetic fields (H fields). Basically, the lowest E-field
exposure limits occur at frequencies between 30 and 300 MHz. The lowest
H-field exposure levels occur at 100-300 MHz. The ANSI standard sets the
maximum E-field limits between 30 and 300 MHz at a power density of 1 mW/cm\2/
(61.4 volts per meter) in controlled environments--but at one-fifth that
level (0.2 mW/cm\2/ or 27.5 volts per meter) in uncontrolled environments.
The H-field limit drops to 1 mW/cm\2/ (0.163 ampere per meter) at 100-300
MHz in controlled environments and 0.2 mW/cm\2/ (0.0728 ampere per meter)
in uncontrolled environments. Higher power densities are permitted at frequencies
below 30 MHz (below 100 MHz for H fields) and above 300 MHz, based on the
concept that the body will not be resonant at those frequencies and will
therefore absorb less energy.
In general, the proposed ANSI guideline requires
averaging the power level over time periods ranging from 6 to 30 minutes
for power-density calculations, depending on the frequency and other variables.
The ANSI exposure limits for uncontrolled environments are lower than those
for controlled environments, but to compensate for that the guideline allows
exposure levels in those environments to be averaged over much longer time
periods (generally 30 minutes). This long averaging time means that an
intermittently operating RF source (such as an Amateur Radio transmitter)
will show a much lower power density than a continuous-duty station for
a given power level and antenna configuration.
Time averaging is based on the concept that the
human body can withstand a greater rate of body heating (and thus, a higher
level of RF energy) for a short time than for a longer period. However,
time averaging may not be appropriate in considerations of nonthermal effects
of RF energy.
The ANSI guideline excludes any transmitter with
an output below 7 watts because such low-power transmitters would not be
able to produce significant whole-body heating. (However, recent studies
show that handheld transceivers often produce power densities in excess
of the ANSI standard within the head).
There is disagreement within the scientific community
about these RF exposure guidelines. The ANSI guideline is still intended
primarily to deal with thermal effects, not exposure to energy at lower
levels. A growing number of researchers now believe athermal effects should
also be taken into consideration. Several European countries and localities
in the United States have adopted stricter standards than the proposed
Another national body in the United States, the
National Council for Radiation Protection and Measurement (NCRP), has also
adopted recommended exposure guidelines. NCRP urges a limit of 0.2 mW/cm\2/
for nonoccupational exposure in the 30-300 MHz range. The NCRP guideline
differs from ANSI in two notable ways: It takes into account the effects
of modulation on an RF carrier, and it does not exempt transmitters with
outputs below 7 watts.
Recently much concern about EMR has focused on low-frequency
energy, rather than RF. Amateur Radio equipment can be a significant source
of low-frequency magnetic fields, although there are many other sources
of this kind of energy in the typical home. Magnetic fields can be measured
relatively accurately with inexpensive 60-Hz dosimeters that are made by
Table 3 shows typical magnetic field intensities
of Amateur Radio equipment and various household items. Because these fields
dissipate rapidly with distance, "prudent avoidance" would mean staying
perhaps 12 to 18 inches away from most Amateur Radio equipment (and 24
inches from power supplies and 1-kW RF amplifiers) whenever the ac power
is turned on. The old custom of leaning over a linear amplifier on a cold
winter night to keep warm may not be the best idea!
Typical 60-Hz Magnetic Fields Near Amateur Radio
Equipment and AC-Powered Household Appliances
Values are in milligauss.
Item Field Distance
Electric blanket 30- 90 Surface
(Source: measurements made by members of the ARRL
Bio Effects Committee)
Microwave oven 10- 100 Surface (1- 10 at 12")
IBM personal computer 5- 10 Atop
monitor 0- 1 15" from screen
Electric drill 500-2000 At handle
Hair dryer 200-2000 At handle
HF transceiver 10- 100 Atop cabinet (1- 5 at
15" from front)
1-kW RF amplifier 80-1000 Atop cabinet (1- 25
at 15" from front)
There are currently no national standards for
exposure to low- frequency fields. However, epidemiological evidence suggests
that when the general level of 60-Hz fields exceeds 2 milligauss, there
is an increased cancer risk in both domestic environments (Savitz et al,
1988) and industrial environments (Matanoski et al, 1989; Davis and Milham,
1990; Garland et al, 1990). Typical home environments (not close to appliances
or power lines) are in the range of 0.1-0.5 milligauss.
DETERMINING RF POWER DENSITY
Unfortunately, determining the power density of the
RF fields generated by an amateur station is not as simple as measuring
low-frequency magnetic fields. Although sophisticated instruments can be
used to measure RF power densities quite accurately, they are costly and
require frequent recalibration. Most amateurs don't have access to such
equipment, and the inexpensive field- strength meters that we do have are
not suitable for measuring RF power density. The best we can usually do
is to estimate our own RF power density based on measurements made by others
or, given sufficient computer programming skills, use computer modeling
Table 4 shows a sampling of measurements made
at Amateur Radio stations by the Federal Communications Commission and
the Environmental Protection Agency in 1990. As this table indicates, a
good antenna well removed from inhabited areas poses no hazard under any
of the various exposure guidelines. However, the FCC/EPA survey also indicates
that amateurs must be careful about using indoor or attic-mounted antennas,
mobile antennas, low directional arrays, or any other antenna that is close
to inhabited areas, especially when moderate to high power is used.
Typical RF Field Strengths near Amateur Radio Antennas
A sampling of values as measured by the Federal
Communications Commission and Environmental Protection Agency, 1990.
Freq, Power, E Field, Antenna Type MHz Watts V/m
Dipole in attic 14.15 100 7-100 In home
Ideally, before using any antenna that is in close
proximity to an inhabited area, you should measure the RF power density.
If that is not feasible, the next best option is make the installation
as safe as possible by observing the safety suggestions listed in Table
Discone in attic 146.5 250 10- 27 In home
Half sloper 21.15 1000 50 1 m from base
Dipole at 7-13 ft 7.14 120 8-150 1-2 m from earth
Vertical 3.8 800 180 0.5 m from base
5-element Yagi at 60' 21.2 1000 10- 20 In shack
14 12 m from base
3-element Yagi at 25' 28.5 425 8- 12 12 m from
Inverted V at 22-46' 7.23 1400 5- 27 Below antenna
Vertical on roof 14.11 140 6- 9 In house 35-100
At antenna tuner
Whip on auto roof 146.5 100 22- 75 2 m from antenna
15- 30 In vehicle 90 Rear seat
5-element Yagi at 20' 50.1 500 37- 50 10 m from
It is also possible, of course, to calculate the
probable power density near an antenna using simple equations. However,
such calculations have many pitfalls. For one, most of the situations in
which the power density would be high enough to be of concern are in the
near field--an area roughly bounded by several wavelengths of the antenna.
In the near field, ground interactions and other variables produce power
densities that cannot be determined by simple arithmetic.
Computer antenna-modeling programs such as MININEC
or other codes derived from NEC (Numerical Electromagnetics Code) are suitable
for estimating RF magnetic and electric fields around amateur antenna systems.
And yet, these too have limitations. Ground interactions must be considered
in estimating near-field power densities. Also, computer modeling is not
sophisticated enough to predict "hot spots" in the near field--places where
the field intensity may be far higher than would be expected.
Intensely elevated but localized fields often
can be detected by professional measuring instruments. These "hot spots"
are often found near wiring in the shack and metal objects such as antenna
masts or equipment cabinets. But even with the best instrumentation, these
measurements may also be misleading in the near field.
One need not make precise measurements or model
the exact antenna system, however, to develop some idea of the relative
fields around an antenna. Computer modeling using close approximations
of the geometry and power input of the antenna will generally suffice.
Those who are familiar with MININEC can estimate their power densities
by computer modeling, and those with access to professional power-density
meters can make useful measurements.
While our primary concern is ordinarily the intensity
of the signal radiated by an antenna, we should also remember that there
are other potential energy sources to be considered. You can also be exposed
to RF radiation directly from a power amplifier if it is operated without
proper shielding. Transmission lines may also radiate a significant amount
of energy under some conditions.
SOME FURTHER RF EXPOSURE SUGGESTIONS
Potential exposure situations should be taken seriously.
Based on the FCC/EPA measurements and other data, the "RF awareness" guidelines
of Table 5 were developed by the ARRL Bio Effects Committee. A longer version
of these guidelines appeared in a QST article by Ivan Shulman, MD, WC2S
QST carries information regarding the latest developments
for RF safety precautions and regulations at the local and federal levels.
You can find additional information about the biological effects of RF
radiation in the publications listed in the bibliography.
RF Awareness Guidelines
These guidelines were developed by the ARRL Bio
Effects Committee, based on the FCC/EPA measurements of Table 4 and other
o Although antennas on towers (well away from
people) pose no exposure problem, make certain that the RF radiation is
confined to the antenna radiating elements themselves. Provide a single,
good station ground (earth), and eliminate radiation from transmission
lines. Use good coaxial cable, not open wire lines or end-fed antennas
that come directly into the transmitter area.
o No person should ever be near any transmitting
antenna while it is in use. This is especially true for mobile or ground-mounted
vertical antennas. Avoid transmitting with more than 25 watts in a VHF
mobile installation unless it is possible to first measure the RF fields
inside the vehicle. At the 1-kilowatt level, both HF and VHF directional
antennas should be at least 35 feet above inhabited areas. Avoid using
indoor and attic-mounted antennas if at all possible.
o Don't operate RF power amplifiers with the covers
removed, especially at VHF/UHF.
o In the UHF/SHF region, never look into the open
end of an activated length of waveguide or point it toward anyone. Never
point a high-gain, narrow-beamwidth antenna (a paraboloid, for instance)
toward people. Use caution in aiming an EME (moonbounce) array toward the
horizon; EME arrays may deliver an effective radiated power of 250,000
watts or more.
o With handheld transceivers, keep the antenna
away from your head and use the lowest power possible to maintain communications.
Use a separate microphone and hold the rig as far away from you as possible.
o Don't work on antennas that have RF power applied.
o Don't stand or sit close to a power supply or
linear amplifier when the ac power is turned on. Stay at least 24 inches
away from power transformers, electrical fans and other sources of high-
level 60-Hz magnetic fields.
Source material and more extended discussion of topics
covered in this chapter can be found in the references given below and
in the textbooks listed at the end of Chapter 2.
W. R. Adey, "Tissue Interactions with Nonionizing
Electromagnetic Fields," Physiology Review, 1981; 61:435-514.
W. R. Adey, "Cell Membranes: The Electromagnetic
Environment and Cancer Promotion," Neurochemical Research, 1988; 13:671-677.
W. R. Adey, "Electromagnetic Fields, Cell Membrane
Amplification, and Cancer Promotion," in B. W. Wilson, R. G. Stevens, and
L. E. Anderson, Extremely Low Frequency Electromagnetic
Fields: The Question of Cancer (Columbus, OH: Batelle Press, 1989), pp
W. R. Adey, "Electromagnetic Fields and the Essence
of Living Systems," Plenary Lecture, 23rd General Assembly, Internat'l
Union of Radio Sciences (URSI), Prague, 1990; in J. Bach Andersen, Ed.,
Modern Radio Science (Oxford: Oxford Univ Press), pp 1-36.
Q. Balzano, O. Garay and K. Siwiak, "The Near
Field of Dipole Antennas, Part I: Theory," IEEE Transactions on Vehicular
Technology (VT) 30, p 161, Nov 1981. Also "Part II; Experimental Results,"
same issue, p 175.
D. F. Cleveland and T. W. Athey, "Specific Absorption
Rate (SAR) in Models of the Human Head Exposed to Hand-Held UHF Portable
Radios," Bioelectromagnetics, 1989; 10:173-186.
D. F. Cleveland, E. D. Mantiply and T. L. West,
"Measurements of Environmental Electromagnetic Fields Created by Amateur
Radio Stations," presented at the 13th annual meeting of the Bioelectromagnetics
Society, Salt Lake City, Utah, Jun 1991.
R. L. Davis and S. Milham, "Altered Immune Status
in Aluminum Reduction Plant Workers," American J Industrial Medicine, 1990;
F. C. Garland et al, "Incidence of Leukemia in
Occupations with Potential Electromagnetic Field Exposure in United States
Navy Personnel," American J Epidemiology, 1990; 132:293-303.
A. W. Guy and C. K. Chou, "Thermographic Determination
of SAR in Human Models Exposed to UHF Mobile Antenna Fields," Paper F-6,
Third Annual Conference, Bioelectromagnetics Society, Washington, DC, Aug
C. C. Johnson and M. R. Spitz, "Childhood Nervous
System Tumours: An Assessment of Risk Associated with Paternal Occupations
Involving Use, Repair or Manufacture of Electrical and Electronic Equipment,"
Internat'l J Epidemiology, 1989; 18:756-762.
D. L. Lambdin, "An Investigation of Energy Densities
in the Vicinity of Vehicles with Mobile Communications Equipment and Near
a Hand-Held Walkie Talkie," EPA Report ORP/EAD 79-2, Mar, 1979.
D. B. Lyle, P. Schechter, W. R. Adey and R. L.
Lundak, "Suppression of T-Lymphocyte Cytotoxicity Following Exposure to
Sinusoidally Amplitude Modulated Fields," Bioelectromagnetics, 1983; 4:281-292.
G. M. Matanoski et al, "Cancer Incidence in New
York Telephone Workers," Proc Annual Review, Research on Biological Effects
of 50/60 Hz Fields, U.S. Dept of Energy, Office of Energy Storage and Distribution,
Portland, OR, 1989.
S. Milham, "Mortality from Leukemia in Workers
Exposed to Electromagnetic Fields," New England J Medicine, 1982; 307:249.
S. Milham, "Increased Mortality in Amateur Radio
Operators due to Lymphatic and Hematopoietic Malignancies," American J
Epidemiology, 1988; 127:50-54.
W. W. Mumford, "Heat Stress Due to RF Radiation,"
Proc IEEE, 57, 1969, pp 171-178.
S. Preston-Martin et al, "Risk Factors for Gliomas
and Meningiomas in Males in Los Angeles County," Cancer Research, 1989;
D. A. Savitz et al, "Case-Control Study of Childhood
Cancer and Exposure to 60-Hz Magnetic Fields, American J Epidemiology,
D. A. Savitz et al, "Magnetic Field Exposure from
Electric Appliances and Childhood Cancer," American J Epidemiology, 1990;
I. Shulman, "Is Amateur Radio Hazardous to Our
Health?" QST, Oct 1989, pp 31-34.
R. J. Spiegel, "The Thermal Response of a Human
in the Near-Zone of a Resonant Thin-Wire Antenna," IEEE Transactions on
Microwave Theory and Technology (MTT) 30(2), pp 177-185, Feb 1982.
T. L. Thomas et al, "Brain Tumor Mortality Risk
among Men with Electrical and Electronic Jobs: A Case-Controlled Study,"
J National Cancer Inst, 1987; 79:223-237.
N. Wertheimer and E. Leeper, "Electrical Wiring
Configurations and Childhood Cancer," American J Epidemiology, 1979; 109:273-
N. Wertheimer and E. Leeper, "Adult Cancer Related
to Electrical Wires Near the Home," Internat'l J Epidemiology, 1982; 11:345-
"Safety Levels with Respect to Human Exposure
to Radio Frequency Electromagnetic Fields (300 kHz to 100 GHz)," ANSI C95.1-1991
(New York: IEEE American National Standards Institute, 1990 draft).
"Biological Effects and Exposure Criteria for
Radiofrequency Electromagnetic Fields," NCRP Report No 86 (Bethesda, MD:
National Council on Radiation Protection and Measurements, 1986).
US Congress, Office of Technology Assessment,
"Biological Effects of Power Frequency Electric and Magnetic Fields--Background
Paper," OTA-BP-E-53 (Washington, DC: US Government Printing Office), 1989.