RF Radiation Exposure
Radio World, 12 December 1990
by Harold Hallikainen
As part of station license renewal, stations are to submit an
environmental assessment if the facilities to be authorized will have a
significant effect on the environment, as specified in 1.1307 of the rules. If
the facilities will not have a significant effect on the environment, an
explanation of why there will be no significant effect is required (see
question 7, FCC form 303-S). Over the next couple months we'll look at the FCC
requirements and how to determine compliance. Get out your copy of FCC
Bulletin OST 65. I think you can get a copy from the FCC Office of Public
Information (phone 202 632 7000).
The instructions for Form 303-S summarize 1.1307 by saying that a station
will have a significant impact if it is located in a sensitive area (wilderness
area, wildlife preserve, flood plain or historic area), if it will involve
significant changes in the surface features, if high intensity strobe lights
are to be used in a residential area, or if workers or the public will be
exposed to RF levels above the ANSI guidelines (ANSI C95. 1-1982). The ANSI
spec is available from ANSI by calling 212 354 3300 or 212 354 3473. The FCC
has also published guidelines for evaluating compliance. These guidelines are
in OST Bulletin 65 (FCC Office of Science and Technology). Many stations are
now required to determine compliance with the RF radiation limitations for the
first time as part of license renewal. Let's look at what is required to
answer "no" to question 7.
The FCC wants to know how you determined the station will not have
significant environmental impact. OST 65 provides background information and
information on predicting RF fields, measuring the fields and controlling
exposure.
Human RF Exposure Regulation
The National Environmental Policy Act of 1969 (NEPA) requires federal
agencies to take into account the potential environmental impact of their
actions. The FCC adopted rules in 1974 implementing NEPA in FCC procedures.
These rules (in part 1, subpart 1) require applicants to supply an
environmental assessment if a facility to be built or operated (since station
renewal is an application to operate an existing facility, it is affected by
these rules) would have significant environmental effect. In 1985, the FCC
added human RF exposure to the list of possible environmental effects. The FCC
adopted the exposure guidelines adopted by ANSI (American National Standards
Institute), since no other scientific widely accepted standards existed.
ANSI Guidelines
The ANSI guidelines were adopted in 1982. These guidelines suggest
maximum human exposure levels to RF radiation in the frequency range of 300 KHz
to 100 GHz. The most restrictive frequencies are between 30 and 300 MHz, where
human "absorbtion" occurs at the highest rate. As the body absorbs energy
(power times exposure time), it converts the electromagnetic energy into heat.
The ANSI quidelines are based on the heating effect of the radiation as opposed
to any possible but not yet detected effect of the radiation itself (induced
currents interference with nerve communications, interference with DNA
replication, etc.). As power is dissipated, temperature will slowly rise. The
rate of temperature rise is proportional to the power and inversely
proportional to the heat capacity of the tissue being heated. Further, as the
temperature of one object or area rises, heat flows away from the warmer area
towards cooler objects. This heat flow (in calories per second) is
proportional to the temperature difference and inversely proportional to the
thermal resistance of the medium. Taking all this into account, ANSI
determined the maximum allowed temperature rise in a six minute (0.1 hour)
period, considered the power absorbtion rate at various frequencies and came up
with suggested power density (ideally, metric units of watts/square meter, but
practically, milliwatts or microwatts per square centimeter) limitations for
these frequencies.
Power Density
Power density is the amount of power that is absorbed in a unit area. If,
for example, we have a 1 watt isotropic (radiates equally in all directions)
radiator and place a sphere around the radiator, the sphere will absorb all the
power being radiated (1 watt). If the sphere has a radius of 1 meter, the
surface area of the sphere is
A = 4 pi R2 = 4 pi square meters, about 12.566 square meters.
Each square meter receives about 79.6 milliwatts. If the distance from
the radiator is doubled, the power density drops by a factor of four to about
19.9 millwatts per square meter.
The power density can be broken down into the product of the electric
field strength and the magnetic field strength (volts/meter times amperes/meter
yields watts/square meter). Far from the radiator (the far field), the
electric and magnetic field have a proportional relationship based on the
impedance of free space (377 ohms). The electric field strength (volts/meter)
is the magnetic field strength (amperes/meter) times the impedance of free
space (ohms). This is another expression of Ohm's law (E=IR with both sides of
the equation divided by distance in meters). Further, once we have a
relationship between electric and magnetic fields, we can determine the power
density (watts per square meter) based on one or the other. These
relationships are:
S = E2/377 = 377 H2 where
S is power density in watts/square meter
E is the electric field strength in volts/meter
H is the magnetic field strength in amperes/meter
377 is the impedance of free space in ohms.
To convert S to milliwatts per square centimeter, multiply the entire
equation by 0.1, which does the meter to centimeter and watt to milliwatt
conversion.
Near the radiator, the magnetic and electric field will not have the
proportional relationship they have in the far field. A current carrying coil
generates a strong magnetic field while a high voltage conductor (such as the
base of a half wave antenna) generates a strong electric field. Since the
limits anticipated by ANSI are generally near field measurements, the limits
assume the far field relationship and put the resulting limits on both electric
and magnetic fields. Even if the product of the electric and magnetic fields
does not exceed the power density limitation, the system is considered
noncompliant if either the electric or magnetic field strength limitations are
exceeded. Since the power density is proportional to the square of E or H, the
limits are specified in terms of volts squared per square meter and amperes
squared per square meter (E squared and H squared).
Time of Exposure
Since the heating effect is slow and the human body is able to compensate
for variations in heat generation its tissues (such as through normal
metabolism), ANSI has applied the power density limits to a six minute
exposure. The limits allow for continuous exposure. If the exposure is at
twice the power density limit (or twice the limit of the squared electric or
magnetic field strength) the exposure must be limited to three minutes. With a
changing power density, the power density is to be integrated over a six minute
(0.1 hour) period. In this six minute period, a uniform field at the limits
would result in energy absorbtion of 144 joules per kilogram of body mass (0.4
watts per kilogram). Where the field is non-uniform (such as with a handheld
radio), the peak absorbtion is limited to 8 watts per kilogram averaged over any
one gram of tissue.
The Limits
AM stations need to consider the limits that apply between 0.3 and 3 MHz.
Here, the electric field strength squared (volts squared per meter squared) is
to be limited to 400,000. The magnetic field strength squared (amperes squared
per meter squared) is limited to 2.5. The power density (milliwatts per square
centimeter) is limited to 100.
FM stations need to consider the limits that apply between 30 and 300 MHz.
Here, the limits are 4,000 volts squared per meter squared, .025 amperes
squared per meter squared and 1.0 milliwatts per square centimeter.
Next Month
Next month we'll look at how you can determine compliance with the ANSI
limits. Meanwhile, get your copy of OST 65!
Harold Hallikainen is president of Hallikainen and Friends, a manufacturer of
transmitter control and telemetry systems. He also teaches electronics at
Cuesta College, San Luis Obispo. He can be reached at 805 541 0200.