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Radiofrequency (RF)
Radiation Part 2
Cell Tower
Life Bluetube Headsets
Cell Phone Towers Health Effects
EM Field Meter
Cell Phone Sensitivity
Broadcast Antennas
Radio and television broadcast stations transmit their
signals via RF electromagnetic waves. Broadcast stations
transmit at various RF frequencies, depending on the
channel, ranging from about 550 kHz for AM radio up to about
800 MHz for some UHF television stations. Frequencies for FM
radio and VHF television lie in between these two extremes.
Operating powers can be as little as a few hundred watts for
some radio stations or up to millions of watts for certain
television stations. Some of these signals can be a
significant source of RF energy in the local environment,
and the FCC requires that broadcast stations submit evidence
of compliance with FCC RF guidelines.
The amount of RF energy to which the public or workers might
be exposed as a result of broadcast antennas depends on
several factors, including the type of station, design
characteristics of the antenna being used, power transmitted
to the antenna, height of the antenna and distance from the
antenna. Since energy at some frequencies is absorbed by the
human body more readily than energy at other frequencies,
the frequency of the transmitted signal as well as its
intensity is important.
Public access to broadcasting antennas is normally
restricted so individuals cannot be exposed to high-level
fields that might exist near antennas. Measurements made by
the FCC, EPA, and others have shown that ambient RF
radiation levels in inhabited areas near broadcasting
facilities are typically well below the exposure levels
recommended by current standards and guidelines. Antenna
maintenance workers are occasionally required to climb
antenna structures for such purposes as painting, repairs,
or beacon replacement. Both the EPA and OSHA have reported
that in these cases it is possible for a worker to be
exposed to high levels of RF energy if work is performed on
an active tower or in areas immediately surrounding a
radiating antenna. Therefore, precautions must be taken to
ensure that maintenance personnel are not exposed to unsafe
RF fields.
Portable Radio Systems
"Land-mobile" communications include a variety of
communications systems that require the use of portable and
mobile RF transmitting sources. These systems operate in
narrow frequency bands between about 30 and 1,000 MHz. Radio
systems used by the police and fire departments, radio
paging services, and business radio are a few examples of
these communications systems. There are essentially three
types of RF transmitters associated with land-mobile
systems: base-station transmitters, vehicle-mounted
transmitters, and handheld transmitters. The antennas used
for these various transmitters are adapted for their
specific purpose. For example, a base-station antenna must
radiate its signal to a relatively large area, and,
therefore, its transmitter generally has to use higher power
levels than a vehicle-mounted or handheld radio transmitter.
Although these base-station antennas usually operate with
higher power levels than other types of land-mobile
antennas, they are normally inaccessible to the public since
they must be mounted at significant heights above ground to
provide for adequate signal coverage. Also, many of these
antennas transmit only intermittently. For these reasons,
such base-station antennas have generally not been of
concern with regard to possible hazardous exposure of the
public to RF radiation. Studies at rooftop locations have
indicated that high-powered paging antennas may increase the
potential for exposure to workers or others with access to
such sites, for example, maintenance personnel. Transmitting
power levels for vehicle-mounted land-mobile antennas are
generally less than those used by base-station antennas but
higher than those used for handheld units.
Handheld portable radios such as walkie-talkies are
low-powered devices used to transmit and receive messages
over relatively short distances. Because of the low power
levels used, the intermittence of these transmissions, and
the fact that these radios are held away from the head, they
should not expose users to RF energy in excess of safe
limits. Therefore, the FCC does not require routine
documentation of compliance with safety limits for
push-to-talk two-way radios.
Microwave Antennas
Point-to-point microwave antennas transmit and receive
microwave signals across relatively short distances (from a
few tenths of a mile to 30 miles or more). These antennas
are usually rectangular or circular in shape and are
normally found mounted on a supporting tower, on rooftops,
on sides of buildings, or on similar structures that provide
clear and unobstructed line-of-sight paths between both ends
of a transmission path or link. These antennas have a
variety of uses, such as transmitting voice and data
messages and serving as links between broadcast or cable TV
studios and transmitting antennas. The RF signals from these
antennas travel in a directed beam from a transmitting
antenna to a receiving antenna, and dispersion of microwave
energy outside of the relatively narrow beam is minimal or
insignificant. In addition, these antennas transmit using
very low power levels, usually on the order of a few watts
or less. Measurements have shown that ground-level power
densities due to microwave directional antennas are normally
a thousand times or more below recommended safety limits.
Moreover, as an added margin of safety, microwave tower
sites are normally inaccessible to the general public.
Significant exposures from these antennas could only occur
in the unlikely event that an individual was to stand
directly in front of and very close to an antenna for a
period of time.
Satellite Systems
Ground-based antennas used for satellite-earth
communications typically are parabolic "dish" antennas, some
as large as 10 to 30 meters in diameter, that are used to
transmit (uplinks) or receive (downlinks) microwave signals
to or from satellites in orbit around the earth. The
satellites receive the signals beamed up to them and, in
turn, retransmit the signals back down to an earthbound
receiving station. These signals allow delivery of a variety
of communications services, including long-distance
telephone service. Some satellite-earth station antennas are
used only to receive RF signals (that is, just like a
rooftop television antenna used at a residence) and, since
they do not transmit, RF exposure is not an issue. Because
of the longer distances involved, power levels used to
transmit these signals are relatively large when compared,
for example, to those used by the microwave point-to-point
antennas discussed above. However, as with microwave
antennas, the beams used for transmitting earth-to-satellite
signals are concentrated and highly directional, similar to
the beam from a flashlight. In addition, public access would
normally be restricted at station sites where exposure
levels could approach or exceed safe limits.
Radar Systems
Radar systems detect the presence, direction, or range of
aircraft, ships, or other moving objects. This is achieved
by sending pulses of high-frequency electromagnetic fields (EMF).
Radar systems usually operate at radiofrequencies between
300 megahertz (MHz) and 15 gigahertz (GHz). Invented some 60
years ago, radar systems have been widely used for
navigation, aviation, national defense, and weather
forecasting. People who live or routinely work around radar
have expressed concerns about long-term adverse effects of
these systems on health, including cancer, reproductive
malfunction, cataracts, and adverse effects for children. It
is important to distinguish between perceived and real
dangers that radar poses and to understand the rationale
behind existing international standards and protective
measures used today.
The power that radar systems emit varies from a few
milliwatts (police traffic-control radar) to many kilowatts
(large space tracking radars).
However, a number of factors significantly reduce human
exposure to RF generated by radar systems, often by a factor
of at least 100:
• Radar systems send electromagnetic waves in pulses and not
continuously. This makes the average power emitted much
lower than the peak pulse power.
• Radars are directional and the RF energy they generate is
contained in beams that are very narrow and resemble the
beam of a spotlight. RF levels away from the main beam fall
off rapidly. In most cases, these levels are thousands of
times lower than in the main beam.
• Many radars have antennas which are continuously rotating
or varying their elevation by a nodding motion, thus
constantly changing the direction of the beam.
• Areas where dangerous human exposure may occur are
normally inaccessible to unauthorized personnel.
In addition to the information provided in this document,
there are other sources of information regarding RF energy
and health effects. Some states maintain nonionizing
radiation programs or, at least, some expertise in this
field, usually in a department of public health or
environmental control. The following table lists some
representative Internet Web sites that provide information
on this topic. The Health Physics Society neither endorses
nor verifies the accuracy of any information provided at
these sites.
Radiofrequency (RF) Radiation Part 1
Arlington, Texas, USA
Zimbabwe, Harare
Holy See, Vatican City
Ethiopia, Addis Ababa
Penrith, Australia
Djibouti, Djibouti
San Marino, San Marino
Hungary, Budapest
Darwin, Northern Territory, Australia
Dominican Republic, Santo Domingo
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