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February 2002, the Federal trade Commission (FTC) charged two companies that sold
devices that claimed to protect wireless phone users from radiation with making
false and unsubstantiated claims. According to FTC, these defendants lacked a
reasonable basis to substantiate their claim.
What are wireless telephone base stations?
Fixed antennas used for wireless telecommunications are referred to as cellular base
stations, cell stations, PCS ("Personal Communications Service") stations or
telephone transmission towers. These base stations consist of antennas and
electronic equipment. Because the antennas need to be high in the air, they are often
located on towers, poles, water tanks, or rooftops. Typical heights for freestanding
base station towers are 50-200 feet.
Some base stations use antennas that look like poles, 10 to 15 feet in length, that are
referred to as "omni-directional"antennas. These types of antennas are usually
found in rural areas. In urban and suburban areas, wireless providers now more
commonly use panel or sector antennas for their base stations. These antennas
consist of rectangular panels, about 1 by 4 feet in dimension. The antennas are
usually arranged in three groups of three antennas each. One antenna in each group
is used to transmit signals to wireless phones, and the other two antennas in each
group are used to receive signals from wireless phones.
At any base station site, the amount of RF energy produced depends on the number
of radio channels (transmitters) per antenna and the power of each transmitter.
Typically, 21 channels per antenna sector are available. For a typical cell site using
sector antennas, each of the three transmitting antennas could be connected to up
to 21 transmitters for a total of 63 transmitters. However, it is unlikely that all of the
transmitters would be transmitting at the same time. When omni-directional
antennas are used, a cellular base station could theoretically use up to 96
transmitters, but this would be very unusual, and, once again, it is unlikely that all
transmitters would be in operation simultaneously. Base stations used for PCS
communications generally require fewer transmitters than those used for cellular
radio transmissions, since PCS carriers usually have a higher density of base station
antenna sites.
Are wireless telephone base stations safe?
The electromagnetic RF signals transmitted from base station antennas stations
travel toward the horizon in relatively narrow paths. For example, the radiation
pattern for an antenna array mounted on a tower can be likened to a thin pancake
centered around the antenna system. The individual pattern for a single array of
sector antennas is wedge-shaped, like a piece of pie. As with all forms of
electromagnetic energy, the power decreases rapidly as one moves away from the
antenna. Therefore, RF exposure on the ground is much less than exposure very close
to the antenna and in the path of the transmitted radio signal. In fact, ground-level
exposure from such antennas is typically thousands of times less than the exposure
levels recommended as safe by expert organizations. So exposure to nearby
residents would be well within safety margins.
Section 4A: Safety
152
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