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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.
11. 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.
Cellular and PCS base stations in the United States are required to
comply with limits for exposure recommended by expert
organizations and endorsed by government agencies responsible for
health and safety. Measurements made near cellular and PCS base
station antennas mounted on towers have confirmed that ground-
level exposures are typically thousands of times less than the
exposure limits adopted by the FCC. In fact, in order to be exposed to
levels at or near the FCC limits for cellular or PCS frequencies an
individual would essentially have to remain in the main transmitted
radio signal (at the height of the antenna) and within a few feet from
the antenna. This is, of course, very unlikely to occur.
Section 4: Safety Guidelines and Warranty Information
4A: Safety 128