Table of Contents
Advertisement
user from RF absorption use special phone cases, while others involve nothing
more than a metallic accessory attached to the phone. Studies have shown that
these products generally do not work as advertised. Unlike "hand-free" kits,
these so-called "shields" may interfere with proper operation of the phone. The
phone may be forced to boost its power to compensate, leading to an increase in
RF absorption. In 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, Sprint 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 Sprint PCS communications generally
require fewer transmitters than those used for cellular radio transmissions, since
Sprint 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
Section 4A: Safety Guidelines
144
Advertisement
Table of Contents
