Re: Microwave Antennas

[Rick Mannix <rcmannix@UCI.EDU>]

At 03:49 PM 08/10/2001 -0400, Andrew Mattox wrote:



>WE are in exactly the same situation with four cell providers on our roofs

>and the need to have our own crew working in close proximity. One of the

>companies posted a warning sign on the roof access door. Just want to make

>sure that you share all of the info you receive with the list. thanks



Several people directed me to the FCC Website. That below was extracted

from there.





SAFETY ISSUES REGARDING "CELL SITES"



[From the Federal Communications Commission Web Site: fcc.gov]



ARE CELLULAR AND PCS TOWERS AND ANTENNAS SAFE? 



Cellular radio services transmit using frequencies between 800 and 900

megahertz (MHz). Transmitters in the Personal Communications Service (PCS)

use frequencies in the range of 1850-1990 MHz. Antennas used for cellular

and PCS transmissions are typically located on towers, water tanks or other

elevated structures including rooftops and the sides of buildings. The

combination of antennas and associated electronic equipment is referred to

as a cellular or PCS "base station" or "cell site". Typical heights for

free-standing base station towers or structures are 50-200 feet. A cellular

base station may utilize several "omni-directional" antennas that look like

poles, 10 to 15 feet in length, although these types of antennas are

becoming less common in urban areas.



In urban and suburban areas, cellular and PCS service providers now more

commonly use "sector" antennas for their base stations. These antennas are

rectangular panels, e.g., about 1 by 4 feet in dimension, typically mounted

on a rooftop or other structure, but they are also mounted on towers or

poles. The antennas are usually arranged in three groups of three each. One

antenna in each group is used to transmit signals to mobile units (car

phones or hand-held phones), and the other two antennas in each group are

used to receive signals from mobile units. 



At a given cell or PCS site, the total RF power that could be transmitted

from each transmitting antenna at a cell site depends on the number of

radio channels (transmitters) that have been authorized and the power of

each transmitter. Typically, for a cellular base station, a maximum of 21

channels per sector (depending on the system) could be used. Thus, for a

typical cell site utilizing sector antennas, each of the three transmitting

antennas could be connected to up to 21 transmitters for a total of 63

transmitters per site. When omni-directional antennas are used, up to 96

transmitters could be implemented at a cell site, but this would be very

unusual. Furthermore, while a typical base station could have as many as 63

transmitters, not all of the transmitters would be expected to operate

simultaneously thus reducing overall emission levels. For the case of PCS

base stations, fewer transmitters are normally required due to the

relatively greater number of base stations.



The signals from a cellular or PCS base station antenna are essentially

directed toward the horizon in a relatively narrow pattern in the vertical

plane. The radiation pattern for an omni- directional antenna might be

compared to a thin doughnut or pancake centered around the antenna while

the pattern for a sector antenna is fan-shaped, like a wedge cut from a

pie. As with all forms of electromagnetic energy, the power density from a

cellular or PCS transmitter decreases rapidly as one moves away from the

antenna. Consequently, normal ground-level exposure is much less than

exposures that might be encountered if one were very close to the antenna

and in its main transmitted beam. 

 

Measurements made near typical cellular and PCS installations, especially

those with tower- mounted antennas, have shown that ground-level power

densities are thousands of times less than the FCC's limits for safe

exposure. 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 transmitting beam (at the height of the antenna) and

within a few feet from the antenna. This makes it extremely unlikely that a

member of the general public could be exposed to RF levels in excess of

these guidelines due to cellular or PCS base station transmitters. 



When cellular and PCS antennas are mounted at rooftop locations it is

possible that ambient RF levels could be greater than those typically

encountered on the ground. However, once again, exposures approaching or

exceeding the safety guidelines are only likely to be encountered very

close to or directly in front of the antennas. For sector-type antennas RF

levels to the side and in back of these antennas are insignificant. 





ARE CELLULAR AND OTHER RADIO TOWERS LOCATED NEAR HOMES OR SCHOOLS SAFE FOR

RESIDENTS AND STUDENTS? 



As discussed above, radiofrequency emissions from antennas used for

wireless transmissions such as cellular and PCS signals result in exposure

levels on the ground that are typically thousands of times less than safety

limits. These safety limits were adopted by the FCC based on the

recommendations of expert organizations and endorsed by agencies of the

Federal Government responsible for health and safety. Therefore, there is

no reason to believe that such towers could constitute a potential health

hazard to nearby residents or students. 



Other antennas, such as those used for radio and television broadcast

transmissions, use power levels that are generally higher than those used

for cellular and PCS antennas. Therefore, in some cases there could be a

potential for higher levels of exposure on the ground. However, all

broadcast stations are required to demonstrate compliance with FCC safety

guidelines, and ambient exposures to nearby persons from such stations are

typically 





Rick Mannix

Health Physicist

Laser Safety Officer

University of California

EH&S Office

4600 Bison Ave.

Irvine, CA 92697-2725



949-824-6098

949-824-8539   fax

rcmannix@uci.edu

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