13 - Laserfiche WebLink

Home Browse Search

• For example,one Symposium participant observed ground noise levels(about 5 to 15 microwatts,or that, for tower-mounted omnidirectional base 1 to 3%of the 1992 ANSI uncontrolled standard), station antennas,measured exposures near ground "bur far,far below any standards that are used to level are less than one microwatt per square protect health." centimeter—less than 0.2%of the 1992 ANSI Another participant shared measurement data standard.The participant also observed that direc- taken in the vicinity of several different base tional antennas result in very low ground-level stations.At a distance of about 100 feet from a exposures.Based on the participant's measure- tower supporting several directional antennas,in a ments of exposures from base stations mounted on shopping-centerlot,typical midday cellular office-building roofs,the only time exposures even PPS parking approach 1992 ANSI limits is when one gets with- power densities were"2 to 15 nanowarts per square in a yard or so of an actual transmitting antennas centimeter...extremely low..."16 That is less than themselves—as opposed to an inert metal pole 0.002%of the 1992 ANSI uncontrolled limit.17 At another directional base station,with roof- which supports such an antenna up in the air mounted antennas about 16 feet up in the air, Other Symposium participants provided similar exposure levels at three to six feet above the roof observations, based on their own measurements. line were 5 to 15 microwatts per square centime- You may recall from Chapter 2 that as cellular ter.This is about 0.9% to 2.6%of the 1992 ANSI traffic increases,base-station antenna heights and uncontrolled exposure limit, but the roof was ERPs decrease.Because exposure levels rise closer inaccessible to the general public.Measured power to an antenna but decrease as ERP falls,the two flux densities were about 0.2%to 0.6%of the 1992 factors tend to cancel each other out.One partici- ANSI controlled limit. pant described surveying the interior of a hospital Because actual measurements require sophisticated adjacent to a building with a roof-mounted base station.The base station's signal levels were at equipment and can be time consuming and costly, 4110 background noise levels in the hospital kitchen, engineers have developed various techniques for and from 1 to 3 microwarts per square centimeter predicting exposure levels.One participant on a terrace and in other parts of the 27th floor— described a sophisticated method of computer very low levels.In another setting,one floor below analysis that takes into account a base station's a roof-mounted base station(a vertical distance of antenna types,antenna heights,transmitter powers, about 10 to 15 feet),exposure levels were 10 number of active channels,and the effects of any microwatts per square centimeter—less than 2% intervening construction materials.This method of the 1992 ANSI limit for uncontrolled settings. allows estimates of exposures to people within,say, an office building that has a base station mounted A Symposium participant also described measure- on its roof.The participant described a good corre- merits at a base station with wall-mounted direc- lation between the predicted exposures and those tional antennas designed to serve the inside of an actually measured. office building.Measured signal levels were within The participant described how he had used this 1992 ANSI limits except in the immediate vicinity method to analyze a proposed base station at a fire and in the main beam of the antenna.The signals station.The proposal had engendered some measured in the equipment room,behind the controversy among area residents.For many years, antenna's reflector and therefore outside the main the fire department had used the tower for its own beam, were extremely weak,essentially at back- antennas.The highest predicted signal levels from ground-noise levels. the base station would be about 3.5 microwatts One participant reported that the EPA had per square centimeter,less than 0.7%of the 1992 surveyed typical background noise levels from all ANSI uncontrolled exposure limit.In fact,the par- sources(including TV and FM broadcast stations) ticipant observed,exposure from the fire depart- in a variety of settings,and had arrived at an average merit transmitter was probably much higher than level of 0.005 microwatts (five nanowarts)per any from the base station. square centimeter.That is less than 0.00190 of the Several participants observed that exposures 1992 ANSI limit for uncontrolled exposures.The • from cellular towers are usually less than those participant observed that,close to a base station, exposures usually are considerably above back- from UHF TV stations,police and fire radio transmitters, and FM broadcast stations.