Print | Need Larger Text? | Share

Report on: Measurement of Cellular Base-station Emissions Using a Newly Developed RF Field Mapping System*

Introduction

With the rapid acceptance of mobile cellular technology by the public there has been a concurrent rise in apprehension about base-station emissions by some segments of society. These concerns have given rise to organized opposition to the siting of existing and proposed base-stations especially near residential and school areas. Various factors contribute to these concerns including reports in the popular press about scientific studies linking electromagnetic energy from mobile handsets (not base stations) to cancer. Though still controversial in the scientific community, these findings are often interpreted as being applicable to exposures from base station emissions as well. It is often argued that, although base stations exposures are at very low levels compared to that from a handset, they are continuous, exposing people for twenty-four hours per day.

In addition to the scientific reports, these concerns are also fuelled by the basic assumption by the public that as cellular demand increases, so does the emission levels from base stations. The concepts of cellular telephone systems are usually misunderstood, and are often confused with the principles of broadcast radio. With broadcast radio (AM and FM), revenues are related to the coverage area, which increases with the effective radiated power of the transmitter. In cellular systems, the limited number of available frequencies must be re-used within a geographic area which is called a cell. Cellular system architecture, therefore, is driven by the need to control radiofrequency interference, which requires the limitation of radiated power levels. Increases in system capacity (i.e. the number of users that can be supported simultaneously) are achieved through the decrease in size and the increase in number of cells in the system.

Accordingly, each cell must transmit lower power in order to limit interference with other cells using the same frequencies.

One aspect of base-station emissions that is usually not well known, except by engineers and system planners, is that signal power (or exposure conditions, from a health standpoint) at a single spot on the ground can vary over time due to the way electromagnetic energy propagates in complex urban environments and in some cases, due to cellular call traffic at the base station . These effects are usually random and can result in slowly varying or fast varying fluctuations in the exposure level [Yacoub 1993, Gajda et. al. 1998] . They also give rise to extremely variable exposure conditions from point to point on the ground. The result is that two closely spaced points may have exposure levels differing by a considerable amount. The end effect on a person in the vicinity of a cellular base station is that one's exposure is highly variable no matter if the person is moving about or is stationary.