7 Spatially Averaged Measurements
Understanding Spatial Averaging
The major standards concerned with human exposure to radio frequency radiation specify maximum exposure levels averaged over the whole body. The co-linear dipole antenna arrays that are very common in modern wireless communications systems, for example, have multiple lobes close to the antenna. The field strength typically varies by 6-7 dB along the length of an array. Therefore, the measured value is highly dependent on not only the distance from the antenna but the height above the ground.
The traditional method of making spatially-averaged measurements is to use a “storypole”. A storypole is a non-conductive pole , often wooden, equal in height to an average adult with distance marks equally spaced along its length. Measurements are made alongside the storypole at each height and then mathematically averaged. The height and spacing of each measurement varies from standard-to-standard. For example, the IEEE C95.1-1999 standard specifies measurements from 0 centimeters (ground level) to 200 centimeters in 20 centimeter increments. Some exposure standards, such as Canada’s Safety Code 6, require that measurements be averaged across two dimensions – vertically and horizontally. Modern wireless communications sites make this manual technique more difficult than ever since field levels at many sites, particularly multi-user sites, are constantly varying. For example, paging systems go on and off and the number of cellular channels in use is constantly changing. Thus, a series of measurements made at varying heights can vary more as a function of time than location.
Chapter 7 Spatially-Averaged Measurements | 39 |