NARDA 8718B - Manual (Page 40)

39 Chapter 7 Spatially-Averaged Measurements
Spatially Averaged Measurements
7
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.