Spatially averaged measurements, Understanding spatial averaging – NARDA 8718B User Manual

Page 40

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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.

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