12 Spatial Averaging
144 SRM-3006 Narda
12.1 About the Spatial Averaging function
The standards and regulations governing the protection of humans from
high-frequency radiation specify limit values which normally have to be
averaged over the entire human body. This is because the fields may vary
greatly within a given volume. This is particularly the case in the vicinity of
the antennas used for mobile telecommunications, which have a primary
lobe in the desired direction of radiation but many secondary lobes in
different directions. As a result, the measured value depends on the spatial
height as well as the distance from the antenna.
The standard method of spatial averaging uses a simplified model of the
human body, often in the form of a dummy made of non-conducting
material. The dummy is the same size as a normal adult and is marked at
specific heights. The field strength is measured at each of these heights,
and the results averaged mathematically. The spacing and heights of these
markings vary from one standard to another. For example, IEEE C95.1-
1999 requires measurements from 0 cm (floor level) to 200 cm at equal
intervals of 20 cm. Other standards, such as Canada’s Safety Code 6
require two-dimensional measurements in vertical and horizontal directions.
The revised ECC Recommendation (02)04 of October 2003 specifies
measurements at 1.1, 1.5 and 1.7 m above floor level.
Such measurements are difficult, particularly in the vicinity of modern
mobile communications sites shared by several providers, because the field
strengths are changing all the time: paging systems transmit sporadically,
the number of occupied mobile phone channels changes continually, and so
on. For this reason, the results of measurements can often depend more on
the time than on the spatial location.
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