DRAFT Recommended Practice for Measurements and ...

1/29/98 75 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
approximately one or two meters in extent. The spatial average of the field within that
area should be considered as the appropriate level for comparison with whatever
protection guide is being employed as a criterion. Measurements near metallic objects
should be made with the edge of the probe at least 3 ‘’probe lengths,’’ e.g., 20 cm, from
the object (see 5.3.6.1).
In an analysis of RF field measurement data obtained during a filed survey in a
metropolitan area adjacent to a large antenna farm, Tell [D] reported on the degree of
variability in the measured field magnitudes obtained at the same measurement location,
using the same instrumentation with measurements performed by the same individual
but with different orientations of the observer relative to the measurement point. A
broadband electric field probe equipped with a datalogger was used to acquire the
minimum, maximum and average square of the RF field strengths along a vertical path
two meters in height with the observer facing the measurement point from four different
directions. Measurements were performed at 171 different locations along neighborhood
streets near a broadcast site with six FM radio antennas. Observer induced field
perturbations accounted for a mean value of 71 percent (2.3 dB) variation in the spatial
average measurement of the RF fields (ratio of maximum spatial average to minimum
spatial average obtained from four successive vertical scans at each measurement
point). RF fields were found to be non-uniform over the body dimension with a mean
value of the maximum to minimum field readings of 10.3 (10.1 dB). These variables in
measured fields should be carefully considered when interpreting measurements used
for showing compliance with exposure limits.
While mounting or holding the measuring antenna or probe, care should be taken to avoid
reflections or perturbations of the field by support structures or by the operator’s body.
Where required, to avoid field perturbation, metallic portions of the measuring device, or
support structure, should be covered with absorbing material of appropriate quality.
Where possible, probe interconnect cables should be oriented normal to the electric field.
When that is not practical, or where severe multipath effects produce fields originating
from multiple directions, metallic cables should be covered with absorber unless tests
demonstrate that the cable position does not affect the measurement. Dielectric fixtures
should be as small as possible (minimum reflection cross section) and should be of low
dielectric-constant material, or be less than one-quarter wavelength in effective thickness
Τ E . The effective thickness is given by:
Τ E = Τ(ε r ) 1/2 (Eq 5.1)
where Τ is the physical thickness, and ε r is the relative permittivity. Even dielectric slabs
(ε r > 2) can significantly alter plane wave fields if the effective thickness is greater than
0.1 wavelength.
For highest accuracy, sources of error can be accounted for so that the true field
strengths may be ascertained with less than ±2 dB of uncertainty. To obtain this level of
accuracy at frequencies above approximately 300 MHz, a scanned measurement or
many fixed-point measurements per wavelength should be performed in order to obtain
information on the variations in field strength in that area due to multipath and other
reflections.
5.3.3 Far-Field, Complex Sources.
When measuring the fields from multiple, distant sources of unknown frequency,
polarization, or direction of propagation, a broadband isotropic probe is required. Since
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.