DRAFT Recommended Practice for Measurements and ...

1/29/98 105 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
Appendix B
Theoretical Calculations of Exposure Fields
B1. Technical Considerations of RF Source Characteristics
Although the prediction of power-density levels in the vicinity of RF sources is
complicated by many factors, useful estimates can be made. The quality of such
calculations will depend on the analytical approach used, as well as on the accuracy of
the values of the peak power, pulse duration, pulse repetition rate, antenna radiation
patterns, antenna placement, and scanning rates that are used in theoretical
computations. Corrections for near-field effects may also be appropriate. The operating
parameters listed below must be specified adequately so that the true average-radiated
power from the antenna, and the resulting power density at a distant point can be
calculated.
For all sources (pulsed or CW), the antenna type and size, gain, antenna pattern
including E and H plane beam widths and sidelobe distribution, antenna height above
ground, operating frequency, antenna beam orientation (all possible cases) and the
attenuation of the transmission line that connects the RF generator to the antenna should
be known or estimated. For the calculation of the expected power density levels of pulse
modulated sources, the maximum possible values of peak power, pulse duration, and
pulse repetition rate that closely approximate, but do not exceed the maximum rated duty
factor of a transmitter should be used. In the case of multiple sources, the contribution of
each source should be considered when estimating the combined effect.
B2. Antennas -- On Axis
The field in front of an antenna can be characterized by the following three regions.
(1) Reactive Near-Field Region. This is the region of space immediately surrounding
the antenna or leakage source where the reactive (nonradiating) components
predominate and energy is stored in the field. The reactive near-field region
extends to a distance of approximately one wavelength from the antenna, except
for the case of electrically large antennas (whose physical size is greater, in any
dimension, than several wavelengths).
(2) Radiating Near-Field Region (Fresnel Region). In this region, which starts at a
distance from the antenna where the reactive field region has diminished to an
insignificant amount, the antenna gain and the angular distribution of the radiated
field vary proportionally with distance from the antenna. This is because the phase
and amplitude relationships of the various waves arriving at the observation point
from different areas of the antenna change with distance. For reflector-type
antennas, such as parabolic dishes, the radiation is somewhat more complex in
its distribution pattern.
(3) Far-field Region (Fraunhofer Region). This region is sufficiently far from the source
that the phase and amplitude relationships of the waves arriving from different
areas of the antenna do not change appreciably with distance. The antenna gain
and angular pattern are essentially independent of distance, and the power density
is inversely proportional to the square of the distance from the source. Although
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