ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 44)

44 Common Diagnostic Techniques www.ets-lindgren.com
Evaluating various solutions requires great skill and awareness, and it is in this
area that the far-field/near-field effects can be the most misleading. The E-field
and H-field vectors are initially determined by the source impedance. As you
move away from the source, these vectors increasingly balance until the
radiating field is isolated as a plane wave with a characteristic impedance of
377 ohms.
In the near-field the field strength can contain, in addition to the radiating field, a
significant non-radiating reactive component. This reactive component does not
propagate far. The radiating field will fall off proportionally with the reciprocal of
the first power of the distance from the source, 1/R. However, the reactive
component will fall off proportionate with the reciprocal of multiple powers of the
distance from the source, 1/RN.
Typically, the reactive field will fall off at a rate approaching 1/R
3
. Two points
should be observed:
1. Often the near-field reading will be dramatically different than would be
expected based on an extrapolation of the far-field reading. Near-field
readings will seem higher than expected due to the presence of the
reactive field; alternately, it may be lower than expected because of
nulls created by the interference pattern set up near the unit.
A reflection pattern is often established near the unit by the direct wave
combining with the reflection off parts of the unit and other items in the
vicinity. A design which reduces field strength by attenuating the
non-radiating, reactive field may show relatively little effect on the
far-field reading.