ETS-Lindgren 7405 E & H Near Field Probe Set User Manual

Page 44

Common Diagnostic Techniques

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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

. Two points

should be observed:

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.