ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 47)

www.ets-lindgren.com Common Diagnostic Techniques 47
The more distant measurement points may lose the signal into the
system noise; a given solution may only redirect the beam. Especially
with narrow beam problems, solutions frequently only shift the beam so
that it radiates in a different direction.
After measurement points are chosen, baseline the unit by measuring
each point with an E-field and an H-field probe. That way, each design
alternative can be implemented and measured over the same set of
points.
2. The two procedures differ here in how they approach the
measurements that have been taken.
 The first method is based upon finding a solution with a large safety
margin. For example, suppose a signal fails the required limit by 3 dB.
Once that signal is found in the lab, it can be measured in the
near-field. The goal is then to reduce in this near-field the 3 dB plus a
safety factor of 6 dB or 10 dB.
This allows a large margin of error due to near-field effects.
Additionally, a solution that passes this must then be confirmed by
far-field measurements.
 The second method identifies several solutions which could be
effective. In the previous example where the signal failed by 3 dB, after
pre-screening in the lab, a variety of solutions may be selected and
tested.
A final benefit of pre-screening is that through the inevitable failures, new
information can be discovered. For example, an attempt to reduce an emission
may fail the following reasons:
1. The diagnosis was wrong.
2. The technique was inappropriate to the diagnosis.