The biggest "chamber effect" in radiated immunity testing is reflections. The
reflections off the walls, ceiling, and especially the floor in semi-anechoic
chambers will add or subtract from the field generated via the direct path. The
main effect is to make the 16 point uniform field calibration difficult or
impossible, since the phase length of the reflection path changes with position,
thus adding to or subtracting from the direct path power (reducing uniformity).
Reflections have a big effect on the necessary forward power. When the
reflections subtract from the field generated via the direct path, additional
forward power is needed.
Reflections back to the antenna do increase the VSWR. Increased VSWR is usually
viewed as a loss of power being radiated by the antenna, since part of it is
reflected back by the mismatch. In the case of an antenna in a chamber and the
resulting increase in VSWR, power is still effectively radiated by the antenna,
but part of the power is reflected back to the antenna by the chamber, and then
back to the source.
Generally, one wants the absorber to be as absorbant as possible so that the
direct path is the only one delivering power to the DUT, simulating free space.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
richwo...@tycoint.com on 09/26/2002 09:34:41 AM
Please respond to richwo...@tycoint.com
To: emc-p...@majordomo.ieee.org
cc: (bcc: Don Borowski/SEL)
Subject: Radiated Immunity Testing
What types of "chamber effects", if any, should one consider when
determining the extra factor to be applied to determine the forward power
applied to the transmit antenna. Potential effects might include loading of
the antenna by the anechoic materials or the chamber itself and resonance's.
If there are such effects, what factor should be applied to these effects in
order to determine the required forward power to the antenna? And before
someone mentions the other factors to consider, I am aware of the need to
account for VSWR and loss in the directional coupler and cable.
Richard Woods
Sensormatic Electronics
Tyco International
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