You cannot perform a characterization that will mean anything. The room will
have standing waves that will be strongly dependant upon the size and
placement of the unit under test, the placement of the antenna and the
frequency. The best that you can do is perform a pretest to find the
frequencies of interest then move to the OATS for a final test. A screen
room can be used for before and after comparison of EMI fixes, as long as
the unit under test is not moved. But once you have a fix, you will still
have to test on the OATS. Actually, you can perform diagnostic tests in a
lab if you set the antenna 1 m away. Just keep other sources a few meters
away from the antenna.
To do what you want to do, you will need a compact semi-anechoic chamber at
a cost of about $140, 000 including the turn table. We just started using
one that complies with the NSA test given the constrant that we can't run
the antenna up to 4 m. We have found up to 6 dB of variation between the
chamber and the OATS. However the variation is small enough that we pretest
and fix in the chamber and only move to the OATS once we have confidence
that we have at least 6 dB of margin. So far so good, but I don't doubt that
some day we will end up out of compliance at the OATS even with 6 dB of
margin in the chamber.
----------
From: [email protected] [SMTP:[email protected]]
Sent: Tuesday, April 27, 1999 12:22 PM
To: [email protected]
Subject: Characterizing a screen room
My company is planning to purchase a screen room for radiated
emissions
precompliance testing.
I'm aware that reflections can cause resonances and drastically
influence
readings. What kind of testing could I do to characterize the room
(aside from
simple experience)?
--
Patrick Lawler
[email protected]
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