Interesting... we are gojng to set-up a pre-compliance semi-anechoic
chamber for 3 m measurements (mainly radiated emissions) on telecom
products and we'll need to correlate it with a 10m full-compliant chamber.
Our DUT's are typically sub-rack or 2m+ high telecom racks. Your idea of
considering a fully anechoic vs semi-anechoic chamber sounds pretty
interesting to me. We are limited in height to about 2 m (chamber internal
space) so we are not able to maximize with a limited antenna height
scanning. So rotating the EUT and adding 6 dB for the missing in-phase
floor reflection sounds reasonable... unless I'm missing something....
I heard some time ago about a correlation study btw fully- and
semi-anechoic rooms performed by CISPR subcommittee A and if I remember
well they came up with disturbing increased uncertainties / variancies
related to cables layout due to missing coupling between cables and ground
floor (image theory and stuff...).
Anyone who can fill me in more with this would be welcome !
Regards,
Paolo Roncone
At 10:37 AM 1/11/01 -0500, David Heald wrote:
Hello all
There are a few variables that need to be addressed to answer this
question. The first is the nature of the chamber. My reply will assume
that this is a fully anechoic chamber (walls, floor, and ceiling all
lined with absorber material). Otherwise, all bets are off due to the
unpredictable reflections from the surfaces in the chamber. In a chamber
this size, I will also assume that the antenna height is fixed, or at
least not very adjustable. Given a fully anechoic room and a fixed
antenna height, theoretically you should be able to extrapolate (about 10
dB from 1 to 3 meters antenna distance and another 10 dB from 3 to 10
meters) with only about 6 dB of uncertainty. In practice this is usually
accurate but real world conditions have slightly more uncertainty so 10
dB is a fairly safe margin to use.
A few things to keep in mind: if the chamber is only semi-anechoic
(walls and ceiling lined) you will have more uncertainty due to possible
cancellation due to floor reflections. At this point, relative change or
frequency identification is about the only thing the chamber is good
for. Also, near field readings can be significantly different from
far-field readings. If you come up with marginal near field readings, be
prepared for the worst when you take 10m readings. Finally, be sure to
check BOTH antenna polarities.
I hope this helps
Usual employer disclaimer . . .
David Heald
Senior EMC Engineer/
Product Safety Engineer
Curtis-Straus LLC NRTL
Laboratory for NEBS, EMC, Safety, and Telecom
Voice:978.486.8880x254 Fax:978.486.8828
<http://www.curtis-straus.com>www.curtis-straus.com
Tudor, Allen wrote:
Greetings:
What's the best way to correlate a pre-compliance chamber (smaller than a 3m
chamber) to a 10m anechoic chamber? Should I use a signal generator and
antenna or should I use a comb generator?
Would the answer be different if I were correlating the pre-compliance
chamber to an OATS?
Thanks in advance.
Allen Tudor, Compliance Engineer
ADC DSL Systems Inc.
6531 Meridien Dr.
Raleigh, NC 27616
phone: 919.875.3382
email: <mailto:[email protected]>[email protected]
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