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 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: allen_tu...@adc.com
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