Greetings again.
I received some questions about this off list and there has been more
discussion in this direction, so I thought I would throw my other two
cents in.
For small fully anechoic chambers with little room for antenna height
adjustment, you should be able to have uncertainty of about 6dB or so
(10dB is much safer realistically) when you apply correction factors for
a 10m site. The reason for this is, as John Barnes pointed out, the
absence of reflected waves being received in addition to the direct
waves. The key importance to a fully lined chamber (including the
floor) is that destructive waves are not present. With a reflective
floor, destructive waves can lower your readings by more than 30dB. Add
this to the 6 dB or so of uncertainty for additive waves and your total
error could be enormous. With an absorber lined floor, the influence of
the destructive waves is eliminated or reduced, so a correlation of 6dB
(again 10dB is safer) should be achievable (this simply accounts for the
absence of constructive interference).
Another important factor to ensure you don't have any surprises when
moving from precompliance to a compliance run is to manipulate the
cables during testing (oh, how much easier our job would be without
cables). Large signal strength changes can be achieved just by moving
cables a few inches.
I also have to agree with Gert's and Ken's comments on far field
measurements. I mentioned this in my original message, but didn't
elaborate at all. These are very important considerations that can
greatly affect any expected correlation to a 10m OATS.
--
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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