Ah yes, theory is one thing. It will give us the ideal approximation, using its stated assumptions. But then there is the real EUT and all of its variables. How one correction factor of one EUT, or a noise source, etc. correlates to the next one is a rather WAG. For example, 3m FAC to 3m OATS might be as much as 6 dB higher neglecting all the previous mentioned reasons why it cannot be that much. And 3m OATS to 10m OATS should drop approximate 10 dB due to the increased distance. The FCC allows below 30 MHz to use a two point method to determine the actual rolloff of the field for the EUT and adjust results accordingly. The rules apply to delineated regions and conditions, but those are arbitrary for us humans. Physics are somewhat more gradual in the transitions. I regularly deal with systems that rolloff at 3rd order, and some up to the 4th order. But that is our EUT specific. ps. I have also tested systems that did not rolloff. The measured levels were the same at 3m and 10m.
Think of launching a plane wave, unexpected as it was at the time. Accidental, that turned into something else by others. ________________________________ From: Charlie Blackham <[email protected]> To: Jim Hulbert <[email protected]>; "[email protected]" <[email protected]> Sent: Monday, December 19, 2011 4:58 PM Subject: RE: Semi-Anechoic Chamber Question Jim As others have suggested, I’d recommend leaving you chamber as it is, HOWEVER, I would perform correlation to OATS using some of your typical EUT, as well as a CNE or other noisy “box” – I’ve seen >10dB difference, relative to limit, between 3m FAC’s and 10m OATS measurement when measuring an EUT that had a cable that dropped down to ground plane. Regards Charlie From:Jim Hulbert [mailto:[email protected]] Sent: 16 December 2011 21:49 To: [email protected] Subject: [PSES] Semi-Anechoic Chamber Question I have an older 3-meter semi-anechoic chamber that is fully compliant for radiated immunity testing per IEC 61000-4-3. Occasionally I also use the chamber to run a radiated emissions "pre-scan". When I use the chamber for the pre-scan up to 1 GHz, I remove the foam absorbers and ferrite floor tiles between the antenna and EUT to get a reflecting ground plane. The aborbers are no problem but the floor tiles are quite heavy. As an easier option, I'm thinking of coming up with a relatively light weight conductive surface that can be easily placed over the floor tiles and easily removed again. Something like a foil that could be rolled into place and then rolled up again when done. I could connect the foil to the shielded room floor to ground it. Does anyone have any experience doing something like this or would I be wasting my time and better off continuing my practice of removing the tiles? I'm not looking for compliance emissions measurements because I'm limited to 3 meter distance and an antenna sweep height of 3 meters. However, I would like to get the best pre-scan data I can to minimize surprises when I do go out to an open area test site for the final test. Thanks. Jim Hulbert - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
