John, you ask why the difference in levels measured between test distances of 3 meters and 10 meters. It’s fairly common for a device to fail at frequencies below 125 MHz at 3 Meter test distance and then pass at a 10 Meter test distance. Besides all the other possible factors (such as was a different chamber and test equipment used), the question becomes, was it a Near Field or Far Field RF signal that was being measured? Near Field RF levels drop faster than Far Field RF Levels. The problem with a 3 Meter test distance is the frequency being measured might be impacted by Near Field, verses Far Field only measurement at 10 Meters.
I have read a number of papers that claim different wave lengths for the Near Field effect. The values I have seen are between 1 and 3 wave lengths (with RF think wave lengths). I suspect it is system dependent and typically 1 to 2 wavelengths and I suspect the primary reason for the effect between the two measurement distances. Here are the approximate possible frequency ranges impacted by Near Field at a test distance of 3 Meters: Three wavelength signal: RF levels up to 280 MHz Two wavelength signal: RF levels up to 140 MHz One wavelength signal: RF levels up to 70 MHz As far as I am concerned 10 meters is the better test distance as it is in the Far Field for the frequencies between 30 MHz and 1 GHz. Although 30 Mhz is close to one wavelength at 10 Meters. Jim Bacher, WB8VSU [email protected] <mailto:[email protected]> or [email protected] <mailto:[email protected]> From: John Woodgate <[email protected]> Sent: Wednesday, October 09, 2024 4:18 PM To: [email protected] Subject: [PSES] Technical musings Reply to Derek @ LF Research, because his post is labelled as SPAM. Yes, adding OATS is always healthy.😉 Is there an accepted explanation for the '3 m excess'? The published results are consistent with the field being diffuse (that term is from acoustics: I'm not sure how widely it's used in EMC circles), i.e the resultant of a large number of direct, reflected and diffracted rays. It is hardly surprising: a cuboid space is 'ideal' for producing a diffuse field above 'eigentone' wavelengths. This might create at least a 3 dB increase over 'inverse square' and maybe more. I suppose things get complicated at wavelengths that cannot be called 'short'. Has anyone tried a spherical chamber? If that's too difficult, a 'quartic sphere [(x,y,z)^4 = r^4, like a Swedish traffic circle] has noticeably rounded corners and edges, so might be close enough for a useful improvement. -- OOO - Own Opinions Only Best Wishes John Woodgate Keep trying <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> Virus-free. <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> www.avg.com _____ 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] <mailto:[email protected]> All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ <https://www.mail-archive.com/[email protected]/%20> Website: https://ewh.ieee.org/soc/pses/ <https://ewh.ieee.org/soc/pses/> Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) <https://ewh.ieee.org/soc/pses/list.html> List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] <mailto:[email protected]> Rick Linford at: [email protected] <mailto:[email protected]> For policy questions, send mail to: Jim Bacher at: [email protected] <mailto:[email protected]> _____ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC <https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1> &A=1 - ---------------------------------------------------------------- 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: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
