And yet, while there is considerable uncertainty with EMC measured quantities this situation doesn’t appear to be causing a general problem. It’s the “nature of beast” and there is little motivation to do anything about it since the status quo appears to be acceptable.
Ralph From: Jim Bacher, WB8VSU <[email protected]> Sent: October 11, 2024 12:17 PM To: [email protected] Subject: Re: [PSES] Technical musings Gert, the FCC based their numbers on preventing interference to police and other emergency services along with not interfering with your neighbors radio or TV. I assume Europe picked their levels for the same reasons. At current levels I occasionally have had issues with near by RF emitting devices interfering with my ham radio activity. The same could be true for other services. What the standards do not take in consideration is what is happening to the noise floor. The noise floor rising makes weak signal detection more difficult. If it gets bad enough it could impact more than just weak signal detection. Raising the RF levels of the standard would raise the noise floor. Raising the levels would not be a good thing. Well designed devices can easily comply with the standards with reasonable margins. Jim Bacher, WB8VSU [email protected] <mailto:[email protected]> or [email protected] <mailto:[email protected]> On October 11, 2024 2:49:00 PM Gert Gremmen F4LDP <[email protected] <mailto:[email protected]> > wrote: Why using numbers (in dBuV/m) if the MU is 200-400 % ? Using number assumes a defined and known uncertainty. All this experiences calls out for a more strict measurement set-up, with extensive cable lay-out description, EUT set-up a uniform test site (5 meter FAR ?) , standard antennas, regular calibration and verification ahead on each test , an intelligent site attenuation calculation per frequency range (such as 30-100 100-300 and 300-1000 MHz) and more, if well done resulting in a lower MU of the total measurement. Focus shall be on reproducibility, a 6 dB offset is not that of a problem als long as we are all 6 dB off. Seen the fact that the current measurements do result in a (more or less) satisfactorily EMC situation in spite of excessive variations in results, we may assume that the current emission limits are on the safe side (= too low), and can be adjusted (say 5 dB) upwards, once a better overall MU is obtained, finally resulting in cheaper EMC testing, cheaper EUT production, and less excessive radiation = less interference. As the determining value for compliance is a QP-value, additional attention shall be paid to the peak pre-scan dwell times (actually defining the frequencies to be measured) and EUT emission cycling times, in order to find all qualifying frequencies for QP-evaluation, an aspect that is too often overlooked. It won't be easy to catch up for all these, but didn't we get to Mars too ? Gert Gremmen On 11-10-2024 20:23, John Woodgate wrote: Yes, there's not only the intractable near-field issue, but all the EM influences between the various pieces of equipment. This all adds to the uncertainty and irrepeatable results. On 2024-10-11 18:08, doug emcesd.com wrote: A criteria I have seen and agree with is that the distance from the EUT to the antenna be 10x the size of the EUT to insure the antenna is seeing a uniform field so it’s calibration is valid. This is not the same as being in the far field. This is a big issue at 3 meters. I have significant issues with many, if not most standards I have read. For instance, the people who wrote IEC 61000-4-4 did not understand the way the "capacitive" clamp works. It is also an "inductive" clamp and as a result it is directive and more energy is sent to the auxiliary equipment than to the EUT, there is no excuse for this. the clamp is positioned backwards in the standard!!!! I have been pointing this out for 30 years now to my clients and others. Here is a link to a paper I wrote on this almost 30 years ago: https://emcesd.com/pdf/esd96-w.pdf In my opinion, neither the clamp nor the standard accurately describe actual EFT although in later years some progress has been made, not nearly enough though. I see problems like this in many standards I read. Another problem that is much harder to control happens over in the ESD side. My personal discharge at 4 kV holding a small piece of metal with a measurement chain with 5 GHz bandwidth has a peak current twice what the standard calls for but the follow-on "hump" is more of a straight line down to the horizontal axis much faster than the standard calls for containing a lot less energy. I think this is due to the fact I have less capacitance (surface area, I am about two meters tall but on the skinny side from running 3,000 miles a year) that what was used for the standard which is probably closer to average than me. I have no idea how to account for variability between people and the actual environment they are in when an ESD event happens. Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: [email protected] <mailto:[email protected]> Website: http://dsmith.org _____ From: John Woodgate <mailto:[email protected]> <[email protected]> Sent: Friday, October 11, 2024 8:58:14 AM To: [email protected] <mailto:[email protected]> <mailto:[email protected]> <[email protected]> Subject: Re: [PSES] Technical musings Thanks, Jim. I wondered whether there was anything other than the 'near-field thicket' involved. Measurement results in the near field can be reliably reproduced only in absolutely identical test set-ups. This is not compatible with 'standardization'. On 2024-10-11 16:48, Jim Bacher wrote: 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 <mailto:[email protected]> <[email protected]> Sent: Wednesday, October 09, 2024 4:18 PM To: [email protected] <mailto:[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 <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.avg.com_email-2Dsignature-3Futm-5Fmedium-3Demail-26utm-5Fsource-3Dlink-26utm-5Fcampaign-3Dsig-2Demail-26utm-5Fcontent-3Demailclient&d=DwMDaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=c9NR2mGfldry-2pM9Bbuww&m=DRzvQBMpj-WwD6xyUgBid8ppyv7j7BkK85n49Ul8bwfkM-CO9V55PArJsLox-xG9&s=1mS2sew-kVjleZxEGVyBEMs2HQ_Mc4tcIFSvC_qxLCU&e=> Virus-free. <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.avg.com_email-2Dsignature-3Futm-5Fmedium-3Demail-26utm-5Fsource-3Dlink-26utm-5Fcampaign-3Dsig-2Demail-26utm-5Fcontent-3Demailclient&d=DwMDaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=c9NR2mGfldry-2pM9Bbuww&m=DRzvQBMpj-WwD6xyUgBid8ppyv7j7BkK85n49Ul8bwfkM-CO9V55PArJsLox-xG9&s=1mS2sew-kVjleZxEGVyBEMs2HQ_Mc4tcIFSvC_qxLCU&e=> www.avg.com _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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 -- OOO - Own Opinions Only Best Wishes John Woodgate Keep trying _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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