This is a point I made (unsuccessfully) to the SAE ARP-958E drafting committee. They now require separate horizontal and vertical polarization antenna factors, but measure them three meters above a ground plane, and purport to say this is value-added relative to a single polarization calibration as earlier, even though these antennas are all used one meter above a ground plane.
Since we are discussing Don White, he made the point in volume 4 of the handbook series clear back in 1971 that a biconical used with one end closer to the floor than two feet would capacitively load the antenna enough to change the antenna factor. So a half-century before the -958 working group hosed this up, the issue was clearly understood. This gets back to Doug Smith. Not saying we should require 0.1 dB uncertainty, but the people running the show don’t even understand the issues. -- Ken Javor Ph: (256) 650-5261 From: Brent DeWitt <[email protected]> Reply-To: Brent DeWitt <[email protected]> Date: Wednesday, October 9, 2024 at 7:42 PM To: <[email protected]> Subject: Re: [PSES] technical musings Good point Ralph. As with many here, I grew up with Don's books and found them a great resource back then. That said, what I meant to examine is not so much the environment, but the characteristics of a receiving antenna in close proximity (fractions of a wavelength) to a reflecting "ground" plane. Depending on dimensions/balun design/spacing and who knows how many other variables, it gets quite messy! This is probably a point where our antenna design experts, rather the I, can chip in. On 10/9/2024 4:13 PM, Ralph McDiarmid wrote: Hi Brent, The handbook series from Don White Consultants, “Electromagnetic Interference and Compatibility, Vol 4” does a nice job of describing the effect of resonances in a bounded space, like a shielded chamber with and without RF absorbing material. Ralph From: Brent DeWitt <[email protected]> Sent: October 9, 2024 12:20 PM To: [email protected] Subject: Re: [PSES] technical musings Hi Ken, I find it interesting that the data "settles" somewhat above 300 MHz, which is where many "hybrid" biconical/LP antennas transition effective elements. Shortened, fat dipoles are rather notorious for varying AF and VSWR, while LP arrays are relatively well behaved. The source antenna is well described and known, not so with the receive antenna. The first question that comes to my mind is whether the same receive antenna was used in both measurement cases. In my past roles, I have put a great deal of "faith" in the AF calibrations done by high quality laboratories such as Liberty Labs, but that data doesn't always tell the whole story. What I'm basically saying is that measurements made in the near-field are difficult to reproduce. Brent On 10/9/2024 2:42 PM, Lfresearch wrote: Hi Ken, Nice article. I would love to have seen you add in an OATS to your measurements. What you publish correlates with what we have found on our 10m OATS, 5m chamber and 3m compact chamber. Namely that below 100 MHz, 3m test distances emissions tend to read high. I hadn’t planned to publish our results, but in light of this series of posts, maybe I should. Take care, Derek. On Oct 9, 2024, at 12:40 PM, Ken Wyatt <[email protected]> wrote: Hi All, I had a chance to make some comb generator measurements at a lab with two 10m chambers. I used the AET USDS spherical comb generator designed by Dr. Bruce Archambeault (no longer available, unfortunately). There was an age difference between the two chambers, with the older ferrite tile lined and the newer, ferrite tiles with absorber cones. One of the charts shows the difference between 3m and 10m test distance. Use of comb generator to compare 3m and 10m measurements: https://www.edn.com/review-the-aet-usds-spherical-harmonic-comb-generator/ One outcome was the linked article, but what was missing in that was the comparison between the two chambers (horizon polarization, only). See second link to the table. In every case, the comb generator was positioned on a tripod 1m above the reflecting floor. There was as much as 15 dB difference between the two. I’ve not published this last chart, but did leave it with the test lab management. Chamber 1 and 2 comparison: https://www.dropbox.com/scl/fi/yln8697jufl7xl8kvybol/Delta-Chamber-1-Versus-2.png?rlkey=n9s1wvzgfe8wybwxfig8jangp&dl=0 Cheers, Ken _______________________ I'm here to help you succeed! Feel free to call or email with any questions related to EMC or EMI troubleshooting - at no obligation. I'm always happy to help! Kenneth Wyatt Wyatt Technical Services LLC 8201 Lighthouse Lane Ct Windsor, CO 80528 Contact Me! New Books! <EMC TS Vol1 Cover.jpg> <EMC TS Vol2 Cover.jpg> <EMC TS Vol3 Cover SM.jpg> Web Site | Blog The EMC Blog (EDN) Subscribe to Newsletter Connect with me on LinkedIn On Oct 9, 2024, at 1:07 AM, doug emcesd.com <[email protected]> wrote: I agree and was involved with the early development of emission standards with with Don Heirman. But still, other areas of engineering are much more accurate/have very low uncertainty than EMC. It is sad that most everyone I knew from the early days of EMC are gone now. Many of them, Heirman and Ott, for example, were friends. I hope to continue to help engineers in the wider field of engineering and physics, including EMC for another 25-30 years. By the way, another of my interests is medicine, which I have studied on my own for 50 years now and have successfully slowed the aging process and successfully treated my spouse’s ovarian cancer (after the surgery to remove two very large tumors) without an oncologist. Just another part of science as EMC is. Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: [email protected] Website: http://dsmith.org From: Brent DeWitt <[email protected]> Sent: Tuesday, October 8, 2024 5:57:46 PM To: doug emcesd.com <[email protected]>; [email protected] <[email protected]> Subject: Re: [PSES] technical musings Doug, Respectfully, I would not agree. I'm not sure what "other areas of engineering" you're referring to. 🙂 First, if we are discussing commercial emissions standards, they are ancient and date back to the days when Apple 2 PCs were interfering with "rabbit ear" TV reception (1979) when Part 47 was drafted. The levels were largely drawn from the VDE 0871 standard. I doubt anyone with knowledge of the methodology has suggested that the overall process is deterministic. The question of measurement uncertainty was championed by Don Heirman for years and continues to be debated. Regulations and standards have done a very good, if somewhat painful, job of quantifying the uncertainty of the measurement process. The most competent and well executed test laboratory cannot fully control a customer's EUT or even have total control over the configuration. That is an advantage for a manufacturer having their own in-house test facility. All that said, I feel that the "state of EMC testing" has evolved to produce very respectable results, given the real world constraints. Best to all and 73 Jim! Brent DeWitt, AB1LF IEEE EMC Society Senior Life Member Milford, MA On 10/8/2024 7:07 PM, doug emcesd.com wrote: Seems like the state of EMC testing is not good compared to other areas of engineering! Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: [email protected] Website: http://dsmith.org From: Jim Bacher, WB8VSU <[email protected]> Sent: Tuesday, October 8, 2024 8:50:49 AM To: [email protected] <[email protected]> Subject: Re: [PSES] technical musings Back in the 1980s I had a variation in system measurements between labs. Management decided the lab I used wasn't any good. In the process of trying to convince management variations were normal, I came to the conclusion that measuring systems at different labs would likely have a +5 to -7 dB variations. None of our test equipment is perfectly flat. No one calibrates every Hz to account for those imperfections. The frequency being measured is typically not on any of the calibrated frequencies. For testing a system, equipment and cable variations can impact their emmisions significantly. I have seen over 20 dB of variation due to the variations. How much effort was made to peak signals? Then is the test equipment measurement operator an avid Ham DXer (chases weak signals to talk to far away stations)? Reason is a Ham will likely do a more accurate peaking of rotation and height of the search antennas. I have had one government lab fail to compensate for a preamp when reporting levels and said the product failed. I had the agent pull it back for a few days, then take it back and it passed with no changes. So some errors can be to using the wrong calibration factors when calculating the levels. Jim Bacher, WB8VSU [email protected] or [email protected] JBRC Consulting LLC Product EMC & Regulatory Consultant https://www.trc.guru/ email:[email protected] IEEE Life Senior Member On October 8, 2024 11:14:28 AM David Schaefer <[email protected]> wrote: Harry Hodes gave a presentation at the C63 meetings last week on proficiency testing. He runs the ACIL PT program, and the data he shared showed a ton of variance. Many labs were in I think +/-3-4 dB range, but there were outliers – the worst cases being one lab reading 30-40 dB high, another 30-40 dB low. If I recall correctly for one PT program 95 labs participated and around 10 were outside the expected tolerance. Doing a check of the measurement system prior to testing is critical. A comb gen scan takes only a few minutes but should catch these egregious errors. <image404476.jpg> David Schaefer Technical Manager Element Materials Technology 9349 W Broadway Ave Brooklyn Park , MN 55445 , United States O +1 612 638 5136 ext. 10461 [email protected] www.element.com <image627505.png> <image882535.png> <image798446.png> <image979275.jpg> From: doug emcesd.com <[email protected]> Sent: Monday, October 7, 2024 6:08 PM To: [email protected] Subject: [PSES] technical musings CAUTION:This email originated from outside of Element Materials Technology. DO NOT click links or open attachments unless you recognize the sender and know the content is safe. Please contact the TOC if you are in any doubt about this email. I have seen differences of 9 dB on the same measurement on an EUT and comb generator by different accredited labs! Doug This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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