Estonian Standards are great If you purchase a 2 user licence (about 25% more), you can copy/paste text and are not restricted to single PC/laptop
Best regards Charlie Charlie Blackham Sulis Consultants Ltd Tel: +44 (0)7946 624317 Web: https://sulisconsultants.com/ Registered in England and Wales, number 05466247 From: Ken Wyatt <[email protected]> Sent: 05 January 2021 15:56 To: [email protected] Subject: Re: [PSES] Draft SAE ARP-958E EMI Measurement Antennas; Calibration Method: Deeply flawed Revision E is nearing completion and balloting Hi Ken, You can get the EN version of CISPR 25 from the Estonian Center for Standardization at the cost of jut 31.80 €. They also sell a host of other EN standards at affordable costs. EN 55025:2017 (latest): https://www.evs.ee/en/evs-en-55025-2017 They have a system similar to PayPal that can handle charge cards. Note that the PDFs are copy-protected and labeled with your company name in the margin. They require Adobe Acrobat DS (free download) to open them. But once opened, they can be printed and rescanned to PDF for your convenience. 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 56 Aspen Dr. Woodland Park, CO 80863 Phone: (719) 310-5418 Web Site<http://www.emc-seminars.com> | Blog<https://design-4-emc.com> The EMC Blog (EDN)<https://www.edn.com/electronics-blogs/4376432/The-EMC-Blog> Subscribe to Newsletter<http://www.emc-seminars.com/Newsletter/Newsletter.html> Connect with me on LinkedIn<https://www.linkedin.com/in/kennethwyatt/> On Jan 4, 2021, at 1:21 PM, Ken Javor <[email protected]<mailto:[email protected]>> wrote: Well, I spent $240 getting pdfs of ARP-958A/B/C (had original and “D”). Going to have to think long and hard about dropping $410 on the CISPR standard... Ken Javor Phone: (256) 650-5261 ________________________________ From: John Woodgate <[email protected]<x-msg://55/[email protected]>> Organization: J M Woodgate and Associates Date: Mon, 4 Jan 2021 20:02:10 +0000 To: Ken Javor <[email protected]<x-msg://55/[email protected]>>, <[email protected]<x-msg://55/[email protected]>> Subject: Re: [PSES] Draft SAE ARP-958E EMI Measurement Antennas; Calibration Method: Deeply flawed Revision E is nearing completion and balloting There is some information in CISPR TR16-3 on this issue which might be helpful. ====================================================================================== Best wishes John Woodgate OOO-Own Opinions Only www.woodjohn.uk<http://www.woodjohn.uk> <http://www.woodjohn.uk<http://www.woodjohn.uk/>> Rayleigh, Essex UK "A nation once again" (with apologies to Eire) People have to be told in stark terms that they can disobey the Covid rules at the risk of their own lives, but disobeying at the risk of others' lives is no less than a crime against humanity. On 2021-01-04 18:56, Ken Javor wrote: Draft SAE ARP-958E EMI Measurement Antennas; Calibration Method: Deeply flawed Revision E is nearing completion and balloting Alcon, Please forward to any you feel might be interested. Bringing up this issue to raise awareness of what I consider to be a measurement practice travesty in the draft SAE ARP-958E – and hopefully stop this during the upcoming and imminent balloting for this revision. The issue is that for the first time they propose to measure antenna factors separately for horizontal and vertical polarizations. There are two distinct problems with that: they are going about it incorrectly, and even done correctly, it simply isn’t necessary. The difference between antenna factors for different polarizations hinges on the presence of a ground plane, so this discussion is largely about ground plane effects. Full disclosure/disclaimer: I seem to be the only person involved in the “E” revision process who had a problem with what follows. I was first instructed to quit messaging on this topic and then after the first two meeting invitations no further invitations followed. My comments on the draft “E” revision submitted through NASA MSFC were rejected. It is reasonable to assume I am considered a crank by the balance of the committee membership. Background. SAE ARP-958 covers calibration of antennas used at one-meter separation, supporting radiated emission requirements in EMI control standards such as MIL-STD-461, DEF-STAN-59-411, RTCA/DO-160 (EUROCAE ED-14G), CISPR 25 and derivatives of these standards. The original 1968 release was only for log-spirals (circular polarization) and didn’t specify antenna height above ground plane when calibrating. In fact, they don’t mention ground planes at all. They do mention an area free of obstructions, so that a free field environment is simulated. They mention the use of tripods, so it is reasonable to assume tripods such as those used during actual EMI testing, meaning that antennas would be set up at a similar height as when used for EMI testing. This means that any reflections off a ground plane during calibration would be similar in magnitude and effect as during EMI testing – but there is no mention of ground planes, and they do mention free field. So how do we deal with reflections off the floor? One entirely empirical answer is to calibrate outside on an open area test site (OATS) without a metallic ground plane. I checked the antenna factors of some vintage log-spirals covering 200 MHz to 10 GHz about ten years ago in my front yard, prior to shipping them off to a customer. The check used the same tripods as during EMI testing, but extended to the maximum height the tripods would provide. Both the log-spirals and the receivers used during the check were multiple decades old, and had not been calibrated in that time. With old and obsolete equipment, measured antenna factors were identical to those published for these log-spirals, back in the 1960s. But that is only partial and circumstantial evidence that the original calibration technique eschewed a ground plane. In order to establish that conclusively, a control measurement would be necessary on an OATS with a metallic ground plane, and showing significantly inferior results compared to the measurement without a ground plane. The 1992 “A” revision specified the use of a metallic ground plane, and also placement of antennas for the antenna factor calibration three meters above it. Now on the face of it, this seems overkill compared to the excellent results I measured without a ground plane. But the A revision (and all subsequent revisions, including draft “E”) apply to not just log-spirals, but also biconicals, tuned dipoles, log-periodics, and horns, not to mention electrically short rod and loop probes. Problem 1: Wire-type antennas, if placed close to a ground plane in vertical polarization, will be unbalanced by differential capacitive loading due to one antenna element being closer to the ground plane than the other. Figure 1 shows the geometry for calibration on the left, and EMI test use, on the right. It is assumed here that differences in capacitive loading between ground plane and elements are proportional to the first power of distance from the ground plane. This is a first-order approximation that suffices for this discussion. One solution to that unbalance is to only calibrate in the horizontal polarization. So the three-meter height is not fully explained by the addition of different type antennas in the “A” revision. But if there is a need/desire to calibrate in vertical polarization, Figure 1 demonstrates that to represent the ground plane-induced unbalance properly, the calibration must be made at the same height as when the antenna is used to make the EMI measurement. Otherwise the unbalancing effect of element-to-ground plane capacity is misrepresented by a factor of 7.* <image.png> Figure 1: Capacitive loading to ground plane unbalances wire-type antenna lower element. Capacity will be based on relative separations from ground plane, as shown. Dimensions based on 1.4 m length biconical. Problem 2: Another advantage of a raised antenna is minimizing the relative magnitude of the ground plane-reflected ray compared to the one-meter path length direct ray. On the one hand revisions A - E enforce a ground plane, which is going to increase reflections vs. grass and dirt or concrete/asphalt less any rebar. On the other hand, they increase separation from the source of the reflections. While these measures seem self-cancelling, there is rationale for it. Since the original release didn’t specify the use of a ground plane, and didn’t prohibit it (except via the free-field description) specifying a ground plane would act as a standardization tool – extremely important in a calibration standard. And as the A – E revisions apply to various antennas, they also apply to a larger frequency range as well, from 30 MHz to 18 GHz. It isn’t clear, without experimentation, that an OATS without ground plane would be as anechoic over the wider frequency range of all these antennas as it was over the log-spiral range of 200 MHz to 10 GHz. Finally, ground in general (not including asphalt/concrete) will have varying conductivity depending on soil composition and degree of moisture content, which variation is again highly undesirable in a calibration standard.** The ground plane and three-meter height requirement don’t change after “A.” It is identical through “B,” “C,” “D,” and draft “E”. What is new in “E” is a requirement to separately measure horizontally and vertically polarized antenna factors for wire-type antennas and possibly aperture antennas, for which it makes even less sense.*** For this discussion, refer to Figure 2. One purpose of the three-meter height is to minimize the effect of reflections.**** Compared to the height at which we make EMI measurements – roughly 1.2 meters over a ground plane – the effect of reflections at three meters is much less (~ 20 log (1.3/3.04) = -7.4 dB). Yet another disconnect is the frequencies at which antenna factors will be most affected by ground plane reflections. Consider the three-meter height. A reflection path will be roughly 6 meters, 5 meters longer than the direct ray. Five meters is a half-wavelength at 30 MHz, so using a three-meter height over ground we see effects down to 30 MHz. Whereas if we measured antenna factors the way we use the antenna for EMI testing, namely at approximately one-meter height, the bounced-ray path length is roughly 2.6 meters, so the path length difference of 1.6 meters is a half-wavelength at around 100 MHz. So the three-meter height underestimates the amplitude effect, but shows perturbations at frequencies where they don’t exist (or exist at a lower level) when measuring at one-meter separation. <image.png> Figure 2: Frequency spectrum of where ground plane bounces interfere depends on antenna height over ground plane. IFF (if and only if) we are going to the trouble of measuring the difference between different polarization reflections, then we ought to be doing it at the height at which we will use the antenna. Otherwise, it is a total waste of time and money, but worse, gives the false impression we are doing something useful, when we aren’t. Problem 3: And that, in my opinion, is the most serious issue. The motivation to measure separate polarization-specific antenna factors reveals a fatal flaw in the understanding of what a one-meter antenna factor actually is. Presumably, if we are going to start measuring and using polarization-specific antenna factors, the motivation is a desire for added accuracy. But that desire will not be satisfied, except in an illusory “feel good” manner. Particularly for wire-type antennas (dipole, biconical, log-periodic array), where the difference between horizontal and vertical antenna factors is expected to be most pronounced (especially if measured as used, with antenna center 1.2 meters above the ground plane), the antenna is not measuring anything approaching a “true” electric field. The basic calibration set-up of e.g., two biconicals facing each other one meter apart means that the field impinging upon the receive antenna is uniquely that created by the transmit antenna, and is not at all the plane wave necessary for an accurate characterization of any other impinging electric field. The “field intensity” measured by any antenna with physical aperture dimensions one meter or larger calibrated at one-meter separation from an identical antenna is an effective field intensity only. This can be seen inspecting published antenna factors for ETS/Lindgren biconical and log-periodic arrays as in Figure 3. If we were in the far field, and there were no reflections, all antenna factors would be identical, and there would be no need for separate measurements thereof. But in fact they all differ, and the one-meter antenna factor differs more from the 3- and 10-meter measurements than the 3- and 10-meter measurements differ from each other, which make sense in that the larger the separation, the closer to the far field, and the closer the antenna factors approach the single asymptotic far field value. It is the fact that we are measuring an effective field intensity and that the reported field intensity is only obtained using the specified antenna that accounts for MIL-STD-461 specifying the types and physical apertures of antennas used for radiated emissions measurements. If a tuned dipole were substituted for a biconical, or a log-periodic array substituted for the double ridge-guide horn antenna, even adjusting for antenna factors, the reported field intensities would differ, because the non-uniform field gradient impinging upon different size and type physical apertures will give outputs at the antenna terminal that cannot be correlated using near field antenna factors. Field intensities measured using different antenna types and/or sizes would only correlate under the plane-wave conditions that are obtained when all the antennas have been calibrated in the far field, and when the EMI measurements were also made in the far field.***** <image.png> Figure 3: Antenna factor as a function of transmit–receive antenna separation. The point is that our one-meter measured antenna factor is a figure-of-merit: valuable as long as we all do things the same way. Tweaking our antenna factor a few dB for one polarization vs. the other does nothing to add accuracy or precision to our measurement. [We are deep into the cave allegory here. Compare Plato’s predicted treatment of someone who pointed out the truth to the cave dwellers to the disclaimer found in the introduction to this discussion.] Solution: From the point-of-view of minimizing the loading effect of the ground plane, it makes sense to only calibrate horizontally polarized antenna factors. But because of ground plane bounced-ray interference, there may be some merit to only calibrating vertical antenna factors at the three-meter height, in that the effect of the bounced ray is minimized for wire-type antennas, and ground plane loading is minimized at that height. Choosing a single best polarization would come down to minimizing the total errors of both loading and reflections. If there must be separate H & V antenna factors, then they should be measured at ~1 meter above the ground plane. If ARP-958E is accepted as-is, then MIL-STD-461 going forward needs to specify only one antenna factor is to be used, and which one. This will not only save time and money on antenna calibration, but it will avoid a new source of measurement uncertainty: variations in the use of horizontal and vertical antenna factors. Left unaddressed, some EMI test facilities will choose one, some the other, and some both. A minor but annoying impact to having multiple polarization antenna factors is the need for separate such antenna factor files tied to each polarization. Near identical files will have to be read in, and the software rewritten to apply the proper file for each polarization. Will we do separate H & V measurement system integrity checks for each antenna above 30 MHz, as well? Lastly, if ARP-958E is passed as-is, expect hourly rates at commercial EMI test facilities to increase to absorb the cost of double the calibration time for each antenna used above 30 MHz. ———————————— *The unbalance factor of seven between 3-meter height calibration and 1.2-meter EMI test height use is arrived at based on the comparison of differential loading in each case. In the calibration case, with the lower element experiencing 1.2x the loading that the top element does, the difference in loading between the two has a value of 0.2 (1.2 – 1). In the EMI test use situation, the lower element experiences 2.43x the loading of the top element, so the unbalance is a value of 1.43 (2.43 – 1). Comparing the 1.43 test use unbalance to the 0.2 calibration unbalance, we arrive at an unbalance ratio between use and calibration of 1.43/0.2 = 7. **If we were starting from scratch today, it might make sense to put absorber on the floor to eliminate any reflection whatsoever. Ferrite tile absorber for practical use at lower frequencies wasn’t available in 1968, and was still rare and expensive in 1992, when revision A was released. To my mind (and the methodology and philosophy of ARP-958 through the “D” revision) ground plane reflections shouldn't be part of an antenna factor anyway. That should be controlled by the test standard and site. ***The reason that different polarizations have different antenna factors due to ground plane reflections is that a vertically polarized ray bouncing off the floor doesn’t change phase, whereas a horizontally polarized wave changes phase by 180 degrees. This is a consequence of the nature of a conductor, which cannot support an electric field internally. ****The 3-meter height is not terribly difficult to set-up for low-gain wire-type and even log-spiral type antennas, but when it gets to horns, it is another matter entirely. The uhf DRG is big and bulky; it is not designed for mast use. The microwave DRG can have pretty high gain at higher frequencies, so that minimizing pointing error becomes critical and much harder to control three meters above ground level. The fact that they went to the three-meter height despite such drawbacks emphasizes the desire to minimize the effects of ground plane reflections. ***** This statement should not be interpreted as an endorsement of far field measurements in lieu of measurements presently made at one-meter separation such as in MIL-STD-461, RTCA/DO-160, CISPR 25 and derivative standards. If the integrated system-level configuration can place a source of EMI at one-meter separation from a victim antenna, that is how the EMI control measurement must also be made. Ken Javor Phone: (256) 650-5261 <image.jpg> ------ End of Forwarded Message - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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