If the antenna selected does the job of properly illuminating the 1.5 m x 1.5 m quiet zone, then it doesn’t really matter what the chamber would do if you were using a less directive antenna. The job is to properly illuminate the quiet zone.
Relative to a horn with too much directivity. If you had the space in the chamber, you could just back it up until you were providing the proper coverage. There is nothing magical about three meters as a maximum separation, unless the chamber size inhibits a larger distance. A highly directive horn could be placed much closer to a back wall or corner than a log-periodic type array, because of the better front-to-back ratio. Finally, I have always been intrigued by an apparent oversight in military, aerospace and commercial RS/RI test procedures. The following is common to all, but it is especially glaring in 61000-4-3, because of the unique pains taken to establish a quiet zone. The issue is the field sensor polarization. When I was working 61000-4-3, back in the late ‘90s, there was no control established saying that the uniformity had to be established with the field sensor polarization mirroring that of the test antenna. Instead, there was nothing saying that the root sum square of all three axes on a three axis sensor couldn’t be used to establish the uniformity. Now it worked out in the chamber I was using, and I expect it is going to be the case in any chamber that will support a quiet zone, that the parallel polarization is going to dominate. But one would think that the uniformity check would be based on the parallel polarization, maybe even with a requirement that the orthogonal field components should be X dB down from the predominant vector. If you don’t have that sort of control, your, ahem, measurement uncertainty increases, because the coupling of the electric field to the test sample and its attached cables depends strongly on the orientation of the field relative to same. Ken Javor Phone: (256) 650-5261 ________________________________ From: Bob Richards <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Fri, 27 Aug 2010 07:04:27 -0700 (PDT) To: <[email protected]> Subject: RE: Field Uniformity Calibration per 61000-4-3 I understand about the beamwidth issue as it relates to covering the 1.5x1.5m area. What I was referring to was the chamber performance. If the beamwidth is enough to cover the 1.5x1.5 UF, but drops off quickly outside that, then the performance of the anechoic material on the side walls/floor/ceiling is not really being tested. An isotropic radiator would be, in my mind, a better measure of the chamber performance as it relates to uniformity. I know you would not want to use an isotropic radiator for testing products, but for the chamber performance check I think it would be worst case. On another note, years ago I was involved with the purchase of a horn antenna for radiated immunity testing above 1ghz. The gain was higher than other antennas I looked at. At 1ghz it was great, but above about 3ghz the beamwidth narrowed enough that I could not get uniformity. Above 4ghz the outside corners of the 1.5m square UF were below the noise floor of the probe. :-( I learned my lesson on that one. Bob R. --- On Thu, 8/26/10, Jason Smith <[email protected]> wrote: The directivity of the antenna could play a role if the chamber’s performance was not the best. The antenna does play a huge role in what the uniform field is and what power is required to reach field. Most bicon and biconilog antennas are poor choices for RI since they use a balun to match the antenna which is a source of lost power. You need a broad beam width to cover the 1.5x1.5m widow at 3 meters which is not to much an issue below 1 GHz but is a concern above this. The right antenna selection makes all the difference. The best solution is an LP that covers 80-1GHz and ether a stacked LP or a double ridge horn to cover above this. A minimum 3dB beam width of 30degres is needed through the frequency range. Thank you, Jason H. Smith ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Bob Richards Sent: Thursday, August 26, 2010 10:11 AM To: [email protected] Subject: Re: Field Uniformity Calibration per 61000-4-3 What would be considered worst case? A bicon - log periodic pair, or a bilog hybrid? I think the less directional the antenna, the worse the uniformity may be, but, I'm no expert on antennas. Bob R. --- On Thu, 8/26/10, John Woodgate <[email protected]> wrote: From: John Woodgate <[email protected]> Subject: Re: Field Uniformity Calibration per 61000-4-3 To: [email protected] Date: Thursday, August 26, 2010, 2:57 AM In message <1347744851.367114.1282781 [email protected] <http://us.mc565.mail.yahoo.com/mc/comp se?to=1347744851.367114.1282781476042.J [email protected]> <http://us.mc565.mail.yahoo.com/mc/comp se?to=1347744851.367114.1282781476042.J [email protected]> .comcast.net>, dated Thu, 26 Aug 2010, [email protected] <http://us.mc565.mail.yahoo.com/mc/[email protected]> <http://us.mc565.mail.yahoo.com/mc/[email protected]> writes: > Shouldn't the chamber user's antenna be used for the field-uniformity test? Otherwise I don't see how the results would apply if a different Tx antenna is later used. The radiation cahracteristics, including the coupling with the chamber, will be different. I think so, too, but there are possibly two stages: - check the chamber for freedom from non-uniformity [1] using a reference antenna; - check that acceptable uniformity can be achieved with the user's antenna. [1] e.g. due to one or more quasi-specular reflections. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK If at first you don't succeed, delegate. But I support unbloated email http://www.asciiribbon.org/ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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