I agree with the guidelines stated. Just wanted to point out a problem I had with VCCI on the last submission for registration of our oats. The initial submission used both horizontal and vertical AF for site attenuation measurements; however, VCCI would only accept horizontal AF for both measurements. We did a second vertical measurement and re-submitted with no problems.
----- Original Message ----- From: Robert Bonsen <rbon...@orionscientific.com> To: <paolo.ronc...@compuprint.it>; <pierre.se...@wanadoo.fr>; <emc-p...@majordomo.ieee.org> Sent: Wednesday, March 15, 2000 1:47 PM Subject: Re: Antenna factors to be used for NSA measurement > > This is a discussion which starts up at least once every year. There are > some issues with measuring NSA which are not very well understood including > antenna factors. > > First of all, how you test and which factors to use should not be a matter > of opinion. Unfortunately, the standards do not completely specify the > requirements on the antenna factors which enhances confusion and introduces > room for data manipulation. Having said that, there are some simple > guidelines which are based on physics, literature and practical experience. > > 1. There is a difference between measuring EUTs and performing site > attenuation measurements. NSA measurements are solely used for verifying > the performance of the site. It uses a simple antenna substitution > technique, in which the site under test is verified against the performance > of the site on which the antenna factors are calibrated. Hence, the better > the cal site on which the antennas are calibrated resembles a "perfect > OATS" (i.e. an OATS with an infinitely large ground plane), the better > quality the NSA measurement will be. As far as measurement method goes, a > dual antenna factor calibration suffices if the same antennas will be used > in the NSA measurement, there is no accuracy advantage when using a > three-antenna method in this case. > > 2. Measurement distance. There are substantial difference in the antenna > factors (and site attenuation) values at various range distances. In order > to perform a correct NSA measurement, use the same range distance for the > AF calibration as for the NSA measurement. The same goes for the EUT > measurements, always use the same range distance. > > 3. Geometry. Although some OATSes and a few semi-anechoic chambers will > pass NSA with using horizontal antenna factors only, this is not a correct > measurement method. Since we're comparing the site-under-test to the AF > measurement OATS, any change in the setup which results in a change in the > physics of the AF or NSA measurement will introduce an artificial > systematic error into the measurement. Both literature and experience has > shown that AF measured at different geometries, with different > polarizations and/or different source antenna heights), produce different > antenna factors. Variations of up to 3 dB and sometimes more are to be > expected for the low frequency regions. These variations will result in > added errors in any site attenuation measurement in which the correct > corresponding antenna factor is not used. In a lot of cases, this will > bring a perfectly ok chamber or OATS out due to the artificial measurement > error. > > 4. Accuracy. Antenna factors provided by the manufacturer or measured by a > cal lab are typically not of sufficient accuracy (with a few exceptions). > Accuracy in the AF measurement is extremely important since the NSA > measurement does not provide for a lot of uncertainty margin for the AF. > Typically, AFs are measured with a 2 dB uncertainty at best, even when > higher accuracies are claimed. Cable layout, padding, equipment, etc. are > extremely important. Also, AFs are typically only measured at one height at > one polarization, which is not sufficient for accurate chamber cals. > > I recently was asked to calibrate a chamber using factors which were > claimed to be better than +/- 1dB, but the frequency steps were so large > that a small bump in the AF was completely missed, resulting in a more than > +/-2dB uncertainty. Had the lab paid attention to setup and the > peculiarities of this antenna, the AF measurement would have been fine, and > we would have saved considerable time and money. Since most labs have a > substantial margin for antenna factor error (up to 3 dB) in their > uncertainty budget for EUT measurements, accuracy and geometry does not > become much of an issue. However, in NSA measurements we're looking at > substantially smaller margins and all of the aforementioned issues become > important. > > On a final note, free-space factors are not an alternative. Simple physics > dictates the presence of the ground plane (which is not present in > free-space measured AF) adds a variable to the equation which in turn may > add systematic errors to your measurement. And this has been demonstrated > in literature. Free-space factors are for free-space measurement facilities > (fully anechoic chambers). Using free-space factors (or single geometry > factors) can only be allowed if the additional uncertainty is included in > the error budget of the NSA measurements. And this is not the case with the > current standards (ANSI C63.4-1992, CISPR22). > > Hope this helps. For a little more detail, check my web site. > > Regards, > -Robert > > Robert Bonsen > Principal Consultant > Orion Scientific > email: rbon...@orionscientific.com > URL: http://www.orionscientific.com > phone: (512) 347 7393; FAX: (512) 328 9240 > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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