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: [email protected] 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. 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