As Gert points out, extrapolation is fraught with hazards of assumptions. If you absolutely must attempt to extrapolate data from extreme distances, like a 1-meter measurement to a 10-meter equivalent field, you should test the sanity of your extrapolation algorithm.
For instance, it has been said that, at 10 meters, the emissions could not be measured (they were below the noise level of the detection system). The same was true at 3 meters distance. However, at 1 meter, signals were detected. My first thought is how certain are the 1 meter data? That is, were all measurements well above (maybe 6 dB) the noise level at 1 meter? However, let’s assume this is true. Since nothing was observed at 3 meters, it’s obvious that the field decay is greater than 6 dB over the 1 to 3 meter distance. It would greatly reinforce your claim of an accurate extrapolation algorithm if you had some empirical data to back up your scheme. For instance, could you show (and plot) the decay of the strongest emission, over the range of maybe ½ meter to 3 meters, at ½ meter increments? Once you have some field decay data, you could then try a regression to a formula for predicting decay. Since your emissions are likely not originating in a precisely defined antenna, the entire physical structure of your EUT is the antenna. Whatever extrapolation model you come up with will likely not be usable with other EUT’s, but it will probably be better than just assuming 1/(r^2) or 1/(r^3). I believe that the White EMC Handbook series had a formula for extrapolating from very near fields to far fields. The decay in the very near field was 1/(r^3), rolling off to 1/(r^2) as the wavelength decreased. The critical parameters were the distance to the EUT at the close distance, the wavelength of the emission and the distance to the EUT at the extrapolated distance. Perhaps this model is discussed in more modern EMC texts also. Ed Price WB6WSN Chula Vista, CA USA -----Original Message----- From: ce-test, qualified testing bv - Gert Gremmen [mailto:g.grem...@cetest.nl] Sent: Monday, December 05, 2016 10:46 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] EN 302 195 Distance Conversion Factor In the close field area E-field/H-field varies with 1/(r^2) OR 1/(^3) depending on the source and nature of it. In addition at close distances similar fields may have an opposite vector polarity (close to EUT) and may partially cancel each other. In general it is not a good idea measuring close field components to draw conclusions on radiated emission components at greater distances, as these components do not actually radiate. That is why you won’t find any conversion factors for frequencies below 30 MHz, at distances shorter than the close-far field transition zone. (lambda/2pi) Of course measurements in this area make sense about the EMI-level at the measurement point, and that is why some standards make measurements in the close field at a predefined distance. Changing that distance will make measurements incomparable. Regards, Ing. Gert Gremmen Approvals manager ------------------------------------------------------------------------------------ + ce marking of electrical/electronic equipment Independent Consultancy + Services Compliance Testing and Design for CE marking according to + EC-directives: - Electro Magnetic Compatibility 2014/30/EC - Electrical Safety 2014/35/EC - Radio & Telecommunication Terminal Equipment 99/5/EC Web: <http://www.cetest.nl> www.cetest.nl (English) <http://www.ce-test.nl> www.ce-test.nl (Dutch) <http://www.cetest.fr> www.cetest.fr (under construction) Phone : +31 10 415 24 26 ------------------------------------------------------------------- This e-mail and any attachments thereto may contain information that is confidential and/or protected by intellectual property rights and are intended for the sole use of the recipient(s) named above. Any use of the information contained herein (including, but not limited to, total or partial reproduction, communication or distribution in any form) by persons other than the designated recipient(s) is prohibited. If you have received this e-mail in error, please notify the sender either by telephone or by e-mail and delete the material from any computer. Thank you for your co-operation. From: Grace Lin [ <mailto:graceli...@gmail.com> mailto:graceli...@gmail.com] Sent: Tuesday 6 December 2016 00:46 To: <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] EN 302 195 Distance Conversion Factor Dear Members, What is the appropriate distance conversion factor per EN 302 195, 9 kHz - 30 MHz? The limits were specified at 10m. Test data was too low to be detected at 10m and 3m. 1m distance was used to collect some data. EN 300 330 provides a chart for the distance conversion factors between 3m and 10m. It seems there is no such information in EN 302 195. Thank you very much for your time and I look forward to hearing from you. Best regards, Grace Lin - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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