The second sentence of the first paragraph below is unclear. I meant to say there was as much difference between your free space antenna factor and the colleague’s ten meter factors as between the three and ten meter factors. Ken Javor
Phone: (256) 650-5261 _____ From: Ken Javor <ken.ja...@emccompliance.com> List-Post: emc-pstc@listserv.ieee.org Date: Thu, 28 Feb 2008 22:06:46 -0600 To: Untitled <emc-p...@ieee.org> Conversation: Free Space Antenna Factor Subject: Re: Free Space Antenna Factor I think the antenna factor data confirmed Mr. Woodgate’s assessment of your antenna salesperson. From a practical point-of-view, I would say that since there is as much difference on average between your free space antenna factors and the ten meter antenna factor of your colleague’s antenna, go ahead and use his three meter antenna factors and don’t sweat the details. I am really interested in Mr. Woodgate’s response to the following assertion, upon which the previous statement was based. If you have two different serial numbers of the same model antenna, and the antenna factors one to the next vary as much as shown below, then either the antenna quality control is very poor, or (much) more likely the antenna calibration tolerances are just that bad (OATS-to-OATS NSA can vary 8 dB, right?). I would have thought that a free space antenna factor would be more like 30 meters, especially at 30 MHz, but I’m not an expert on that. But if the comparison is valid, meaning your colleague’s ten meter antenna factor should correlate to your free space factor, then just use your colleague’s three meter numbers and be done with it. You will gain nothing by paying for a three meter calibration of your antenna, aside from appeasing some accreditor whose understanding is limited to checking calibration stickers and record books. Ken Javor Phone: (256) 650-5261 _____ From: Grace Lin <graceli...@gmail.com> List-Post: emc-pstc@listserv.ieee.org Date: Thu, 28 Feb 2008 21:28:45 -0500 To: Luke Turnbull <luke.turnb...@trw.com> Cc: <emc-p...@ieee.org> Subject: Re: Free Space Antenna Factor Dear John, Ken, Luke and others, Thank you so much for your comments. I printed out the document from the link Luke provided. I ran out of time in the office to read it. Please allow me to explain my question more detail. One of antennas I purchased came with free space antenna factors. According to ANSI C63.5: 2006, free space antenna factors are done at 10-meter distance. I will perform radiated emission measurement at 3-meter. Should I use the free space antenna factors without any calculation? Or, is there any equation to convert free space antenna factors for measurement distance other than 10-meter? The antenna vendor told me to use the free space antenna factors, then compare limit to 10-meter limits. I am not convinced to do so as this will have about 10 dB difference. To confirm my concern, I had a friend, who works at a well-known commercial lab and has the same model of the antenna, fax me his antenna factors at 3m and 10m. I list some data below for your reference. Free space antenna factors are from my antenna. 3m and 10m data are horizontal polarization. 30MHz-19.6 (3m), 18.7 (10m), 17.8 (free space) 40MHz-14.2 (3m), 13.7 (10m), 11.9 (free space) 50MHz-9.9 (3m), 9.1 (10m), 7.3 (free space) 60MHz-6.8 (3m), 7.1 (10m), 5.45 (free space) 70MHz-7.2 (3m), 7.3 (10m), 5.2 (free space) 80MHz-8.7 (3m), 7.4 (10m), 6.5 (free space) 90MHz-10.2 (3m), 9.4 (10m), 8.75 (free space) 100MHz-11.8 (3m), 11.3 (10m), 10.5 (free space) 200MHz-10.9 (3m), 10.3 (10m), 9 (free space) 300MHz-14.5 (3m), 13.2 (10m), 13 (free space) 400MHz-17.1 (3m), 16.2 (10m), 15.9 (free space) 500MHz-18.6 (3m), 17.5 (10m), 17.25 (free space) 600MHz-19.9 (3m), 18.6 (10m), 18.5 (free space) 700MHz-20.2 (3m), 18.8 (10m), 19.18 (free space) 800MHz-21.3 (3m), 19.9 (10m), 19.6 (free space) 900MHz-22.2 (3m), 20.8 (10m), 20.35 (free space) 1000MHz-22.9 (3m), 20.9 (10m), 20.9 (free space) >From the above data, I don't find any difference between 3m and 10m close to 10 dB. Thank you and look forward to your help. Best regards, Grace On 2/28/08, Luke Turnbull <luke.turnb...@trw.com> wrote: Grace, Have a look at the following guide from NPL, Page 41. (Sorry, you have to give them your name and address to download it). The correction is because the actual receiving element of a log-periodic may vary in distance between about 2.5 and 3.5m. The correction gives the field that would be at 3m distance from the product. http://publications.npl.co.uk/npl_web/pdf/mgpg4.pdf Good Luck, Luke Turnbull >>> "Grace Lin" <graceli...@gmail.com> 27 February 2008 18:48 >>> Dear Members, Could someone please teach me how to scale free space antenna factors for my 3-meter distance measurement? One antenna vendor told me that I could scale free space antenna factors. I couldn't reach this vendor at the moment. Thank you and look forward to hearing from you. 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