Now you've done it Bill - I've heard some pretty interesting stories about the first test sites. In my case we were class A verification and while we needed to test we could do it ourselves. Companies never have a dime for stuff they don't want to do - usually eaten up by the annual sales binges etc. So on a shoe string I set up a 3 meter test site (yes class a was either 10 or 30), but we went with 3 because the reflective plane was less costly and used the exception for testing at closer distances in noisy environments, and we used the 20log(d1/d2) to extrapolate to the specified distances.
The ground plane was the luxury part of the set-up. The test equipment shed was an old well house about 6 X 6 feet X 8 feet, made of tin siding and roof. The sole source of heat was a 100 watt light bulb. (I upgraded at my own expense from the existing 60 watt bad boy. The walls didn't even seal up. There was a gap of about 4 to 6 inches that was used as ventilation I think. It was large enough that playful but obnoxious cohorts would occasionally throw snow on me through the openings. In the winter the snow that blew in through the ventilation didn't melt. Just the place you want to put a $60,000 dollar analyzer, with a temperature sensitive crystal oven. I was able to use that as a crutch for upgrading after the first year. Mind you it had absolutely nothing to do with the health and welfare of the test personnel. The snow was so deep that we were graciously allowed to use facilities tractor and trailer to drag the analyzer and the equipment under test between the indoor facilities and the OATS. One of the first things to remember in the winter was to go to the bathroom before going out to the test site, because the long walk back in a foot or better of snow wasn't pleasant. The EUT test table was an very robust antenna rotator sandwiched between a couple of wooden plates. The table itself was a heavy duty garbage can turned upside down with another wooden sheet on top of that. We finally got to upgrade that during a hot summer day when the garbage can heated up and got soft and couldn't support the EUT. From inside the shed we heard the crash and found a broken prototype laying on the ground. In the summer the heat was well over a 100F. and I had a older test technician that wore a "uniform". The company didn't require one but he always wore heavy blue pants and white long sleeve shirt. He had one for every day of the week. I mention it only so you can get an idea that this guy wasn't your basic slacker who would just throw on anything before going to work. Then you might be able to appreciate how hot it was when I went out to see how the test was going to find him stripped down to his tee shirt, sleeves ripped off and he'd cut his pant legs off to make a pair of shorts. He had made a sweat band from the now ripped off shirt sleeves. When he heard me coming he stepped out of the test hut all of his sartorial splendor. He complained only by pointing out that prisoners of war had better living conditions. (and I don't think he was completely kidding) The first analyzer wasn't even a nice HP. It was it was the cheapest thing we could find with spectrum analyzer in the title. But I had to do hand calculations for every frequency. It came with a template that you could put over the screen with the limits in reverse fashion, and you could manually go through the frequency spectrum to find suspects, and then do all of the hand calculations to account for transducer affects, cable losses and then use slope/intercept to see what the limit at every frequency of interest was etc. It took days to do a single test - and obviously I'm not claiming much accuracy. This all lead, eventually to a very nice OATS site (except for local ambient) that included an actual heated and air conditioned test house , modern HP equipment with QP adaptors, automated turn table and antenna tower, a small indoor precompliance and troubleshooting chamber, a full 30 meter site, which met the FCC requirements, and was eventually certified - about 10 years later by A2LA. I suspect there are many other "dawn of FCC testing" stories and sites out there. (I had to chase cows from an official site at a test vendor location once) Gary From: Bill Owsley [mailto:[email protected]] Sent: Wednesday, October 17, 2012 9:39 AM To: [email protected] Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits Way back in the old days, so goes the tale as it was told to me, for the FCC,broadcast receivers were determined to have a certain level of sensitivity for reliable reception of the intended broadcast. So Limits were set capriciously and arbitrarily just below that sensitivity level. Measuring distance was determined in a similar fashion, 3 meters being the home environment, and 10 meters being the work or non-home environment. I vaguely recall a 30 meter distance. All this are tales of the dark side when there were only OATS and testing was all day long in the blistering summer sun, or all night while feeding mosquito's. The automotive industry declined to play along and took care of themselves, as did the military, and the airlines, And they do have some near field testing and get to use comfortable test environments like indoors for a large portion. We got so envious of those comfortable conditions, we ginned up a fine story about ambients interfering with our tests, and weather interfering with test time, etc. that we got to build a 3 meter chamber, the first one recognized by the FCC as an alternative to the OATS. ________________________________ From: John Woodgate <[email protected]> To: [email protected] Sent: Wednesday, October 17, 2012 8:45 AM Subject: Re: [PSES] OATS vs FAR Radiated Emissions Limits In message <of583e7385.c0c56cf9-on86257a9a.0040152b-86257a9a.00418...@mmm.com<mailto:of583e7385.c0c56cf9-on86257a9a.0040152b-86257a9a.00418...@mmm.com>>, dated Wed, 17 Oct 2012, [email protected]<mailto:[email protected]> writes: > And has any of this OATS, SAR, FAR, and TEM cell data differences been > correlated to actual interference problems? Is the EMC industry crying "wolf"? The only practicable way to check is to look at the number of complaints of interference, but many countries now don't collect them, and the number of interference cases probably exceeds the number of complaints by a large factor. It is certain that if any manufacturer or industry association heard any alarmist cries, representations would be made for speedy changes. > > Limits and test methods should be based in reality. They should not be > academic exercises. For example, much of the world's products are in the > near-field of each other (cockpits, OR, control rooms, etc.). Why aren't > there near field test procedures? Yes, I know the problems but those are just > excuses. Methods need to be developed (and alas, I'm not smart enough). The problems are not excuses, any more than an inability to develop anti-gravity is an excuse. Ye canna change the laws o'physics, Cap'n! Near-field measurements are horribly non-repeatable and, in almost all cases, cannot be relied on in a regulatory context. -- OOO - Own Opinions Only. See www.jmwa.demon.co.uk<http://www.jmwa.demon.co.uk/> The longer it takes to make a point, the more obtuse it proves to be. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]<mailto:[email protected]>> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]<mailto:[email protected]>> Mike Cantwell <[email protected]<mailto:[email protected]>> For policy questions, send mail to: Jim Bacher: <[email protected]<mailto:[email protected]>> David Heald: <[email protected]<mailto:[email protected]>> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]<mailto:[email protected]>> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]<mailto:[email protected]>> Mike Cantwell <[email protected]<mailto:[email protected]>> For policy questions, send mail to: Jim Bacher <[email protected]<mailto:[email protected]>> David Heald <[email protected]<mailto:[email protected]>> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
