All the various standards specify that a metal ground plane be used for an OATS. This is to alleviate the problem with variations in the conductivity of the ground, due to water table variations at any one site or due to different soil and base materials among sites. If the level of the water table is affecting your site characteristics, this suggests that the metal ground plane is too small.
Regarding snow accumulation on top of a metal ground plane, this will also affect the characteristics of the site. Measurements at any one particular turntable azimuth and antenna height will show great deviations around a null; scanning the antenna height removes this great uncertainty by measuring the peak. The real test of the effect of snow accumulation is to perform two NSA measurements, one in dry weather and one with the maximum snow coverage, then compare these results. The height at which the peak occurs may change (and indeed ought to correlate with the change in height of the reflecting plane due to snow, taking into account the conductivity and dielectric constant of the snow compared to the conductivity of the metal plane), but as long as the magnitude falls within the +/- 4 dB tolerance, the site is okay. Mike Heckrotte Wan Juang Foo wrote: > > Dear All, > Does anyone know how to measure the level of the water table around a > OFTS/OATS? I believe the answer is in the radiated emission measurements. > > This thread made me to recall my experiences with snow in the North of > England. I have found out that ( if the snow does not start to accumulate > on the top of the enclosure,) the presence of snow on the ground is akin > to a shifting ground plane (variable 'ground water table' levels) which > affects the 'null' points. So we have to be careful to interpret results > if measurements are done at say some restrictive heights. On the whole I > have measured the deviation from the original radiated emission > measurements (when the ground was not soaking wet and without snow) and > compared the results of the radiated emission measurements at certain > frequencies (especially around the nulls ) in my short study and have found > the difference to be very substantial! The maximum difference between > sample means are about 23 dB around the frequencies where at one time was > the nulls (found during fair weather). Having accounted for the effect of > temperature changes on the EUT the conclusion was the shifting ground water > table, which was not readily measurable as snow pile up on the grass and it > became more complicated as they starts to melt the next day. The peak > radiated emissions outside the nulls are relatively stable. > > I was using a smallish 'outhouse-wardrobe sized' enclosure to protect the > EUT. BTW, the OFTS (aka OATS) site is a certified site. :-) Of course > the measurements for the certification was done during fair weather! > > Tim Foo > > "Darren Pearson" <dar...@genesysibs.com> > > 12/14/01 06:01 PM Subject:Re: Enclosed OATS facilities > Hi > On the subject of OATS, I need to get one set up in the UK. > Can any one recommend a company that will install an compliant OATS in the > UK? > > Regards Darren > > "Ehler, Kyle" <keh...@lsil.com> > > 12/14/01 05:53 AM > I have the 'dome' (well, my employer does). Actually, it is an 8 meter > dia. radome, white fiberglass throughout. With door, HVAC and rotating > floor that serves as ground plane. > We pipe in fibre optics for PC host to EUT control from a receiver shelter > located 50 meters perpendicular to the elipse. It is part of our $5M > facility. > > It looks like a giant golfball sitting on a large, grounded, concrete pad. > -not the sort of place to be in a raging electrical storm... > > If we could simplify the RE emissions data collection, it would truly be a > golden opportunity. Worthy of the expense to upgrade. Would this serve as > a really large, spherical GTEM or more like a spherical magnetic field > antenna, with the EUT on the INSIDE? > > Food for patents... > > geor...@lexmark.com > > 12/13/01 05:39 AM > Chris, > I like your innovative thinking! When I once managed our acoustics lab, we > had both a semi-anechoic chamber and a reverberation chamber. The SAC was > similar to those for EMC, in that the walls were designed to absorb all > sound waves striking them, so that sound pressure at a point near the DUT > could be measured. > > The reverberation chamber sides (6) were to reflect all sound waves > striking them so the total noise energy emitted from the DUT could be > measured. > > This might suggest that your "igloo" EMC chamber not include the initial > absorber layer. With a good conductive inner liner, all EMC energy would > eventually be conducted away by this liner, and measured with a low > impedance meter between the liner and ground. It really does not matter > how many reflections take place, as all wave energy must eventually leave > via the conductive path. > > The problem is how to fully electrically isolate this conductive > "collector" from additional outside EMI. Perhaps a very thick di-electric > (low C) between the collector and an outer conductive layer to absorb > in-coming EMI. > > If you ever build one that works, just allow me to visit you at your new > house in the Bahamas, bought with your profit from the "Igloo Chamber". > > George > > Subject::Enclosed OATS facilities in snow country > > "Jim Conrad" <jc...@shore.net> > > 12/13/01 09:25 PM > We plow the snow away from around the building. Unfortunately the building > is up on a small rise but it does make it easy to get the snow line below > the ground plane. > > Jim > > "Chris Maxwell" <chris.maxw...@nettest.com> > > 12/13/01 04:45 AM > Hmmmm, > > This conductive layer of snow reminds me of a daydream/ thought experiment > that I had for measuring emissions... > > What if you put a DUT inside a chamber that looked like a hemisphere. The > chamber would be hollow (otherwise, how would the DUT get in). The chamber > "skin" would be a sandwich with a thin layer of absorber on the inside and > a good conductor (conductor 1) then a dielectric then another good > conductor (conductor 2) on the outside. > > Why these layers? > > The inner layer would offer just enough attenuation to reduce reflections, > while letting some energy get to the conductor 1 behind it. > > The conductor 1 layer would effectively be a "integrating measurement > antenna" which picks up and integrates all emissions from the DUT. > > The dielectric layer would insulate conductor 1 from conductor 2. (maybe > this layer would need to be RF absorbant as well, not sure). > > The conductor 2 layer would be grounded all the way around and would serve > to block ambients. > > What would happen? > > Would conductor 1 capacitively couple to the DUT such that a simple swept > RF voltage measurement between the DUT and conductor 1 would show the total > interference produced by the DUT? > > Who's with me? Let's go to K-mart and get: > > A large dome tent. > About 50 square yards of tin foil > Some Tokin flexible ferrite stuff ** > A DUT. > An RF voltmeter/spectrum analyzer and a stub cable. ** > > **probably not available at K-mart...maybe Wal-Mart? > > Might make a fun experiment, or maybe give the neighbors the idea that > you're building an escape pod to the mother ship. > > Any immediate pitfalls that can be foreseen by the collective gurus? > > Chris Maxwell | Design Engineer - Optical Division > email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 > 8024 > > NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA > web www.nettest.com | tel +1 315 797 4449 | > > Bill Owsley <ows...@cisco.com> > > 12/13/01 03:09 AM > > The IBM Greenock lab had an OATS with a very steep roof line, sharp A > frame, and the heat to keep folks warm inside kept the lower edges somewhat > free of snow. I don't recall any concerns, because the snow was not on > top. > > - Bill > > geor...@lexmark.com > > 12/13/01 02:06 AM > > Once upon a time, before we were spun off from IBM, and I was the EMC > manager here, I faintly recall that the IBM Boebligen lab in > Germany had an OATS facility. I also faintly recall that snow on the > rooftop did impact the measurements needed. > > Note that OATS structures are normally constructed with non-conductive > materials, e.g. wood, plastic, etc. A layer of snow represents a plane of > conductive material, albeit not a great conductor. > > However, these are memories from the distant past. Surely there are some > still using OATS facilities where winter snow is a problem. > > George > > plaw...@west.net (Patrick Lawler) > > 12/13/01 01:40 AM > I saw some photographs of an enclosed OATS facility in an area subject to > snow. > > How does snow accumulation on the roof affect performance measurements? > Does it affect the NSA figures? > Is the effect significant enough that attempts are made to keep the roof > snow free? Or does the normal attempt at keeping the inside test area warm > enough for people take care of snow build-up? > > > plaw...@west.net > (Patrick Lawler) To: EMC-PSTC > <emc-p...@ieee.org> > Sent by: cc: (bcc: Wan Juang > Foo/ece/staff/npnet) > owner-emc-pstc@majordo Subject: Enclosed OATS > facilities in snow country > mo.ieee.org > > > 12/13/01 01:40 AM > Please respond to > plawler > > > > I saw some photographs of an enclosed OATS facility in an area subject to > snow. > > How does snow accumulation on the roof affect performance measurements? > Does it affect the NSA figures? > Is the effect significant enough that attempts are made to keep the roof > snow free? Or does the normal attempt at keeping the inside test area warm > enough for people take care of snow build-up? > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > 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: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old > messages are imported into the new server. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
