Many, I appreciate the references. In the meantime, I found another reference by Wilson that fits my simple-minded needs:
ANTENNA GAIN-FACTOR EQUIVALENT FOR TEM CELLS Perry F. Wilson National Institute of Standards and Technology 325 Broadway, Boulder, CO 80305, USA If I can¹t get it to work, I will look at the more complex approaches, but so far it appears this one works. By which I mean, I can understand it and it seems to be in agreement with empirical data. Ken Javor Phone: (256) 650-5261 From: Manny Barron <[email protected]> Reply-To: Manny Barron <[email protected]> Date: Wed, 14 Aug 2019 17:07:44 -0700 To: <[email protected]> Subject: Re: [PSES] GTEM cell used for RE measurement Hi Ken, Here's the source for one algorithm used in GTEM radiated emission testing. There may be more but this is what I found. ETS-Lindgren GTEM Manual: http://www.ets-lindgren.com/sites/etsauthor/ProductsManuals/TEM_Devices/3992 50%20REV%20j_MANUAL_GTEM.pdf See page 38 "Radiated Emission Testing - General" for an overview, which describes a 3 position correlation algorithm derived by Wilson, et al. at ABB. Here's one paper that provides the algorithm details: P. Wilson, "On Correlating TEM Cell and OATS Emission Measurements", IEEE Transactions on Electromagnetic Compatibility, Vol 37, No. 1, Feb 1995. Couldn't find this paper as a freebie online, but it's available for download on the IEEExplore.org website (free for IEEE EMC members). After reviewing this algorithm I see it is not a simple transducer function like it is for immunity, but instead it involves significant math computations thereby requiring a computer algorithm. One interesting note is that on page 40 of this same ETS-Lindgren manual it states an MS-DOS utility program is available for download on their website to assist with the calculations (but I've been unable to locate it). NPL/York Measurement Good Practice Guide 65: https://www.npl.co.uk/special-pages/guides/gpg65_gtem This link was previously provided on this listserv thread in an earlier e-mail. See page 16-17 "GTEM to OATS Correlation", which references the following paper for the theoretical details: Angela Nothofer, "Cross-polar coupling in GTEM cells used for radiated emission measurements", PhD thesis, University of York, Department of Electronics, December 2000. Here's a link to this thesis but it requires login credentials to download: http://yorsearch.york.ac.uk/primo_library/libweb/action/display.d o?tabs=requestTab&ct=display&fn=search&doc=44YORK_ALMA_DS21221784140001381&i ndx=1&recIds=44YORK_ALMA_DS21221784140001381&recIdxs=0&elementId=0&renderMod e=poppedOut&displayMode=full&frbrVersion=&frbg=&&vl(138704577UI0)=any&dscnt= 0&scp.scps=scope%3A%2844YORK_ALMA_DS%29%2Cscope%3A%2844YORK_WREO_YORK%29%2Cs cope%3A%2844YORK_ATOM_DS%29%2Cscope%3A%2844YORK_DIGLIB_DS%29&tb=t&vid=44YORK &mode=Basic&srt=rank&tab=tab1&dum=true&vl(freeText0)=angela%20nothofer&dstmp =1565825463456 <http://yorsearch.york.ac.uk/primo_library/libweb/action/display.do?tabs=req uestTab&ct=display&fn=search&doc=44YORK_ALMA_DS21221784140001381&indx=1&recI ds=44YORK_ALMA_DS21221784140001381&recIdxs=0&elementId=0&renderMode=poppedOu t&displayMode=full&frbrVersi> <http://yorsearch.york.ac.uk/primo_library/libweb/action/display.do?tabs=req uestTab&ct=display&fn=search&doc=44YORK_ALMA_DS21221784140001381 &indx=1&recIds=44YORK_ALMA_DS21221784140001381&recIdxs=0&ele mentId=0&renderMode=popp> Lastly, here's another downloadable paper by this same thesis author, which on page 4 references the same P. Wilson paper addressed above (plus also references IEC 61000-4-20): https://pdfs.semanticscholar.org/2e32/99409062dc3c683db97281d09891d7419518.p df In summary, the desired transfer function is not a simple equation but instead is a math intensive computer algorithm. Hope this is helpful. Manny Barron [email protected] On Tue, Aug 13, 2019 at 11:00 AM Ken Javor <[email protected]> wrote: > This isn¹t exactly what I was looking for, which was an algorithm, but it is > empirical data for particular GTEM cell, and it shows raw data in dBuV and the > predicted far field intensity at three meters. The 20 dB/decade increase with > increasing frequency is likely what prompted my analogy to t e difference > between antenna gain transmitting vs. antenna effective aperture receiving. > > > > Ken Javor > Phone: (256) 650-5261 > > > > From: "Heckrotte, Michael" <[email protected]> > Reply-To: "Heckrotte, Michael" <[email protected]> > Date: Tue, 13 Aug 2019 16:34:58 +0000 > To: <[email protected]> > Conversation: [PSES] GTEM cell used for RE measurement > Subject: Re: [PSES] GTEM cell used for RE measurement > > Hi Ken, > > TX and RX equations are reciprocal. The measurements themselves are referenced > to different locations, however we can address this. While the conversion > between antenna gain and antenna factor does involve wavelength this will also > be consistent between TX and RX after taking distance into account. Free-space > path loss also involves wavelength, and is also reciprocal. > > Free-space theory, different distance references can be resolved using simple > equations assuming far-field path loss conditions: > * For emissions we measure the field strength at [received by] the antenna > * For immunity we want to know the field strength at some distance D from the > antenna > > OATS theory: > * Add in the ground plane and associated coupling > > GTEM theory: > * Associated coupling is different than OATS > * The measurement distance is not 3/10/etc. meters so need to develop the > equivalent of Friis equation for free-space path loss > > Finally, in practice we don¹t take the same coupling factors into account when > performing these two measurements: > * For immunity we measure the field strength using a (small) somewhat-ideal > battery-powered isotropic probe with a fiber optic interface thus there are no > conducting cables. Next we put a (larger) product in place of the probe and > the coupling is different due to device size, cables, etc. Then we claim that > the product ³sees² the previously measured field strength but we don¹t > actually make a new field strength measurement with the product and associated > coupling mechanisms in place. Hence, we artificially simplify immunity, though > this isn¹t necessarily bad just as our simulated test field will change due > to device coupling, an actual field in the real-world environment will also > change, just not by the same amount. > * For emissions the measurement is made with all the various device coupling > mechanisms in place. > > Admittedly this doesn¹t solve your task at hand, but perhaps it can shed some > light on where you might make some simplifying assumptions. > > > Best Regards, > Mike > > > From: Ken Javor <[email protected]> > Sent: Tuesday, August 13, 2019 7:27 AM > To: [email protected] > Subject: Re: [PSES] GTEM cell used for RE measurement > > I understand how the GTEM cell works for immunity. That is quite simple, as > you say. It¹s not as simple for emissions. I think an analogy is the equation > for transmitting from an antenna is not reciprocal with receiving. One is > simply a function of gain, the other involves frequency/wavelength. I too > worked with GTEM cells in the past and understood better, and I too did a > search and found the very complex formulae (!) but now I need to be able to > get from a field intensity limit to the coupled potential at the cell coax > connector, and I¹m not finding that, nor a way to get there. > > Ken Javor > Phone: (256) 650-5261 > > > From: "James Pawson (U3C)" <[email protected]> > Reply-To: "James Pawson (U3C)" <[email protected]> > Date: Tue, 13 Aug 2019 07:47:56 +0100 > To: <[email protected]> > Subject: Re: [PSES] GTEM cell used for RE measurement > > Hello Ken, > > Thinking aloud here: would such a factor exist as a generic formula? If I > think about this the other way round, putting 1V of RF into the coax connector > will generate 1V between the septum and ground (or between the septums? > septii? in the case of a differential GTEM like a Laplacell). > > This 1V will be spread over the distance between septum and ground, dictating > the overall field strength. This means cells with different dimensions will > have different factors. This is leaving aside the frequency response / VSWR of > the cell. > > I do recall seeing some formulae for correlation between GTEM and OATS/SAC but > that was a while ago. A quick google on the subject shows a few papers with > lots of big equations. > > I no longer have my old copy of EN 61000-4-20 but there might be some > interesting reference material in there. This standard also notes that many of > its provisions only apply to ³small EUTs² which is anything below a certain > size in relation to the GTEM volume or anything with cables attached (which > are ³under consideration²) > > > More practically, you could generate your own factors using an RF generator > and an isotropic field probe and using reciprocity to flip this around to get > a Volts out field inside factor. > > > Additional: There¹s also this NPL / York EMC guide on ³The Use of GTEM Cells > for EMC Measurements² which has some interesting info in. Its been a long > while since I¹ve read it in any depth though. > https://www.npl.co.uk/special-pages/guides/gpg65_gtem > > > Hope some of this is useful. > All the best > James > > > > James Pawson > EMC Problem Solver > > Unit 3 Compliance > Design for EMC / Pre Compliance / Problem Solving / EMC Testing / Consultancy > / Environmental & Vibration > www.unit3compliance.co.uk <http://www.unit3compliance.co.uk> > <http://www.unit3compliance.co.uk> <http://www.unit3compliance.co.uk/> -- > 07811 139957 > Opening Hours: Tuesday to Friday, 0830 to 1800. Closed Monday. > > > From: Ken Javor <[email protected]> > Sent: 12 August 2019 15:44 > To: [email protected] > Subject: [PSES] GTEM cell used for RE measurement > > Does anyone out there have a simple transducer factor that yields rf potential > at the GTEM cell vertex coaxial connector for a given vertical electric field > component coupling to the cell septum? Purpose is to calculate gain/noise > figure specs for a preamplifier used between spectrum analyzer and GTEM cell > to measure RE. > > Thank you. > > Ken Javor > Phone: (256) 650-5261 > > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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