Well, I already know the basics of antennas and propagation and OATS measurements, but what I don¹t know is how the vertically polarized electric field from a small source couples a potential to the septum relative to the GTEM enclosure, and how all that relates to a prediction of a radiated field at some far field distance. Somewhere I have Myron Crawford¹s original treatise on this subject, but I can¹t put my hands on it.
Ken Javor Phone: (256) 650-5261 From: "Heckrotte, Michael" <[email protected]> Date: Tue, 13 Aug 2019 16:34:58 +0000 To: Ken Javor <[email protected]>, "[email protected]" <[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/> -- 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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