Agreed Ken. In this case the e-field conversion is irrelevant, and the specified antenna factor is what it is.
From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Monday, February 27, 2017 11:11 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] CORRECTION (wrong page) Passive Loop Emissions [General Use] If we go all the way back to the OP: The customer has requested an extended magnetic field emission test over the range 100kHz to 2MHz with a limit defined in dBpT. The antenna to be used is an EMCO 6512 which has it correction factors provided in dBS/m which the emission software used does not recognise. So is there a conversion factor that enables the right correction factor to be entered or is the conversion factor only used once a result is obtained?? We can see that whether we are in the far field or not doesn't matter at all. If we were using an electric field probe (say a 41" rod) to measure to a limit expressed in dBuA/m or dBpT, then we would absolutely have to make an assumption about the impedance of the wave in order to use a pure electric field measurement to get at magnetic field characterization. But that is not the case: we are using a shielded loop which not only inherently measures the magnetic field, but rejects the electric field. So we have a limit expressed in units of magnetic flux density, which in air is directly proportional to magnetic field (the 2 dB factor discussed previously) and we have a loop antenna factor that gets us from the EMI receiver measured rf potential to the magnetic field impinging on the loop that induced it. Problem solved; case closed. P.S. For any who aren't convinced or don't follow the logic, look at the antenna factors here: http://www.ets-lindgren.com/charts/6512 Both factors are the same except for the 51.5 dB ohm offset, discussed previously and both clearly exhibit the Faraday Law response of a loop to a magnetic field as a function of increasing frequency. One can even derive the loop inductance by looking at where the Faraday Law proportionality to frequency flattens out: that is where the 50 ohm receiver impedance begins to load the inductive output impedance of the loop. It is not the magnetic antenna factor that is in question; it is the electric field antenna factor that makes the assumption of a plane wave. The magnetic field will be measured precisely; the electric field measured using this loop is a guesstimate based on a far-field assumption. Ken Javor Phone: (256) 650-5261 > From: Cortland Richmond <k...@earthlink.net <mailto:k...@earthlink.net> > > Reply-To: <k...@earthlink.net <mailto:k...@earthlink.net> > > Date: Mon, 27 Feb 2017 21:13:53 -0500 > To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > > Subject: Re: [PSES] CORRECTION (wrong page) Passive Loop Emissions [General > Use] > > On 2/27/2017 7:53 PM, Brent DeWitt wrote: >> I think Ken's rational makes sense to me, since the 51.5 is derived from >> 20*log(377). >> >> > Sure, but now we're back to how close we are -- wavelengths -- to the emitter. > 20*log(??) > > Low frequencies can be tricky, and I once had to double-check a test > lab (not yours) results at a vendor, dragging the EUT out to their > parking lot then wheeling a cart with a 6510 loop antenna, battery, AOR > AR5000 receiver and RMS voltmeter [all mine] away to see how fast the > signal dropped with distance compared to 3m. > > A fun time was NOT had, but I probably came across as a mad scientist. > Again. At least no one ran out of a chamber... > > > Cortland Richmond KA5S > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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To post a message to the list, send your e-mail to <emc-p...@ieee.org> 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://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
