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## RE: [RFI] EMC active loop antennas

 To: RE: [RFI] EMC active loop antennas "Hare,Ed, W1RFI" Thu, 12 Feb 2004 10:28:36 -0500
 ```> I would also use a rod antenna to measure the incident > E-field, and not rely on the FCC's assumption of free-space conditions. The FCC's assumption of free-space conditions is based on standard methodology. The methods in ANSI C63.4 are similar to those in CISPR-22, etc. Although the FCC rules are expressed in terms of electric field: on HF, 30 uV/m 30 meters from the source (+29.54 dBuV/m), most measurement antennas on HF are magnetic loops. The standard test methods permit H field measurements to be made and extrapolated to an equivalent E field reading. This does presume that the measurement is being made in the far-field region of the radiator, where E/H = 377 ohms. When I first learned of this test method, I was sceptical, because most HF measurements are made in the near-field regions of the radiating source. I know that E/H is probably not 377 ohms in the near-field regions of a radiating element. And, for any particular set of points, NEC/EZNEC near-field modeling shows quite well that the E and H fields generally vary up and down in opposite ways from each other in the near-field region. So, if a single measurement is made, one can be off by 10 or more dB in doing the calculation. But a closer look shows something more significant. Although at any particular point, the E and H fields are not E/H=377 ohms, if one finds the point of maximum E field radiation near the radiating element and the nearby point of maximum H field, Emax/Hmax = 377 ohms, within a few dB or so in all the cases I have looked at using EZNEC. Seeing as the rules do seem to imply that the maximum point of emissions should be found, finding maximum H will give a suprisingly accurate indication of what maximum E will be. For an example of an EZNEC near-field graph showing E and H at distances of 3 meters and 30 meters along an axis parallel to a long conductor, see the file I snipped from one of ARRL's BPL filings: http://www.arrl.org/~ehare/temp/fields.doc Ed Hare, W1RFI ARRL Lab 225 Main St Newington, CT 06111 Tel: 860-594-0318 Internet: w1rfi@arrl.org Web: http://www.arrl.org/tis Member: IEEE C63 - Accredited Standards Committee on Electromagnetic Compatibility Chairman: C63 SC-5, Immunity Chairman: C63 ad hoc PLC/BPL Working Group Member: SAE EMC/EMR committees Member: ICES/IEEE SCC-28 International Committee on Electromagnetic Safety Member/staff: ARRL, The National Association for Amateur Radio Member/Board of Directors: QRP Amateur Radio Club International > -----Original Message----- > From: Cortland Richmond [mailto:ka5s@earthlink.net] > Sent: Wednesday, February 11, 2004 7:29 PM > To: Hare,Ed, W1RFI; rfi@contesting.com > Subject: RE: [RFI] EMC active loop antennas > > > At a former employer we used the EMCO 6502 with integral amplifier and > rechargeable battery pack. It worked quite well. > See > http://www.emctest.com/productpage.cfm?model=6502&producttype= > Antennas > > I don't remember ever running the battery down in a day's testing. Of > course, we didn't run it all day long. > > I have an EMCO 6511 (1 KHz - 5 MHz) > http://www.emctest.com/productpage.cfm?model=6511&producttype= > Antennas , > an old ALR-25M for both of which I have the only the "usual" antenna > factors, and an even older Empire LP-105 for which I have not > even general > antenna factors. The 6511 is not bandswitching, the ALR-25 > is for its 10 > kc - 30 mc range, and the LP-105 from .15 -30 mc. > Bandswitching networks > complicate antenna factors. > > The signal pickup of a loop antenna depends on its area and > the incident > H-field, as you know, and the calibration charts usually add a 51.5 dB > free-space factor for equivalent E-field. This can be seen in > the curves > for passive loops such as the EMCO 6511 above and the EMCO 6512 at > http://www.emctest.com/productpage.cfm?model=6512&producttype=Antennas > > A loop antenna properly built to the same dimensions and > number of turns > will necessarily have substantially the same antenna factors. > This suggests > that we might build our own and get reasonably accurate readings even > without lab calibration. > > I would also use a rod antenna to measure the incident > E-field, and not > rely on the FCC's assumption of free-space conditions. > > Cortland > > > > [Original Message] > > From: Hare,Ed, W1RFI > > To: > > Date: 2/11/2004 1:02:55 PM > > Subject: [RFI] EMC active loop antennas > > > > Do any of you have experience with any particular active HF > loop antennas > calibrated for EMC measurements? If so, if you have any > recommendations for > an economical choice, please let me know on this list or by > private email. > I want something reasonably small, but sensitive. > > > > Ed Hare, W1RFI > > > > _______________________________________________ RFI mailing list RFI@contesting.com http://lists.contesting.com/mailman/listinfo/rfi ```
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