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RE: [RFI] EMC active loop antennas

To: <rfi@contesting.com>
Subject: RE: [RFI] EMC active loop antennas
From: "Hare,Ed, W1RFI" <w1rfi@arrl.org>
Date: Thu, 12 Feb 2004 10:28:36 -0500
List-post: <mailto:rfi@contesting.com>
> 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. 
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:


Ed Hare, W1RFI
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 
      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 <w1rfi@arrl.org>
> > To: <rfi@contesting.com>
> > 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
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