> 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 <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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