I successfully use a home brew 0.5-meter diameter shielded and untuned loop
for DFing and RFI sleuthing. I couple that through a broadband transformer
to a receiver. I let the receiver do the frequency selectivity. The small
untuned loop is far simplier to build and much easier to use. The
perpendicular to the plane of the loop null is extremely sharp.
Dave - W ØLEV
On Tue, Jul 20, 2021 at 11:23 AM Jim Brown <jim@audiosystemsgroup.com>
wrote:
> The typical home in the US has dozens of electronic noise sources,
> mostly switch-mode power supplies (SMPS), but also devices that have
> microprocessors built in -- everything from refrigerators to home
> entertainment products to computers to battery chargers. Our antennas
> hear those sources and those of our neighbors. The clearly defined peaks
> and straight verticals (stable frequency) tell us that it's running on a
> clock, so most likely something with a microprocessor.
>
> RFI, however, is like peeling an onion -- eliminate the strongest noise
> sources, and we'll see many additional weaker ones. This app note ran in
> National Contest Journal several years ago, and the slide deck was for a
> talk at the Visalia DX convention.
> http://k9yc.com/KillingReceiveNoise.pdf
> http://k9yc.com/KillingRXNoiseVisalia.pdf
>
> NA6O has shared his design of a loop for direction finding here.
> http://wb9jps.com/Gary_Johnson/RFI_files/Handheld_DF_Antenna.pdf
>
> And DXEngineering is preparing to sell a much larger one designed by
> WD8DSB. https://www.dxengineering.com/parts/dxe-noiseloop
>
> 73, Jim K9YC
>
>
> On 7/20/2021 9:18 AM, Ramakrishnan Muthukrishnan wrote:
> > I have been experiencing QRM from some unknown source for the past
> > several months, a picture of it is here:
> > <http://rkrishnan.org/files/40m-qrm.png>
> >
> > This happens in every band and is repeats at 20khz. The signal is steady
> > and not wobbly like one sometimes see in SMPS created QRM. When
> > listening to SSB, I turn on the notch filter function on the SDR to get
> > rid of the "carrier" like signal. But that only does a bit.
>
> _______________________________________________
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> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
--
*Dave - WØLEV*
*Just Let Darwin Work*
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