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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