[RFI] periodic peaks in the spectrum

[Jim Brown]

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.