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By Bill Calderwood, K1CT  

I would like to offer a few tips on isolation of radio frequency interferences
noises.  I would assume such noise is a local phenomena.

First I would arrange to have my HF receiver to be battery powered.  Assuming
you hear the noise while the radio is battery powered, flip the
master breaker to the house and see if the noise goes away.  If it does that's
a pretty good indication it is coming from your own equipment/appliances.  
Flip all of the tributary circuit breakers and turn the main breaker back on.  
Sequentially turn on each breaker while checking for the return of the QRN.  
Once the QRN returns you've found which circuit contains the likely
offending device, unplug everything on that breaker's circuit and see if the
noise goes away - hopefully it will.  Then plug in your equipment/appliances
sequentially sampling for return of the QRN.  Hopefully you'll be able to
isolate it to a definite offender.  

If the source is not from your house, I recommend seeing if you can hear
the QRN on a portable radio.  If so move around with the radio until you pick
up the signal strongly.  If you can use an AM radio for this you can try DFing
the noise with the ferrite loop internal antenna - beware there are two nulls
which are 180 degrees different.  Use the nulls rather than the peaks since
the nulls are much narrower than the peaks.  Don't even think about using an
FM receiver since FM receivers are essentially immune to this type of noise.  
You might try listening to files at for a clue of your
potential interference cause.  Search the web for RFI location techniques
and solutions......a lot has been written about this.

Some of the common sources of such noise are cable TV, Power Lines,
broadband internet over power line, your neighbor's plasma TV, your
neighbor's computer/display and almost anything with a switching power
supply (including the one powering your radio).  Fluorescent lights, RF lights,
LED lights and neon lights are other possibilities.

Some of the thing I've found that I originally never expected... my
Panasonic telephone answering machine.  When I lived in Washington my
neighbor had this bogus device for magnetically cleansing his water pipes...
it put out a lot of evenly spaced carriers throughout HF.

I hope this has been of some help.  I'd be curious to hear what you find
out from your investigations - let us all learn from what you have learned.

                                                                          73, Bill K1CT