Tom has given us some excellent suggestions.
Let me also suggest using a SPECTRUM ANALYZER to identify suspicious
signals. There are industrial sources of tens of kW at 6.78, 13.56 and 27.12
MHz that sometimes escape their shielding. There are high power military and
commercial transmitters in unexpected places, too. Overhead power, CATV and
phone lines can propagate these sources a long distance, adding their own
intermods. Common things like TV preamps and cable line amps occasionally
become low frequency power oscillators.
These problems are not theoretical. I had a 13.56 source make my receiver
deaf on 20M (during Sweepstakes, no less), and have seen a military
transmitter pump enough power into a nearby FM station that it looked like
it had a bad SWR. The owners had spent $$$$ attempting to fix a perfectly
good antenna system. Both of these problems were identified quickly after
looking at the "RF soup" with a spectrum analyzer.
73, Gary
K9AY
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