IMD can be caused by sources external to your antenna. A neighbor installed a cheap chain link fence, sections rusted within a year. The two strongest local stations mixed and produced spurs up to at least 10 times their fundamental.
The fence looked so ugly after a winter they tried painting it and it looked even uglier, lipstick on a pig ugly.
They took it down and installed a fancy wood privacy fence and my low level, wide band grunge, decreased by about 10dB.
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Owen Duffy has an article on intermod and colorbond. Colorbond is an Australian product, paint baked on sheet metal.
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https://owenduffy.net/blog/?p=12086
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He also has an article on "how much power does it take to produce a S9 signal 1km away."
https://www.owenduffy.net/bpl/S9QQ.htm
3 microwatts EIRP.
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Very weak RF mixing can produce some interesting signals.
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When tracking down external intermod, one must use at least two receivers with different mixer/IF scheme to insure any signal isn't generated in the receiver.
This topic really deserves an IO group of it's own.
