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Old 08-20-2017, 02:41 PM
Tim R.'s Avatar
Tim R. Tim R. is offline
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Join Date: Mar 2011
Location: Houston
Posts: 92
Originally Posted by jr_tech View Post
I suspect harmonics of the local oscillator are beating with very strong FM signals and converting them to the if passband, even though the antenna coil is not tuned to anywhere close to the FM station frequency.

Fascinating stuff. Thanks for the explanation!

Out of curiosity I did a bit of research into how an FM signal could be received by an AM radio. In the process I learned about slope detection, and also why the audio signal was so poor.

And as it so happens, the 4th harmonics of the FM stations fall into that section of the shortwave band, so what you is say is probably correct - those harmonics are beating with ones from the local oscillator. I was using a long wire antenna as well, so perhaps that helped overcome some of the antenna coil mismatch.

Appreciate your help with solving this mystery. The great thing about this hobby is you never stop learning!

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