FM radio reception on shortwave bands

[jr_tech]

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.

jr

[Tim R.]

Originally Posted by jr_tech

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.

jr

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!


-Tim

[old_coot88]

Not to be picky, but it was more likely harmonics of the radio's local oscillator (LO) that was heterodyning (beating) with the FM stations.

[Tim R.]

Originally Posted by old_coot88

Not to be picky, but it was more likely harmonics of the radio's local oscillator (LO) that was heterodyning (beating) with the FM stations.

Just thinking about what you said, and it occurred to me that it would be highly unusual for any (licensed) radio station to radiate harmonics, let alone the entire FM band. Thanks for correcting my thought process - sometimes it takes a second viewpoint to make things clear!


-Tim

[Electronic M]

Originally Posted by old_coot88

Not to be picky, but it was more likely harmonics of the radio's local oscillator (LO) that was heterodyning (beating) with the FM stations.

A lot of the cheapie SW radios on the market have sloppy enough front ends that you can get both FM and AM broadcast band images in the SW bands. I've seen both in some models.

Best to get a mid to high end offering from a well respected brand if you wish to do serious SW listening.

[jr_tech]

Many older single conversion, single rf tuned radios are quite poor at rejecting "image" signals. They usually do not trap out higher frequency signals (there was no need in 1938 or so because the modern TV and FM bands did not exist).
These sets were "good enough" at the time, but that time is long gone.

Agree, if you want to do serious SW listening, get a decent modern radio.

jr