FM radio reception on shortwave bands
I noticed a rather odd phenomenon while exploring the upper end of the shortwave band. For the most part that area is quiet, except for a small section from about 17-20mc. Tuning this range brings in a number of faint, garbled radio stations. At first I thought I was listening to the AM band, until I was able to make out someone announcing the station frequency and call sign. By comparing what I was hearing to an FM radio, I verified that I was indeed picking up the FM band.
Any idea what's going on here? How is this even possible? Something malfunctioning perhaps? Thanks, Tim |
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. :scratch2:
jr |
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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 |
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.
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-Tim |
I live about 5 miles from a NOAA station on 162.55mhz and, occasionally...I will get an image of it down around 21mhz.
If you've got really strong signals making in to your receiver...you can get stuff like this. It could be hetrodyning with a local oscillator...if you've got one in the radio running that high. In older radios it's usually 10mhz; my IC-725 transceiver from the late 80's has an IF running in the 70mhz range; but even then I don't often hear broadcast FM imaging. You can also get hetrodyning from adjacent strong signals as well. |
At 21.647 mHz if you are in SSB mode? :scratch2:
162.55 - 2x 70.4515 = 21.647 jr |
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Best to get a mid to high end offering from a well respected brand if you wish to do serious SW listening. |
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 |
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The radio is Philco's first model that received shortwave bands, from 1932. It uses a 450kc IF. Quote:
For what it is, though, the set is an excellent performer. With a 50' antenna shortwave comes alive at night. I can pick up several dozen stations with little difficulty. Right now I have it tuned to WTWW, which actually has some decent programming. Tim |
All my shortwave radios that tune above 23MHz pick up an FM station,
except the fully shielded boatanchors. In my previous location I could pick up second and thirds harmonics of AM broadcast stations. One was 5 kW about 250 yards off the end of my antenna, the other was 10 kW about 1/4 mile away. These were verified by preselectors. |
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