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Old 08-18-2017, 08:39 PM
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Question 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
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Old 08-18-2017, 08:49 PM
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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
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Old 08-20-2017, 03:41 PM
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Quote:
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.

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
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Old 08-20-2017, 04:12 PM
old_coot88 old_coot88 is offline
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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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Old 08-20-2017, 04:51 PM
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Quote:
Originally Posted by old_coot88 View Post
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
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Old 08-20-2017, 05:22 PM
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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.
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Old 08-20-2017, 06:05 PM
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At 21.647 mHz if you are in SSB mode?

162.55 - 2x 70.4515 = 21.647
jr
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Old 08-21-2017, 10:33 AM
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Quote:
Originally Posted by old_coot88 View Post
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.
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Old 08-21-2017, 05:39 PM
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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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Old 08-22-2017, 03:56 PM
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Quote:
Originally Posted by dewdude View Post
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.
I live in an urban area, between 10-20 miles from most of the transmitters.

The radio is Philco's first model that received shortwave bands, from 1932. It uses a 450kc IF.


Quote:
Originally Posted by 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.
I think you sum up the phenomenon nicely. The radio is a product of its time and as a result, some quirkiness should be expected.

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
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Old 08-24-2017, 09:40 PM
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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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