Annoying FM radio interference on ground freq's

dwalt

dwalt

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DWalt
For some reason, whenever we're on a ground frequency, we're picking up staticy, broken, annoying FM radio interference.

It happens on both radios (COM1 - GNS430, COM2 - SL-30).

Any insights?
 
Sounds like a low power illegal or broken station. Report to the FCC. Maybe CAP can help find it if you hear it on 121.5. That might actually be a useful urban DF exercise, even if it's not the usual ELT search.

Badly designed transmitters can bleed power into other wavelengths.

If it really is FM, it's not an engine and it's not a standard aviation radio (though some of the emergency services radios are FM). I don't know how you can distinguish that with an AM VHF radio, though.
 
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MAKG1 said:
Sounds like a low power illegal or broken station. Report to the FCC. Maybe CAP can help find it if you hear it on 121.5. That might actually be a useful urban DF exercise, even if it's not the usual ELT search.

Badly designed transmitters can bleed power into other wavelengths.

If it really is FM, it's not an engine and it's not a standard aviation radio (though some of the emergency services radios are FM). I don't know how you can distinguish that with an AM VHF radio, though.
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Agreed....

The OP is hearing a Frequency modulated signal on a Amplitude modulated radio....:confused:...:confused:.
 
If you are close to a high power transmitter, it is not uncommon to have it break through on an aviation frequency. KHWO has this problem.
 
N801BH said:
Agreed....

The OP is hearing a Frequency modulated signal on a Amplitude modulated radio....:confused:...:confused:.
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AM receivers can demodulate FM albeit poorly. One possibility is called "slope detection". In simple terms this happens when the receiver center frequency is offset from the FM carrier. As the frequency shifts towards the receiver's center frequency the amplitude reaching the detector goes up.

And FWIW the problem could be the receiver. It's not unusual for a receiver to become overloaded by a strong out of band signal.
 
creweite said:
If you are close to a high power transmitter, it is not uncommon to have it break through on an aviation frequency. KHWO has this problem.
Click to expand...

I have experienced this with an AWOS at a certain airport. I can even hear it on my car stereo in the parking lot.
 
The engine is in operation when it's happening. It may be AM radio -- not sure really -- I just guessed FM because I heard some recent music and assumed as much
 
If it's intermittent and "staticky" it could also be an intermodulation problem. Two signals can mix on any bimetal or corroded surface which acts as a diode. If you're hearing it on a variety of radios, that would be my guess.

This could be an AM broadcast station mixing with an FM station on a bad fence post or rusty hangar door. Tracking down the problem will start with identifying the source of the music and its broadcast frequency. If you can't make out an ID, listen to ground on a handheld radio while scanning the broadcast bands in your car until you find the match. Then, add and subtract that station's frequency from ground, and you'll find the first two frequencies to check for the other transmitter.

If it's intermod, the mix is usually happening at a location where both signals are strong. If it's near the airport, you're usually looking for two transmitters near the airport. But it could also be a mix happening miles away, with the weak mix byproduct having enough range to reach the airport.

If you find the match, don't get members of your EAA chapter to head out with torches and pitchforks to the broadcasters involved. Intermod is not usually a defect in anyone's transmitter. You're more likely to be looking for poorly-maintained metal objects near the airport.
 
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