Modify an AM/FM radio to an airband receiver:what if any, changes to the demodulati

Hello,

It seems to be possible to modify a cheap AM/FM radio to receive air traffic control (ATC) communication just above the regular FM band. One range used for ATC is at 108...137 MHz.

Some quick projects seem to de-tune the local oscillator that mixes the incoming signal down to the intermediate frequency (IF). To make the frequency of the local oscillator higher, an air coil is widened (reduced inductance) and a variable capacitor is turned such that its value becomes lower.

Examples can be watched here https://www.youtube.com/watch?v=zDN9fZDGBBg or here https://www.youtube.com/watch?v=5kvxjNeWMxs.

So far, I get everything. However, when looking at the data sheet in TDA1083 - TEMIC - IC Chips - Kynix Semiconductor Hong Kong Limited.of one of the ICs commonly used in such radios, and turn to page 6 for the typical application circuit, it seems that it should be a good idea to modify the demodulator as well, because the ATC signal is not broadcast in FM, but in AM.

Do the little hobby projects still work just because they use a very strong signal close to an airport and somehow, something will get through the demodulator anyway, or does it really not matter if the circuit is not optimized for AM instead of FM in the VFR range?

Edit:

I just read on a German forum that the trick has to do with using the ratio detector in a way it was not designed for. Seems like this way, a device designed for FM-demodulation will work for AM. I would be grateful if someone could explain how this works.

Best regards!

Receiving an AM signal on an FM receiver is called "Slope demodulation". With a scope and service monitor I could probably show you what is going on, not so easily explained by an old radio tech.
If you are doing the mod just for grins and not expecting to use the radio for serious listening, it might be an interesting project. For serious listening it will be a poor excuse for a radio. You will probably hear many overlapping transmissions and have a very hard time differentiating between various transmitters. The bandwidth of the FM rx is 20KHz wide, the bandwidth of an airband AM tx is around 3KHz (don't hollar at me about AM bandwidth, just using the easiest explanation I can think of ;-). The front end of the FM radio will look like a barn door with a flock of sheep coming through. Airband transmitters are in the range of 5 to 15W. FM transmitters are generally in the megawatt range don't expect to hear very distant signals at all.
I can see a use for this mod if you live in a country or area that is paranoid about listening to airband signals and happen to be an aircraft fan. If you are sitting outside the airport boundary listening to a "transistor radio" the security probably won't give you a second look as opposed to having a scanner or dedicated airband radio.

Have fun with your project.

Bruce

The band width of an FM stereo broadcast is roughly 150KHz wide and AM aircraft is closer to 6KHz wide. You can easily slope detect an AM signal on a narrow band FM receiver designed for a 15KHz or 20KHz wide signal as described below, but AM slope detection on a wide band FM broadcast radio will not work very well.

You could possibly retune the LO portion of the FM broadcast radio to get the receiver in the AM aircraft band but there are secondary parts of the tuning capacitor or circuit that also tune the radio's front end so you have to diddle that for optimum reception. Then you could possibly route the retuned front end and IF to the AM demodulator in the AM/FM radio and you would have very good AM quality aircraft reception if the circuits were in your favor.

Then you would want a squelch circuit, which you would have to build and implement from scratch.
prcguy

n0nhp said:

Receiving an AM signal on an FM receiver is called "Slope demodulation". With a scope and service monitor I could probably show you what is going on, not so easily explained by an old radio tech.
If you are doing the mod just for grins and not expecting to use the radio for serious listening, it might be an interesting project. For serious listening it will be a poor excuse for a radio. You will probably hear many overlapping transmissions and have a very hard time differentiating between various transmitters. The bandwidth of the FM rx is 20KHz wide, the bandwidth of an airband AM tx is around 3KHz (don't hollar at me about AM bandwidth, just using the easiest explanation I can think of ;-). The front end of the FM radio will look like a barn door with a flock of sheep coming through. Airband transmitters are in the range of 5 to 15W. FM transmitters are generally in the megawatt range don't expect to hear very distant signals at all.
I can see a use for this mod if you live in a country or area that is paranoid about listening to airband signals and happen to be an aircraft fan. If you are sitting outside the airport boundary listening to a "transistor radio" the security probably won't give you a second look as opposed to having a scanner or dedicated airband radio.

Have fun with your project.

Bruce

Click to expand...

Since it's an AM/FM radio, it already has AM demodulation capability. You just need to force that mode while tuned to airband frequencies.

Easier said than done in most cases. Your typical AM/FM receiver has its AM demodulator located within the AM receiver circuit and the FM demodulator located within its FM receiver circuit. To splice one demod circuit into the other receiver may not be all that easy.


To me the whole project just seems like too much work for the reward. Get an air band receiver and be done with it.

Yes, you would have to tap the FM receiver side at the 10.7MHz IF but before any limiter circuits. Then the AM detector that usually operates at 455KHz would have to work at 10.7MHz. It would probably be easier to cobble together a tunable downconverter from VHF to the AM broadcast band.
prcguy

ElroyJetson said:

Easier said than done in most cases. Your typical AM/FM receiver has its AM demodulator located within the AM receiver circuit and the FM demodulator located within its FM receiver circuit. To splice one demod circuit into the other receiver may not be all that easy.


To me the whole project just seems like too much work for the reward. Get an air band receiver and be done with it.

Click to expand...

you cannot slope demodulate an AM signal with an FM receiver. It works the other way around, An FM signal may be demodulated with an AM receiver if tuned carefully.