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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 or here.

So far, I get everything. However, when looking at the data sheet 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.

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In Belgium you're not allowed to listen in on ATC or police, etc communications. I think the law says something like "communications which are not addressed to you". Something like that. Also includes mail. I guess most countries have a law like that. –  stevenvh Jul 10 '11 at 16:54
    
I believe it is the same here in the .de domain. But this is just a "what if" sort of question, not that I would dare to really try, ever. –  zebonaut Jul 10 '11 at 17:16
    
Oh no, of course not! I wouldn't dare thinking you'd do a thing like that! –  stevenvh Jul 10 '11 at 17:21
    
@Stevenvh, I think it varies by state in the US. Just had a number of Belgians visit us in the states, quite a few things different in the realm of what we can do, very interesting. –  Kortuk Jul 12 '11 at 4:21
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3 Answers 3

up vote 1 down vote accepted

All of the cheap conversions I have heard of use slope detection. There are, of course, separate FM and AM RF/detector sections inside every AM/FM radio but this method works well enough without going to the extra complexity that would come from utilizing the AM detector.

For most radios, this "modification" is just retuning the LO slug to a higher frequency.

It is not uncommon to hear aircraft comms on a cheap FM radio if you're near the airport, due to poor image rejection and overload.

It won't be as good as a purpose-designed receiver, but the point is cheap and quick, not perfect, and this is good enough for casual monitoring.

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Using an FM radio to receive AM transmissions depends on the circuitry having very poor AM rejection.

It may work on cheap radios, but don't expect to successfully modify a good-quality radio the same way.

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I think you answered it yourself in the first line of your question:

It seems to be possible to modify a cheap AM/FM radio

It'll be the AM portion of the radio that is being modified, not the FM portion.

By the looks of that chip you mentioned it has both AM and FM demodulators in the same chip, so no need to modify anything outside, other than the reference frequency.

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The oscillator that's modified should be the one that's used to mix the incoming FM signal down to 10.7 MHz, not the one that brings the AM signal to 455 kHz. These are two different ones. What would a combined AM/FM demodulator look like? My books say AM and FM demodulators operate in a quite different way. –  zebonaut Jul 9 '11 at 21:18
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there is a kind of FM demodulation where you feed the IF into a filter such that the IF center lands in a rising slope of the filter; this effectively turns higher frequency into greater amplitude, thus achieving FM demodulation. If your 10.7MHz IF happened to be carrying an AM modulated signal, this same demodulation filter would, I think, let the AM signal through, but would emphasize the higher frequency components of it, i.e., would just have an effect like a 'tone' control. This is supposed to be one of the least complex ways to demod FM so it seems a strong possibility. –  JustJeff Jul 10 '11 at 17:03
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