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Why would we want aviation radios to operate in AM? Is it just that we standardized on AM before the 60s, it works well enough, and migrating would require a wo

Why would we want aviation radios to operate in AM? Is it just that we standardized on AM before the 60s, it works well enough, and migrating would require a worldwide effort? Or is there some interesting advantage to AM for planes? Maybe superior range?

K7PJP 2y ago
AM doesn’t exhibit the capture effect, so you can hear multiple transmissions at once. https://en.m.wikipedia.org/wiki/Capture_effect https://en.m.wikipedia.org/wiki/Capture_effect
- gwbas1c 2y ago
  > The ability to receive multiple signals simultaneously is in some cases considered beneficial and is one reason that the aviation industry, and others, have chosen to use AM rather than FM for communications.
- retrac 2y ago
  It's also possible to detect the presence of an AM transmission (if not necessarily decode) at a much lower received power than with FM. With FM there's little in the way of graceful degradation. As signal strength falls it can go from full fidelity to silence with very little transition.
- dumbo-octopus 2y ago
  This makes AM radio more fun to listen to, as you get knobs for both your radio and your mental “tuner”. At night, you can pretty consistently hear 3+ broadcasts at once, and listen to them all simultaneously a la overhearing many conversations at once in a busy room.
nsxwolf 2y ago
One reason is that AM signals blend together whereas with FM the stronger signal will tend to "capture" the others and be the only one heard. This matters a lot in busy airspace where multiple people may be talking simultaneously.
kmbfjr 2y ago
Audio loudness is correlated to signal strength, so it is clear a very strong signal is from a source that is close, important to know in aircraft. Two transmissions will produce a heterodyne (whine) due to the offset of carrier frequencies, so you hear both and the tone to know there are two transmitters. On FM, you’d hear only one, the strongest who captured your receiver.
mlyle 2y ago
To answer you in a different way: Today we would build these systems with FM or digital voice. But there are enough advantages we've gotten used to (graceful degradation, better handling of doubling, etc...) that it's not really unequivocally a win to switch to FM. There would be downsides that anger existing users, and it would be expensive, and the net benefit would be relatively small.
upofadown 2y ago
>Is it just that we standardized on AM before the 60s This one mostly. As otherwise mentioned, there is some advantage in the case of multiple transmissions at once for AM, but in practice both are often entirely garbled. The loud tone caused by any slight difference in transmit frequencies is the big indicator that something has gone wrong. If one signal is much stronger than the other then that tone will be very faint and will likely be missed. So AM has something like FM capture effect in that case. AM has allowed for very narrow channels on the aviation band. In Europe mostly, the channels are only 8.33 kHz apart. Typical narrow band FM voice communication requires 12.5 kHz. Since the end points in aviation are usually within line of sight of one another, radio communication is very easy. Any modulation method would work well. Of course in practice this means that the equipment is allowed to degrade that much more before anyone gets around to fixing it...

K7PJP 2y ago

AM doesn’t exhibit the capture effect, so you can hear multiple transmissions at once. https://en.m.wikipedia.org/wiki/Capture_effect https://en.m.wikipedia.org/wiki/Capture_effect

gwbas1c 2y ago
> The ability to receive multiple signals simultaneously is in some cases considered beneficial and is one reason that the aviation industry, and others, have chosen to use AM rather than FM for communications.
retrac 2y ago
It's also possible to detect the presence of an AM transmission (if not necessarily decode) at a much lower received power than with FM. With FM there's little in the way of graceful degradation. As signal strength falls it can go from full fidelity to silence with very little transition.
dumbo-octopus 2y ago
This makes AM radio more fun to listen to, as you get knobs for both your radio and your mental “tuner”. At night, you can pretty consistently hear 3+ broadcasts at once, and listen to them all simultaneously a la overhearing many conversations at once in a busy room.

nsxwolf 2y ago

One reason is that AM signals blend together whereas with FM the stronger signal will tend to "capture" the others and be the only one heard. This matters a lot in busy airspace where multiple people may be talking simultaneously.

kmbfjr 2y ago

Audio loudness is correlated to signal strength, so it is clear a very strong signal is from a source that is close, important to know in aircraft. Two transmissions will produce a heterodyne (whine) due to the offset of carrier frequencies, so you hear both and the tone to know there are two transmitters. On FM, you’d hear only one, the strongest who captured your receiver.

mlyle 2y ago

To answer you in a different way: Today we would build these systems with FM or digital voice. But there are enough advantages we've gotten used to (graceful degradation, better handling of doubling, etc...) that it's not really unequivocally a win to switch to FM. There would be downsides that anger existing users, and it would be expensive, and the net benefit would be relatively small.

upofadown 2y ago

>Is it just that we standardized on AM before the 60s This one mostly. As otherwise mentioned, there is some advantage in the case of multiple transmissions at once for AM, but in practice both are often entirely garbled. The loud tone caused by any slight difference in transmit frequencies is the big indicator that something has gone wrong. If one signal is much stronger than the other then that tone will be very faint and will likely be missed. So AM has something like FM capture effect in that case. AM has allowed for very narrow channels on the aviation band. In Europe mostly, the channels are only 8.33 kHz apart. Typical narrow band FM voice communication requires 12.5 kHz. Since the end points in aviation are usually within line of sight of one another, radio communication is very easy. Any modulation method would work well. Of course in practice this means that the equipment is allowed to degrade that much more before anyone gets around to fixing it...