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Why does aviation use AM radio instead of superior FM?

Posted: Thu Mar 16, 2017 8:43 pm
by FTMCPIUS
At least as far as audio quality is concerned. Probably covered previously but I am a relatively recent subscriber.

Thanks for your insight.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Thu Mar 16, 2017 9:34 pm
by oOfredOo
Because a stronger FM signal completely supresses a weaker signal. It's called the capture effect. With AM you just receive both.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Thu Mar 16, 2017 9:48 pm
by NameOmitted
FM and AM use different parts of the frequency spectrum with very different radiation properties. FM is line of site. AM shoots through obstacles. You can get a LOT further with AM than you can with FM.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Thu Mar 16, 2017 10:09 pm
by timz
Is that true? FM is line-of-sight and AM at the same frequency isn't?

Re: Why does aviation use AM radio instead of superior FM?

Posted: Thu Mar 16, 2017 10:14 pm
by NameOmitted
If you put AM on the same frequency, the actual radio waves would respond the same way. However, the AM (Amplitude Modulation) would still require less bandwidth, and could therefore be pushed further on the same frequency than FM (Frequency Modulation). So, the line of site issue would not be the same, but you can still punch AM a lot further with the same power, since you are using less bandwidth.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 1:01 am
by Starlionblue
NameOmitted wrote:
FM and AM use different parts of the frequency spectrum with very different radiation properties. FM is line of site. AM shoots through obstacles. You can get a LOT further with AM than you can with FM.


No. The line of sight issue doesn't stem from modulation. It is a byproduct of wavelength.

You're thinking of FM vs AM broadcast radio. FM broadcast radio uses frequencies between 87.5MHz and 108MHz. AM broadcast radio uses different bands depending on wavelength. 148.5 kHz to 283.5 kHz, 530 kHz to 1710 kHz and 3 MHz to 30 MHz. The "Air band" reserved for civil aviation is 108 to 137 MHz.

The reason AM radio, FM radio, Air Band and so forth are separated is not a technical limitation. It is a regulatory limitation to prevent transmitters from interfering with each other. There's nothing technical stopping you from transmitting AM or FM in any of those frequencies as long as you have the equipment for it.

Back to line of sight. The higher the frequency, the more you're line of sight limited. Which is why VHF works fine with line of sight, but we have to use HF for oceanic sectors.

timz wrote:
Is that true? FM is line-of-sight and AM at the same frequency isn't?


No.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 10:12 am
by flyingturtle
Another reason is audibility. While FM has a better audio quality, human speech can still be understood when AM reception is bad.

And yet another reason is that AM transmitters and receivers are pretty easy to build.

And as you know, in aviation everything has to follow certain standards - and it isn't easy to change them.


David

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 10:51 am
by gloom
Actually lots of false items in the topic. I think someone should straighten that out.

First - as already said, range depends on wave length. The shorter wave, the more line-in-sight range. Long wave (low frequency) is reflected in stratosphere, and therefore is able to go for thousands kilometers, depending of course on transmitter power and receiver antenna gain).
Second - AM vs FM. Amplitude modulation vs Frequency Modulation. Frequency Modulation is less affected by other electromagnetic waves. However, it is wide spectrum (since it changes frequency around base frequency), and as such it is performing better where audio wide spectrum is required. Voice does not, telephony limit is 4kHz for that reason (you can easily check how bad a music via phone is). AM is performing way better on long range, since singal strength loss is vital to FM (you start to get complete drops), while for FM it is just a voice strength reduction (usually it means "still readable", when FM would have like 50-80% dropped).
Third - getting AM signal out is simpler in terms of equipment, than FM. And also less dependant on outside conditions (when generating/decoding FM singal, any other signals may influence by its harmonic).

I think someone could be more elaborate and precise, it's already over 20 years since my studies :)

Cheers,
Adam

https://en.wikipedia.org/wiki/Frequency ... -en-de.gif

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 5:15 pm
by WIederling
oOfredOo wrote:
Because a stronger FM signal completely supresses a weaker signal. It's called the capture effect. With AM you just receive both.


This.

Traffic collisions are audible using AM ( well trained/adept listeners can even make out both messages.)
With FM the receiver decodes the strongest signal only.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 5:48 pm
by strfyr51
NameOmitted wrote:
FM and AM use different parts of the frequency spectrum with very different radiation properties. FM is line of site. AM shoots through obstacles. You can get a LOT further with AM than you can with FM.

Actually? All the Navigation VORs are on the FM band and all the Non-directional Beacons are AM

Re: Why does aviation use AM radio instead of superior FM?

Posted: Fri Mar 17, 2017 10:23 pm
by WIederling
strfyr51 wrote:
NameOmitted wrote:
FM and AM use different parts of the frequency spectrum with very different radiation properties. FM is line of site. AM shoots through obstacles. You can get a LOT further with AM than you can with FM.

Actually? All the Navigation VORs are on the FM band and all the Non-directional Beacons are AM


VOR needs the high frequencies to achieve the desired narrow Antenna pattern.

NDB were "borrowed" from shipping and were introduced much earlier.
Using LF and MF frequencies long before technology pushed into the HF ranges.

Re: Why does aviation use AM radio instead of superior FM?

Posted: Sun Mar 19, 2017 8:07 am
by CXfirst
strfyr51 wrote:
NameOmitted wrote:
FM and AM use different parts of the frequency spectrum with very different radiation properties. FM is line of site. AM shoots through obstacles. You can get a LOT further with AM than you can with FM.

Actually? All the Navigation VORs are on the FM band and all the Non-directional Beacons are AM


VORs have two signals, the reference signal is FM and the variable is AM. There is no "FM band", we have VHF bands, MF bands, HF bands and many more, but they could all be modulated by AM or FM (or pulse modulation), with each having advantages and disadvantages.

It seems a lot of users here are confusing modulation with frequency ranges. It doesn't help that in car radios they are described as AM and FM, but displayed as frequencies.

-CXfirst

Re: Why does aviation use AM radio instead of superior FM?

Posted: Sun Mar 19, 2017 1:28 pm
by planewasted
oOfredOo wrote:
Because a stronger FM signal completely supresses a weaker signal. It's called the capture effect. With AM you just receive both.

This is the reason I learnt when I studied radio electronics. Same reason why radio stations prefer FM. You will not hear other stations that are close in frequency or far away with weak reception.