JustinPinnix
Pre-Flight
I've been playing around with my scanner a bit and have noticed that most other VHF radio services are FM (broadcast, marine, amateur, etc). Does anyone know the history/reason why the aircraft band uses AM?
Why are aircraft radios AM?
- Thread starter JustinPinnix
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Steve
En-Route
http://www.bandradio.com/radio/
Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
SkyHog
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Everything Offends Me
Why can you pick up FM 107.9 on channel 108.000 in most airplanes then? Will AM normally pick up FM stations?
CJones
Final Approach
FM Freq's actually drift a bit. Modern FM receivers will compensate for this by following and retuning to the shifted freq. If you've ever used an old FM receiver, you'll notice that you have to retune very slightly every once in a while.
gismo
Touchdown! Greaser!
That's called "slope demodulation". Because your receiver is tuned near but not on the FM signal's center frequency your radio's AM demodulator produces more output when the FM frequency is higher (closer to your frequency) and less when the FM freq is lower. IOW your audio varies with the frequency of the FM signal which is how you demodulate FM. Since the relationship between the FM frequency and the output of your demodulator isn't linear, it sounds quite distorted.
gismo
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Steve, I think you've got that backwards. If aircraft VHF comm radio was FM then we wouldn't have such problems with "collisions" on the CTAFs for uncontrolled airports. It would also mean that when one pilot stepped on the transmission of another, at least one would be received fairly clearly most of the time.
AFaIK, AM was used because FM was considered "advanced" technology (costs more, weighs more, and is less reliable) when aircraft to ground communication was first deployed. It also takes about twice the transmit power to cover the same range with FM vs AM. I'm pretty sure that if we were starting from scratch today, we'd use FM (or better yet, digital).
Steve
En-Route
Hell, Lance, I just cut and pasted a description from a website which discusses radio applications. It's not my description, take it up with the webmaster if you have a problem with it.
gismo
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LeonardMack
Pre-takeoff checklist
You would still have the same "squeal" effect. The problem with FM could be that if your radio "locked" onto a different signal first, it might stay locked onto that when someone closer might be transmitting.
It does? The distance radio waves travel has much more to do with the frequency range (In aviations case VHF) than the type of modulation used. It is line of site, no matter whether it is AM or FM, and when you are at 5000 feet, there is a lot within the line of site. On the ground, you are least have the curvature of the earth to stop the signals from going to far in flat areas. Amplitude modulation (AM) takes up much less bandwidth than frequency modulation (FM), thus you can fit more "frequencies" in without interfering with a nearby frequency.
A digital spread spectum system is the ideal solution. We could give every airport their own frequency. It would also require every plane replace its radio with new and exspensive technology.
Like you said, the reason we still use AM is because a switchover would require everyone to replace their radio. Eventually, it will probably happen.
gismo
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I have a fair amount of experience using FM two-way radios and have encountered "competition" on the frequency in use quite often. The result can be any of the following:
1> Both transmissions are heard but garbled.
2> The stronger signal gets through with little or no interference.
3> The receiver flips back and forth between the two transmissions.
Notice that a "squeal" isn't one of the possibilities listed. The only way you'd get a hetrodyne (squeal) is if the receiver's demodulator was letting some of the carriers AM through. That's quite plausible with a poor quality receiver but also quite easy to eliminate with a good demodulator.
The "squeal" you hear when to AM transmissions on the same channel occur is due to the slight difference between the two transmitted carrier frequencies. The closer the two AM signals are to each other in strength the stronger the "squeal" will be although it's still quite objectionable when one is considerably weaker than the other.
You are correct that the carrier frequency has more effect on the transmitted range than the modulation, especially when you compare "Shortwave" down to LF with VHF since the latter isn't sufficiently refracted by the atmosphere/earth boundary to "bend" around the earth.
But the reason that FM requires more power than AM is simply that the FM carrier must be generated at 100% power for the entire transmission while the average power for simple AM is 50% of the peak. With SSB (or even DSB) the average power for AM is reduced even further depending on the modulation signal itself.
AFaiK, the frequency spectrum of FM and AM always includes the frequency range of the modulation (around 3Khz for voice). Channel spacing on the FM broadcast band is 100 Khz for several reasons but it's quite possible to reduce that spacing to several Khz for voice transmission.
DSS doesn't necessarily provide more channels than other formats, it's main attractions are resistance to jamming, interference, and eavesdropping. Digital modulations in general provide opportunites for better communication because they make it possible to send redundant information that can be used to reconstruct the original modulation even when parts of the signal are lost.
Perhaps we'll get a better format when the FCC and FAA decide to allow and provide for duplicate formats on the ground and in the air. The actual cost for this shouldn't be a significant obstacle.
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Graueradler
Pattern Altitude
The squeal is the difference between the two carrier frequencies. The closer they are to the same frequency, the lower the tone of the squeal. It is the same effect as the low frequency sound you hear when the two engines on a twin are not perfectly in sync. If they are perfectly in sync., the sound goes away because the difference between the two frequencies is zero.
Skip Miller
Final Approach
Not necessarily so! If they are perfectly in sync but 180 out you get mucho annoying noise.
-Skip
gismo
Touchdown! Greaser!
I think that's pretty much what I said. Did you disagree with anything in the portion of my post that you quoted?
Graueradler
Pattern Altitude
Strength of the squeal vs tone or frequency of the squeal
gismo
Touchdown! Greaser!
Ah, then I wasn't clear enough. I thought the first sentence (The "squeal" you hear when to AM transmissions on the same channel occur is due to the slight difference between the two transmitted carrier frequencies.) covered that but perhaps the second created confusion.
The relative strength of the signals determines the strength/volume of the squeal, not the frequency/tone. Two strong signals = loud squeal, two weak signals = not so loud of a squeal, one weak and one strong = almost no squeal. But the frequency is always just the difference between the carrier frequencies
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Ghery
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Ghery Pettit
I are one. And this whole thing made perfect sense to me. But my wife thinks I'm a nerd, too.
jkaduk
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Because they are simpler and cheaper. Demodulator in AM: a diode and a cap. Demodulator in FM: A lot more parts than that. At least in the old days.
jesse
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The same reason that people still build machines powered by a 486. Why put the time and money into upgrading something that works? (This is why Vista isn't selling).
It'd be pretty hard to suddenly try to force every pilot into new radios for 10AMU or whatever ungodly price they would be.
It'd be pretty hard to suddenly try to force every pilot into new radios for 10AMU or whatever ungodly price they would be.
ScottM
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iBazinga!
No longer true about absolute parts counts. Most FM demods are now on silicon in the chip. Pretty cheap and easy.
Want to hear about how I designed an FM demodulator using a ceramic resonator that was de-Q'ed a bit for a cell phone?
JustinPinnix
Pre-Flight
Does Doppler effect play into the squeals at all? Seems like that could be a reason why two aircraft's carriers are at different frequencies (besides the "cheap oscillator" explaination).
Cap'n Jack
Final Approach
Lets see-
delta f = f*v/c where f=frequency, v= speed of the transmitting object in meters/sec, and c=speed of light (3E8 meters/sec)
Let's assume you are flying an SR-71 at 3000mph-> 1341 meters/sec. Also assume a frequency of 110 MHZ.
This gives you a respectable 492 Hz change in frequency.
Anything I'm likely to fly (100 mph -> 44 meters/sec) gives a shift of ~16 hz- hardly noticable.
Conclusion- I want to fly Justin's plane and make annoying radio noise.
delta f = f*v/c where f=frequency, v= speed of the transmitting object in meters/sec, and c=speed of light (3E8 meters/sec)
Let's assume you are flying an SR-71 at 3000mph-> 1341 meters/sec. Also assume a frequency of 110 MHZ.
This gives you a respectable 492 Hz change in frequency.
Anything I'm likely to fly (100 mph -> 44 meters/sec) gives a shift of ~16 hz- hardly noticable.
Conclusion- I want to fly Justin's plane and make annoying radio noise.
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JustinPinnix
Pre-Flight
Ahh, but you forgot to take the other plane into account. If you and I are both flying towards each other at 100mph, our shift would be a whopping 0.4 Hz
Thanks for crunching the numbers. I had always been curious...
Cap'n Jack
Final Approach
Justin- sorry- I just edited it again since 100 mph is more than 0.4 meters/sec. Our combined frequency shift would be a whopping 32 Hz- just getting in the range of human hearing.
gismo
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gismo
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Yep, FM is about as inexpensive as AM on the receiver side (high quality modulators are a bit tougher to come by cheaply, but what's needed for voice comm isn't a problem). Heck you can get most of a TV in a single chip these days.
jkaduk
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John Kaduk
That's why I said in the old days. When the system was created.