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 can you pick up FM 107.9 on channel 108.000 in most airplanes then? Will AM normally pick up FM stations?
Why can you pick up FM 107.9 on channel 108.000 in most airplanes then? Will AM normally pick up FM stations?
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.
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).
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.
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.).
It also takes about twice the transmit power to cover the same range with FM vs AM.).
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.
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.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 expensive 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.
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.
Not necessarily so! If they are perfectly in sync but 180 out you get mucho annoying noise.If they are perfectly in sync., the sound goes away because the difference between the two frequencies is zero.
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.
I think that's pretty much what I said. Did you disagree with anything in the portion of my post that you quoted?
Strength of the squeal vs tone or frequency of the squeal
Now I know why I didn't become an EE. ;-)
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.
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
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 -> 0.04 meters/sec) gives a shift of 0.2 hz- hardly noticable.
Conclusion- I want to fly Justin's plane and make annoying radio noise.
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.
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?
That's why I said in the old days. When the system was created.No longer true about absolute parts counts. Most FM demods are now on silicon in the chip. Pretty cheap and easy.