Why AM?

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w4rez

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I've been a scanner listener/comm buff for about 17 years and I've often wondered about this: Why are VHF and UHF aircraft comms AM when most everything else that uses these band FM?

I would imagine that the answer would have something to do with AM being less susceptible to electrcal noise or that it works better at the higher speeds but I'd like to hear an "official" or semi-official explanation.
 

jrplmil

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This is not official but I feel that thats all they had or new about when it was started. I did not see my first am/fm radio till the 60's Ah the good old days
 

KR4BD

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One reason AM is used for aircraft communication is because with AM, you can hear two signals on the same frequency - at the same time. With FM, the "capture effect" allows you to ONLY hear the strongest signal reaching your receiver on a particular frequency. If you tune to an AM radio station, you can often hear weaker signals under the stronger one, where with FM, the stronger one blocks out the weaker ones. Don't get confused with FM overloading of strong signals where you might hear two FM signals at the same time on cheaper receivers where signals mix with each other (strong signal overloading).
 
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N_Jay

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KR4BD said:
One reason AM is used for aircraft communication is because with AM, you can hear two signals on the same frequency - at the same time. With FM, the "capture effect" allows you to ONLY hear the strongest signal reaching your receiver on a particular frequency. If you tune to an AM radio station, you can often hear weaker signals under the stronger one, where with FM, the stronger one blocks out the weaker ones. Don't get confused with FM overloading of strong signals where you might hear two FM signals at the same time on cheaper receivers where signals mix with each other (strong signal overloading).
While this sounds good, and is often quoted as the reason, the first answer is correct.

History, plain and simple!:cool:
 

KR4BD

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N_Jay ...

If the answer is "history" as you say, why hasn't the aircraft industry switched over to FM?

I will stick with my answer above. FM's "capture effect" will block out weak signals. If an aircraft is far out over water delivering a weak AM signal to land, it can still be heard UNDER a stronger AM signal on the same frequency. With FM, it would not even be heard under a stronger signal.
 
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N_Jay

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KR4BD said:
N_Jay ...

If the answer is "history" as you say, why hasn't the aircraft industry switched over to FM?

I will stick with my answer above. FM's "capture effect" will block out weak signals. If an aircraft is far out over water delivering a weak AM signal to land, it can still be heard UNDER a stronger AM signal on the same frequency. With FM, it would not even be heard under a stronger signal.
Too many planes, owned by too many individual people and companies registered in too many countries.

There have been upgrades over the years and they have been long, slow and painful.

Stick with your myth all you like.
 

KR4BD

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N_Jay

It is true that AM is older (more historic, in your words), but if FM is so much superior, why haven't the world governments mandated a change to FM? Please don't say it would cost too much. Anyone flying a plane can afford to buy a new radio if it is going to be the standard. The answer remains that AM is safer and can be heard in weak signal situations.

In 2009, the U.S. is forcing everyone to have a digital TV if they want to continue watching over-the-air TV. The date has been announced for the change, giving everyone time to convert. I have not seen any announcements to make FM the "modulation of choice" in the aircraft world due to the reasons I have cited.
 

w4rez

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KR4BD said:
One reason AM is used for aircraft communication is because with AM, you can hear two signals on the same frequency - at the same time. With FM, the "capture effect" allows you to ONLY hear the strongest signal reaching your receiver on a particular frequency. If you tune to an AM radio station, you can often hear weaker signals under the stronger one, where with FM, the stronger one blocks out the weaker ones. Don't get confused with FM overloading of strong signals where you might hear two FM signals at the same time on cheaper receivers where signals mix with each other (strong signal overloading).
After thinking about this, I would agree, especially with regards to nav aids and ILS. I don't think a localizer would work very well if it were FM, would it?
 

kikito

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KR4BD said:
If the answer is "history" as you say, why hasn't the aircraft industry switched over to FM?
The FAA has actually been in the planning process for years now, for switching to digital of all things! It might start happening in the next 6 years BUT like with every other Government deadline, that remains to be seen. Other parts of the world are already using something other than AM (digital included) in conjunction.

But like N_Jay mentioned, it IS a slow and painful process, especially when the industry wants modernization but they want it now and CHEAP too, preferably better if they don't have to spend money. Go figure!

Anyway, do a search for FAA NEXCOM, that'll give you an idea of what's coming "down the road"....

Interesting enough, DVSI, Inc. the provider for the P25 vocoder and others, also seems to be the provider for FAA's future digital vocoder.....
 
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N_Jay

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KR4BD said:
N_Jay

It is true that AM is older (more historic, in your words), but if FM is so much superior, why haven't the world governments mandated a change to FM? Please don't say it would cost too much. Anyone flying a plane can afford to buy a new radio if it is going to be the standard. The answer remains that AM is safer and can be heard in weak signal situations.

In 2009, the U.S. is forcing everyone to have a digital TV if they want to continue watching over-the-air TV. The date has been announced for the change, giving everyone time to convert. I have not seen any announcements to make FM the "modulation of choice" in the aircraft world due to the reasons I have cited.
Find me one reference. Do not include the unsupported Wiki and About articles.

On what date would they make this change WORLDWIDE?

Look at how long it has taken to make the TV change, and that is one country.

Look at the work that went into the, now on-hold, "Next Generation" aviation digital radio system change. Look into the reasons it is on-hold.

Unfortunately we are arguing a negative, so I doubt either of us will find any solid sources.
(There is no reason to document the lack of a change)

Having been in radio for over 30 years, watching the slow and painful progress of technology and the slower and even more painful changes once a technology is adopted and in common use, I stand by my belief.
 

trainman111

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And they expect to have this system operational by December 2009? Something tells me its not going to happen for a while...
 

Lobstah_Guy

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Am

bob1857 said:
So when scanning Milair...220.000 to say 400.000 is the mode AM or FM ?
Mil-air is also AM in the UHF range (225 to 380). The top 20 mhz (380-400) is
now being used for Federal/Military trunked FM systems.
 

bob1857

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Got my 785D just set up, finally. Searching in my area for milair....now I know to use AM mode. Also read to search in 50 steps...what range should I use ? 225 to 380 ?
Bob
 

morfis

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bob1857 said:
Got my 785D just set up, finally. Searching in my area for milair....now I know to use AM mode. Also read to search in 50 steps...what range should I use ? 225 to 380 ?
Bob
Still worth doing up to 400 as mil are still using it. Also suggest you use 25KHz steps.
Don't forget that you also have a/a freqs at VHF as well
 

TinEar

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Military air freqs are assigned in 25 kHz steps in both the VHF and UHF bands in AM mode. A 50 kHz step will miss a lot of freqs. Only on low band VHF, used by aircraft such as the A-10 and various helicopters, will you need FM mode.
 

ka3jjz

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Bob, while searching will get you some hits you might not get otherwise, just to get your feet wet and test your installation, check out your local ARTCC freqs - many of the civil air stuff also has a UHF milair component as well. I would also check your local airport; many times they too will have UHF milair freqs along with your civil air ones you would normally expect.

A little homework would also be in order. Take a look at the DoD FLIPs for your area - altho not 100% accurate - nothing ever is - it's certainly better than blindly searching.
Finally, let us know where you are (city/state will do fine), as well as jumping onto your local state's forum here.

Re the 380-400 Mhz range - Larry Van Horn wrote recently that some 380 mhz freqs are not being converted for trunking. I'd have to dig thru my MT pile to find the article, but it would be worth keeping a spare bank for these as well.

There's just a few of the resources available. With your 785 and a good antenna mounted up with good coax, you should start hearing stuff in no time. 73s Mike
 

kc4jgc

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trainman111 said:
And they expect to have this system operational by December 2009? Something tells me its not going to happen for a while...
I think you're confusing aircraft & television.... Analog TV is to cease in February 2009.
Most full power stations are transmitting digital/HD now.
 
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