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Author Topic: Why AM for Aviation?  (Read 1794 times)
KC0RDG
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« on: February 08, 2006, 02:06:31 PM »

I could not find an answer to this question and thought someone might know.  I am curious, why is AM the mode of transmission used in aviation?  Why not FM or SSB?

73
Elijah
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WB6BYU
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« Reply #1 on: February 08, 2006, 02:39:22 PM »

FM has a capture effect - you hear only the strongest station
when several are calling.  With AM you can at least hear
that a weaker signal is in there behind a strong one.  This
makes it more likely that a weak distress call will be
heard.

SSB is used for long distance HF communications, but the
frequency stability for fixed-channel VHF just wasn't
possible until comparatively recently.  You still don't
see it used for land mobile VHF because it is harder to
maintain a constant audio output level without a carrier
to drive the AGC.
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WA9SVD
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« Reply #2 on: February 08, 2006, 02:41:20 PM »

AM was the first voice modulation method developed (not sure if "invented" is the proper term.)  And was the easiest to detect, so it came into common use for such applications as aircraft comms, as well as standard broadcast.  FM modulation and detection was developed later, and while it had some advantages over AM, it still requires a stronger signal strength than AM to be detected efficiently.  SSB wasn't developed until the technology of radio had advanced to the point that filters, phase shift networks, and the like could be built reliably, both from the standpoint of circuit design as well as components.
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KC8AXJ
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« Reply #3 on: February 08, 2006, 02:44:11 PM »

Can you use an AM radio in the evening ??
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WT0A
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« Reply #4 on: February 08, 2006, 03:02:29 PM »

sure, why not.
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AA4PB
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« Reply #5 on: February 08, 2006, 03:30:10 PM »

I'm sure *part* of the reason is legacy. Can you imagine what a mess it would be to change the aviation industry from AM to FM? Everyone would have to monitor both modes during the changeover period. FM would have to have a real clear advantage to make it worthwhile. It doesn't for the reasons others have stated.
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W8JI
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« Reply #6 on: February 08, 2006, 03:38:16 PM »

The reason is AM was used first. Since it was used first, even though it requires heavier gear, it remains in use.

Imagine the problems changing a system like that over, once something has been established!! No easy way to change it, although I'm sure they would like to.



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WI7B
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« Reply #7 on: February 08, 2006, 03:50:31 PM »

Aircraft used commercial AM radio stations for navigation. This continued through the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts).

Since pilots needed AM receivers, the leap to using AM transceivers was not great.  AM was, of course, the first means of avionic telephonic communication.

VOR itself uses a main signal that is AM and broadcast in phase-arrays with a secondary FM signal. The phase difference between them the AM and FM signals give angular direction to an airfield.


However, SSB is used in the Air Force.  The former General LeMay was a major advocate of using SSB in the NORAD's B-52 squadrons in the 1950's.

73,

---* Ken
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WA9SVD
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« Reply #8 on: February 08, 2006, 04:04:43 PM »

 by KC8AXJ on February 8, 2006    Can you use an AM radio in the evening ??    

    Use AM in the PM?  Does that mean you've phased out FM?
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AG4RQ
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« Reply #9 on: February 08, 2006, 04:25:30 PM »

The reason why AM is used instead of FM and SSB is because when more than one signal is being transmitted, AM is the only mode that will cause heterodyne. With FM, the weaker signal is covered up by the stronger one. This is known as "Capture Effect". With SSB, most times the stronger signal will mask the weaker one. Two signals heterodyning lets those monitoring know there's another signal in there. This is important. The weaker signal being detected lets all know there is another aircraft trying to transmit. That transmission may be important enough to avert a crash in mid-air.
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K6AER
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« Reply #10 on: February 08, 2006, 05:30:16 PM »

Legacy was a major portion of why AM is used in aircraft but other factors have kept it their as well. Frequency stability was never a problem for the AM bandwidth was sufficient that nearby use took car of the frequency problem. Even the old King KX-170 radios had +/ 2 PPM stability. Medium bandwidth AM has always been the preferred mode for aircraft due to the Doppler effect. On VHF 118-136 MHz you can see as much as a 10 KHz shift at 450 knots. On Military 225-400 MHZ the shift can be twice as high. This is why the narrow band modes have never been adopted. I had 6 & 2 and 450 in my plane and when flying to a repeater I would have to shift my frequency depending on wither I was flying away or towards the repeater site.

FM, as mentioned, also suffers from the capture effect but bandwidth vs. Doppler is the main reason aviation has stuck with medium bandwidth AM
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KB5DPE
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« Reply #11 on: February 08, 2006, 06:54:51 PM »

"AM in the PM".  Sounds like a good name for an evening radio talk show.
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KB5DPE
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« Reply #12 on: February 08, 2006, 06:56:02 PM »

"AM in the PM".  Sounds like a good name for an evening radio talk show.
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N5DZ
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« Reply #13 on: February 08, 2006, 09:32:04 PM »

It is my understanding that aviation was scheduled to change over to FM along with the other radio services. At that time private planes out numbered commercial planes several thousand to one so most of the expense would fall on hobby fliers. Many complained that they could not afford new radios and rather than have a lot of planes flying around with no radio, the FCC relented and dropped the change.
As for capture effect, it barely exist at 5khz deviation.  If capture effect was such a safety consideration, why did police, fire or any other radio service where a radio transmission could mean life or death change to FM. I don't think capture effect was even a considration at the time.  
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K5DVW
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« Reply #14 on: February 09, 2006, 06:09:45 AM »

Doppler effect!? That's a new one.

The doppler shift for a 135 MHz carrier for a plane traveling at 500 MPH would only be 100 Hz, not 10 KHz and that's only if it's flying at 0 or 180 degrees from the emitter.
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