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Old 28th Jan 2023, 3:31 pm   #1
Iangebbie
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Default AM Transmissions

AM transmissions on Air-Band

Many may have wondered why amplitude modulation is still used for air-band communications, and will it still be used in the future.

From the technical side, issues relating to two transmissions (Different transmitters) on the same frequency can still be heard by ATC (Air Traffic control) if FM was used the stronger transmission would mask the weaker transmission (Referred to as capture effect)

The Air-band frequencies cover from 108 MHz to 137 MHz but the channel spacing is different on different parts of the band, 108 to 118 MHz was allocated 50kHz (Used for VOR beacons)

118 to 137 MHz uses 25kHz spacing for voice channels, there is an option to reduce the spacing now to 8.3 kHz (This option was available from 2012)

Typical equipment I have encountered (From the offshore Oil and Gas industry) using A3E modulation, Walter Dittel hand portables (Dates back to the 1980s) see https://www.manualslib.com/manual/87...tel-Fsg-5.html This was built like a Land rover defender, metal case, bullet proof knobs, simple to operate, survived any rain downpours, or just being dropped. About the only mainanance (Besides TX RX checks) was replacement of the rechargeable battery pack, and issues with the combined headset microphone lead.

Another company well know for Base stations was Jotron see https://jotron.com/?s=TR-7750
This base station was normally populated with two transmitters at 50W along with two receivers (O+G requirement because it was a mandatory system) it would also be fed from two UPS supplies

Locations for base stations / Heliadmin and control room/ In my life time I have never had to repair any of these items, only carry out routine check on TX and RX with a radio test set.

I believe that military still use A3E on the UHF for military aircraft, will AM be replaced on Air band, there is no common agreement in place between any countries at this time so AM will stay for a long while yet.

Regards Ian G
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Old 28th Jan 2023, 4:05 pm   #2
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Default Re: AM Transmissions

"118 to 137 MHz uses 25kHz spacing for voice channels, there is an option to reduce the spacing now to 8.3 kHz (This option was available from 2012)"

I started working on air band comms for a manufacturer back in 2000.

ISTR that in the early to mid 2000s' it was offered as an optional build in the (then) already established range of tx-rx of that time. I certainly worked on them prior to 2005.
Rob
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Old 28th Jan 2023, 4:19 pm   #3
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Default Re: AM Transmissions

I thought I read somewhere, years ago, that AM was safer because the squelch on FM doesn't always lift in certain conditions?

Cheers

Aub
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Old 28th Jan 2023, 4:20 pm   #4
Graham G3ZVT
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Default Re: AM Transmissions

When I started in Amateur Radio in the early '70s AM was the mode-of-choice in the 144- 146MHz band. NBFM tended to be derided by operators as an inferior option, giving thin quality audio compared to the full bodied sound of proper anode + screen modulation

On the other hand, our homebrew FM efforts sometimes were no more sophisticated than the addition of a varicap diode at the sending end, and slope-detecting on an AM set at the receiver.
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Old 28th Jan 2023, 4:26 pm   #5
G6Tanuki
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Default Re: AM Transmissions

Quote:
Originally Posted by Aub View Post
I thought I read somewhere, years ago, that AM was safer because the squelch on FM doesn't always lift in certain conditions?

Cheers

Aub
I always understood that it was because of the FM "Capture effect" whereby in the presence of two FM signals, the stronger one essentially destroys the modulationn of the weaker and so to a listener there is no evidence that the weaker signal is present[which could be problematic in an emergency].

With AM you don't get this capturing of the receiver, so the presence of a weaker station is still rather noticeable.

From memory, most civil-air-band gear on sale these days is 8.33KHz-capable.
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Old 28th Jan 2023, 4:36 pm   #6
Iangebbie
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Default Re: AM Transmissions

Hello Robin, yes the proposed 8.3 kHz spacing was know well before 2012, it was mandatory after 2012 / should have stated this in my original post (Not as optional)

Regards Ian G
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Old 28th Jan 2023, 6:48 pm   #7
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Default Re: AM Transmissions

Quote:
Originally Posted by robinshack View Post
"118 to 137 MHz uses 25kHz spacing for voice channels, there is an option to reduce the spacing now to 8.3 kHz (This option was available from 2012)"
8.33kHz channel selection is now a requirement. Aircraft in many countries have been forced to re-equip.

THis brings in a load of requirements for switched channel bandwidths, adio filters etc.

Also look into chains of offset transmitters spaced within a channel bandwidth. You fly along a route, tuned to one channel and the offset transmitters fade across from one to the next. You don't need to re-tune. All carry the same modulation. You have audio filtering to cut heterodyne whistles.

This works on AM, but FM can't do it. Anoother reason why AM is entrenched.

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Old 2nd Feb 2023, 4:02 am   #8
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Default Re: AM Transmissions

Was the original choice of AM for aviation VHF voice communications based upon a rigorous study of the AM vs. FM issue, or was there an element of inertia, stemming from prior, and perhaps limited use of AM before FM moved out of its experimental phase? And if the latter obtained, was the more rigorous and compelling support for AM developed post facto.

I understand that in the USA, the VHF airband frequency allocation was an outcome of the 1945 FCC deliberations on VHF and UHF frequency allocations (which notably kicked broadcast FM “upstairs” to 88-108 MHz), and that the whole nationwide system was set up in the immediate post-WWII era.

That was concurrent with the extensive expansive of the US VHF land mobile network using FM (in those days typically with ±15 kHz deviation, so narrow relative to broadcast, but not really narrow.) Anyway, it would appear that suitable FM technology was readily available in the same period that the VHF airband network was established. That provides some circumstantial evidence that the choice of AM for the aviation case was technically based, and not simply inertial.

In the marine ship-to-shore VHF case, the decision was made in favour of FM during the 1950s, although I understand not without some debate. Evidently whatever drawbacks FM had in the ground-to-air case were not considered to be persuasive negatives for the ship-to-shore case.

That suggests a question – what VHF mode is used by the rescue services, when using both aircraft and watercraft, for air-to-vessel and vessel-to-air communications? Presumably that case was considered when the marine VHF modulation choice was made, so the question does have a vintage context.


Cheers,

Steve P.
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