AM or FM, Question, Please

[u2brent]

Good work @u2brent!

LOL,
The truth is, I googled it, and that was the best result/explanation.

 

[ATCTech]

Still, it's the perfect document to share. Thank you for your effort!

 

[ATCTech]

Quick correction to my post (#19) with the photos. It should read "although in Canada we did not use the VHF variant.", not the AM variant. They're ALL AM.

 

[majoco]

If anyone can think of a perfectly valid reason for changing to FM for the civil airband compared to what we already have with AM I'd be glad to hear it - and perhaps someone would like to chime in with the costs of purchase of often three radios for each aircraft, control heads, wiring etc and also consider three perfectly good AM radios which are of no value for resale. Also the down time in flight-hours lost and for removal/installation/testing and re-certification. We had enough trouble persuading all operators to install ADs-B and TCAS. There would be unseen costs for the FAA, UK NATS and all ICAO countries for Flight Inspection to make certain that the coverage we have now will be maintained. You can't suddenly find that there are dead spots on a busy cross-country route that previously had total coverage.
All in all, I think the suggestion is laughable. Yes, the towers my have VHF FM com but not for civilian aircraft.
....and to add to ATCTech's years of experience, I'll put in my 2 cents worth - 5 years UK Merchant Navy Radio Officer, 6 years UK CAA installing Instrument Landing Systems and LF/VHF beacons, NZ CAA 6 years Technical Instructor, 10 years Flight Inspector all round the South Pacific, 19 years Licensed Avionics Engineer.

 

[AM909]

Learning time... Who can answer why are most ATC comms still in AM?

FM has a capture effect..... if a plane captures the frequency and is stronger, another one will not be able to be heard. That doesn't happen in AM.... the signal may have a chance to be heard.

Well, that's what I remember, too, but every time I hear people double, one of them is understandable and the other is not at all, whether it's two aircraft or aircraft and ground. It may be theoretically possible, but the right conditions (signal strengths?) for it to happen don't seem to occur often. Anyone else actually hear it in practice? (Talking AM full-carrier here, not SSB here).

 

[spanky15805]

majoco... ever worked with climax? What do you think of it?

 

[majoco]

Who/what/where is/was climax?

 

[alcahuete]

Why AM? That I can tell you in one word.....Tradition.

 

[ATCTech]

If and when voice comms change it will be digital in today's world without doubt. Dual mode capability would be mandated during the transition, which would be years (if not a decade or more in certain parts of the globe), not months. Look at how long it takes multi-user services (Police, Fire, EMS and so forth) to transition technologies and multiply the cost and complexity by millions.

 

[sonm10]

So why not SSB?

 

[nd5y]

So why not SSB?

Frequency tolerance and drift.

 

[spanky15805]

The ICAO standard the EU is using for offset carrier. How they picked the name is beyond me...

 

[AM909]

So why not SSB?

Frequency tolerance and drift.

I don't know about that. Stability has improved incredibly in the last few decades. Sufficient accuracy at VHF is not that expensive to accomplish, especially if you require GPS while you're at it. There's also ACSB's pilot tone approach to freq alignment (OT: anyone know if there were reasons other than marketing that it died in the US LMR market?).

 

[Echo4Thirty]

There is not really any reason to convert to SSB from AM in the AC band. 8.33 gives them plenty of additional spectrum and its very easy to implement with attrition.

That said, I am not aware of any facilities in the US that are using an 8.33 center frequency, am I correct that EU is where this has mainly been rolled ot?

 

[nd5y]

I am not aware of any facilities in the US that are using an 8.33 center frequency, am I correct that EU is where this has mainly been rolled ot?

It's not used for air traffic control in the US but there are other users.

87.137 (a) 17
In the band 117.975-137 MHz, the Commission will not authorize any 8.33 kHz channel spaced transmissions or the use of their associated emission designator within the U.S. National Airspace System, except, on an optional basis, by Aeronautical Enroute Stations and Flight Test Stations, or by avionics equipment manufacturers which are required to perform installation and checkout of such radio systems prior to delivery to their customers. For transmitters certificated to tune to 8.33 kHz channel spacing as well as 25 kHz channel spacing, the authorized bandwidth is 8.33 kHz when tuned to an 8.33 kHz channel.

There are other rule parts with details on 8.33 kHz channels in the US.

 

[SmitHans]

Well, that's what I remember, too, but every time I hear people double, one of them is understandable and the other is not at all, whether it's two aircraft or aircraft and ground. It may be theoretically possible, but the right conditions (signal strengths?) for it to happen don't seem to occur often. Anyone else actually hear it in practice? (Talking AM full-carrier here, not SSB here).

As mentioned, AM does not have the capture effect. So when there are multiple aircraft transmitting at once, likely only 1 will be intelligible, but ATC will know that more than one aircraft keyed up. So ATC will handle the intelligible call, & once they finish up with that they will often ask for the other aircraft to identify & repeat their comms.

A common example I hear, ATC will finish with the aircraft it can understand, then ask who's calling.

 

[krokus]

For the record, here are photos of the actual UHF air band radios we have used in ATC in Canada since the late 1970s. They were also available in VHF models, although in Canada we did not use the AM variant. The ITT GRT-22 exciter (10W nominal) and the GRR-24 receiver. There was also a 100W RF amplifier available. (Photos "borrowed" from online sources, these ones have seen better days!)

View attachment 121949View attachment 121950

This series is what we used for the few VHF freqs my ship utilized, which was basically for IAD/guard. (The UHF freqs were with AN/WSC-3 models.)

Less surprising would be that we used this series for VHF & UHF at the military airfield. We had racks full of them.

 

[majoco]

Exactly what SmitHans posted +1 happens here often - especially as we have two flight training schools at our local airport - at weekends it's hard to get a word in edgeways!

 

[samka4att]

Well, that's what I remember, too, but every time I hear people double, one of them is understandable and the other is not at all, whether it's two aircraft or aircraft and ground. It may be theoretically possible, but the right conditions (signal strengths?) for it to happen don't seem to occur often. Anyone else actually hear it in practice? (Talking AM full-carrier here, not SSB here).

But, at least you knew someone was trying to get through. You could clear all other traffic, and ask for a repeat.

 

[ATCTech]

"Two on at once there" is a very common thing to hear from ATC in busy airspace like here at Toronto Canada. From ground control all the way to ultra-high en route airspace it happens all the time.