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Old 30th Sep 2017, 8:51 am   #1
crestavega
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Default Why is aeronautical VHF AM?

Why is aeronautical VHF in AM?
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Old 30th Sep 2017, 9:33 am   #2
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Default Re: Why is aeronautical VHF AM?

I've wondered that - but not enough to do any research. Presumable it worked OK so they have kept it. Being VHF there's not to much noise about anyway, especially in an aircraft or the middle of an airfield.

I had AM PMR for many years and it seemed to work as well as FM.

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Old 30th Sep 2017, 9:34 am   #3
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Default Re: Why is aeronautical VHF AM?

Because they want hetorodynes to show that two signals were transmitted at once.
No capture effect.
Simulcast from multiple sites is simpler too.
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Old 30th Sep 2017, 9:43 am   #4
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Default Re: Why is aeronautical VHF AM?

Very weak signals have better S/N for AM than FM. I suppose this is another manifestation of capture effect: for a weak signal the FM receiver can be largely captured by noise.

The intention is that a weak signal is more likely to be noticed with AM.
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Old 30th Sep 2017, 12:49 pm   #5
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Default Re: Why is aeronautical VHF AM?

"Capture effect" is the usually-given reason, so that someone calling MAYDAY or PAN-PAN won't be totally blotted out by a stronger signal.

The other issue I suspect is that it would be a vastly-complex issue to refit the entire global aviation-fleet with FM radios: there would probably have to be a significant overlap during which aircraft would need an equivalent of the Pye Whitehall in order to operate satisfactorily.

[If there was to be a conversion there would be much to be said to switching to pilot-tone-SSB rather than FM: this would improve on the channel-spacing benefits of AM over FM, be more-resistant to i nterference, and unlike 'normal' SSB there'd be no need for any fine-tuning].

Intercontinential aviation switched from 'classic' AM to SSB some time in the late-1950s/early-1960s.

There were many fewer planes around then than there are today!
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Old 30th Sep 2017, 12:56 pm   #6
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Default Re: Why is aeronautical VHF AM?

Quote:
Originally Posted by Peter.N. View Post
Being VHF there's not to much noise about anyway, especially in an aircraft or the middle of an airfield.
The biggest sources of interference are the aircraft's own ignition system [shielded HT leads/spark-plugs are ubiquitous] - which of course travels everywhere with you - and the problem of front-end-overload if flying near to FM broadcast-band transmitters.

To comply with international aviation-standards the front-end design of modern VHF-airband radios is most intriguing: multiple varactor-tuned circuits (designed more like cavity-filters or helical resonators) with D/A converters and ROM-based lookup-tables fed from the frequency-synthesizer in order to generate the varactor voltages. It's strange at first 'aligning' a radio's front-end by entering 8-bit values into a computer, but it does give you 'correct' tracking at 254 points across the receiver's coverage!
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Old 30th Sep 2017, 2:24 pm   #7
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Default Re: Why is aeronautical VHF AM?

Quote:
Because they want hetorodynes to show that two signals were transmitted at once.
Indeed, having flown in (and driven the thing too, great fun, "take control I need to make a 'phone call") the bosses helicopter there is the occasional on channel signal, the usual reply is "I have been stomped on, please repeat" with a hetorodyne as said before, it is also simplex for the same reason.

The data rate does go down but the parity check is supurb.
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Old 30th Sep 2017, 2:30 pm   #8
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Default Re: Why is aeronautical VHF AM?

Several reasons:

Aviation went onto VHF rather early simply because of size of antennae and available frequency range. This was before FM got going for PMR and VHF marine users. There is so much infrastructure in aviation VHF AM radio and it would be such a nightmare to change it all simultaneously that they never bothered. Most aircraft wouldn't have space for duplicating equipment over a change-over period.

Along the way, the heterodyne tone was cited as a reason because it's an indicator that there were two (or more) transmissions overlapping. This isn't somethiing to rely on. The beat note could easily be below the 300Hz LF cutoff most radios have, or above the 2.8kHz cutoff the later standards require them to have.

The reason for the 2.8kHz lowpass rolloff is that someone invented a scheme for multiple transmitters on the ground, say along a long valley, all on the same channel (I didn't say frequency) The transmitters are actually offset several kHz from each other and transmit at the same time. The plane RX gives a horrendous heterodyne whistle as the plane flies in the regions where there are two strong signals, but the frequency is above the required audio frequency. The old '25kHz spacing' channels can fit 5 different TXs in, the new 8.33kHZ spacing channels can still fit three in.

The logistics of changeover would be inconvenient. There is already a change underway of replacing the radio in every aircraft with an 8.33kHz model, but that can be done gradually with dual-capability radios. The beat note means someone's doubling is useful but not reliable. The multiple ground station trick is the real killer.

David (designing aircraft radio gear is the day-job!)
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Old 30th Sep 2017, 2:49 pm   #9
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Default Re: Why is aeronautical VHF AM?

A question for David, do modern transmitters (in aircraft) have a deliberate (random) offset to make a heterodyne audible, OK it wouldn't be perfect but better than not noticing a stomp.
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Old 30th Sep 2017, 4:17 pm   #10
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Default Re: Why is aeronautical VHF AM?

No, nothing so deliberate. There's simply an accuracy requirement of a few ppm and that's it.
It got tightened up for the new 8.33kHz channel spacing sets.

The occasional transponder transmissions on 1090MHz for ADS-B out (Charmingly called 'Squitters' - Honest!) have deliberate randomisation of the periods between repeats, to that if one aircraft's squitter tramples another's, then next time they probably won't.

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Old 30th Sep 2017, 5:17 pm   #11
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Default Re: Why is aeronautical VHF AM?

Here's an extract from the chapter in the first edition (1942) of "Principles of Aeronautical Radio Engineering" by P.C. Sandretto that discusses the then about-to-be-introduced VHF radio system. Apparently both AM and FM were trialled, but it was found in comparative tests that FM offered no significant advantage. AM was chosen to get the system up and running with minimal technical risk, but it was envisaged that FM might be used in the future " should frequency modulation prove, upon further tests, to have outstanding advantages.".
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Old 30th Sep 2017, 7:00 pm   #12
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Default Re: Why is aeronautical VHF AM?

Interesting - but it sounds like this is the American version of history - which as we all know can be unreliable at best, and outright lies and disinformation at worst.

I will offer an alternative version - which may be no more true than the American version, indeed the two stories may well have run in parallel on opposite sides of the Atlantic without either party being aware of what the other was doing.

The story I know is the British one - and no airlines were involved as far as I know. It was a purely military matter, and that is fairly typical that the military sets the standard since they are often more advanced than any commercial operation - particularly when a war is in progress.

I refer to WWII, of course, and at the time USA was sitting on the side-lines. VHF AM for airborne radio came into being in the UK sometime prior to the outbreak of the Battle of Britain, so we are talking July 1940. The equipment I am referring to is the TR1133 airborne transceiver, with the T1131 and R1132 on the ground. These were designed by the Royal Aircraft Establishment I believe, and the first samples went into use during the Battle of Britain.

There is an IEE paper about the development of these sets - but I can't lay hands on it right now. It described the innovative design work needed to tackle RF circuitry operating at over 100MHz, which was pretty high for anyone pre-1940. Those familiar with the R1132 receiver (which is the only common item of the three) will be aware, for instance, of the silver plated chassis in the RF section.

Why did we suddenly move to VHF? Well, the problem with the earlier sets was that they were on HF and were primitive (e.g. R1082/T1083 system with plug-in coils). HF was a problem because of too much range, and thus too much interference, plus potential jamming from afar.

Why AM? Because FM was unknown in the UK (well it was to the decision makers in the RAF). That contrasts with the USA, which I believe was already broadcasting using FM. No test equipment existed to measure basic things like FM deviation. Walter Farrar (G3ESP) once told me that he designed the very first FM deviation meter in the UK when he started work at SRDE during WWII. Marconi then adopted it, and produced it commercially.

If such contrasting stories sounds slightly incredible, then remember that the world back in 1940 was not "connected" in the way we are now. The internet did not exist. All communications was by post - and to a lesser extent phone. If you worked in business, you would probably have to get a signed chitty from your boss to phone to USA, so no-one just called USA without serious intent. So I submit that its perfectly possible to have quite different stories developing in the UK and the USA.



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Old 30th Sep 2017, 7:40 pm   #13
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Default Re: Why is aeronautical VHF AM?

Subsequent pages of the chapter give some details of the circuitry used in what the chapter makes clear were experimental systems being developed by commercial airlines, and also discusses some design possibilities for anti-collision warning systems that could operate at these frequencies. The concluding paragraphs of the chapter look forward to a time when frequencies of around 500MHz might become usable as the technology develops. It's not a field in which I have any particular expertise, but one of the engineers I worked under at Plessey who was working on avionics systems, thought highly of Sandretto's book.
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Old 30th Sep 2017, 8:21 pm   #14
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Default Re: Why is aeronautical VHF AM?

Yes, I can well believe the commercial world was looking forward to 500MHz operation. At the same time, some people were already doing it - when money was effectively no object. The Germans started a radar development on 380MHz around 1938. And they had the Wurzburg ground radar operating on 500MHz and in service by 1940.

That rather proves my point that the military were usually in advance technically of anything the commercial world could do - and usually because the commercial world always has its eye on the pounds and pence, whereas the military world just says "We need it" (and they get it).

What the question of why AM for airborne comms really bears on is a more tricky historical matter of who influenced the final decision? Clearly various people were working on this. But who decided to set a world standard, that has been set in stone ever since (regardless of how antiquated it now is)?

A further clue as to who really influenced this decision is the choice of frequency band. Sandretto says the frequency range of interest was 140 to 144MHz. The band chosen by the RAF in the UK was 100 to 124Mc/s. The airband now is 108 to 137MHz. So that suggests the commercial airline developments didn't get to choose the frequency band.

Interestingly, when you examine the early British developments, with the TR1133 transceiver, that rapidly progressed to become the TR1143. Both of them become eclipsed though when the Americans took a TR1143 and turned into the much better known (and now far more common) SCR-522 transceiver. That set operated over a much wider frequency band of 100 - 156Mc/s, (another words the Americans modified the original British design) suggesting that the question of where the airband would end up was an open question at that time during WWII (1942 I think).


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Old 30th Sep 2017, 8:27 pm   #15
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Default Re: Why is aeronautical VHF AM?

On the question of AM vs FM, I recall some interesting work I undertook at Pye Telecomms in the late 1970s. I was involved in developing a new mobile VHF radio for the police in the UK. It had to cover both AM and FM, and 12.5KHz and 25kHz channelling.

You could change mode at a flick of a switch. So it became possible to road tests using prototype equipment. We had a direct comparison between FM and AM in the same channel widths. When constrained in this way, there really was nothing to choose between the two modes, and that was probably because the capture effect of FM was not really coming into play (very little interference around on VHF channels), and the deviation was rather low in FM mode, which made the quieting effect rather minimal.

FM really does well when the deviation is high, and that is obviously the case in things like FM broadcasts with its wide deviation. You get a vastly better ultimate S/N ratio, than you can get with AM, and that is very noticeable. But constrain the two systems to the same narrow channel width and this advantage disappears.

Even worse, FM has a threshold effect (whereas AM does not) which means in very weak signal situations, AM is still usable, whereas the FM has failed. In comms work that can be a decisive advantage in favour of AM.


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Old 30th Sep 2017, 8:56 pm   #16
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Default Re: Why is aeronautical VHF AM?

Huh? I thought I'd written without any 'version' to it. "Aviation went onto VHF rather early.." doesn't credit any particular nation or date, but it was enough detail for explaining why aviation still uses AM on VHF.

David (100% Yorkshireman, quite fond of teasing Americans, and who counts as good friends a number who tease right back)
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Old 30th Sep 2017, 9:16 pm   #17
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Default Re: Why is aeronautical VHF AM?

The current revisions of ED-23 and DO186 still count the aviation NAV/COM bands as 108 to 154MHz.... but don't tell the inhabitants of above 136.975MHz

The 'FM advantage' with 25kHz channels is not going to be a lot, but it was enough to convince maritime and PMR users.

Fortunately the aviation lot when they were feeling the pinch on space didn't opt for grabbing more MHz, but opted to split some (not all) of their existing 25kHz spaced channels in three. NBFM packed into such a narrow channel has no real benefit over AM, but the not to change to FM decision has been made and remade from the era of 100kHz channels and onwards.

Tests have been run at various times to compare the two formats. I think the possibility of changing was last seriously considered in the run up to the channel split from 50kHz to the 25kHz '760 channel' plan. The people I know on both committees weren't involved back then.

Anyway, I wonder how many other people in the 21st century have designed and put into production AM transmitters?

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Old 30th Sep 2017, 11:26 pm   #18
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Default Re: Why is aeronautical VHF AM?

In my youth I spent a lot of time using AM CB radios and when FM came along it wasn't popular amongst diehard AM CB enthusiasts. For late night operation, AM was much nicer, there was no need to use any squelch and distant signals could be heard by turning up the volume as required. When multiple signals used the same channel there was a nice beating effect. But FM was much more tiring on the ears in this scenario because of the need to use squelch to drown out the FM background noise, and this meant weak signals were lost. Also there is a noise problem with FM if an interfering signal is nearly as big as the wanted signal. This can cause carrier screeching effects that are hard on the ears. Any weak signals that could break the squelch had the classic noisy harshness of a weak FM signal... again, very tiring for an operator. FM CB was the best for local contacts where the S/N ratio was good and the capture effect could drown out distant unwanted traffic on the same channel. The squelch could be used to good effect here too.

But for sporadic operation where distant signals were the norm then AM was the best. It was a great band to use late at night. Very peaceful and easy on the ears. But during the daytime 27MHz AM could get very noisy due to propagation effects and other services.
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Old 1st Oct 2017, 1:05 am   #19
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Default Re: Why is aeronautical VHF AM?

I didn't know there was an issue re replacing all the existing equipment but I'd always understood [from when this question had been asked previously
elsewhere] that AM had been retained, as opposed to anything more seemingly sophisticated eg SSB or FM, to maximise operational safety and reliabilty-in the same way that the english language was standardised.
It's not hard to imagine the difficulty involved in trying to achieve Air Traffic Control within a variety of reception conditions, especially on the ground
I can't claim any specialist knowledge though.

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Old 1st Oct 2017, 7:21 am   #20
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Default Re: Why is aeronautical VHF AM?

Squelch is difficult. The radios are required to have their squelch open for any signal of -93dBm or stronger, and they should give at least 6dB SINAD at that level. You'd suppose that the spec was set intelligently to give enough sensitivity and therefore range, so you'd want the receiver to be a bit better to give some margin for manufacturing tolerances and environmental conditions. But no, most pilots reckon that's a bit on the deaf and noisy side and want to be able to have a squelch level several dB lower and have several dB better SINAD. When choosing a new radio, it has to meet the Minimum Operational Performance Standard spec and if the aircraft is certified, the radio has to have the right approvals certificate and have a certificate for fitment to that model of plane. It has to match the appearance of other gear in their avionics stack. Display and button illumination has to track the other equipment as the single panel dimmer knob is turned, then they start comparing numbers for sensitivity, weight, power output and power consumption. This is a very technically-literate market. Once they've got the chosen radio installed they want it to just work. Squelch isn't an easily accessible panel control. ATC do not want people turning it up and becoming deaf to radio traffic. Pilots really really do not want squelch that keeps opening and blasting their eardrums with QRM and QRN. They do not want squelch that chatters with traffic from aircraft so far away that they are of no concern.

With such narrow channels and low deviation, the 'FM advantage' is not much and capture effect is not much either, so the performance of noise squelch systems isn't wonderful, and there is still that spec on squelch being open at an absolute level of input power. Now chuck in the issue that some engines are well known for less effective screening of their ignition than others...

Yep, squelch is difficult.

Not that long ago, A gentlemen's agreement got torn up and governments started licensing new FM broadcasters all the way up to 107.9 MHz.They saw it as a new revenue stream. Aviation radios saw it as a new source of stress. Required standards were updated and planes were given a deadline to comply with them. This meant either replacing all radios or fitting anti-broadcast filters in the antenna feeders and then doing certification tests on the installation. It was seen as easier to fit a new radio which was certified already to the new standard.

We are now in the run up to another deadline and all aviation radios have to be replaced by models which are narrower band and can tune in 8.33 kHz steps.

There is considerable opposition to repeated, forced replacement of equipment, voiced through commercial airline bodies and private pilot associations. America has decided that it doesn't need to change to 8.33kHz radios.

The change-overs have been made manageable by new standards maintaining compatibility with the old. there are still some '25kHz' channels left in the band plan because not all have been split in three. But aircraft outside the US are still required to fit new 8.33kHz radios and those radios have to know which channels are '25kHz' ones and to switch in wider IF filters.

This looks like an easement for ground stations and not aircraft, but the wide channels are needed for retaining area coverage via multiple, offset frequency, transmitters.

FM in such narrow bandwidths would lose the advantages of wideband FM, and not offer much difference. The offset transmitter trick would be lost and this is a major issue. Above all the politics of another forced replacement of equipment and an incompatible one at that would be horrendous.

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