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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 1st Oct 2017, 7:22 am   #21
Peter.N.
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Default Re: Why is aeronautical VHF AM?

My word, another education. I know about the capture effect and hetrodynes ( my call is G0HET ) but didn't appreciate the significance. At least I learn something every time I show my ignorance on here.

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Old 1st Oct 2017, 9:13 am   #22
Jon_G4MDC
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Default Re: Why is aeronautical VHF AM?

Most of the offsets for simulcast I can see in use around here are +/-5kHz

Any ideas how they are chosen?

Last edited by Jon_G4MDC; 1st Oct 2017 at 9:34 am.
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Old 1st Oct 2017, 12:22 pm   #23
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Default Re: Why is aeronautical VHF AM?

It's almost designed to be confusing.

The channel plan was designed around radios with a display resolution of 5kHz yet the channels are on multiples of 8.3333333..kHz

When the radio displays 118.000MHz, its transmit frequency is 118.000MHz, the receiver is centred on 118.000MHz and the wide receiver filter is selected (the bandwidth previously used with 525kHz channel spacing) this channel is one of the '25kHz' ones. The receiver is thus wide enough to receive multiple signals on carriers offset +/- several kHz... there is room enough for the centre and four offsets to be used. So ground stations

When the radio displays 118.005MHz, its transmit frequency is still 118.000MHz and the receiver is still centred on 118.000 MHz but now the narrow receiver filter is selected. This is an '8.33kHz' channel! There is just enough receiver width for one offset ground transmitter on each side of the channel centre one.

When the radio displays 118.010MHz it's transmitter is on 118.008333..MHz and the receiver is centred there and is narrow.

118.015 gives you 118.016666..MHz, narrow.

You won't get 118.020 it should just skip it

118.025 is a return to sanity. You get 118.025 and wide receive.

118.030 gives you 118.025 narrow

118.035 gives you 118.033333..MHz narrow
118.040 gives you 118.041666..MHz narrow

118.050 is a return to WYSIWIG and is wide.

And so on and so on

So the indicated frequencies aren't real frequencies at all, they've become just channel names. As you wind upwards through the channels, the radio switches between wide and narrow receiver filters. If you're lucky, the radio has an 8.33/25 switch and in the 25 mode, it just gives you the wide channels on their 25kHz spaced centres and you need only bother with the 8.33kHz channels when you need to use one.

It's no problem for pilots. When told to dial up a channel, they just do it and the radios take care of getting the right frequency and IF filter.

If you're listening to aviation comms using a non-aviation radio, they you're tuning in real megahertz and the discrepancies between channel names and actual frequencies will show up.

Where the ground stations use multiple offset transmitters to increase their coverage, they plan which is on what offset so that if offsets are re-used, at least the transmitters are so far apart that an aircraft at an altitude which will need that ground station will not be getting strong signals from more than one at a time. On '25kHz' channels you may find offsets up to +/-7.5kHz in use. Radios are required to have a lot of audio attenuation above 3.2kHz.

David
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Old 1st Oct 2017, 12:38 pm   #24
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Default Re: Why is aeronautical VHF AM?

I'm quite a fan of AM. It is easier on the ear. FM is harsh, and as has been said, a lot of the time you can get away with defeating the squelch altogether on AM. The background noise is soft. The squelch even when enabled seems to have a smooth transition, although I dare say this could be down to receiver design. There was at least one PMR radio (Kyodo/Key KM150) which employed a delay circuit on the audio side to get rid of that horrible noisy squelch tail you get with FM. I seem to remember that AM was much favoured over FM with CB enthusiasts. I agree with them. Why change things if they work ok?
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Old 1st Oct 2017, 2:52 pm   #25
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Default Re: Why is aeronautical VHF AM?

Thanks for the clear explanation about how the channels work David! That has been mystifying to say the least until now.
Please can you reveal the two RX bandwidths in use? 25kHz PMR sets usually had 15kHz BW IFs - I wonder if Aviation was a bit wider?

The PYE Whitehall also included a sharp low pass in the Rx AF, cut off about 2.5kHz, to remove heterodynes for exactly the same reason. Actually I find that filter helps intelligibility and is nice to listen to whether or not there is staggered carrier working going on.
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Old 1st Oct 2017, 4:53 pm   #26
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Default Re: Why is aeronautical VHF AM?

The MOPS doesn't specify a bandwidth.

It gives a spec for audio flatness, a spec for maximum variation between different offset charriers within the channel and a spec for adjacent channel rejection. It leaves you to find a way to meet all
at once.

With the halfway points to the next channels at +/- 12.5kHz with an 8-pole crystal filter you might manage 17kHz at -6dB points. With DSP selectivity, you can get a steeper shape factor and push it out a bit. There are audio distortion specs for offset carriers so you don't want to trim the outlying offsets to VSB. For the adjacent channel rejection you also run into synthesiser phase noise broadening the skirts of the effective selectivity..

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Old 1st Oct 2017, 7:30 pm   #27
Jon_G4MDC
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Default Re: Why is aeronautical VHF AM?

Hehe - nice. The spec writers leave the designers to juggle all the balls.
I bet they were designers once, then they got promoted to the politicking + conference circuits. I hope they were happy flying in their concoctions.
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Old 1st Oct 2017, 9:36 pm   #28
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Default Re: Why is aeronautical VHF AM?

No, it's not quite such a product of evil intent. The specs are written around what the receiver has to achieve, not how it has to do it. Sometimes this approach leaves you free to find a new technique other times it leaves you wondering how you can walk a knife-edge and sometimes wondering if they've asked something just on the impossible side.

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Old 2nd Oct 2017, 10:20 am   #29
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Default Re: Why is aeronautical VHF AM?

A very interesting and informative thread...

For the General Aviation and Light Aviation worlds (including gliders and balloons) this has been an expensive imposition. It's not cheap to a) buy and b) have a new radio fitted in a CAA-approved aircraft by an approved installer. I know - I have recently installed a new Trig 25/8.3 radio in my son's syndicated LAA aircraft (although the CAA are providing a small % reimbursement on the cost of the radio).

Will the military have to have an 8.33k capability on their VHF boxes as well?
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Old 3rd Oct 2017, 9:18 am   #30
Peter.N.
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Default Re: Why is aeronautical VHF AM?

Regarding squelch and sensitivity, the Pye Westminster radios had a variable gain control on the IF strip, this could be set so that with just enough gain that the squelch could be left open with just detectable noise, and that was how my base station operated for many years.

The only time we had any noise was in the sporadic 'E' season when we had eastern European radio stations breaking in, but they would have done that anyway even if the squelch was operative.

You can't possibly use all the gain that the receiver had, you could only hear signals above what would have been the squelch level anyway so I can't see why that system was not used universally - but I'm sure someone will tell me.

Peter
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Old 3rd Oct 2017, 11:12 am   #31
M0FYA Andy
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Default Re: Why is aeronautical VHF AM?

Quote:
Originally Posted by Jon_G4MDC View Post
Hehe - nice. The spec writers leave the designers to juggle all the balls.
I bet they were designers once, then they got promoted to the politicking + conference circuits. I hope they were happy flying in their concoctions.
A tad unfair - speaking as a 'spec writer'!

I was a systems engineer throughout my career, working on Fly-by-Wire flight control systems. We were responsible at the system level, but computers, actuators, sensors etc. were supplied to our specifications by companies who specialised in such items. We certainly didn't just write specifications and then toss them over the wall, joint discussions ended up in specs acceptable to all parties.

Andy
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Old 3rd Oct 2017, 10:19 pm   #32
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Default Re: Why is aeronautical VHF AM?

I read an Air Ministry account of the VHF development which said that it was intended to be an FM system, but they had problems with making a good FM modulator. So short of time and desperate to get the system into service, the pragmatic decision was to abandon FM since an AM modulator would easily work OK.

In my flying days (70's) the received wisdom was that they stuck with AM because of the lack of capture effect which was seen as a safety advantage so that weak transmissions could be heard in spite of stronger ones or at least would be noticed. I seem to remember that the effect of doubling was very variable so I guess the various radio's carriers were not so precise.
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