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.
Richard
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