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G6Tanuki · 30th Sep 2017, 1:49 pm

"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!

30th Sep 2017, 1:49 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	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!