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Old 1st Oct 2017, 12:26 am   #18
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
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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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 1st Oct 2017 at 12:38 am.
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