In message , Brian Reay writes
On 07/02/15 17:20, Jeff wrote:

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

The 'capture' effect of FM is rather limited with NBFM. While you may
not be able to understand if two transmissions are present (just as you
may not on AM) you can often tell if there are.

To really gain (or perhaps not in this application) from the capture
effect, you don't really what NBFM.

The capture effect was mentioned as one of the reasons for UK CB being
FM but it was rather a dubious one, certainly a 'make weight' in the
RA's argument.

I thought that the RAs insistence on FM was that the constant signal
envelope level was less likely to interfere with 'things' (apart from a
click at start and end of a transmission).
--
Ian