[Milsurplus] Capture effect and aircraft radios

[Charlie L.]

The FAA holding on to the use of AM dates back to the days of wanting to
maximize reliability by having only a simple single point of receiver
failure, that  being the AM detector diode, and the greater chance with
more components used in FM demod, more components to possibly fail. AM also
gave an audible indication of two aircraft talking at once, and also to
pilots who might get their message jammed by the air traffic call and
another aircraft at the same time.  The stronger FM station does take
over., or capture, the discriminator and will cover up the other station
completely to marginally.  But, in today's aviation radios. all the freq
control is uProc based and the transmit freqs are very accurate.  What you
don't hear today is a heterodyne, you will hear both transmitters exactly
on freq, and it sounds just like the old party line phone, both pilots
talking at the same time perfectly readable, but confusing to listen to,
which generates a request for a repeat.  I still run a 6 meter FM rig on
52.525, an old RCA Fleetfone base station and I used to run it at 15KC,
it's designed deviation.  When talking to other wide band rigs, the audio
was always land line quality in fidelity.  I had to lower the deviation as
with today's modern gear with 5KC, most of my modulation was sweeping right
out of the bandwidth of a transistor rig.  I have issues with the wideband
receiver in that those same modern xstr rigs with only 5KC deviation,
barely fill the discriminator so the audio is low, but all that is moot as
I have found the modern ham has no clue 52.525 is the national
calling/working freq for 6, or how a discriminator works.  My old tube
Motorola on 2 meters with its discriminator meter added to it, (accuracy
verified with a calibrated CSM) watching the repeater input and noting the
number of ops with modern gear with their output freq as much as 2KC off
and them wondering why they were having problems with a repeater at a
distance is a common thing, and the concept that their TX is a bit off freq
is hard for them to grasp because their digital readout says it is on freq.

Charlie in NC
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