On 23-Jan-2004, Nathan Young wrote:

I'm not an avionics expert, but if the COM1 and COM2 antennas are near
each other (which they generally are on a GA plane), the COM2 antenna
is going to be receiving a ton of energy (relatively speaking) when
the COM1 radio transmits. Even if this energy is on a different
frequency (ex. COM1 on 120.55, COM2 on 124.8) the channel selection
fillters will not be able to supress the transmit energy from COM1.

I've always wondered how aviation radios deal with this. I suspect
they have some sort of level detector on the output of the 1st
(receiver) amplifier stage which shuts off the input if it reaches a
threshold.

If so - this would explain why your COM2 receive cuts out when COM1
transmits.

Well, enough speculation. Perhaps someone who actually knows how the
radios work will respond.


You've pretty much nailed it, Nathan. However, the problem is not in the
"channel selection filters" as you call them. Channel selectivity is
achieved with filters at the "intermediate frequency" or "IF" stages.
Before the received signal gets to that point it must be amplified by the RF
preamplifier and frequency-shifted in the first mixer. Both of these stages
have a limited "dynamic range", which means that they cannot pass a
relatively weak signal (as in one from a ground station) in the presence of
a vastly stronger one (as in one emanating from another nearby com antenna
on the same airframe).

--
-Elliott Drucker