On Fri, 23 Jan 2004 21:54:40 -0600, "O. Sami Saydjari"
wrote:

Does anyone out there have experience with Garmin's 340 audio panel and
its feature that lets you split the Com 1 and Com 2 so that the pilot
can talk/receive on one while the co-pilot can talk and receive on a
separate frequency on another? It is called the "Com 1/2" button.
On my installation, the co-pilot experienced that Com 2 reception was
blocked with I was talking on Com 1. That is not how it is supposed to
work. I have been told by one avionics shop that it is difficult to not
have Com1/Com2 interference and that I should just write-off this
feature. Another shop said that the Com 1 and Com 2 antenna's had to be
relocated so that they were on opposite sides of the plane (one op top
and one on bottom). I am not sure what to believe at this point. Any
advice or experience that folks have had would be appreciated.

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.

-Nathan