REFLECTOR: Com interference with Autopilot

[Sid Knox]

----- Original Message ----- 
From: "Tom" <tomcat05 at comcast.net>
To: <reflector at tvbf.org>
Sent: Tuesday, September 04, 2007 7:59 PM
Subject: REFLECTOR: Com interference with Autopilot


> Recently replaced a Navaid autopilot with a Trutrak. When the Navaid was
> engaged-it would blink and go offline any time I transmitted. Ripped out
> all of the navaid wiring when I put the Trutrak in and I still have the
> same issue. The only thing that I used for both is toggle type breaker
> and the AP is the only thing on it. Any ideas as to what I should be
> checking for to resolve this?...thx Tom

This problem may be a cousin to Kirk's COM radio problem.
One cause of rf getting into places where it shouldn't is trouble with a COM 
antenna or coax feed line.
In a proper coax-antenna combination, essentially all the power from the 
transmitter is radiated into space.
If there is an impedance mis-match for some reason, a portion of the rf that 
should be radiated instead is reflected
back down the coax line (standing waves) causing the coax itself to radiate. 
This is not good.  The worse the
mis-match, the more of the rf that gets radiated into the
inside of the aircraft which then finds its way into other equipment either 
directly or via the wiring.  Obviously, even with a "perfect"
antenna system, some of the radiated rf will penetrate the airframe but 
generally not enough to cause problems (unless somehow
the autopilot wiring is very close to the antenna... not a likely 
happening).
There are methods of "hardening" equipment to reduce susceptibility to stray 
rf (ferrite rf chokes, shielded wiring, ...) but
they are labor intensive to implement, may be of near-zero benefit,  and 
only a band-aid fix if the root cause of the
problem is something wrong in the antenna system.
Find someone with a good SWR meter who knows how to use it and check-out the 
transmission line/antenna for the radio
that is giving you the problem.   Might as well check all the antennas... 
it's quick and easy.
You didn't say whether the problem occured with both COMs or just one (??).
The "toggle type breaker" is a non-issue.

$0.02

Sid Knox
Oklahoma

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