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ka6dan

Well Known Member
Perhaps I didn't word my previous post well.

I thinking of installing a com antenna in near proximity to my transponder. My GTX 23 transponder is mounted under the baggage compartment floor and I am considering mounting a com antenna less than a foot away from the unit. Will this create any interference issues? The transponder antenna is several feet aft of the location I've selected for the com antenna and should not be a concern. But com antenna will be less than a foot from the GTX 23. The object is to get the antenna away from the 9A landing gear.
 
ka6dan said:
The transponder antenna is several feet aft of the location I've selected for the com antenna and should not be a concern. But com antenna will be less than a foot from the GTX 23. The object is to get the antenna away from the 9A landing gear.
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I'm not sure what "several feet" means. Follow the manufacturer's recommendation.

Having a com antenna outside the aircraft and mounting the transponder "box" inside the aircraft near the antenna should not be a problem. They are effectively shielded from each other by the structure, which is ground.

Keep the coaxial cables as far apart as possible to preclude coupling. It's possible, but not likely.

The proof is in the operation.

Don
 
ka6dan said:
Perhaps I didn't word my previous post well.

I thinking of installing a com antenna in near proximity to my transponder. My GTX 23 transponder is mounted under the baggage compartment floor and I am considering mounting a com antenna less than a foot away from the unit. Will this create any interference issues? The transponder antenna is several feet aft of the location I've selected for the com antenna and should not be a concern. But com antenna will be less than a foot from the GTX 23. The object is to get the antenna away from the 9A landing gear.
Click to expand...

Hello Dan,

This is a good question. As it sounds like you know, the GTX 23ES manual recommends that the transponder antenna be placed a minimum of 3 ft from the transponder. Since the GTX 23ES is a 250W transmitter, there is some concern of RF interference without this separation.

In reviewing both the GTX 23ES manual and a COM radio manual (in this case the GTR 200) there is no specific guidance limiting the distance from a COM antenna to a transponder main module, so it would seem that the separation from the COM antenna to the transponder module is not often a concern (especially given the shielding provided by the bottom of your RV), and what you describe should be adequate separation.

Let us know if you have additional questions.

Thanks,
Steve
 
One problem with having a transponder antenna a minimum distance from the unit is lead length. Many years ago we had a system in a Pitts Special where the transponder antenna was less than a foot from the unit. The transponder continually replied to itself because of SWR. We put a 3' antenna lead on it and solved the problem.

It took us quite a bit of hair pulling to find the problem.
 
My 23ES is mounted under the GIB left thigh (under the floor). My comm antennas are maybe 18 inches or so forward of the box (at most) and I've had no problems. The 23ES antenna is located further aft per manufacturer recommended distances.
 
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