VHF Com antenna placement near transponder antenna

Samuel Seidel

Pre-takeoff checklist
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Sam
I'm adding a second radio to my plane, planning on putting a bottom mounted antenna on the bottom of the plane. Is there any problem placing the com antenna near the transponder fin? I can't find anything that suggests a minimum spacing.
 
I can't find anything that suggests a minimum spacing.
Define "near." Most radio/antenna installation instructions will give a recommended distance from obstructions or other antennas. However, the general rule listed in FAA ACs and antenna docs is 36 inches from other antennas.
 
I believe the Gtx 327 install manual specs 3 feet, but you will likely find some on VAF who have had success with less separation as low as 2 feet.
 
I think the best I can do is 2 feet.
I've seen installs with less than 36 inch spacing so you might get away with the 2 feet. Perhaps call support for the radio and xspndr OEM and get their input. I've done that in the past with some tricky installs with limited antenna real estate and they usually gave me a workable solution or options.
 
I flagged an RV down who I saw had both com antenna under there plane and one 2 inches from there transponder antenna and they said they had no problems with that radio. I’m probably just going to install it across from the transponder antenna about 20-25 inches away.

My plane is small and I can either put it up near the engine. In the middle where the transponder antenna is. Or in the rear where it will be hit on a tail strike.

The frequencies are way off too 130mhz vs 1090mhz. But I’m not an rf engineer, so maybe someone can chime in.
 
I’m a long inactive Amatuer Radio Operator. AKA ham. However, I don’t see any chance of frequency bleed between the two antennas, just a chance of any transmitted energy from one affecting the other from desensitization to outright destruction. But, I think at aviation power levels, you should be fine.
 
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