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Another transponder antenna question

pgroell

pgroell

Well Known Member
Friend
Hi all,
I've searched here and found some answers, but I still have some decision to make about the placement of the transponder antenna in my RV-7A.
It's a Garmin GTX-330 and I intend to use RG-400 coax.
The installation manual (revision J, march 2005) states:
  1. To prevent RF interference, the antenna must be physically mounted a minimum distance of three feet from the GTX 330.
  2. for RG-400, maximum cable length : 6' 1.3" (was 8'8" in earlier editions of install manual)
  3. make the coax cable as short as practical.
This gives for some nice planning. The transponder will sit low on the panel, inboard of the right panel rib.
There is no indication given as to where to measure the distance between antenna and GTX-330, so I assume the measurement has to be taken from the antenna to the nearest point on the box (rear corner if antenna near firewall or faceplate corner if antenna is under the seats or baggage compartment).

So my options are :
  1. Antenna near the firewall and the fuel venting point, distance to GTX-330 is 33" (Garmin wants 36"), coax length is 4 feet.
  2. Antenna under right seat in front of rear spar, outer bay. Distance to GTX 330 is 36" and coax length : 8'8"to 9'
  3. Antenna under left seat, middle between front and rear spar, outer bay : Distance to GTX-330 36" and coax length: 8'8"
So either I compromise on distance between transmitter and antenna, or I compromise on the coax length.
The position under the seats might also prove problematic due to the gear legs.
What is you opinion?
Best regards
 
Last edited:
Not being an expert or anything,
I mounted my antenna on the center line just forward of the main spar (under the fuel select). Ran coax along center section and then up firewall. I'm using the GTX327 and have had no problems. Coax run is close to limit but I think the run length is to minimize signal loss and another foot out of 6ft 0r 8ft shouldn't amount to very much. I tried to keep the antenna away from the firewall since there is a bunch of metal (engine, exhaust, engine mount, etc) that could interfere with the signal. IMHO any location that is clear of major obstructions and should be just fine.
 
Thanks

Thanks Bill and Chris, where did you then install your comm antenna and what type did you use? I intended to put it near the main spar, but with the transponder antenna there it would violate the 36" between antennas rule.
Regards
 
Rami com...

I used a Rami bent com antenna ahead of the spar on one side, and the Xponder antenna on the other. Only later did I learn that they were supposed to be separated more than they are in my installation.
All I can say is, no trouble from either com or Xponder.
I have seen a great many installed exactly the same way.

Hope this helps,
Chris
 
Used same antenna as Chris but placed it under baggage compartment still on center line. Using a BNC elbow keeps it below the elevator control tube. I think it's about 2 feet aft of transponder antenna. I was actually more concerned about getting it far away from main gear (9A).
 
1K Words

RV6ABottomAntennae.jpg


Hans
 
I put mine in the same spot on the opposite side....Hope it works but as others have mentioned, it should work just fine...:eek:
 
I know this thread is about antenna location and not the antenna itself, but here is one other thing to consider: Pay a bit more money and upgrade to the RAMI AV-74 or similar style antenna.

At some point in the future you will likely be on your back underneath your plane, wiping off some oil or whatever, and you will break off that stubby wire style antenna. Ask me how I know.

just a thought

erich
 
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