Transponder antenna mounting.

[Mogas]

Hi Zulu1
Please could you give me some advice.
I will shortly be installing a Garmin GTX320A transponder and Ameriking AK350 30K encoder to my Bushbaby.
The antenna I will use is the +/- 10cm little whip type.
I plan to mount the Garmin close to my IcomA2 radio.
I have heard that the antenna needs to be mounted on a ground plane of a minimum size and should point down. The BB is tube and rag and I did not plan for a transponder when I covered it.
My questions are:
1. Are there any problems with mounting the transponder overhead near my radio and the encoder under my panel somewhere?
2. Can I mount the antenna on an alluminium plate, properly grounded to my fuse, near my seat, with the actual antenna poking down through the fabric pointing as near verical as possible?
3. Should I shield the fiberglass seat with foil or something?
Thanks
Mogas

[Griffin]

Garmin GTX320A is a very good choice. Is a very simple reliable design. The Ameriking AK350 30K altitude encoder is probably a bit of an overkill in your non pressurised cabin but again a good reliable bit of kit. If you can mount the two units fairly close together it will make the wiring a lot simpler but mounting the transponder overhead near the radio and the encoder under the panel should be fine. This antenna will need a good ground plane but a piece of aluminium about the size of a CD or bigger will be fine. You will need to ensure the braid of the co-ax cable feeding the antenna makes a good connection with the ground plane but screwing the antenna to the bare aluminium should be sufficient. The aluminium can be painted but make sure there is a good electrical connection between the antenna and the aluminium. The ground plane its self does not have to be electrical bonded to the fuselage as long as the transponder is properly grounded. The transponder transmits 200Watts and you only need 70Watts at the antenna so you can use a longer piece of co-ax cable and put the antenna a lot further back in the tail if you want to.

[RV4ker (RIP)]

I fitted similar setup to the KR when I still had delusions of granduer of crossing controlled air space in a single bound.... 5mm alu plate (literally) with small nipple type antenna poking through skin below the plane. Seems to work fine and reg'd on big iron TCAS at FABB. Am told that with rag and tube it may even work if it is mounted in the aft fuse behind the seats thus it does not penetrate the exterior skin.

2c
Not technically minded so would get confirmation of this. Check with Peter How at composite technologies. He has vast experience with antenna's and their placing etc in his many composites over the years.

[Mogas]

Thanks Guys
Also got a PM from Zulu1.
Will post results when it is in.