Mounting the Transponder Antenna

Mounting the transponder antenna started out as Operation Question Mark. I asked around to find out the best way to mount the antenna, what size the ground plane plate needed to be, what cable do i need to use... etc. And I either got no answer, or a use what you got kinda of answer.

I was told to use RG-58, then later found out that RG-400 was needed, but there are limitations to distance of the cable reach and so on.

So, this is what I was able to come up with.

1. You need to mount the antenna as far away from your body as possible. The microwave radiation can do you harm... I noticed that some people mounted the antennas near their instrument panel just over their legs... I hope cancer is not in your plans!

2. There are specific choices of cable you can use for the transponder, an most are not cheap. Plan the distance you want to mount the antenna, and then do a little research before buying that cable.

3. The antenna needs to be mounted on a metal ground plane on a fiberglass glider. Cooking foil can work from what I'm told, but I used a metal cookie lid that Robert suggested that I could use.

4. You need to find a way to mount the antenna, and it's orientation.


In regards to bullet point 4, Robert stated that their are two way you could mount the antenna. Pointed up, where Airliners could see you, then, ATC might not. And then you have the reverse, where if the antenna is mounted down, ATC can see you, and other traffic might not... You decide!?

I chose to mount the antenna down, since the location was under my O2 tank, and I did not want to reduce the signal coverage.

I also found that I could use RG-400, and that a section at the maximum distance for the transponder cable was 8 feet, which placed it a safe distance, yet under the tank.

The next question, was how to mount it? I became concerned when I found that there was a Tech Note from Streifeneder on how to mount the transponder antenna. But after waiting a few weeks after inquiring about the TN, and receiving no answer, I assumed that I was worrying about nothing.

Again, I was pondering about a mount that would work. I showed Robert an aluminum mount that I planned to mount off of the O2 tank bracket, but he handed me a mold he made from wood, and stated, "do you think this would fit?"

It was a simple and brilliant design!

It required a few layers of fiberglass to make.

I made this stand overnight, and cut the mounting hole in the center. I also drilled a few holes in the stand tabs to allow the epoxy cement to ooze through and help anchor the mount.



Due to the distance and reach of where I needed to mount the stand, I knew that I could not reach far enough into the fuselage to hold the antenna, and screw it into the ground plane plate. I would need to mount the plane and antenna first and then glue it into the fuselage.

(You might notice, that I have the antenna mounted down... I hope ATC sees me.)

I applied a thick layer of thickened epoxy to the mount tabs, and then placed the completed antenna and mount into the fuselage. It required some weigh to be placed on the tabs and antenna top to hold it in place while the epoxy cured.

With the antenna mounted, I terminated the end of the RG-400 with the correct BNC connector, and carefully routed the cable though the fuselage to prevent any sharp bends that could compromise the structure of the cable.







The trick now is to be able to attach the other end to the transponder in the panel.