Transponder antenna coax help

trib · #1 07-08-2008, 06:35 PM

I bought a used KT-76 transponder and the tray has the antenna connector with about 9" of coax attached. The coax doesn't have an identifier, but it is stiffer and a bit larger than RG-58u, which I have. I'm trying to figure how to connect the antenna. Can a length of RG-58u be spliced to the existing coax, and if so, any tips on how? Is there a source for the connector plug for the KT-76 to install new cable to, and if so, is RG-58u acceptable for this? thanks for the help

CESSNADON · #2 07-08-2008, 07:00 PM

You could probably use RG58 to extend the cable. I'd ckeck with the manufacturer. However, you can't splice the coax. You need to install female connector on the short coax and a male coax connector on the extension.

mannanj · #3 07-08-2008, 07:13 PM

I would discard the old cable and replace it with RG-400. You will have to de-solder the old connection in the tray, then re-solder the RG-400 to it. You will then have to connect a new "BNC" connector on the antenna end.

All this isn't hard to do if you've done any soldering before, but I'm guessing that you will have to have a friend help, or take it to someone who can do the work for you.

Stein, at SteinAir who advertises on this web site can do it for you. Just contact him, tell him the length from the tray to the antenna and send him the tray.

trib · #4 07-09-2008, 09:38 AM

Having finished about 95% of the electrical systems installation, I'm fairly comfortable with soldering, so I'll tackle the unsoldering of the connector and installing new cable. What I'm unfamiliar with is which coax to use, particularly since it involves antenna cable and the potential losses. What is the difference between using the RG-58 and the RG-400 cable? The antennas I purchased for the nav and com antennas had RG-58 supplied with them. Do the characteristics of the transponder require something different? Thanks for the help. This is definitely what is meant by classifying building and airplane as an educational process!

Mel · #5 07-09-2008, 09:44 AM

For the lengths we use in our airplanes, RG58 works just fine. My transponder antenna is aft of the baggage area in my -6 and my transponder test shows max power out at every biannual check. Been working this way for over 15 years.
Also I agree that you should remove the short length of coax and make one continuous cable.
A male/female connection works fine, but it's just one more place for a failure.