Transponder certification - interesting situation

[Neal]

Today I had my first 24 month transponder cert (non IFR). The tech struggled to get a signal and tried various things as his unit kept showing a fail, no signal. I pointed on the side panel to my antenna where the BNC connector was and he was able to connect right into that and got a pass, then after reconnecting the BNC the antenna passed. I haven't flown around ATC (other than 1200) for probably a year so who knows if it may have quit working. My antenna is also relocated due to my cargo pod and he had mentioned it's already low powered signal as it is, the additional cable length could be part of the problem. But it seems disconnecting and reconnecting solved the issue, but also the tip I'm posting here is how he used a direct cable connection for the test.

Shows the value of the test and I'm glad it wasn't something I had to chase down as I'm about out of drama tokens on this plane.

 

[Cactus Charlie]

My GTX 345R passes with no issues with normal separation between aircraft antenna and test set antenna.

You have the option of having the transponder bench tested which will eliminate any aircraft installation issues. Of course that would require the test shop to have the appropriate test harness for your transponder.

You certainly should not require a direct connection between the coax and the test set to pass the installed test. It's possible the transponder is low output. It's also possible the coax from the the transponder tray to the antenna is defective.

 

[Neal]

The moral of my story is that it could have been a bad connection. After reconnecting the BNC it worked fine. I reported in my coil pack thread how I was getting intermittent ADS-B issues on my GNC 355. Maybe this could be part of that cause? So the lesson learned there is maybe check your antenna connection if you're getting errors like I've seen and this transponder test may have found the issue.

 

[Cactus Charlie]

It's almost impossible to mate a BNC incorrectly. However, I have seen BNC terminated cables in which the pin had pulled back from the fully inserted position and would not mate fully with the female pin.

The relative length of the inner conductor and the shield can change as the cable is flexed and the pin is not latched. The pin only stays in the fully inserted position if the outer shield was positioned correctly before it was crimped.

Take a close look at the position of the pin compared to other BNC terminated cables. This should be checked after the cable is built but it's not impossible for a bad one to get past inspection.