Transponder antenna integrity?

We were wiping some oil off of the transponder antenna and noticed that the antenna can turn within its sleeve/housing - stiff, but turns. Is this normal, or does it indicate a possible problem?

Especially curious, because we're still hunting for the source of issues with our mode C and the wireless sniffing of our uAvionix Echo ADS-B.

The transponder turning within it's housing may or may not be a problem, BUT if you are having other transponder related problems, the antenna probably needs replacing. These antennas are relative cheap and easy to change.

I have disassembled my TED antenna a couple of times to repair it for spare pusposes. I seem to remember that the rod base is firmly soldered into the BNC connector at the base. Regardless of the attachment method, I am ceratain that the rod was firmly attached to the BNC connector and was not able to spin within it. I venture a guess that if you pull while spinning, the rod will come straight out of the nylon sleeve.

If yours is spinning, that is not normal and should be high on the list for probable problems. You should be able to disassemble it and solder the rod back onto the BNC connector, if you make a jig to hold it straight, while you wait for a new one. I have had good success with this repair.


Larry

What Larry said. It shouldn't rotate. Those spike antennas are only $30 so, if it were me, I'd just replace it. You don't want to have to worry about it.

When I said it "may" rotate without causing a problem, I've seen a few cases where the entire inner component will rotate as an assembly. Not likely to happen with the BNC connector in place.

As Mel has stated above..."not likely". However, the antenna BNC female center conductor of the connectors well as the male pin of the mating connector are gold plated. If these gold surfaces are allowed to rotate within the mateing connector, then over time the gold flashing will be worn away. The gold is used as a coroshion inhabiatant. If the gold is worn away, corrosion will develop between these surfaces and can lead to degraded performance.

Thanks for the input. I pulled the passenger seat panel to get to the antenna. It's an AV-22. The BNC female connector and its insulation rotate when I rotate the antenna, and there is still continuity between the antenna and the female center connector...but I can't say what the condition is of the gold flashing.

Given the possibility of degrading performance, the mode C & ADS-B problems we're trying to chase down, and the fact the the seat pan is out, I'm guessing replacement is warranted. A couple questions:

1. Is it OK to replace with the $30 TED antenna, instead of the $75 AV-22? We have a Garmin 327.

2. Replacing the coax transponder cable would be a bear, because of how its routed. OK to leave in place? Connector cleaner?

If the antenna lead is RG-58, I would replace it. If it is RG-400 I would inspect the BNC connector very closely.
And yes, the lower cost "TED" antenna will work just fine.

Thanks, Mel & all. I'll order the TED, get some help inspecting the BNC connector (the cable is RG-400), and report back.

Closing the loop on this...

The RG-400 looked good, so we installed the new TED antenna and took a short flight over to KABQ (a class C) and back with the UAT ADS-B circuit breaker pulled.

ATC didn't report any mode-C issues, so we feel we have narrowed down our troubleshooting to the uAvionix Echo unit (it is showing and outputting "0 ft" pressure altitude).

NM Doug said:

Closing the loop on this...

The RG-400 looked good, so we installed the new TED antenna and took a short flight over to KABQ (a class C) and back with the UAT ADS-B circuit breaker pulled.

ATC didn't report any mode-C issues, so we feel we have narrowed down our troubleshooting to the uAvionix Echo unit (it is showing and outputting "0 ft" pressure altitude).

Click to expand...

I'm having issues with ATC intermittently not seeing me as well. I also have uAvionix. I'm a little unclear about your post - are you saying that the uAvionix ADSB possibly could be causing the problem?? Maybe it's somehow interfering with the transponder transmission and causing ATC to not see you?

I had a KT76A and never once had a problem. I installed the GTX 327 recently along with the uAvionix and that's when my intermittent problems started.

Brian -

We think that some ATC facilities detect if an airplane is sending ADS-B out and then use the ADS-B reported altitude instead of the transponder's mode C broadcast. In that event, if the Echo is sending out an invalid pressure altitude (instead of falling back on GPS altitude), ATC might report a "mode C problem."

We are speculating here... ATC has not complained about our mode C when we've had the Echo circuit breaker pulled.

Doug

1flyingyogi said:

I'm having issues with ATC intermittently not seeing me as well. I also have uAvionix. I'm a little unclear about your post - are you saying that the uAvionix ADSB possibly could be causing the problem?? Maybe it's somehow interfering with the transponder transmission and causing ATC to not see you?

I had a KT76A and never once had a problem. I installed the GTX 327 recently along with the uAvionix and that's when my intermittent problems started.

Click to expand...