Transponder noise?

[akarmy]

Thanks for checking in. Unfortunately I have not yet made any further progress. We had an amazing string of 5 days of clear warm weather here and when faced with such adversity I turned off the xpnder and headed out!

I'll continue to update this thread as I try some more things. Next up is to get a longer coax and move the antenna farther away from the transponder unit.

[akarmy]

Ok, quick update. No solution yet...

I made up a new 10ft RG400 cable with a clear routing away from all wires. Still have the noise. Then moved the antenna out of the airplane and had a helper hold it 8 feet outside the plane. Still the noise.

So, I think I need up update a bit of info. It's noise I notice in the headphones, however with the noise the transponder light flickers. I think this is significant. As the transponder is painted by ATC instead of a steady long blink I get stuttering flashing of the light until it goes out for the next cycle. The noise I hear is in sync with the flickering of the light.

So it's not the wire, not the placement of the antenna, not the grounding of the antenna, not the antenna wire connectors... The next two areas to investigate. First the antenna could be bad from the start. Maybe is has some internal resistance or something. Or maybe the transponder is going bad. I'll maybe check with the local guys and see if someone has a GTX320 I could swap in for a test.

[Av8r_Sed]

Hi Andy,
You need to take care with these transponders that you always use a good quality grounded antenna. The final stage output transistors are not so forgiving and can burn out with a bad load. Also, I would't advocate having an assistant hand hold a transponder antenna given the microwave frequency you're working with.

Your idea of swapping transponders is a good one, but you might consider putting your transponder in someone else's ship with a known good setup. That would let you know pretty quick if your GTX320 is still healthy.

Another thought, you might want to kick this over to the Aeroelectric forum on the Matronics site. http://forums.matronics.com/viewforum.php?f=3. There's a lot of expertise over there.

-- Paul S

[jtpitkin06]

Andy,
From one of your earlier replies…
“The only other noise I get is a bit of strobe power supply whine. It's not very loud, but when the engine is at idle on the ground you can just hear it when you turn on the strobe power pack.”


This is a clue that your problem is probably not a transponder fault… it is an audio problem. You hear the squitter (an avionics term) in the audio when the transponder reply light flashes, but you are also hearing other noises and whines.



If your transponder is replying on the ground, I assume you have a radar site on the field. Sometimes even the best system will hear transponder/radar noise if it is very close to the radar site. In that case you are probably hearing the incoming signal, not the outgoing reply. The time difference is measured in milliseconds so you can’t tell which by looking at a reply light.


I’m still focusing on a ground loop as the culprit, so let’s proceed in that direction.


To prevent ground loops your audio cables should have two insulated conductors with a separate shield. The shield should be connected to ground at the receiver end only. Do not connect the shield to ground at the audio jack ends. If you are using single conductor audio cable with a shield you might consider changing it to double plus shield.


Push-to-talk switch wiring can also be a source. Again, use double conductor plus shield. Ground the shield at the receiver and not at the switch.


It’s very hard to diagnose these problems without being there. See if you can get help on your local field. Find a good Ham operator with an oscilloscope. A scope makes it much easier to track and find the noise.


I’m pretty confident it is an audio problem and there is nothing wrong with your transponder or your VHF receiver. Like you, I’m hoping for the simple fix without the dreaded visit to the avionic$ $hop. Keep sniffing. These things sometimes take a while.

John Pitkin
Greenville, TX

[akarmy]

Ok, here's the latest update... (at this point I'm really only updating for the archives as these types of problems tend to come up)

First off, John you are right! I wish I could say I had the solution, but that will come over time. I put out the call to the local members for a transponder, 30 min later I had another GTX320 in my hands and multiple airplanes to test mine in. Short answer, no problems with my transponder or my antenna & wiring setup. This is where I got tripped up as of course we are all taught that when you make a change and then have a problem, look closely at your change. Well it appears unrelated.

The rapid transponder lights (normal) I'm based less than 10 miles from KSEA, there's lots of "painting" going on in the area. I get hits with my hangar door's just open a few feet.

So now it's down to chasing the noise down. I have good headphone lines, with good shields, per John's guidelines. Next clue, if I turn off the intercom the noise goes away. My intercom has direct pass through so i can clearly hear the radio and my ship powered Zulu's are quiet as can be (no strobe noise etc), just when the intercom is on the noise is present.

Now my PTT lines are two basic lines, not shields. (might be an area to look). I'll have to take some time and start going through the list of areas one by one and see what I can find.

Thanks for the clues and knowledge everyone!

[jdmcbean]

Andy,
Do you have an audio plug ? or an audio wire from an XM source like a Garmin 496 ? If so check that source.

[kitfox2009]

Andy
What radio do you have? On my ICOM A210 I find there is a very fine line between noise and no noise when using the internal intercom. The various programable features such as mic audio input level, intercom squelch level, mic gain, and sidetone seem to require careful adjustments to reduce noise. I have had no luck setting mine when on the ground. I go flying,then turn the flight controls over to my passenger and spend some time playing with these adjustments until the noise is gone and still maintain acceptable audio levels. With the intercom shut off, there is very little noise regardless how the radio is programed. I also installed a push button switch right beside the radio that disconnects the intercom. Flying solo, no intercom. Also handy to isolate the pilot if the passenger is yaking too much!
Good luck, these little glitches can be a real "bear to figure out.
Don

[akarmy]

Andy,
Do you have an audio plug ? or an audio wire from an XM source like a Garmin 496 ? If so check that source.

I do have an audio input, but nothing connected when testing. I do not have the garmin audio wired in.