Transponder noise?

[kitfox2009]

Hi Andy
That was me with the feedback from the ANR control box near the PTT switch. Just for fun I wrapped the box in aluminum foil (NO difference). Now I just hang my controllers on each side,under the panel. Keeps them out of the way,easy to adjust and NO noise!! It sure was a "bear" trying to figure out what was happening though.
Good luck.
Don
PS The old non ANR D/C`s did not have this issue. They do attract a lot of engine/charging system/cabin noise however. I bring along a spare set just in case the ANR`s act up again.

[jtpitkin06]

Try connecting a spare coax (radio shack) to the transponder and antenna and routing it away from the other cables and wires. If things quiet down then you are on the right track.

The strobe light noise in the audio also is a big clue. Although Lightspeed Zulus are known to eliminate transponder noises, not pick them up, you might check anyway just to see if they are picking up any extra noises. Using just one headset, move about the cabin to see if the location of the headset or wires changes the noise level. If the audio cables are picking up noise through the wire bundles you might have to move them as well.

JP

[jtpitkin06]

Andy,

I don't want to leave you hanging.... what's the status of your noise? Did you find anything new or eliminate anything?
Finding squeals and clicks is tough, so we're here for the duration.

John

[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