Transponder noise?

[akarmy]

Yea, I knew you would have some good ideas John!

First up, I removed the antenna from the plane today and just held it free hand. Same noise. So there's no help or hurt from being grounded to the airframe at the antenna connector location.

The transponder seems to work, as the light continues to blink when I get painted in flight, however every time it's flickering and with the flickering I get the clicking noise.



Looking in the Garmin install manual it only calls out that shorter is better and with RG400 it should be no longer than 8.5 feet. Mine is about 4 ft.



Fair enough. Thus far my audio system has been very quiet, but we will see.



Com antenna is on the top of the fuselage on the back turtle deck (bent whip) New transponder antenna is on the hard point under the passenger seat a little bit back from the stick. (should be common on the newer fuselages)



Um, this one could be a problem. My transponder coax run is ganged up with all sorts of wires including the headphone audio and power cables. They are shielded, but still. Of course this has been the case before with the old antenna and never had any issues with it. I may have to look at moving the cables apart as I continue troubleshooting.



I have a single ground source with a forrest of tabs on the firewall. EVERYTHING comes back to this spot. No airframe grounds for local equipment are used. The panel also has ground and the transponder install ground goes there too.


The only other noise I get is a bit of strobe powersupply whine. It's not very loud, but when the engine is at idel on the ground you can just hear it when you turn on the strobe power pack.

The first place to look for your noise is at the receiver to make sure your audio grounds are all connected at the same place close to the receiver. All audio grounds must come to “Mecca” (the receiver or chassis ground very close to the receiver). Do not think grounding to a convenient spot on the frame or to a grounding bar on the DC electrical system is the same. It isn’t.

All audio jacks and mic jacks must be mounted with shouldered isolation washers to prevent the jack body from grounding to the frame.

Check audio jacks are connected when you check the grounds between them. They have Mecca in common. The tips should not be connected if you are running them through an intercom with separate volume controls.

Yep, all this is done and clean.

If you check all this and still have the clicks or static, then you might try moving the antenna.

So while looking at the install manual I came across this line...

"D. To prevent RF interference, the antenna must be mounted a minimum distance of three feet from the
GTX 320/320A."

The old antenna was behind the baggage sack, the new one is about 2 feet diagonal from the transponder in the panel. I wonder if it's interfering with itself. Sigh, of course this would mean "moving" the hole I now have drilled in the bottom of the fuselage.

Next up I think I'll build a longer cable and try and move it aft without mounting it to see if that helps.

The only other thing is possibly the new coax bnc end was installed badly? Maybe I got a stray ground wire mixed up with the center conductor? Of course it tests fine showing no conductance between center and ground with the bench meter, but looking at things that have changed this is one of them.

The Mystery continues...

[Av8r_Sed]

Andy,

Can you borrow a stanard non-ANR headset to test with? I recall a recent string discussing the controller on a set of Lightspeeds picking up noise from the stick mounted PTT switch. In that case, relocating the controller fixed the issue.

-- Paul S

[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.