Transponder noise?

[akarmy]

Ok here's a new puzzle for you all. Maybe someone will have an idea I haven't yet thought of.

I have a Garmin GTX 320A transponder. I was installed and working fine with an internal antenna like this:

http://www.cumulus-soaring.com/misc/aae/L2-5T-5I.jpg

However it had bad range and since it was installed on one side of the fuselage it only worked when the airport was on the right side :confused:

So I replaced it with one of these:

http://www.cumulus-soaring.com/rami/AV-22-150h.jpg

Mounted under the seat on the metal hard point that the series 7 has. The little stub is grounded to the airframe with it's mounting.

Now for the problem. After the upgrade every time the transponder fires I get some scratching audio sound in my headsets. Also the transponder "light" on the front of the unit seems to flicker when it gets activated by the sweep. If I turn it to standby all is quiet. If I unhook the antenna all is quiet. Same antenna cable with both setup's just I shortened about 5 feet and installed a new crimped end. It's RG400 cable. I've tested with a meter and there is no connection from the center of the antenna to ground. There is good grounding from the outside of the bnc to the airframe via the antenna mounting. Thinking it's using the airframe as the ground plane right?

Headsets are Lightspeed Zulu and are powered by the plane's electrical system.

That's all I can think of off hand. Any thoughts about what to test next to resolve?

[DanB]

Andy,
Do an experiment and make a 2" diameter ground plane out of aluminum, insulate the frame as ground and change the ground of the antenna to only use the disc. Just a thought.
Dan

[jtpitkin06]

Andy,

There are several issues going on here. Most manufacturer do not recommend shortening the coax on transmitters, elts or transponders. The total coax length is a tuned length. Most of the time you just coil up the excess cable. Changing the length may change the transmitter output strength. That is not your reported problem, here, but it may be when you get things quieted down in the audio system.

The big issue is noise. Maybe you are looking in the wrong direction. Yes, the transponder is the source, but the rest of the audio system should filter it out.

You don’t state the location of you receiving antenna in relation to your transponder antenna. If they are close to each other (on the same mounting plate) you could get interference known as inter-modulation and you will hear it in the audio. If the antennas are separated by a couple of feet, we can rule it out for now. Also, Confirm your transponder coax does not zip tie or lie next to audio cables or dc power cables.

You stated you have a good ground on the airframe to the antenna body. You checked the ground to bnc connector at the antenna. Did you check that you have a good ground from the airframe to the chassis ground at the transponder? Some people have installed shock mounted panels and forget to run a grounding strap from the airframe to the panel.

Then there is the ever elusive ground loop. Ground loops can be in the form of static, squeal, whines or clicks. They are some of the hardest things to diagnose in an aircraft.They can be triggered by tranmitters, flashing bulbs, strobes, electric gyros, alternators or almost anything electrical.


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.

Check microphone jacks are grounded when you check between them. They also connect at Mecca.
Check the ring and tip of the mic jacks are not grounded.
Check the tip of the phone jacks are not grounded.
Check grounds with headphones and mics both plugged in and disconnected.
Check with only one of the headsets plugged in.


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

Audio problems are not an easy fix on a forum. I recently had a ground loop and it took 2 electronic nerds and an IA half a day to find it. It was a burned trace on an edge connector of a PC board caused by a short in a headphone jack that only occurred when you were plugging it in. Once plugged in the short no longer existed.

This should get you started. Report back how it helps or not.

John Pitkin
Greenville,

[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.

[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

[jtpitkin06]

OK, we're getting somewhere. You say no noise when you switch the intercom off to pass-through. That's a HUGE help. This means the bulk of the installation is OK.

What make/model intercom? I'll look at the schematic and see if I can find some potential feedback areas.

John Pitkin

[kitfox2009]

G`morning John
I am using the integral intercom that is part of the ICOM A210. I removed the PMI 3000 (sp) and sold it to a friend for $40 when I installed the new radio last year. At first I wondered if this was a good idea,but now are OK with the results. I am not connected to any navigation aids or music sources, though I understand this is all possible with these radios.
Now that I have learned to keep the ANR control boxes on the Lightspeed headsets away from the PTT switches on the sticks, everything is cool.
Good luck.
Don:D:) sometimes:confused:

[akarmy]

Sigtronics Sport 200S

http://www.sigtronics.com/air/sport200.html

http://www.teamkitfox.com/Forums/picture.php?albumid=143&pictureid=1803

[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.

[akarmy]

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

I too have the A210, however I'm not using the internal intercom, I have an external Sigtronics.

[kitfox2009]

Hi Andy
I was unable to figure out how to wire in the PMI intercom when I installed the new ICOM. That is why I went with the built-in.
Good luck.
Don

[jtpitkin06]

Andy,

Is your audio input jack isolated from ground with shouldered fiber washers?
If it grounds, even with nothing plugged in, it can create a ground loop.

I'm looking at the wiring diagrams to search for other sources of noise. Give me a day or two.

John Pitkin

[jtpitkin06]

Andy,

If you have a hand mic jack, in addition to the headset mic jacks, the barrel must be grounded back to Point A (Mecca)

The Sig wiring diagram does not show a connection to the hand mic barrel.

Try installing a .01mfd capacitor acrpss J1 pins 7 and 4 [red and black] to smooth out input power noises.

The only thing I can suggest from here is to get an oscilloscope and look at the power supply voltage coming into the unit. Place the probe on the 12 volt input with the scope set to AC. You should see no more than 5 milivolts ripple. Note the frequency of the ripple and see if it varies with the engine RPM. [alternator noise]

Other than that you're going to need local help. I'm sorry we can't fix it over the internet. Some things you just have to be there.

JP

[SomeDay7]

MGL has EMI Suppressors for sale on their site for this problem, along with a handy guide to solve this type of problem. Would this be a good fix?

[jtpitkin06]

SD7,

Ferrite beads can be effective when there is a direct link from the transmitter via an audio cable to the intercom. In this case the offending noise is in sync with the transponder, which has no audio cable. A ferrite bead on the transponder ground alone might help, but would not likely be effective. One could certainly try it as the beads are cheap. MGL does not have a lock on ferrite beads. Go to Digikey or Mouser. Even Radio Shack has them.


There is some evidence of an offending ground loop somewhere in Andy's system. Other noises are reported to be audible. Finding the ground loop is the problem. It can be as simple as a phone jack body rubbing the frame or an improper ground. When found, there is usually one of those “AHA!!!” moments.

Other than coaching someone through on common installation errors, it is almost impossible to diagnose a ground loop problem via emails or forums. You really need to be there with a scope and a meter. Once the common errors are exhausted, it’s time to get help or go to the Avionic$ $hop.

John Pitkin

[jgills240]

Andy,
Since this thread died out, i'm assuming you found the source of the problem. Was it the unshielded PTT wires? I have the identical problem in my aircraft, using different components, and hope whatever your fix was will save me some searching time... thanks in advance!

Justin

[akarmy]

Alas, I have not yet solved it for myself. I guess I got lazy and said
"eh, it ain't that bad" and have ignored it for the time being. I got as far as tracing down the ground leads, all manor of transponder and com antenna cabling and routing, replaced the transponder with another known working unit and have still not found the source as of yet. I do notice that it's worse sometimes and then better at other times. I've wondered if it could be my intercom being the problem, but I doubt it. Like others have pointed out it's most likely somewhere in the wiring or shielding / grounding.

Good luck in tracking it down. I guess I'll wait for you to find the next clues and then maybe start working on mine again.

[jgills240]

Bummer!
Subsequent flights seem to point to the ANR circuitry on my BOSE headset. It's still inconclusive, but the problem "went away" over the last 3 flights. Then on the most recent, I had forgotten to activate the ANR prior to takeoff, then when it powered on the interference was back. It quit when I shut down the ANR and didn't come back on subsequent restarts, so....

[akarmy]

Do you have your Bose headsets powered via the plane power, or batteries? If Plane powered, then maybe a grounding issue with the power supply to them?

These noise issues are hard to find that's for sure. Mine comes and goes these days. I've learned to ignore it for now.

[jhmcglynn]

I was searching for radio noise and came across this post. I am having a similar problem with my kitfox IV. I have a rotax 912 and a Val 360 comm. I bought 3 ferrite beads. I placed them as follows:
1. on the positive side of the capacitor near the relay
2. second one on the lead to the power bus
3. third one on lead from the power bus to the radio.

I monitored the transmitted signals on my headheld with ear buds under my David Clarke ANR headset. Definite improvement but now I am hearing a fast thumping noise in my headset. Still researching and working the problem. i have had the aircraft for about year and the problem developed shortly after I bought and got so bad that my transmissions were unreadable. Could the capacitors be a problem?
regards
John McGlynn KCRG Jacksonville, FL

[jgills240]

My headsets are battery powered. In recent flights, the interference has become extremely intermittent and sporadic, but nothing I do seems to have any influence on it. Cycling power on the headset ANR may help the problem, but it's unsubstantiated at this point.

[jrevens]

jgills240,

It may be a long shot, but I've experienced very annoying and intermittent noise in my ANR headsets, and finally realized the source was my cell phone - it was in a pocket & the headset cables ran in close proximity to it.