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Darinh

Well Known Member
Sorry if this is posted in the wrong place...I couldn't find another forum specifically for Transponders or general avionics. I guess it could go in the Comm forum.? Here is my dilema. I have noticed that whenever my transponder (GTX327) is interrogated I get a little short buzz in my headset. It is not major but enough to really bug me. I know it is from the transponder because I hear the noise at the exact second the little "R" for receive lights up on the readout. I can also set it to "Standby" and the noise stops. I have checked all my grounds including to the antenna and they seem to be good (checked with ohm meter for continuity). Anyone have any ideas what could be the problem? I consider myself a complete novice in the avionics and aircraft electrical department so any and all help and advice is appreciated.

Also, I have read a couple of times about the snap together ferrite rings you can buy to reduce noise. Anyone every used any of these and are there and rules of thumb about how to use them (i.e. on the power wire of a device, signal wire, etc.)?
 
It sounds like your comm and transponder antenna leads may be running together somewhere. You should try to separate them as much as possible. The ferrite beads might help, but they also attenuate the signal somewhat. Not the best solution.
 
My guess is that your noise is coming from RF interference (RFI), although there is a remote possibility that it could be a ground loop or other power supply issue.

What type of coax and connectors did you use on the transponder? I would recommend good quality RG-400 or RG-142 with crimp type BNC connectors. RG-58 might work, but it has a much greater chance of the signal leaking. I had a friend that tried to use Radio Shack right angle BNC connectors (they had a screw terminal for the center pin and a plastic case) for his transponder connections...talk about a recipe for disaster.

I know you said you checked grounds with an ohm meter, but that won't always identify the problem (there may be multiple ground paths). Check out the connection on the back of the transponder tray, make sure the coax shield is properly soldered to the connector. Also disconnect the coax at the antenna and check for a good ground connection between the outside of the BNC and airframe ground. If you have a bad ground caused by paint or a gasket at the antenna, the coax will become part of the antenna and radiate.

RFI can be a real pain the rear. I generally try to make sure the RF on the transmit side is only radiating from the antenna, and then I move on to the other side of the equation. The RF could be getting in through your radio, through the marker beacon, through the intercom (or any inputs connected to it), or even your ANR headset itself. You just need to be systematic, and eliminate one at a time

Good Luck,
Paige
 
Thanks guys for the info. All my cables were done by King Avionics (local avionics shop) and are RG-400 with crimped BNC connectors. Also, my comm cable is on the left side of the fuse and the Tx cable is in the center of the fuse. The do obviously come close to each other where the enter into the comm and Tx at the panel.

I checked the outside of the antenna connector and airframe but have not checked if the cable connector (while disconnected) has good ground...I will try this and also check the backside of the Tx tray. It may be ground looping...

Question: What is the best way to check for ground loops? Is it simply going through the wiring and visually checking for multiple paths to ground or is the a better way?
 
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