Radio Hash Noise with Dynon Transponder When Turned on

[Bardample]

I have a small problem .Recently I have noticed hash noise in my headphones once the HDX unit is fired up ( before starting engine). After turning lots of switches on and off, checking wires, headsets , grounds etc . I discovered that the Dynon transponder when on ( auto mode prior to take off ) causes the hash noise. When the transponder is turned to Standby, the noise diappears and returns when any mode selected other than Standby is chosen. I checked the Txp ground and antennae connections at aerial and Txp, aswell as wriggling wires etc at rear.

Can any one suggest a reason and has any one else had this issue. Thanks JeffR Australia

 

[PaulSS]

How close is the transponder coax running to the radio cables?

 

[Rhino]

And how close are the antennas? Antennas do go bad over time, especially the connections, and if they're in close proximity, you notice it faster.

 

[jakej]

Jeff, check your DM .
Jake.

 

[GKC Aviation]

Since it has only just started doing it, I think we could rule out installation problems.
I reckon a good place to start would be the transponder coax shield, and the antenna bonding to its ground plane.
I've seen many cases of the shield wire all broken off around the BNC connector of antennas, due to either vibration, or people leaning on them while doing maintenance on other things (laying in the empennage, etc).
Hard to see damage inside the connectors. For the low cost of BNC connectors, I just replace them before I go any further.
Also, pull the antenna off, and check for its bond to the fuselage skin, or ground plane if composite.
If it was a problem with the center conductor open circuit, or shorted to the shield, the Dynon transponder will give you warnings for low output.

 

[jakej]

Yeah, all the ‘standard’ stuff - a clue is the rod type antenna & it being bumped whenever bottom cowl pins are pulled. Hopefully he will give an update so others can learn.