Radio Hash Noise with Dynon Transponder When Turned on

Bardample

Bardample

I love flying!
Joined
Aug 18, 2013
Messages
46
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
 
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.
 
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.
 
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.
 
I've never heard RF noise referred to as "hash". I associate that term with data-processing. Just wondering where the use of the word "hash" in that context comes from? Not trying to be a smart-aleck, just curious.
 
It's just slang for interference or noise. I've only heard it a few times, and I've been working avionics and radios for a long time.
 
Just to update post.

I have finally isolated the hash (as we aussies call it) source. The noise occurs after booting up my HDX , before engine start. The hashdisappears after I pull the blade power fuse to my FData Sys AP 60 Audio Mixer ( that takes all audio outputs and channels to the PS Engineering 1000 Intercom. The hash disappears when I remove the Dynon audio out put wires that go to the audio mixer.

Dynon have a R and L output as well as a ground wire as a twisted set of wires to output audio. I have soldered the R and L audio out wires to make a mono input to the audio mixer and connected the black gnd to the audio mixer. I am not sure why this hash occurs . It disappears when I switch off the HDX , remove power to the audio mixer or remove the HdX audio from tne mixer. I recently checked all the earth connections from my panel to the earth bus at firewall , and replaced my HDX chassis ground with a braided wire to my earth buss. The hash noise is a little diminished when I use my ANR headphones in flight.

I have a request into Dynon Support to see where the source if my 'hash' is coming from. I hope to get more info in chasing this down and will update the forum when I have made progress. Jeff
 
Try disconnecting the ground from the audio mixer. Both Dynon and Flight Data Systems tell you to only have it connected at one end, and presumably it's connected at the Skyview end.

Out of curiosity, does your audio system use a common ground point? If not, you might want to look at this:

Rhino

Thread 'Ground Loops and Noise in Aircraft Systems'

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those paths can cause oscillations that result in noise in your data or audio systems. As a result, considerable effort should be applied to ensure common grounds are used whenever possible, and you’ll see that mentioned in aviation forums with some regularity, especially those associated with homebuilt...
  • Like
 
Per Rhino -“Try disconnecting the ground from the audio mixer. Both Dynon and Flight Data Systems tell you to only have it connected at one end, and presumably it's connected at the Skyview end.”

The general principle here is to have ALL your audio grounds at one place, I use a ‘Forrest of tabs’ which is also connected via a #10 wire to the ships battery ground point ( battery on firewall). That method has never given any “issues” on all the installations I’ve done.
 
Rhino, my noise where ever its coming from is inconsistent, and so is my initial report in this thread. On first report above, I felt the Txp was the source. Then I found the noise ( hash to me) came from my audio line from my HDX going into my Flight Data Syst AP60 audio mixer. This hash stopped once I turned off the HDX or removed the audio line from HDX to the AP 60.
At your suggestion, Rhino, I went to the plane to check if disconnecting the gnd line from the Dynon to the AP 60. The hash was almost non existent at that time and disconnecting the gnd line at the AP 60 end made no difference.

I do have a common ground ' buss' in cabin on firewall where all my grounds from panel gear goes. I think the AP 60 manual states that all input sources to the audio mixer should be be shielded, but o ly grounded at either end. The audio output from the dynon is 3 twisted wires R , L and one black gnd wire, so I suppose Rhino you are saying the black gnd should not be connected to the AP 60 gnd.

It seems I do have hash and its probably due to a ground issue, its intermittent ( the worst to find). I have pulled all fuses sequentially whilst listening in. Its obviously related to the Dynon HDX ,as its not there when HDX is off.

Any other thoughts are welcome.

I
 
It should only be connected at one end, whether that be the HDX or the mixer, but audio is different from power ground. All audio grounds should ideally go to the same place. They are distinct from your power grounds, which is what a bus is typically used for. In most aircraft, the common audio ground is the audio panel or intercom, but that isn't set in stone. That was all discussed in the link I posted. However, if this is a new problem that just started happening, any lack of a common audio ground is not likely the reason. You're probably correct in assuming an intermittent connection somewhere.
 
Last edited:
You must log in or register to reply here.
Back
Top