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The ADF in my plane works very well until I switch the transponder to ALT. At that moment, the ADF needle is stuck in a random position and will not show the direction to the selected NDB anymore (the transponder works flawlessly).

Where is the interference coming from? Is it because the two boxes sit on top of each other on the dashboard? Or is it due to a bad placement of their respective antennas on the fuselage? Would seem strange to me, as their respective frequencies are far apart (kHz vs. MHz).

Any norm / guidance on how to install these two systems so that they do not interfere with each other?

Pondlife
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Ze Jibe
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  • What are you getting on the ADF's audio channel when they are both on? I might take an AM radio and move it around the units and coax runs listening for odd broadcast sounds. The xponder or its antenna cable is radiating low freq RF somehow. – John K Oct 13 '19 at 23:44
  • I finally got a chance to do some additional checks. Listening to the audio channel, I can clearly hear the morse code for a given NDB. Switching the transponder to ALT does not modify the audio (at least in a way that I can notice). The problem is also not apparent for all NDBs. I checked with more of them (different frequencies and ranges), and only one was being followed correctly when the transponder was on. – Ze Jibe Jan 05 '20 at 08:54

1 Answers1

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ADFs and transponders are often right next to each other in the dashboard, it's not the placement that's the issue. The two systems work on different sets of frequencies, so antenna placement shouldn't be an issue. If the ADF is being getting interference from the transponder in ALT mode the most likely issue is that one or the other is malfunctioning, or there is a short in the wiring between the two systems.

GdD
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