Transponder interfering with AP

[Rick Moss]

I have been trying to track down an issue with my AP. For a bit of background, it has worked flawlessly for a year and 60 hours of use. Recently it started intermittently reporting “Roll Servo Offline” and more rarely the same for pitch.

Trying to fix the issue, I rewired the servos from the hub to each servo using the Dynon genuine kit. The roll servo no longer drops out, but the pitch goes offline far more often. I have now figured out that this does not happen with the transponder turned off (at least on the ground; will try whilst flying at the weekend). Turning the transponder on gets the pitch offline message within seconds.

So, I’ve rerouted the pitch servo cables further away from the transponder antenna, and wrapped them in copper RF shielding. No difference, fault is exactly the same.

I’ve also replaced the AP circuit breaker, just to rule that out.

Any ideas gratefully received! My current belief is that there is a servo fault, as nothing on the aircraft changed prior to the fault appearing, and it had worked well for many many hours before.

 

[Rhino]

Antenna cables and connections are somewhat known for deteriorating over time. That might be a good avenue to investigate if your inflight tests don't produce satisfactory results. Also, I have to ask if the copper shielding you installed was connected to ground. I don't mean to insult your intelligence in any way, but even the best of us can miss the obvious sometimes.

 

[airguy]

Change your transponder coax to RG400.

 

[Rick Moss]

Antenna cables and connections are somewhat known for deteriorating over time. That might be a good avenue to investigate if your inflight tests don't produce satisfactory results. Also, I have to ask if the copper shielding you installed was connected to ground. I don't mean to insult your intelligence in any way, but even the best of us can miss the obvious sometimes.

Lol, you didn’t insult my intelligence until you said “I don’t mean to” and asked

No, the shielding is not grounded. I shall do that and replace the coax with RG400 and new connectors.

Thanks both

 

[djones]

To add another data point, make sure the transponder antenna and coax is not near any SkyView network cables and particularly that the coax is not routed in a bundle with network wiring and the antenna shouldn't be anywhere near a servo.

 

[Rick Moss]

To update this, I have replaced the servo, sent the transpponder (Trig TT21) back to the manufacturer for a check, installed RG400 cable and new connectors from the transponder to the antenna; no improvement in the issue.

I have now ordered a new transponder antenna and longer length of RG400; will reposition it to the aft end of the airframe.

 

[jakej]

"Turning the transponder on gets the pitch offline message within seconds". - that's a major clue IMO.
Maybe some other basic info would help ?
Airfame type ie Aluminum , fabric or composite ?
Txpndr antenna - above or below the servo ? a picture of that area may be real useful here.

 

[Rick Moss]

"Turning the transponder on gets the pitch offline message within seconds". - that's a major clue IMO.
Maybe some other basic info would help ?
Airfame type ie Aluminum , fabric or composite ?
Txpndr antenna - above or below the servo ? a picture of that area may be real useful here.

Thanks Jake:

The aircraft is a Europa (which I appreciate is not so common outside of Britain), a low-wing Rotax powered 621kg SLA of fibreglass sandwich construction. The XPDR antenna is mounted on the underside between the seats, with an aluminium ground plane on the inside. The servos are either side of the antenna, and above, and shielded by the ground plane at a distance of around 400mm/17".

What really foxes me, is that it was faultless for a year of flying before issue appeared.

 

[jakej]

Was having problems 'posting' - trying again.

I'd suggest moving the Transponder antenna, it's too close IMO, to another location & use RG400 Coax cable if not using it already. All bets are off if using the old RG58. A ground plane of ~6" square will work.
PM me if you wish.
Ps -trying to understand these sorts of issues will do your head in .

jake j
Glasair in Oz

 

[Rick Moss]

Just to update with the solution… As @jakej suggested: Relocated the transponder antenna to just forward of the fin, RG400 cable (again), problem sorted.

 

[jakej]

Thanks for your post re your resolution of the issue - I know that others in the home built community will appreciate it.
jake j