MGL Avionics User Forum

Hi all

I have a jabiru with 2 axis A/P that works a treat, until I broadcast on my second comm.

The antenna for this comm utilises crossed copper tape on the floor of the fuselage which continues around the Pitch Servo. The servo is effectively inside the ground plane, bit not electrically connected to it.

When I transmit on this antenna for longer than a couple of seconds the pitch servo fails and requires a reboot of the A/P.

Would the addition off an aluminium sheet over the copper tape shield the servo from the RF that is causing the failure?

Thanks in advance

Mike

Hi Tasmag,

Mike I am guessing with 4 weeks having gone past since your post, hopefully you have already solved your problem.

Here are some thoughts, I would check your servo cable is twisted pairs with a braided shield. Any type of shielding for the elevator servo data cable should help, but may not solve the problem by itself, but it's good practice to make sure it shielded. Also try putting some ferrites on the servo cable at each end of the servo cable, once again this may not solve the issue by itself, but it's also a good practice.

If this problem has just started to show up, I would consider checking the cable shielding connection to the earth with a multimeter to make sure the servo cable shield, still has a good earth.

If all this checks out with your servo cable and your Jabiru is like mine with it's second VHF antenna and elevator servo just behind the sound curtain, consider doing the following.

Get your second VHF radio and antenna checked out with SWR meter. If that is not easily done, then at first, start by checking out the BNC connectors and the antenna cable conductors are not shorting out between each other at the BNC connector. Then check if the BNC centre pin has connectivity between each end and then the same for the BNC outer part.

If this all checks out and you still have an issue, get the SWR metre going, because if your servo cabling and antenna cable are running through the Jabiru console together with a poorly tuned antenna, this could induce EMI to your servo's data cable and cause the EFIS to disconnect the servo.

Cheers
JimJab

I haven't had a chance to check the outcome, but have added some clip on ferrites in various locations, as advised elsewhere.

I'll let you know the outcome.

Mike

This sounds like a pretty challenging installation - the actual voltage at your transmit frequency on your coax can reach more than 50V and of course it "leaves" the antenna and will couple onto anything that looks even remotely like a wire - if close by you can induce a really big, sometimes damaging amount of energy onto a signal wire.
It's not a big surprise that your servo get's nailed if this gets into it via power rails or CAN bus wires.
Ferrites can of course dampen the RF traveling on wires - effectively increasing the "resistance" of the wire at high frequencies. While this may give you the idea that the problem is fixed - it is a band-aid. It would be better to seek a more robust solution.

Hi Mike,

I had the chance the other day to test my Jab's 2nd VH radio (V-10) whilst doing a cross country flight. This radio's antenna is on the bulk head just behind the sound wall near the pitch servo, I actually never use this 2nd radio.

I was transmitting on the 2nd VHF radio with the autopilot activated, the pitch servo is a Trio Gold and it did not drop out.

The VHF antenna inside the fiberglass airframe, looks like it has two antennas, one is a normal 1/4 wave antenna and the other is a a short rubber ducky type antenna facing down which is the ground plane I was told, maybe that's the trick.

Sometime back, I got this 2nd VHF radio tested with an SWR meter just after I purchased the aircraft, the antenna SWR reading was found to be good or pretty good.

So it looks like it's possible to get this issue sorted in a Jabiru. PM if you like to have yarn about how our Jab's are set up.

Cheers
JimJab

JimJab

Thanks for the reply, I had the same antenna that you have behind the sound curtain and binned it for the one I have now, just couldn't get it to work well enough.

I think I have fixed the issue with the Servo by re-routing the wiring and adding some ferrites. It has been behaving itself ever since. Ill et you know when I have had a few more hours behind it.

Mike

Sounds like your Antenna has a much better ground plane with the copper/tin foil.

Cheers
JimJab