Comm/autopilot problems


Bruce Crain
 

For you builders and flyers hopefully this will help you avoid problems with respect to RMI or RFI even when you think the RG400 coax is bullet proof. My comm coax is run down the right side of the fuz and nothing else is run down that side with it. The only place it came close to is where the problem began which is the autopilot servo. In the past if I had a problem with a certain freq (I now understand it is the higher freqs above 127.0 on the comm freqs that were giving me the problems) I would just ask Center or approach if they had another freq as this one is messing with my autopilot.

I did some trouble shooting with the comm and autopilot interaction with respect to the higher frequencies on the radio. Particularly on Wichita approach. The frequency 134.85 (Wichita approach) would bank the Q left on this frequency. I did some testing and it seemed that anything above 127.0 comm frequency would make the autopilot turn and bank. I took a look in the tail cone and noticed 2 holes in the bulkhead before the split line in the fuz. The hole I was using to run the RG400 comm coax through was only about 4" away from the autopilot servo. So I drilled another hole about 10 to 12 inches away from the autopilot and re-ran the RG400 to the comm antenna in the tail cone. Before I flew I tested the autopilot with the comm set at the offending 134.85 freq. It seemed solid!

Then I flew up toward Wichita and tuned in their freq (134.85) and talked with them and the autopilot stayed solid as a rock. Even RG400 has it's limits I suppose.

Well that's it in a "nut shell".

Bruce Crain

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Rick Hole
 

Bruce, you will remember me from my years at Velocity. I made and installed
countless antennas before retiring. RG-400 is indeed superior and all but
eliminates RF radiating from the center conductor through the outer layers
of shielding. There is another issue at play however. If the antenna is
not a perfect impedance match for the transmitter/coax system some RF will
return on the outside of the shield. We measure that as Standing Wave Ratio
(VSWR). The VHF Com band is wide and no antenna will have a perfect match
from the low to high end. You are likely seeing less interference with your
autopilot in the frequency range where your antenna is best matched (lowest
SWR).
I always threaded the coax through three ferrite donuts as close as possible
to the antenna connection, leaving an air gap between each. These act as RF
chokes and prevent the RF from travelling back on the coax. If you don't
have these installed already I recommend adding them to yours and any future
VHF antennas.
I don't have the specs on these donuts any more, but I think a call to
Velocity parts might provide a source.

Rick Hole


Bruce Crain
 


Rick Hole
 

Transponders are in the 1 GHz (microwave) range.  Whether the same ferrite beads will be effective there I can’t predict.  It wouldn’t hurt to try it.  My guess would be since the transponder is a fixed frequency device, and in general we use commercial antennas for them, so it is likely that the impedance match is quite good and the ferrites may not improve it much.  If you home-brew the TXP antenna all bets are off considering a small fraction of an inch in length would change the center frequency considerably.  SWR meters for microwave may be available, at a price. 

 

You might take a look at the ground for you’re A/P servo.  Since you are seeing RFI you may need a low impedance ground.  That means a heavier wire or better yet braid strap.  The ½” copper foil tape commonly used for antennas should work fine. 

 

Rick Hole