Comm/autopilot problems


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

Join main@Q-List.groups.io to automatically receive all group messages.