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.
