VHF Interference from Autopilot

I noticed in the web log stats that someone had searched for “VHF interference tiller pilot”. This reminded me that I had a problem which occurred soon after the new electrical panel was wired: whenever I turned on the autopilot, there was a constant buzzing interference audible on all VHF channels.

The source was as I had expected: an earthing problem. I used the Seatalk cable to connect to a NMEA multiplexer and the NMEA input to connect to my chartplotter, both of which were earthed, but the primary power input negative (earth) was not connected (d’oh!). The resulting high currents on the signal earths for Seatalk and NMEA caused RFI which was picked up by the VHF antenna.

Many years ago I built an audio (hi-fi) amplifier which had earth loops because of multiple return paths to earth – the AC mains hum was unbearable! Fortunately I remembered that lesson.

The best strategy is always to run all earths (and power supply grounds) to a single earthing point such as the main negative terminal on your electrical distribution panel.