What is a suitable RF ground connection point?

[peaman]

I have installed a new autopilot controller in my starboard cockpit locker, and I ran a new +/- power feed from a circuit breaker in the panel. I find that I need to have a RF ground connection from the controller. Can I just connect the power negative to the RF ground, or would that be too "dirty" electronically? What would be a good RF ground connection point?

 

[toddster]

There are whole books written on this subject, which I have not fully digested, so don't assume this is definitive...

I generally figure that if hooking up the chassis ground to the DC negative were OK, they would have done so inside the box. RF ground should go to your boats bonding system - the network of wires, or preferably flat copper ribbon (RF travels along the outside of a conductor, not the middle), that connect the through-hulls and such together and by definition are at zero volts with respect to sea water. Otherwise, RF noise would feed back into everything else connected to your power supply.
Somewhere, I have a recording of what that sounds like... you can hear all the NMEA data sentences pinging around when you're trying to listen to the SSB radio . The subwoofer for the stereo picks up and amplifies every bit of noise. And you can hear some of it when you turn down the squelch on the VHF.

Sure, the device would probably "work" most of the time if you hooked it up that way, but it's a heck of a lot easier to hook it up right the first time than it is to try to track down a problem connection later.

 

[goldenstate]

When I installed all new electronics two years ago I included a RFI drain separate system connecting the new devices to my common bonding negative post at the engine block.

Installing a NMEA2000 network on a cruising sailboat

sailboatrefit.com

Repeated googling at the time suggested a lot of people attach the RF wires to black negative ground wires and call it good.

Your mileage may vary…

 

[peaman]

When I installed all new electronics two years ago I included a RFI drain separate system connecting the new devices to my common bonding negative post at the engine block.

Installing a NMEA2000 network on a cruising sailboat

sailboatrefit.com

Repeated googling at the time suggested a lot of people attach the RF wires to black negative ground wires and call it good.

Your mileage may vary…

Nice post, including the link with diagrams. I'm doing a very similar level of work on my 32-3.

You wondered about the purpose of the RF drain wires. The very sensitive communication wires in the Seatalk and NMEA2000 cables can transmit and receive RF noise, so those conductors are enclosed within a shield. Any RF noise from within or outside of that shield can potentially create a current in the shield which could, in turn, transfer RF noise to the connected or other devices. The purpose of the RF ground is to literally drain away any potentially problematic current so the "shield" can literally shield communications within from and against any signals from without. The shield acts as a "Faraday Cage".

 

[Kenneth K]

When I installed all new electronics two years ago I included a RFI drain separate system connecting the new devices to my common bonding negative post at the engine block.

This all assumes, I think, that the engine block is actually "grounded" to the sea. If one has a nylon drivesaver installed between the transmission and the shaft coupling, then there is no such external grounding (absent a bonding strap across the drivesaver or a grounding brush on the shaft).

shaft grounding brush

Without a drivesaver, there is undoubtedly metal-to-metal contact between engine block and prop shaft, but if you think about it, all those geared connections through the transmission are heavily slathered in grease or oil. Maybe that's why some of the purists put a dedicated grounding plate somewhere on the hull.