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RF Grounding

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Bill Eggbeer
Active Member
Joined: 6 years ago
Posts: 3
Topic starter  

I am replacing a Raymarine C95 MFD on my Tartan 3700 with an Axiom+.  The MFD is on a pedestal in the cockpit.  The Axiom+ installation instructions call for connecting the RF Ground wire that is part of the power cable to a dedicated RF ground or directly to the boat battery negative terminal.  The C95's RF ground was left unconnected, as is the RF ground on the radar power cable which also connects in the pedestal.  As far as I can tell, the boat does not have a dedicated RF ground, though it does have a system ground.  What should I do?  Leave as is, and don't worry about an RF ground?  Run a ground wire connection down to the battery (not a simple undertaking, but doable)?  Connect the RF grounds for the MFD and Radar together, and connect the Radar RF ground to the system ground (There is a bus at the bottom of the mast where the mast cable connects with different cable running aft to the cockpit - I could run a ground wire from this bus to the system ground).  Thank you!

Jeff Stennett
Active Member
Joined: 3 years ago
Posts: 6

If there's one thing I've learned in my almost 20 years of's that just because that's the way you got the boat, doesn't mean that's the way it's supposed to be.

#2: RF ground is NOT the same thing as "System" or "House" ground, though just from an electrical point of view, they are often treated the same by those not into radio, such as just having your standard VHF radio, vs. Short Wave and SSB radios and their more customized antenna transmission lines.....
....and then there's lightning....
Sounds like your boat's proper, intended RF Ground is that bus connected at the bottom of the mast...and if it consists of a "bunch of wires going aft," that is your multi-RF-Freq/wavelength ground-plane RF ground, intended for efficient transmission on HF, SSB, etc.
S/V "Listen

Hartley Gardner
Eminent Member
Joined: 6 years ago
Posts: 19

What Jeff said.. 🙂  Some further thoughts - it seems like your vessel has some sort of bonding/grounding system installed, though it may have flaws in it from years of exposure to seawater.  Underwater metals (thru-hulls, prop&shaft, struts (keel?) rudder shaft, and any separate zincs should all be bonded (connected) together - the proper wire color is green, but black is not uncommon.  This is what should be connected to the "RF" or protection ground points on your equipment.  You should NOT connect the DC negative buss to this, nor should you connect the shore power ground wire to it (you DON'T want your vessel's bonding system to become part of the local electrical grid!).  To re-iterate:  the negative side of your DC system is NOT a ground - yes, they are often found to be connected (usually through the starter/alternator and engine block) but you DON'T want fault currents (or worse, lightning current!) using your negative leads to get to ground.

  Some marinas will have a problem with your boat if the shorepower ground connects to the ocean - we ran into this ourselves last year.  One problem area is things like inverters, battery chargers and other AC-powered pieces of equipment, which may have a "grounding" terminal or lug on them - if its connected internally to the AC ground wire, leave the other one NOT connected to your vessel's grounding!  Inverters are a particular offender in this area.


S/V Atsa

Louis Carver
New Member
Joined: 1 year ago
Posts: 1

Underwater metals, including through-hulls, props, shafts, struts (keels?), rudders, and any separate zincs, should be bonded (connected) together. The ideal wire color is green, however black is not unusual. This is what needs to be linked to the equipment's "RF" or protective ground points. The shore power ground wire and the DC negative buss should NOT be connected to this. funny shooter 2

This post was modified 1 year ago by Louis Carver