Re: VHF antenna issues?
I think he meant "ground" as in "battery negative". He raises a good point in that voltage drops in the positive supply are easy to find, but when "ground" isn't really "ground", weird things happen. Not only should there be 12V (more like 12.5 to 13.6) at the radio, but the "negative" supply lead should be very close to the same voltage as the "negative" battery terminal - especially during transmit, when the radio is drawing (or supposed to be) several amps.
If there is a problem with the official netgative/ground wire, it's possible that the radio is getting its ground instead via the coaxial cable shield, via the antenna, via the mast, via other grounding or even the nav light wiring - very roundabout, and not a good ground.
Make sure the radio still operates (at least in receive) with the antenna disconnected (i.e. when it is *not* grounded via its antenna cable). Testing with an (ungrounded) emergency antenna as suggested earlier would also show this.
...and check the U/I/C setting.
1998 Jeanneau SO45.2