Why mount the radar dome on an independent spar instead of the mast? Just the weight up high changing the center of gravity?
Exactly - the J/30 is a fairly light boat to begin with, and on top of that San Francisco gets pretty breezy and I'm usually shorthanded with not a lot of weight on the rail. I thought it would be prudent to minimize weight aloft - every pound counts and 25 lbs of radar counts a lot. The pole is only 8' off the deck.
The pole at the stern will have other uses as well. I plan on adding a floodlight to light the cockpit at night, and possibly a davit for general use. I might even move the stern light onto it for better visibility.