Fabricated a backing plate out of .064 flat plate (was a panel cutout for a Dynon...). Match drilled the plate to the bottom skin behind the baggage bulkhead, with the center of the antenna 4" to the left of centerline. Measured this distance from centerline about 5 feet aft, and hung a plumb-bob. Used this as a reference to align the antenna to the slipstream. Attached 4 x #8 nutplates (to the backing plate with flush rivets), and tested everything for fit. Counter-sunk the mounting holes in the backing plate. I have yet to dimple the skin and rivet in place -- will need some help with that step. With the backing plate attached to the skin with clecos, and the antenna fastened to the backing plate, I measured the ohms resistance at all of the screw heads to ground to be less than .003 ohms. I'm satisfied that once mounted in place, the antenna will have a good ground plane (I will also prep the mating surfaces between the backing plate and skin to make good contact).
