If you can't contribute anything positive to this question, then stay out of
it, with your trash!

I have a 10m base loaded mag mount antenna, that I had to put a piece of
felt over the magnet as not to scratch a new vehicle.

When I did this I apparently changed the capacitance between the mag mount
and the roof of the vehicle. Someone suggested that I might want to replace
the felt with either a large balloon or a large prophylactic as it is
thinner and the capacitance would then return to almost where it should be.

Positive comments only!