Denny wrote:
Hmm, all this presumes that whip is the radiator and the car is merely
the ground return for the radiated RF currents...

Assume an ideal vertical antenna with elevated radials
fed by coax with no common-mode current on the coax.
Install a large meshed variable capacitor between the
coax braid and the radials so nothing really changes.
Then start reducing the capacitance to the point where
common-mode current on the feedline becomes a problem.
This is akin to the magmount problem at lower frequencies.
Some solve the problem with a direct strap from the
magmount frame to the vehicle chassis, i.e. by shorting
out that variable capacitor.
--
73, Cecil http://www.w5dxp.com