Hmm, all this presumes that whip is the radiator and the car is merely
the ground return for the radiated RF currents...
What happens if we change our point of view and say that the whip plus
car is all part of an off center fed dipole that is insulated from the
earth by rubber donuts, and the RF currents on the external coax are
merely the equalizing flow of RF returning via unequal capacitive
coupling of the car body and whip to the earth? At that point we are
measuring the return flow between two unequal halves of a dipole...

denny - here there be dragons...

Cecil Moore wrote:
David wrote:
An earlier posting said that a magmount works because the coax braid (rather
than the metal body of the magmount) capacitively couples to the car.

If the coax braid is coupling much RF to chassis ground,
then the capacitive coupling provided by the magnets is
too small. That tends to happen when magmounts are used
on 75m. I will make some common mode current measurements
for magmount coax for 75m and 17m and post them here.
--
73, Cecil http://www.w5dxp.com