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