On Wed, 13 Dec 2006 13:34:10 -0000, "David" nospam@nospam wrote:

An earlier posting said that a magmount works

Hi David,

There are a lot of mystical assertions made in behalf of magmounts.
You need only closely examine one statement to consider the mystical
quotient potential:

because the coax braid (rather
than the metal body of the magmount) capacitively couples to the car.

The "capacitor" is trotted out like it was a mythical animal on public
display. And yet all capacitors have at least two leads. In the
forced arguments of magmounts, obviously one lead is the RF and
presumably the other is the car body. However, those same forced
arguments clam up when the question begs "where do those leads go? (as
all cap leads do eventually complete a circuit to a source).

Very long paths are involved, and those forced arguments failing to
walk that walk (much less do that talk) are not going to explain the
complexities of reactances and resistances that dominate that
dimension.

If current flows on inside of coax braid, and braid is an effective shield,
how can the braid capacitively couple to the metalwork of car?

This statement betrays a misunderstanding of the feed point
relationships and the role of Common Mode.

Either the
braid is shielding or capacitively coupling but not both at same time?

There is no either/or offered in the first place. Your mistake of
feed point relationships has overlooked the "third" wire of the
seemingly two wire load. That "third" wire (the coax shield) runs in
very close proximity to the car body for as great a distance as any
capacitor lead described above. This shield/body relationship offers
vastly more capacitance than any mount.

Review that archives for discussions of the need for feedpoint
decoupling, Common Mode, Chokes, and 1:1 BalUns. This study will
correct the mistake of feed point relationships in your statement
above.

However, the metal body inside the plastic cover does seem rather small to
capacitively couple to the car roof especially for HF.

Indeed.

73's
Richard Clark, KB7QHC