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 current flows on inside of coax braid, and braid is an effective shield,
how can the braid capacitively couple to the metalwork of car? Either the
braid is shielding or capacitively coupling but not both at same time?

Perhaps capacitive coupling is from open unshielded body of magmount, and
coax braid adds to area of 'capacitor plate'.

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

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

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

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

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

On Wed, 13 Dec 2006 11:47:17 -0800, Richard Clark
wrote:

....
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.

The "third wire" (being the current flowing on the outside of outer
conductor of the coax) is more properly a transmission line itself,
possibly a leaky (ie radiating) transmission line.

For example, were you to place a magmount on a large metal ground
plane (that is a sheet, not wires), and lay the coax straight from a
magmount to the source whose ground terminal is bonded to that ground
plane, the effect of the coax will depend on the electrical length
formed by the outside of the shield of the coax and the ground plane.
Consider the effects of the transmission line so formed were it an
electrical quarter wave, and an electrical half wave (taking into
account any bulk shunt capacitance at the magmount due to the mount
itself..

Who knows what happens in actual magmount installations? They truly
fall into the category of "works" on lower frequencies, whatever
"works" means to the individual.

Owen

....
--