butch is right, the ground is capacitively coupled...it's an RF ground, 
instead of an electrical ground.
You will not be able to get continuity between the magnit and the shield of 
the coax...the magnit itself isn't conductive anyway, so it wouldn't be 
directly grounded no matter what.
the metal shell that goes over the magnit is connected to the coax shield, 
and the edges of the shell will sometimes come into contact with the metal 
mounting surface and you can get a potential direct ground that 
way...however, if it's going on a vehicle, the paint is not conductive, so 
again, there will be no direct ground to the vehicle body or chassis.
with mobile antennas, capacitive ground seems to be as effective in the 
performance of the antenna as a direct ground to the vehicle via a permanent 
mount.
A 5/8's magnit mount antenna appears to work just as well as a 5/8's wave 
fender mounted antenna that has the shield grounded right to the vehicle 
body.
Where this becomes an issue is with HF installations where there needs to be 
a counterpoise or grounding system of some kind for the radiator to work 
against.
HF antennas that use magnetic mounts don't seem to radiate as well as HF 
antennas that are hard mounted to the chassis or body of the vehicle.

73
Colin, V A6BS
----- Original Message ----- 
From: "Kevin Minor" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, November 22, 2011 2:11 AM
Subject: Re: How good a ground plane is a tripod?


> Hi Howard.
>
> Interesting thought of the magnet being electricly insolated from the 
> thing
> you're using as a ground.  If this is true, then why worry about a ground 
> in
> the first place?  I'm not questioning you.  I've always been told to give 
> a
> mag mount as much ground as you can, preferably a quarter wave.
>
> You never know what you'll learn from this hobby.
>
> 73
> Kevin Minor, Lexington, KY
> [log in to unmask]