On Tue, 28 Jun 2005 14:10:10 -0400, Scott in Baltimore
wrote:

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.
****

If the shield is the counterpoise for the antenna, then the antenna is
installed with no ground and thus inproperly installed period. In a
properly install antenna system you should not have common mode
currents residing on the shield of the coaxial transmission line.

All to often on cars today, there is far to much plastic and not
enough metal to offer a sufficient RF ground. The vehicle frame and
body, if metal, should be the uppper plate of a capacitor that is
formed with the Earth below. The metal under the antenna is important
for radiation. The more the better.

james