Larry,

The principle is very simple for vhf/uhf antennas. A pair of rectangular
metal plates, usually brass or copper, a couple of inches square are fixed
either side of the window glass. The two plates form the plates of a
capacitor with the glass acting as a dielectric. The coax centre is
connected to the centre of the plate inside the vehicle, with the screen
being bonded to vehicle earth via the transceiver or wired directly, the
screen is also connected to the edge of the inside plate via a small
preset
capacitor. The capacitor is adjusted for lowest SWR. A wire antenna is
connected directly to the plate on the outside of the vehicle. At vhf/uhf
frequencies, the capacitance between the two metal plates passes a fair
amount of the signal through.


That doesn't quite tie up with the ones that I have taken apart. On those
the matching box contained a parallel tuned circuit with the coax screen
connected to the cold end and the 'inside' plate to the top, 'hot', end of
the tuned circuit. The coax inner was tapped a short way up the coil.

The whip is something close to a half wave, which is end fed, and of course
high impedance at that point, so it requires very little capacitance through
the glass to achieve very good coupling.

Through glass antennas do work very well, but they are sensitive to the
composition of the glass; tinting and reflective metal coatings have a
detrimental effect on performance. I have used and made several home brew
versions with results similar to a mag mount on the roof.

73
Jeff
G8HUL