On Wed, 13 Jun 2007 00:07:45 +0100, "Mike Kaliski"
wrote:


"larya" wrote in message
roups.com...
Sorry if I used the wrong words... but I am curious as to how those
glass mounted antennas work... Is there a toutorial that explains
it?...Larry ve3fxq


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

Typically you are looing at a half wave (or even 5/8ths) fed with a
variable cap. The cap tunes out the reactance. Contrary to popular
belief I've had good results with them. No they don't match the big
Diamond colinear I use on top of my 4-Runner, but they did work.

frequencies, the capacitance between the two metal plates passes a fair
amount of the signal through.

There are a few things to look out for. Don't fix the plates over areas of
glass containing demist wires. Vehicles with metallised/solar glass screens
may not be suitable for this type of antenna. At high transmitter powers,
the plates heat up and can cause damage to laminated glass. Larger plates
disipate more heat but 25 watts is probably the maximum power it is
advisable to use.


My transmitter runs 50 watts into a 160 watt mirage and I had no
trouble using such antennas for over 12 years on a T-Bird and Transam.

I never saw the mount get more than warm enough to just tell it had
changed temp.


The ARRL website contains design details, unfortunately for members only.
Details also appear in the ARRL Antenna Book.


Calculate a half wave for the frequency, find a ball park cap value
and have at it.



It is probably fair to say that a glass mount antenna should be used as a
last resort, when no other type of antenna installation is possible, loaned
or hire vehicle, vintage vehicle etc. A magnetic mount antenna system
usually offers far better performance all other things being equal, with
fixed antenna systems being the most efficient of all.

The external mounting point (position and design) may be subject to local
vehicle construction and use regulations relating to the security of
external fittings and risks poses to other road users.

Mike G0ULI