Sunday, July 4, 2010

I’m considering an on-glass VHF antenna for my car ...

Q Randy, KF4PPC, asks, “I’m considering an on-glass VHF antenna for my car. I don’t want to drill holes in the car body, nor do I want to risk scratching the finish with a magnetic mount, so an on-glass antenna seems like a decent compromise. What’s your opinion?”

A For most applications an on-glass antenna will perform adequately. This is particularly true if you do most of your VHF/UHF mobile operating well within the coverage of a sensitive repeater system. If you think you’ll be operating in fringe areas, however, the “compromise” aspects of an on-glass antenna versus a more traditional mobile antenna may be something to consider. (And, by the way, if you use ample car wax and other methods—such as encasing the magnetic base in a plastic bag— a magnetic-mount antenna doesn’t have to present a danger to your car’s finish.)

An on-glass antenna system functions by passing RF to and from the interior of the vehicle through the window glass. The “secret” of the system is found in the two adhesive plates that you attach to the window, one inside and one outside. With the glass between them they create a capacitor that passes RF.

The most important thing to know about glass-mount antennasis that you must follow the installation procedures to the letter. Read and re-read. Check and double check. You get only one chance to attach the adhesive mounting hardware on both sides of the glass. Once they touch the surface, you are committed to their placement. I cannot stress this point enough. The two halves must match per the instructions and once attached, they can’t be moved.

Every installation is different, but I’d suggest that you avoid installing the antenna over embedded heating elements. In addition, there is the issue of passivated glass. Passivation is a process by which tin, copper, magnesium, iron, titanium or gold is applied to the finished glass, or during the manufacture of the glass. Some amateurs report difficulty installing and tuning on-glass antennas that are attached to passivated glass, but others say it isn’t a problem.

From QST October 1999