antenna mounting locations for a vertical whip on a vehicle are on the roof or on the trunk. If you mount the whip
antenna on one of the bumpers or in the bed of a pickup, much of the antenna length will be below the height of the
vehicle. Then a lot of the signal will get sucked down by all of that steel.
You have to get power from the transceiver to the antenna. To do this, you need a transmission line. As stated
previously, use a high quality RG8, RG8X or RG58 coaxial cable with at least a 95% outer braid.
This is a very critical issue because at the base of the antenna high voltages (up to 10,000 volts) can develop.
The feedthrough hole must withstand this high voltage. Generally, a high voltage porcelain feed connector, which
is very common in the marine industry, is used. For a vehicular installation, the only way to go about this properly
is to make a large (2 inch in diameter) hole with a fiberglass plughole. The wire will go through the center, with the
minimum distances between the metal structure and the wire at least 1.5 inches.
The feedline has a few peculiar relationships with the antenna elements. In the case of most antennas, the
feedline should be run perpendicularly to the antenna element. In most mobile or
portable installations, the feedline is short and it runs either perpendicular to the
antenna or up to the antenna, so there is no problem.
Obviously, if the feedline at a base station should be run perpendicularly to a
horizontal antenna, you can have some real problems with running the feedline in a safe and appropriate location.
Fortunately, unlike unbalanced ladder line feedline, coaxial cable can be run along buildings and along or even
under the ground. This makes feedline runs much more convenient. Also, the feedline doesn't have to run
perpendicularly for its entire length; it's just best to have the last quarter wavelength (as compared to the operating
frequency of the antenna) run this way.
81
The SGC Building, 13737 S.E. 26th St. Bellevue, Washington 98005 USA
P.O. Box 3526, 98009 Tel: (425) 746-6310 Fax: (425) 746-6384
© 1997 SGC Inc.
FEEDLINES
FEEDTHROUGH CONNECTORS OR HOLES
FEEDLINE ROUTING
In most cases, the
feedline should be run
perpendicularly to the
antenna element.
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