I am mounting a KW40 11/10
meter antenna on the rear of my 40 ft. diesel pusher motor coach. In
case you are not familiar with the KW40 it is a center loaded short
antenna with a 3KW + rated coil by the manufacturer.
My Predator screwdriver, mounted on the front of the Coach, works very
well but the 3 inch diameter coil doesn't want to hit 10 meters, thus
the attempt to use the KW40 on the rear.
There is no satisfactory ground plane on a fiberglass coach which creates
matching problems especially for an antenna high up on the rear of the
coach . Our Coach is 13 feet tall because it has basement air
conditioning. The height of the short center loaded antenna suffers from being
so high with inadequate ground plane.
I have used everything from Hustlers and bug Catchers to Screwdriver
antennas that were bumper mounted and have worked the world with them.
No matching problems even when running a kilowatt mobile. I would like
to hear from Hams who have matched and used mobile antennas on
fiberglass vehicles and mounted at least eight feet above ground.
I have spent the last several days climbing up and down a step ladder
trying to get a match. I have a MFJ 249 SWR analyzer but not one that
analyzes the base impedance. I am attempting to use an inductive match
to keep the antenna at DC ground but I am about to put a variable cap to ground
and determine the capacitance needed to match the base.
After 55 years of hamming and having a Commercial
General Radio Telephone license, I thought this would be easy, but a
Motor coach is a different animal when it comes to antenna tuning.
If you have experience with POOR ground planes for mobile antennas, please
share your experience. Thanks and 73, John, K5PGW
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