[TowerTalk] short mobile antenna matching

John King k5pgw at yahoo.com
Sun Jul 7 23:38:31 EDT 2013


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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