John, I have no direct experience with your particular matching problem but
have a couple of comments.
We all assume that matching of short vertical radiators requires an
impedance step up to match 50 ohm coax. In general, that would be the case.
With
the feed point elevated as you described, it is conceivable that the base
impedance is already at or higher than 50 ohms, especially if there are some
losses thrown in.
In that case, an impedance step down would be needed and that would require
a different matching scheme than what is normally used. It would be
helpful if the actual R +/-j of the antenna feed point could be determined.
73,
Gerald K5GW
In a message dated 7/7/2013 10:40:54 P.M. Central Daylight Time,
k5pgw@yahoo.com writes:
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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