Add a matching stub in-line with your antenna. IT WORKS! Here's how to make one. You need to start with a 6 foot length of coax with these specific properties:
Before I made the stub, I was a bit skeptical about what a little piece of coax would do to improve my set up but here are my numbers before and after the installation of the stub.
So, for a minimal outlay of cash, about $6, you can get a marked improvement in the operation of your screwdriver antenna without taking it apart.
I am not an electrical engineer and will not begin to tell you why this works, but it does. If anyone knows why this matching stub works so well, please tell me and I will include the explanation here.
Here's an explanation I received from Lee - AB7ON.
"[The] matching stub works because the input base impedance to the vertical antenna is very low compared to [the] impedance of the transmission line itself. If you are using RG59 cable, its impedance is 75 ohms. The typical base input impedance of a short vertical mounted on a vehicle can be as low as 1/2 ohm. 1/2 to 5 ohms is typical. Adding your stub matches the 59 ohm cable to the much lower base impedance of the vertical antenna on your vehicle. It is important to remember that the base impedance can vary from one vehicle installation to another. However, I have never experienced an impedance greater than 12 ohms. I would like to point out that because impedances of vehicle installations vary, it is also likely that the length of the matching stub would also have to be adjusted to effect perfect matching. Regardless, the stub you manufactured should improve any situation greatly. "
My antenna is mounted to the frame supporting the rear bumper of a 1996 Ford Explorer. The main coil of the antenna is therefore lower than the roof of the vehicle. This, I am sure, affects my SWR readings. Your mounting situation may be different from mine, so I cannot guarantee the results you will receive. The effects of using this stub may vary considerably due to your own particular circumstances. If your results are not satisfactory, you will still have a short jumper, which always comes in handy for other ham purposes. Good luck. I would enjoy receiving feedback on your successes using this stub.
Burness F. Ansell, III