[Barrykgerdes]

The magnetic base is to make the aerial sit nicely on the roof of a car. A dipole or extended dipole like you have can be thought of as half of the aerial the other half is the "reflection" from the ground plane.

The theory of radiation from that aerial is a bit too complex for a simple reply but essentially it will behave like 3 half waves in phase and give a theoretical gain over a basic isotropic aerial of about 8db. Or about 5db over a dipole.

If you don't have a metal roof on the van the side of an old computer case will make quite a reasonable ground plane. That's what I use and it gives a good match to the cable.

Barry