With mobile verticals what is ALWAYS forgotten about is that the car
body is just as much a part of the radiating system as the antenna.

The body is just the lower half of a somewhat off-centre-fed,
mishaped, vertical dipole.

It is ridiculous to describe its operation in such laymen's terms, but
the body radiates as much useful power as the antenna itself. And it
is fed with coax whereas, according to perfectionists, to prevent
radiation from the feedline, it ought to be off-centre fed with
450-ohm ladderline. But from wherever it supposedly comes radiation is
never wasted.

I've often thought of modelling a mobile loaded whip in a computer
program. Input data would include body dimensions and height above
ground. With a lot of patience you could do it with an EZNEC type of
program.
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Reg, G4FGQ