In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

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.

Some people refer to mobile verticals as "short tuned counterpoises,
working with a large untuned radiator".

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.

There are probably some not-quite-obvious gotchas to be considered.

I've observed that a significant number of vehicles (e.g. my Ford
Aerostar minivan) don't weld the body panels to the frame. Instead,
the side panels and roof are _glued_ to the framework using a
structural adhesive, and there's no DC connection at these points...
just a capacitive one (probably with high capacitive reactance).
There's DC continuity in some places, but not at all contact points.

Depending on how the vertical is grounded to the vehicle (to the
panels, to the frame, or to both), the RF "ground" currents are likely
to vary quite a bit.

Similar issues are likely to exist in recent-model cars which have a
lot of fiberglass or other composite-plastic panels and structure.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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