On Sat, 06 Oct 2007 17:22:07 -0700, Roy Lewallen
wrote:

The gain increase of 1/2 or 5/8 wavelength antennas over shorter
monopoles comes about by a narrowing of the lobe pointing toward the
horizon. Unfortunately, though, radiation at the low angles is severely
attenuated by reflection from real ground. And this is just where most
of the power from longer verticals is going. So a 5/8 wave HF vertical
usually won't exhibit the gain over a shorter antenna you see with a
perfect ground simulation. Likewise, a finite ground plane like a car
roof impacts low angle radiation, so it has more of an effect on a 1/2
or 5/8 wave radiator than a shorter one, and once again you often won't
see the gain you might expect.

A few minutes with the demo version of EZNEC or a similar program shows
the effect of finite ground on various antenna heights very clearly. Use
MININEC-type ground to eliminate the separate effect of ground system
resistive loss. The full EZNEC program will let you model an antenna on
a car top (by using a wire grid to simulate the car top).


It more that just the car's roof that impacts the antennas' radiation
patterns. The car's whole body has a significant effect and there will
be variations - sometimes large - between different vehicles.

http://k6mhe.com/files/mobile_vhf_ant.pdf

Danny, K6MHE