JosephKK wrote:
On Mon, 16 Feb 2009 10:21:56 -0800, Roy Lewallen
wrote:

Michael Coslo wrote:
Roy Lewallen wrote:
At HF, a vehicle isn't "ground" or a "counterpoise", but the bottom
half of an asymmetric dipole. It radiates at least as much as the
"antenna" due to currents flowing downward along the outside. Calling
a vehicle "ground" or "counterpoise" doesn't impart magical properties
-- it's a conductor carrying currents whose fields don't cancel. In
other words, it's an integral, radiating portion of the antenna. You
can't leave this significant part of the antenna out of a model and
expect the model to give correct results.
I'm assuming that there is a capacitor formed by the car body being some
few inches away from the physical ground also?
Yes. This alters the current distribution on the vehicle, and can make
it an even more effective radiator than the "antenna".

Maybe, maybe not. Roadway surfaces are rarely conductive. More like
static dissipative materials. While the area is significant the
opposite conductive pole plate is missing, computing the effective
capacitance may be challenging.

Much more than the roadway surface is involved. The skin depth in
average soil varies from about 12.6 feet at 30 MHz to 15.9 feet at 3.5
MHz. So significant current flows to depths of several tens of feet,
well below the road surface. Within this distance of the surface you'll
usually find strata with various conductivities and permittivities, as
well as possibly buried pipes, rebar, and who knows what else. Computing
an "effective capacitance" is virtually impossible, and useless if the
vehicle is to be moved even a short distance. So only generalizations
are possible. But any reasonable assumption about the characteristics of
the ground under the vehicle points to it having a significant impact on
the current distribution.

Roy Lewallen, W7EL