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.


And modeling a vehicle can be challenging because of the proximity of
conductors, particularly the whip and vehicle. You have to follow the
rules for closely spaced parallel conductors, and watch the average
gain. You might need considerably more segments than normal where
conductors are very close.

Given my limited experience, it's gotta be very difficult to model. My
setup was worst case, as far a sensitivity to bandwidth goes, a
bugcatcher. Best of a bad lot, I guess, but that makes the tuning very
sharp and sensitive. I'm assuming that the antennas that have fixed
elements "work" and tune by being pretty inefficient.

Which makes me suspect that we won't find any Hi-Q HF antennas that
aren't manually tuned in some fashion.

Yes again. Manufacturers discovered long ago that hams like antennas
that are small, broadband and quiet. No problem -- small coils, small
wire, and bingo -- the ideal antenna. Rotten efficiency, but I've heard
countless hams over the years fussing and bragging about low SWR, and
nary a one who said a word about efficiency. Fortunately most hams don't
realize how many QSOs you can have with a watt or two of radiated power,
otherwise they'd be more concerned that that's all they're getting with
their 100 watt rig.

Roy Lewallen, W7EL