Roy Lewallen wrote:

The best arrangement, as others have pointed out, is to mount the
antenna right at the center of the top of the car. This makes the car
"wire" vertical, a much more efficient arrangement, which the
"shootouts" consistently show. You'll also find that larger trucks,
which effectively form a longer vertical "wire" for the car part,
outdo smaller ones for the same whip.

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Well, maybe. The problem is, the higher the mounting point, the shorter
the whip has to be to be legal to drive down the road. If your car was
13 feet five inches tall, your whip could only be one inch long. How
efficient would that be on 80 meters? The point being, everything is a
tradeoff of one thing for another. If the shootouts say a rooftop is
best, ok, but I have to say I'm surprised. At a relatively low
frequency like 80 meters, the car body is more of a coupler to the
earth rather than a real "ground" of it's own. Given that, then the
longer the whip part, the better.

HF mobile antennas are a fascinating subject and one of these days I
will set up a "shootout" range on my 2.5 acres here in the desert and
do some shooting of my own.

Bill, W6WRT