Jerry wrote:

Roy is certainly right! The vehicle is the other half of the "dipole". It
is why if you stick a bugcatcher at random on a broomstick, it won't work:
SWR will be thru the roof.. Or you could hang a dipole for a given band
(your choice), then snip off the side that went to the coax shield (or vice
versa). So, yes, the vehicle is the other half of the antenna. You could
even look at it as an right-side up "L"!

You have to look at where the currents go. If you had a completely
symmetrical vehicle and put the antenna at the very center of the top,
the horizontal currents along the top would produce fields that almost
completely cancel, just like a bunch of radials. But when they reach the
edge and flow downward, the fields are all in the same direction and
add. So it radiates just like a very fat wire.

But if you put an antenna on the edge of the roof, then the fields from
the horizontal currents won't cancel and you'll have some radiation from
the top as well as the sides. And so forth.

Roy Lewallen, W7EL