Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"I`m not sure why, but most amateurs don`t seem to realize that a whip
isn`t an "antenna" and the car "ground", but each is half of a
dipole-like antenna."

Not exactly..

In a common balanced dipole, each half has the same current quantity and
direction, though in one half the current flows toward the feedpoint
while it flows away in the other half.

From such a dipole, both its halves contribute equally to its radiation.

Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.

A ground plane is a poor model of how currents flow along a car body.

Consider an antenna mounted on top of a car. From the base of the
antenna, the current flows equally in all directions away from the base
of the antenna, like a ground plane. This current doesn't contribute
much radiation, for the reasons you state. But then it reaches the edge
of the top of the car and flows downward. All the portions of the
current are now flowing the same direction, and their fields don't
cancel but add in phase. The net result is the same as if it were just
flowing down a fat wire the height of the car. If the car is eight feet
high, the field from the car will equal the field from an eight foot
whip. In fact, unless the whip is top loaded to make the current
uniform, the car will radiate more than the whip, because the
capacitance of the car to ground will tend to give the car a uniform
current distribution, like a top hat does to a whip. This will increase
the radiated field strength from the car.

Now consider a bumper mounted antenna. The current will spread from the
base and proceed around the car. More will probably flow on the bottom
than the top and sides due to coupling with the ground, but all portions
will be flowing in the same direction and all will radiate. There is no
place on the car where the current distribution or flow pattern
resembles current on a ground plane.

An antenna is also called an aerial. It is defined as that part of a
radio station which radiates or receives radio waves into or from space.

An antenna ground system is defined as that portion of an antenna system
closely associated with the earth and including an extensive conducting
surface which may be the earth itself.

The problem here is that the currents don't care how you define things.
They flow where the physical laws dictate. Defining "ground" doesn't
make them behave differently.

Most radio amateurs have it right.

If your view represents that of most amateurs, they don't.

Roy Lewallen, W7EL