Ed,
You're on it like a cheap suit! A ground plane must have an area at least
1/4 lambda ^2 to be effective. The top of a car doesn't have sufficient
area to work as ground plane at broadcast freqs. Essentially, ground plane
radials provide an artificial ground that is elevated to the effective
height of the antenna wherever it is above earth. It is generally
understood when we speak of "ground plane" re. communications antennas we
are referring to 50 ohm antennas.

Bob Swinney
"Ed Huntress" wrote in message
...
"Jerry Martes" wrote in message
news:wY5Ud.56428$uc.36861@trnddc04...

What would you consider the minimum length of a conductor that fits the
definition of "ground plane"?

Jerry, unless the terminology has changed in recent years, the term ground
plane refers to an artificial, highly conducting ground, not to the
radiator. Typical ground planes are crossed or radially displaced
conductors, wire meshes, or metal sheets that lie in a horizontal plane
and
are grounded as effectively as conditions permit. If the grounding is very
effective, the ground plane doesn't radiate at all. That's the condition
that's usually desired.

As for the radiator, it can be any length. But the usual purpose is to
have
effective radiation perpendicular to the radiator (vertical radiator;
horizontal radiation, with a single lobe pointing just slightly upwards,
shaped like a donut cut off just below its greatest periphery). At
frequencies up to the edge of UHF, that usually dictates a quarter-wave
radiator.

Is this what you're talking about, or am I on the wrong page here?

--
Ed Huntress