That question has been answered several times in this newsgroup. For one
of the most recent answers, go to groups.google.com and find my posting
on July 21, 2003 in the thread " efficiency of horizontal vs vertical
antennas".

Roy Lewallen, W7EL

Serge Stroobandt, ON4BAA wrote:

This conversation is starting to get interesting :-)

INVERTED GROUNDPLANES:
What about using an inverted 1/4wave groundplane in order to avoid ground
losses?
Then you have the high impedance point of the antenna close to the lossy
(low impedance) earth.
Due to the severe mismatch less power will couple into the earth.

The low impedance point (feed point) of the antenna is safely high up in the
(high impedance) air then.