well it looks like this thread has died on the table waiting for answers, so i will try to do my part of the thread CPR.
The following paragraphs are direct quotes from The Department of the Army manual TM 11-666 titled "Antennas and Radio Propagation".
Ground-Plane Antenna:
A ground plane antenna consists of a vertical radiator which, in effect, carries its own artificial ground. the artificial ground or ground plane consists of a flat disk of metal or a number of metal rods or spokes located at the bottom of the radiator and usually at right angles to it.
Since the metal disk or spokes are not connected directly to ground, they may be referred to as a counterpoise. This term is used rarely, however, this part of the antenna usually being called simply an elevated ground plane.
(now we will back up to address the term 'counterpoise')
Counterpoise:
A counterpoise may replace the usual direct ground connection in which current actually flows to and from the antenna through the ground itself. the counterpoise consists of a structure made of wire erected a short distance off the ground and insulated from ground.
The counterpoise operates by virtue of its capacitance to ground. because of this capacitance, the ground currents which flow normally and usually are collected by conduction now are collected in the form of charge and discharge currents. the end of the antenna which normally is connected directly to ground now is connected to ground through the large capacitance formed by the counterpoise.
If the counterpoise is not well insulated from ground, the effect is much the same as that of a leaky capacitor. Leakage currents flow between the counterpoise and ground so that a poorly insulated counterpoise introduces more losses than no counterpoise at all.
(now back to ground plane antennas)
Any desire to operate such an antenna in conjunction with the actual ground would create high ground losses and would prevent efficient radiation or reception. The ground plane antenna, on the other hand, is usually well elevated so that ground losses are minimized. (yes, this does mean that the higher in the air you get your antenna, the less this all matters)
The elevated ground plane also prevents circulating currents from flowing in a vertical mast that might be used to support the antenna. these currents, if not prevented, would cause the vertical support itself to radiate in the same manner as a long-wire antenna. As a result, undesired high angle radiation would be produced.
As always, i could be misinterpreting something, and am open to being shown the light.
LC