Essentially, the ground plane acts as the "missing half" of a dipole two element, half wave long, center fed antenna. It can be thought of as the "return current" path for the radiating antenna.
There are a variety of ground planes, including drooping ground planes, and flat circular ground plane antennas.
Aground plane may consist of a natural surface, such as the earth or an artificial surface of opportunity (such as the roof of a motor vehicle).
As radials get fewer and shorter, the ground plane becomes lossier.
The efficiency of a ground plane can be measured in ohms as the loss component (i.e., fewer ohms are better) of an antenna's total input impedance.
In VHF (30 to 300 MHz or wavelengths 10 to 1 meter) used in mobile communications, the metal of a car body begins to act as an efficient quarter-wavelength groundplane.
