From AMS Glossary
(Also called antenna pattern, lobe pattern, coverage diagram.) A diagram showing the intensity of the radiation field in all directions from a transmitting radio or radar antenna at a given distance from the antenna.
It usually refers to the geometrical solid in space that encompasses all of the lobes of the antenna. For a receiving antenna, it is the response of the antenna to a signal having unit field strength and arriving from all directions. The transmitting and receiving radiation patterns for a single antenna are identical. Two types of radiation patterns should be distinguished: 1) the free space radiation pattern that is the complete lobe pattern of the antenna and is a function of the wavelength, feed system, and reflector characteristics; and 2) the field radiation pattern that differs primarily from the free space pattern by the formation of interference lobes whenever direct and reflected wavetrains interfere with each other, as is found in most surface-based radars. The envelope of these interference lobes has the same shape, but, for a perfectly reflecting surface, it has up to twice the amplitude of the free-space radiation pattern.