The ratio, usually expressed in decibels, of the power required at the input of a loss free reference antenna to the power supplied to the input of a given antenna to produce, in a given direction, the same field strength of the same power flux-density at the same distance. When not specified otherwise, the gain refers to the direction of maximum radiation. The gain may be considered for a specified polarization.
Depending on the choice of the reference antenna, a distinction is made between:
(a) absolute or isotropic gain (Gi ), when the reference antenna is an isotropic antenna isolated in space;
(b) gain relative to a half-wave dipole (Gd ), when the reference antenna is a half-wave dipole isolated in space whose equatorial plane contains the given direction;
(c) gain relative to a short vertical antenna (Gv ), when the reference antenna is a linear conductor, much shorter than one quarter of the wavelength, normal to the surface of a perfectly conducting plane which contains the given direction.