The ratio of the power required at the input of a loss -free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance. Note 1: Antenna gain is usually expressed in dB . Note 2: Unless otherwise specified, 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:
absolute or isotropic gain (G i ), when the reference antenna is an isotropic antenna isolated in space;
gain relative to a half-wave dipole (G d ) when the reference antenna is a half-wave dipole isolated in space and with an equatorial plane that contains the given direction;
gain relative to a short vertical antenna (G r ), 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. [ RR ] ( 188 ) Synonyms gain of an antenna, power gain of an antenna.