In
antenna design, gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an
isotropic antenna, the gain is often expressed in units of dBi (decibels over isotropic). For example, a
dipole antenna has a gain of 2.14 dBi
[1]. Sometimes, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to produce), in which case the gain of the antenna in question is measured in dBd (decibels over dipole).
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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 (Gi), when the
reference antenna is an
isotropic antenna isolated in space;
gain relative to a half-wave dipole (Gd) 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 (Gr), 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.
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