Antenna gain indicates the power radiated from antennas.
This article explains the definition of gain and the difference of absolute gain and relative gain.
Definition of gain
Antenna gain is defined as the ratio of the power radiated from the antenna to the power from reference antenna.
Reference antenna is usually isotropic antenna or half-wave dipole antenna.
It is called absolute gain when reference antenna is isotropic antenna, and relative gain when half-wave dipole antenna.
Absolute gain
The ratio of the power radiated from the antenna to the radiated power from isotropic antenna is called absolute gain.
Although the unit of gain is decibel (dB), the unit of absolute gain is dBi.
Relative gain
The ratio of the power radiated from the antenna to the radiated power from half-wave dipole antenna is called relative gain.
The unit of relative gain is dBd.
Absolute gain of half-wave dipole antenna is 2.14 dBi, thus the following relation is obtained.
\[0\ \mathrm{dBd}= 2.14\ \mathrm{dBi}\]