The antenna gain (G) is a relative quantity which refers to a reference antenna. The reference quantity is the received field strength of the antenna in the receive direction to the received field strength of the reference antenna, also in the receive direction. The reference antenna is either an isotropic antenna or a half-wave dipole.
An isotropic antenna radiates evenly in all directions and has an evenly distributed energy field. The level of an isotropic antenna is specified in decibels isotropic( dBi), where 0 dBi is the reference level for calculating the antenna gain. The dBi reference technique is mainly used for microwaves, in other frequency ranges the reference is the half-wave dipole, and the reference level is decibel dipole( dBd). The logarithmic ratio of the field strengths of the measured antenna to the reference antenna is the antenna gain, expressed in decibels( dB). In general, the antenna gain depends on the radiation pattern of the antenna.
The more pronounced the main lobe of the radiation pattern is, the higher is the received field strength and thus also the antenna gain. This is confirmed by looking at the antenna gain of antennas with different radiation patterns. According to this, a Discone antenna has an antenna gain that is between 3 dBi and 4 dBi. The measured field strength of a Yagi antenna, which has a much more pronounced radiation pattern, is much higher and the antenna gain is between 5 dBi and 15 dBi. Even greater is the received field strength of a parabolic antenna, whose field strength may well be 100 times higher than that of the isotropic radiator. Accordingly, the antenna gain is between 15 dBi and 25 dBi and depends on the size of the parabolic reflector.
Besides the antenna gain, there is also the antenna gain factor, which is directly related to the antenna gain. The antenna gain factor (G) is a dimensionless value which results from the quotient between the maximum power density of the directional antenna and the power density of an isotropic omnidirectional antenna.