In antenna technology, the radiation power of the antenna is specified in decibels isotropic (dBi). The reference value of 0 dBi refers to the antenna gain of an ideal omnidirectional antenna that receives or transmits RF radiation isotropically from all directions.
Since omnidirectional antennas, Yagi antennas, parabolic antennas or dipole antennas are not used in WLANs and other radio technologies, their dBi value is much higher than that of omnidirectional antennas. For example, dipoles have an antenna gain of about 2.5 dBi, and with yagi or parabolic antennas, antenna gains of 20 dBi and more can be realized. This means that the received radiation or transmitted radiation is 100 times stronger than that of the omnidirectional antenna.
The dBi value is given in data sheets as antenna gain value. Since at lower frequencies the antenna gain is determined with a half-wave dipole and specified in decibel dipole( dBd), there is a direct relationship between the two dB values, which allows conversion from dBi to dBd and vice versa: 0 dBd corresponds to 2.15 dBi.