decibel dipol (dBd)
For antenna gain, the reference antenna is usually an isotropic antenna that radiates the same RF energy in all directions. The antenna gain for the isotropic antenna is specified in decibels isotropic( dBi).
However, a half-wave dipole can also be used as a reference antenna. Such a half-wave dipole has a higher antenna gain than an isotropic antenna. If the half-wave dipole is used as reference antenna, the antenna gain is given in decibel dipole (dBd) and the values of other antennas are related to the dBd value. The dBd value is 2.15 dB higher than the dBi value. So, for example, if an antenna has an antenna gain of 3 dBd, then this corresponds to 5.15 dBi.