Antenna Gain (dBi and dBd)
Antenna gain measures how much an antenna concentrates power in its main direction, expressed in dBi (against a theoretical isotropic antenna) or dBd (against a half-wave dipole).
What is Antenna Gain (dBi and dBd)?
An isotropic antenna radiates equally in all directions and exists only in theory. A dipole has a gain of 2.15 dBi, so a figure in dBd plus 2.15 gives dBi.