What is the difference between dBi and dBd?

Antenna gain is measured in decibels. It is the ratio between the gain of the antenna compared to the gain of an isotropic antenna. An isotropic antenna is a theoretical antenna which radiates power uniformly in all directions. When we calculate the gain of an antenna and compare it to an isotropic antenna the unit of the gain is dBi (i stands for Isotropic antenna). An Isotropic antenna has a 0 dB power rating, i.e it has no gain/loss when compared to it self.

Dipole antennas have a different radiation pattern compared to isotropic antennas. Though they are known to be omni-directional, their radiation pattern is 360 degrees in the horizontal plane and 75 degrees in the vertical plane (if standing vertically). It resembles the shape of a donut. Because the beam is slightly concentrated, dipole antennas have a gain over isotropic antennas of 2.15 dB in the horizontal plane. Which means they have a gain of 2.15 dBi.

Some antennas are rated in comparison to a dipole antenna. In this case the unit of the gain would be dBd. So an antenna that has 0 dBd gain will have a gain of 2.15 dBi.

Many manufacturers specify antenna gain in dB. It is important for you to check if this gain is dBd or dBi.