What is dBi?

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.

Isotropic Antenna Pattern

So if an antenna has a gain of 5 dBi in a particular direction, that means when compared to an Isotropic antenna (which will have a gain of 0 dB in that direction), that antenna has a gain of 5 dB.