An antenna focuses the radio waves in a certain direction. Usually, this is called the main direction. Because of that, in other directions less energy wil be emitted. The gain of an antenna, in a given direction, is usually referenced to an (hypothetical) isotropic antenna, which emits the radiation evenly strong in all directions. The antenna gain is the power in the strongest direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power. In this case the antenna gain (Gi) is often specified in dBi, or decibels over isotropic.