The dBi specification is based on the decibel , a logarithm ic measure of relative power . Suppose an antenna A produces an electromagnetic field of intensity I A microwatts per square meter ( I A µW/m 2 ) in its favored direction at a point located some distance away. Also, suppose an isotropic antenna Q produces an electromagnetic field of intensity I Q µW/m 2 at the same distance. Then the gain G of antenna A , in dBi, is:
G = 10 log 10 ( I A / I Q )
A dipole antenna has a gain of 2.15 dBi. An isotropic radiator has a gain of 0 dBi.