A measure of the effectiveness of a directional antenna in a given direction compared with a standard reference, usually an isotropic antenna. Specifically, the antenna gain is the ratio of the intensity radiated in a given direction from the antenna to the intensity of an isotropic source emitting the same power as the real antenna. Sometimes the term antenna gain is used in place of axial gain, which is the value of the antenna gain in the direction of the axis of the main lobe. See antenna pattern.