This page describes antenna directivity and gain. The directivity of an antenna is the ratio of the power density S (radiant intensity per unit area) of the real antenna in its main direction to a hypothetical but non-existent isotropic radiator that would radiate uniformly in all directions. The power density of the isotropic radiator is thus the radiated power evenly distributed on a sphere surface. Every real antenna has a more or less strong directivity. The directivity is defined by the ratio of the power density of the real antenna in its main direction to the power density of the isotropic omni-directional antenna.
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