Directive gain
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In telecommunication, the term directive gain has the following meanings:
1. Of an antenna, the ratio of (a) 4π times the radiance, i.e., power radiated per unit solid angle (watts per steradian), in a given direction to (b) the total power, i.e., the power radiated to 4π steradians.
Note 1: The directive gain is usually expressed in dB.
Note 2: The directive gain is relative to an isotropic antenna.
Note 3: The power radiated to 4π steradians is the total power radiated by the antenna because 4π steradians constitute an entire sphere.
2. Of an antenna, for a given direction, the ratio of the radiance, i.e., the radiation intensity, produced in the given direction to the average value of the radiance in all directions.
Note 1: If the direction is not specified, the direction of maximum radiance is assumed.
Note 2: The directive gain is usually expressed in dB.
Source: from Federal Standard 1037C and from MIL-STD-188