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