Of an antenna, the ratio of (a) 4 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 4 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 4 steradians is the total power radiated by the antenna because 4 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.