directivity factor[də‚rek′tiv·əd·ə ‚fak·tər]
(1) For a transmitting antenna, a number indicating the factor by which the radiated power would have to be increased if the antenna were replaced by an isotropic radiator (assuming the same field intensity for the antenna and the isotropic radiator).
(2) For a receiving antenna, a number indicating the factor by which the input power of the receiver for the direction of maximum reception exceeds the mean power obtained by averaging the power received from all directions of reception (if the field intensity at the antenna location is equal for any direction of wave incidence).
The directivity factor is a quantitative characterization of the capacity of a transmitting antenna to concentrate the radiated energy in a given direction or the capacity of a receiving antenna to select signals incident from a given direction.
O. N. TERESHIN and G. K. GALIMOV