The directivity factor of an antenna is the ratio of the square of the field strength generated by the antenna in a given direction to the average value of the square of the field strength over all directions. The DF indicates how many times the radiated power must be increased when replacing a directional antenna with a non-directional one so that the field strength they create at the observation point remains the same.
Directivity factor is a dimensionless quantity and is expressed in decibels (dB, dBi, dBd). The Latin letter D (from the English term “Directivity”) is used to denote the DF.
Typically, the DF value D0 is considered in the direction of maximum antenna radiation. In this case, the DF becomes a measure of the antenna’s ability to concentrate electromagnetic radiation energy into a narrow beam. By definition, the directivity factor is directly related to the shape of the radiation pattern diagram (RP) of the antenna (more precisely — to the shape of the antenna’s directional characteristic).
It is important to distinguish between the DF and the antenna gain (AG): the DF is entirely determined by the shape of the RP of the antenna and does not take into account the antenna efficiency, i.e., it does not account for energy losses in the structural elements of the antenna or in objects located in the antenna’s near field.
Depending on the antenna design and the operating wavelength, the DF value in the direction of maximum radiation can range from units to millions. The narrower the main lobe of the RP and the lower the level of the side lobes, the greater the DF.