The directivity is a parameter which quantifies the radiation focusing capability of an antenna. It is a measure of how the antenna guides power in the desired direction compared to the other directions.

The directivity is one of the very important parameters used for comparing the performance of different antennas.

The directive gain of an antenna is defined as

Where is the radiation intensity defined as the power per unit solid angle, and is the average value the radiation intensity over solid angle. We then have

The maximum value of the directive gain is called the directivity of an antenna. The Directivity therefore is

The directivity is parameter solely defined by the radiation pattern of an antenna.

For the Hertz dipole the directivity is

For the large antennas with single main beam like the parabolic dishes, the main beam is very narrow and the directivity can be approximately written as

The directivity is generally given in dB ( = 10 log D).

For an antenna with circular beam, the directivity is approximately