The maximum of directivity function is called the directivity.

In defining directivity function total radiated power is taken as the reference. Another parameter called the gain of an antenna is defined in the similar manner which takes into account the total input power rather than the total radiated power is used as the reference. The amount of power given as input to the antenna is not fully radiated.

.......................................................(7.28)

where is the radiation efficiency of the antenna.

The gain of the antenna is defined as