Directive gain

In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in all directions) radiating the same total power.

An antenna's directivity is a component of its gain; the other component is its (electrical) efficiency. Directivity is an important measure because many antennas and optical systems are designed to radiate electromagnetic waves in a single direction or over a narrow angle. Directivity is also defined for an antenna receiving electromagnetic waves, and its directivity when receiving is equal to its directivity when transmitting.

The directivity of an actual antenna can vary from 1.76 dBi for a short dipole, to as much as 50 dBi for a large dish antenna.

The directivity, D, of an antenna is the maximal value of its directive gain. Directive gain is represented as ${\displaystyle D(\theta ,\phi )}$ and compares the radiant intensity (power per unit solid angle) ${\displaystyle U(\theta ,\phi )}$ that an antenna creates in a particular direction against the average value over all directions:

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