# Effective Area

Gain and directivity of an antenna in transmitter or receiver operation are equal (reciprocity).

The received power ${P}_{r}$ is obtained from power density $S$ (electromagnetic field).

Effective area of an antenna ${A}_{e}$ is defined according to:

(1) ${\mathrm{P}}_{r}={A}_{e}\cdot S$

Isotropic antenna:

(2) ${A}_{ei}=\frac{{\lambda }^{2}}{4\pi }$

Real antenna with directivity, $D$ :

(3) ${A}_{e}=D\cdot {A}_{ei}$