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) P r = A e S

Isotropic antenna:

(2) A e i = λ 2 4 π

Real antenna with directivity, D :

(3) A e = D A e i