NOTE 1 – If no direction is specified, the direction corresponding to maximum available power is implied.

NOTE 2 – The total effective area is the sum of the partial effective areas for any two orthogonal polarizations of components of the antenna polarization.

NOTE 3 – In free space, the effective area A_e for a given direction of an antenna when used for reception is related to the absolute gain G_i for the given direction when the same antenna is used for transmission by the equation:

${\displaystyle G_{\text{i}}=\frac{4\pi A_{\text{e}}}{{\lambda }^2}}$

where λ is the wavelength.

NOTE 4 – When an aperture illumination function is uniform in amplitude and phase and the given direction is perpendicular to the aperture, the total effective area of a radiating aperture of a lossless antennas is equal to its geometric area.