| in a given direction, the ratio of the available power at the terminals of a receiving antenna to the power flux density of a plane wave incident from that direction, the wave being polarization matched to the antenna
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 Ae for a given direction of an antenna when used for reception is related to the absolute gain Gi for the given direction when the same antenna is used for transmission by the equation:
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.