A useful parameter calculating the receive power of an antenna is the effective area
or effective aperture.
Assume that a plane wave with the same polarization as the receive antenna is incident upon the antenna. Further
assume that the wave is travelling towards the antenna in the antenna's direction of maximum radiation (the direction from
which the most power would be received).
Then the effective aperture parameter describes how much power is captured from a given plane wave.
Let p be the power density of the plane wave (in W/m^2).
If P_t represents the power (in Watts) at the antennas terminals available to the antenna's receiver, then:
Hence, the effective area simply represents how much power is captured from the plane wave and delivered by the
antenna. This area factors in the losses intrinsic to the antenna (ohmic losses, dielectric losses, etc.).
A general relation for the effective aperture in terms of the peak
antenna gain (G) of any
antenna is given by:
aperture or effective area can be measured on actual antennas by comparison with a known antenna with a given
effective aperture, or
by calculation using the measured gain and the above equation.
Effective aperture will be a useful concept for calculating received power from a plane wave. To
see this in action, go to the next section on the Friis transmission formula.