Let's define the effective aperture of an antenna,
, through the
relationship between the received spectral power and the sky brightness
| (75) |
with the assumption
that the power pattern is independent of infinitesimal surface area of the
receiving antenna in order to be able to factorize the integral on this
infinitesimal surface area. Moreover the factor 1/2 corresponds to the case where the radiation is incoherent
and unpolarized, and the receiver is only sensitive to one
polarization.
This effective aperture corresponds to the fraction of the power density of
a plane wave that is intercepted by the antenna. It has the unit of a
surface [
]. It thus resembles a cross-section in particle
physics. We thus define the aperture efficiency,
, as the ratio of
the effective aperture by the geometric aperture of the antenna
| (76) |