# Aperture (of an antenna)¶

#A surface, near or on an antenna, on which it is convenient to make assumptions regarding the field values for the purpose of computing fields at external points. Note: The aperture is often taken as that portion of a plane surface near the antenna, perpendicular to the direction of maximum radiation, through which the major part of the radiation passes [29].

#The power received by an antenna can be described in terms of a collecting area known as the effective aperture. This may be associated with a physical aperture but even a linear wire antenna can be described in terms of a notional collecting area. If the electromagnetic field has a power density of $$P_d$$, and the receiving antenna has an effective aperture of $$A_e$$ then it will capture a power of $$P_d A_e$$. The effective area depends only on the wavelength of the radiation and the directivity or gain [23].