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Concept
Aperture (antenna)
Summary
In electromagnetics and antenna theory, the aperture of an antenna is defined as "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. 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."
The effective area of an antenna is defined as "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 on the antenna from that direction, the wave being polarization matched to the antenna." Of particular note in this definition is that both effective area and power flux density are functions of incident angle of a plane wave. Assume a plane wave from a particular direction , which are the azimuth and elevation angles relative to the array normal, has a power flux density ; this is the amount of power passing through a unit area normal to the direction of the plane wave of one square meter.
By definition, if an antenna delivers watts to the transmission line connected to its output terminals when irradiated by a uniform field of power density watts per square meter, the antenna's effective area for the direction of that plane wave is given by
The power accepted by the antenna (the power at the antenna terminals) is less than the power received by an antenna by the radiation efficiency of the antenna. is equal to the power density of the electromagnetic energy , where is the unit vector normal to the array aperture, multiplied by the physical aperture area . The incoming radiation is assumed to have the same polarization as the antenna. Therefore,
and
The effective area of an antenna or aperture is based upon a receiving antenna. However, due to reciprocity, an antenna's directivity in receiving and transmitting are identical, so the power transmitted by an antenna in different directions (the radiation pattern) is also proportional to the effective area .
Official source
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