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In electromagnetics and antenna theory, antenna aperture, effective area, or receiving cross section, is a measure of how effective an antenna is at receiving the power of electromagnetic radiation (such as radio waves). The aperture is defined as the area, oriented perpendicular to the direction of an incoming electromagnetic wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point x {displaystyle mathbf {x} } , a beam of electromagnetic radiation has an irradiance or power flux density S ( x ) {displaystyle S(mathbf {x} )} which is the amount of power passing through a unit area of one square meter. If an antenna delivers P o {displaystyle P_{o}} watts to the load connected to its output terminals (e.g. the receiver) when irradiated by a uniform field of power density S {displaystyle S} watts per square meter, the antenna's aperture A e {displaystyle A_{e}} in square meters is given by: A e = λ 2 4 π {displaystyle A_{ ext{e}}={lambda ^{2} over 4pi }} In electromagnetics and antenna theory, antenna aperture, effective area, or receiving cross section, is a measure of how effective an antenna is at receiving the power of electromagnetic radiation (such as radio waves). The aperture is defined as the area, oriented perpendicular to the direction of an incoming electromagnetic wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point x {displaystyle mathbf {x} } , a beam of electromagnetic radiation has an irradiance or power flux density S ( x ) {displaystyle S(mathbf {x} )} which is the amount of power passing through a unit area of one square meter. If an antenna delivers P o {displaystyle P_{o}} watts to the load connected to its output terminals (e.g. the receiver) when irradiated by a uniform field of power density S {displaystyle S} watts per square meter, the antenna's aperture A e {displaystyle A_{e}} in square meters is given by: So the power received by an antenna (in watts) is equal to the power density of the electromagnetic energy (in watts per square meter), multiplied by its aperture (in square meters). The larger an antenna's aperture, the more power it can collect from a given electromagnetic field. To actually obtain the predicted power available P o {displaystyle P_{o}} , the polarization of the incoming waves must match the polarization of the antenna, and the load (receiver) must be impedance matched to the antenna's feedpoint impedance. Although this concept is based on an antenna receiving an electromagnetic wave, knowing A e {displaystyle A_{e}} directly supplies the (power) gain of that antenna. Due to reciprocity, an antenna's gain in receiving and transmitting are identical. Therefore, A e {displaystyle A_{e}} can be used to compute the performance of a transmitting antenna also. Note that A e {displaystyle A_{e}} is a function of the direction of the electromagnetic wave relative to the orientation of the antenna, since the gain of an antenna varies according to its radiation pattern. When no direction is specified, A e {displaystyle A_{e}} is understood to refer to its maximum value, with the antenna oriented so its main lobe, the axis of maximum sensitivity, is directed toward the source. In general, the aperture of an antenna is not directly related to its physical size. However some types of antennas, for example parabolic dishes and horn antennas, have a physical aperture (opening) which collects the radio waves. In these aperture antennas, the effective aperture A e {displaystyle A_{e}} is always less than the area of the antenna's physical aperture A phys {displaystyle A_{ ext{phys}}} , otherwise the antenna could produce more power from its terminals than the radio power entering its aperture, violating conservation of energy. An antenna's aperture efficiency, e a {displaystyle e_{a}} is defined as the ratio of these two areas: The aperture efficiency is a dimensionless parameter between 0 and 1.0 that measures how close the antenna comes to using all the radio wave power entering its physical aperture. If the antenna were perfectly efficient, all the radio power falling within its physical aperture would be converted to electrical power delivered to the load attached to its output terminals, so these two areas would be equal A e = A phys {displaystyle A_{e}=A_{ ext{phys}}} and the aperture efficiency would be 1.0. But all antennas have losses, such as power dissipated as heat in the resistance of its elements, nonuniform illumination by its feed, and radio waves scattered by structural supports and diffraction at the aperture edge, which reduce the power output. Aperture efficiencies of typical antennas vary from 0.35 to 0.70 but can range up to 0.90. The directivity of an antenna, its ability to direct radio waves in one direction or receive from a single direction, is measured by a parameter called its isotropic gain G {displaystyle G} , which is the ratio of the power P o {displaystyle P_{o}} received by the antenna to the power P iso {displaystyle P_{ ext{iso}}} that would be received by a hypothetical isotropic antenna, which receives power equally well from all directions. It can be seen that gain is also equal to the ratio of the apertures of these antennas

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