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In electromagnetics and antenna theory, antenna aperture or effective area is a measure of how effective an antenna is at receiving the power of radio waves. The aperture is defined as the area, oriented perpendicular to the direction of an incoming radio wave, which would intercept the same amount of power from that wave as is produced by the antenna receiving it. At any point, a beam of radio waves has an ''irradiance'' or ''power flux density'' ''(PFD)'' which is the amount of radio power passing through a unit area of one square meter. If an antenna delivers an output power of ''Po'' watts to the load connected to its output terminals when irradiated by a uniform field of power density ''PFD'' watts per square metre, the antenna's aperture ''Aeff'' in square metres is given by: : . So the power output of an antenna in watts is equal to the power density of the radio waves in watts per square metre, multiplied by its aperture in square metres. The larger an antenna's aperture is, the more power it can collect from a given field of radio waves. To actually obtain the predicted power available ''Po'', 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 a radio frequency wave, knowing ''Aeff'' directly supplies the (power) gain of that antenna. Due to reciprocity, an antenna's gain in receiving and transmitting are identical. Therefore ''Aeff'' can just as well be used to compute the performance of a transmitting antenna. Note that ''Aeff'' is a function of the direction of the radio 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, ''Aeff'' 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. ==Aperture efficiency== 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 horns, have a physical aperture (opening) which collects the radio waves. In these ''aperture antennas'', the effective aperture ''Aeff'' must always be less than the area of the antenna's physical aperture ''Aphys'', as can be seen from the definition above. An antenna's ''aperture efficiency'', ea 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 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 ''Aeff'' = ''Aphys'' 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Antenna aperture」の詳細全文を読む スポンサード リンク
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