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In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction. As a receiving antenna, the figure describes how well the antenna converts radio waves arriving from a specified direction into electrical power. When no direction is specified, "gain" is understood to refer to the peak value of the gain. A plot of the gain as a function of direction is called the radiation pattern. Antenna gain is usually defined as the ratio of the power produced by the antenna from a far-field source on the antenna's beam axis to the power produced by a hypothetical lossless ''isotropic antenna'', which is equally sensitive to signals from all directions. Usually this ratio is expressed in decibels, and these units are referred to as "''decibels-isotropic''" (dBi). An alternative definition compares the antenna to the power received by a lossless half-wave dipole antenna, in which case the units are written as ''dBd''. Since a lossless dipole antenna has a gain of 2.15 dBi, the relation between these units is: ''gain in dBd'' = ''gain in dBi'' - 2.15 dB . For a given frequency, the antenna's effective area is proportional to the power gain. An antenna's effective length is proportional to the ''square root'' of the antenna's gain for a particular frequency and radiation resistance. Due to reciprocity, the gain of any antenna when receiving is equal to its gain when transmitting. Directive gain or directivity is a different measure which does ''not'' take an antenna's electrical efficiency into account. This term is sometimes more relevant in the case of a receiving antenna where one is concerned mainly with the ability of an antenna to receive signals from one direction while rejecting interfering signals coming from a different direction. ==Power gain== Power gain (or simply gain) is a unitless measure that combines an antenna's efficiency ''Eantenna'' and directivity ''D'': : The notions of efficiency and directivity depend on the following. 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 gain」の詳細全文を読む スポンサード リンク
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