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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Directivity ： ウィキペディア英語版 Directivity In electromagnetics, directivity is a figure of merit, usually for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in all directions) radiating the same total power. An antenna's directivity is a component of its gain; the other component is its (electrical) efficiency. Directivity is an important measure because most emissions are intended to go in a particular direction or at least in a particular plane (horizontal or vertical); emissions in other directions or planes are wasteful (or worse). The directivity of an actual antenna can vary from 1.76 dBi for a short dipole, to as much as 50 dBi for a large dish antenna.〔 ==Definition== The ''directivity'', ''D'', of an antenna is the maximum value of its ''directive gain''. Directive gain is represented as $D(\backslash theta,\backslash phi)$, and compares the radiation intensity (power per unit solid angle) $U(\backslash theta,\; \backslash phi)$ that an antenna creates in a particular direction against the average value over all directions: :$D(\backslash theta,\backslash phi)\; =\; \backslash frac$ Here $\backslash theta$ and $\backslash phi$ are the standard spherical coordinate angles, $U(\backslash theta,\; \backslash phi)$ is the radiation intensity, which is the power density per unit solid angle, and $P\_\backslash mathrm$ is the total radiated power. The quantities $U(\backslash theta,\backslash phi)$ and $P\_\backslash mathrm$ satisfy the relation: :$P\_\backslash mathrm\; =\; \backslash int\_^\backslash int\_^U\backslash sin\backslash theta\backslash ,d\backslash theta\backslash ,d\backslash phi;$ that is, the total radiated power $P\_\backslash mathrm$ is the power per unit solid angle $U(\backslash theta,\; \backslash phi)$ integrated over a spherical surface. Since there are 4π steradians on the surface of a sphere, the quantity $P\_\backslash mathrm/4\backslash pi$ represents the ''average'' power per unit solid angle. In other words, directive gain is the radiation intensity of an antenna at a particular $(\backslash theta,\; \backslash phi)$ coordinate combination divided by what the radiation intensity would have been had the antenna been an isotropic antenna radiating the same amount of total power into space. ''Directivity'', then, is the maximum directive gain value found among all possible solid angles: : & = \frac \int _^\int _^U(\theta,\phi) \sin\theta \, d\theta\, d\phi}\end The word ''directivity'' is also sometimes used as a synonym for directive gain. This usage is readily understood, as the direction will be specified, or directional dependence implied. Later editions of the IEEE Dictionary〔 specifically endorse this usage; nevertheless it has yet to be universally adopted. 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「Directivity」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.