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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 Friis transmission equation ： ウィキペディア英語版 Friis transmission equation The Friis transmission equation is used in telecommunications engineering, and gives the power received by one antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The formula was derived in 1945 by Danish-American radio engineer Harald T. Friis at Bell Labs. ==Basic form of equation== In its simplest form, the Friis transmission equation is as follows. Given two antennas, the ratio of power available at the input of the receiving antenna, $P\_r$, to output power to the transmitting antenna, $P\_t$, is given by :$\backslash frac\; =\; G\_t\; G\_r\; \backslash left(\; \backslash frac\; \backslash right)^2$ where $G\_t$ and $G\_r$ are the antenna gains (with respect to an isotropic radiator) of the transmitting and receiving antennas respectively, $\backslash lambda$ is the wavelength, and $R$ is the distance between the antennas. The inverse of the third factor is the so-called free-space path loss. To use the equation as written, the antenna gain may ''not'' be in units of decibels, and the wavelength and distance units must be the same. If the gain has units of dB, the equation is slightly modified to: :$P\_r\; =\; P\_t\; +\; G\_t\; +\; G\_r\; +\; 20\backslash log\_\backslash left(\; \backslash frac\; \backslash right)$ (Gain has units of dB, and power has units of dBm or dBW) In addition to the usual derivation from antenna theory, the basic equation also can be derived from principles of radiometry and scalar diffraction in a manner that emphasizes physical understanding.〔http://dx.doi.org/10.1119/1.4755780〕 The simple form applies only under the following ideal conditions: *$R\backslash gg\backslash lambda$ (reads as $R$ much greater than $\backslash lambda$). If , then the equation would give the physically impossible result that the receive power is greater than the transmit power, a violation of the law of conservation of energy. *The antennas are in unobstructed free space, with no multipath. *$P\_r$ is understood to be the available power at the receive antenna terminals. There is loss introduced by both the cable running to the antenna and the connectors. Furthermore, the power at the output of the antenna will only be fully delivered into the transmission line if the antenna and transmission line are conjugate matched (see impedance match). *$P\_t$ is understood to be the power delivered to the transmit antenna. There is loss introduced by both the cable running to the antenna and the connectors. Furthermore, the power at the input of the antenna will only be fully delivered into freespace if the antenna and transmission line are conjugate matched. *The antennas are correctly aligned and have the same polarization. *The bandwidth is narrow enough that a single value for the wavelength can be assumed. The ideal conditions are almost never achieved in ordinary terrestrial communications, due to obstructions, reflections from buildings, and most importantly reflections from the ground. One situation where the equation is reasonably accurate is in satellite communications when there is negligible atmospheric absorption; another situation is in anechoic chambers specifically designed to minimize reflections. 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「Friis transmission equation」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.