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In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line. The SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio or VSWR (sometimes pronounced "viswar"〔 〕〔 〕). For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's currents, electric field strength, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical. The power standing wave ratio (PSWR) is defined as the square of the VSWR,〔Samuel Silver, ''Microwave Antenna Theory and Design'', p. 28, IEE, 1984 (originally published 1949) ISBN 0863410170.〕 however this terminology has no physical relation to actual powers involved in transmission. The SWR can be measured with an instrument called an SWR meter. Since SWR is defined relative to the transmission line's characteristic impedance, the SWR meter must be constructed for that impedance; in practice most transmission lines used in these applications are coaxial cables with an impedance of either 50 or 75 ohms. Checking the SWR is a standard procedure in a radio station, for instance, to verify impedance matching of the antenna to the transmission line (and transmitter). Unlike connecting an impedance analyzer (or "impedance bridge") directly to the antenna (or other load), the SWR does not measure the actual impedance of the load, but quantifies the magnitude of the impedance mismatch just performing a measurement on the transmitter side of the transmission line. ==Impedance matching== (詳細はimpedance matching of a load to the characteristic impedance of a transmission line carrying radio frequency (RF) signals. This especially applies to transmission lines connecting radio transmitters and receivers with their antennas, as well as similar uses of RF cables such as cable television connections to TV receivers and distribution amplifiers. Impedance matching is achieved when the source impedance is the complex conjugate of the load impedance. The easiest way of achieving this, and the way that minimizes losses along the transmission line, is for both the source and load to be real, that is, pure resistances, equal to the characteristic impedance of the transmission line. When there is a mismatch between the load impedance and the transmission line, part of the forward wave sent toward the load is reflected back along the transmission line towards the source. The source then sees a different impedance than it expects which can lead to lesser (or in some cases, more) power being supplied by it, the result being very sensitive to the electrical length of the transmission line. Such a mismatch is usually undesired and results in standing waves along the transmission line which magnifies transmission line losses (significant at higher frequencies and for longer cables). The SWR is a measure of the depth of those standing waves and is therefore a measure of the matching of the load to the transmission line. A matched load would result in an SWR of 1:1 implying no reflected wave. An infinite SWR represents complete reflection by a load unable to absorb electrical power, with all the incident power reflected back towards the source. It should be understood that the match of a load to the transmission line is different from the match of a ''source'' to the transmission line or the match of a source to the load ''seen through'' the transmission line. For instance, if there is a perfect match between the load impedance ''Z''load and the source impedance ''Z''source=''Z'' *load, that perfect match will remain if the source and load are connected through a transmission line with an electrical length of one half wavelength (or a multiple of one half wavelengths) using a transmission line of ''any'' characteristic impedance ''Z''0. However the SWR will generally not be 1:1, depending only on ''Z''load and ''Z''0. With a different length of transmission line, the source will see a different impedance than ''Z''load which may or may not be a good match to the source. Sometimes this is deliberate, as when a quarter-wave matching section is used to improve the match between an otherwise mismatched source and load. However typical RF sources such as transmitters and signal generators are designed to look into a purely resistive load impedance such as 50Ω or 75Ω, corresponding to common transmission lines' characteristic impedances. In those cases, matching the load to the transmission line, ''Z''load=''Z''0, ''always'' insures that the source will see the same load impedance as if the transmission line weren't there. This is identical to a 1:1 SWR. This condition ( ''Z''load=''Z''0) also means that the load seen by the source is independent of the transmission line's electrical length. Since the electrical length of a physical segment of transmission line depends on the signal frequency, violation of this condition means that the impedance seen by the source through the transmission line becomes a function of frequency (especially if the line is long), even if ''Z''load is frequency-independent. So in practice, a good SWR (near 1:1) implies a transmitter's output seeing the exact impedance it expects for optimum and safe operation. 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「standing wave ratio」の詳細全文を読む スポンサード リンク
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