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Standing wave ratio (SWR) is the measurement of a type of impedance mismatch that can cause poor transmission efficiency in radio engineering. Impedance mismatch may cause standing waves along the transmission line.
Standing wave ratio is also known as voltage standing wave ratio (VSWR).
A standing wave, in the context of its physical definition, oscillates in time, but includes a peak amplitude profile that does not move in space. Standing waves were first observed by Michael Faraday in the 19th century.
An instrument called an SWR meter can measure standing wave ratio and interpret impedance in relation to the goals of a radio technology. SWR assessment is often used as a type of operational procedure at a radio station.
Standing wave ratio is an example of how traditional radio technologies were engineered and optimized for the radio age. While radio is still useful in many wireless designs, it has been replaced in other sectors by network technologies directing data packets that do not have the same vulnerabilities or measurement standards.