Editing Standing wave ratio (section)

==Impedance matching==
{{Main|Impedance matching}}
SWR is used as a measure of [[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 [[transmitter|radio transmitters]] and receivers with their [[Antenna (radio)|antenna]]s, as well as similar uses of RF cables such as [[cable television]] connections to TV receivers and [[distribution amplifier]]s. 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 the imaginary part of the [[complex number|complex impedance]] of both the source and load to be zero, 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 wave]]s 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 {{mvar|Z}}<sub>load</sub> and the source impedance {{nobr| {{mvar|Z}}<sub>source</sub> {{=}} {{mvar|Z}}<sup>*</sup><sub>load</sub>,}} 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 {{mvar|Z}}<sub>0</sub>. However the SWR will generally not be 1:1, depending only on {{mvar|Z}}<sub>load</sub> and {{mvar|Z}}<sub>0</sub>. With a different length of transmission line, the source will see a different impedance than {{mvar|Z}}<sub>load</sub> which may or may not be a good match to the source. Sometimes this is deliberate, as when a [[Quarter-wave impedance transformer|quarter-wave matching section]] is used to improve the match between an otherwise mismatched source and load.

However typical [[radio frequency|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, {{mvar|Z}}<sub>load</sub> {{=}} {{mvar|Z}}<sub>0</sub>, ''always'' ensures 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 ({{mvar|Z}}<sub>load</sub> {{=}} {{mvar|Z}}<sub>0</sub>) 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 {{mvar|Z}}<sub>load</sub> 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.