Editing Standing wave ratio (section)

{{Short description|Measure used in radio engineering and telecommunications}}

[[File:SWR-Verlauf an der 7MHz HB9XBG-Antenne.jpg|thumb|SWR of a vertical [[HB9XBG Antenna]] for the [[40 meter band|40m-band]] as a function of frequency]]

In [[Radio-frequency engineering|radio engineering]] and [[telecommunications]], '''standing wave ratio''' ('''SWR''') is a measure of [[impedance matching]] of [[Electrical load|load]]s to the [[characteristic impedance]] of a [[transmission line]] or [[waveguide]]. Impedance mismatches result in [[standing wave]]s 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 (physics)|node]] (minimum) along the line.

'''Voltage standing wave ratio (VSWR)''' (pronounced "vizwar"<ref name="Knott">
{{cite book
  | last1 = Knott     | first1 = Eugene F.
  | last2 = Shaeffer  | first2 = John F.
  | last3 = Tuley     | first3 = Michael T.
  | year  = 2004
  | title = Radar cross section
  | series = SciTech Radar and Defense Series
  | edition = 2nd
  | publisher = SciTech Publishing
  | isbn = 978-1-891121-25-8 
  | page = 374
  | url = https://books.google.com/books?id=0WuGjb8sqCUC&pg=PA374
}}
</ref><ref name="Schaub">
{{cite book
  | last1 = Schaub  | first1 = Keith B.
  | last2 = Kelly   | first2 = Joe
  | year  = 2004
  | title = Production testing of RF and system-on-a-chip devices for wireless communications
  | series = Artech House microwave library
  | publisher = Artech House
  | isbn = 978-1-58053-692-9
  | page = 93
  | url = https://books.google.com/books?id=26RfoKg2UxIC&pg=PA93
}}
</ref>) is the ratio of maximum to minimum [[voltage]] on a transmission line . For example, a VSWR of 1.2 means a peak voltage 1.2 times the minimum voltage along that line, if the line is at least one half wavelength long.

A SWR can be also defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's [[Current (electricity)|current]]s, [[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,<ref>
{{cite book
 |first=Samuel  |last=Silver 
 |year=1984 |orig-year=1949
 |title=Microwave Antenna Theory and Design
 |publisher=IEE
 |isbn=0863410170
 |page=28
}}
</ref> however, this deprecated term has no direct physical relation to power actually involved in transmission.

SWR is usually measured using a dedicated instrument called an [[SWR meter]]. Since SWR is a measure of the load impedance relative to the characteristic impedance of the transmission line in use (which together determine the [[reflection coefficient]] as described [[#Relationship to the reflection coefficient|below]]), a given SWR meter can interpret the impedance it sees in terms of SWR only if it has been designed for the same particular characteristic impedance as the line. In practice most transmission lines used in these applications are [[coaxial cable]]s with an impedance of either 50 or 75&nbsp;[[ohm]]s, so most SWR meters correspond to one of these.

Checking the SWR is a standard procedure in a radio station. Although the same information could be obtained by measuring the load's impedance with an [[impedance analyzer]] (or "impedance bridge"), the SWR meter is simpler and more robust for this purpose. By measuring the magnitude of the impedance mismatch at the transmitter output it reveals problems due to either the antenna or the transmission line.