Editing Standing wave ratio (section)

==Methods of measuring standing wave ratio==
[[File:Slotted_line.png|200px|thumb|Slotted line. The probe moves along the line to measure the variable voltage. SWR is the maximum divided by the minimum voltage]]Many different methods can be used to measure standing wave ratio.  The most intuitive method uses a [[slotted line]] which is a section of transmission line with an open slot which allows a probe to detect the actual voltage at various points along the line.<ref>
{{cite book
 |first=Fredrick E.  |last=Terman
 |title=Electronic Measurements
 |publisher=McGraw Hill
 |year=1952
 |lccn=51-12650
 |page=135 ff
}}
</ref>
Thus the maximum and minimum values can be compared directly. This method is used at VHF and higher frequencies. At lower frequencies, such lines are impractically long.

[[Power dividers and directional couplers|Directional couplers]] can be used at HF through microwave frequencies. Some are a quarter wave or more long, which restricts their use to the higher frequencies. Other types of directional couplers sample the current and voltage at a single point in the transmission path and mathematically combine them in such a way as to represent the power flowing in one direction.<ref name="Schulz">
{{cite web
 |last=Schulz |first=Glenn B., (W9IQ)
 |date=January 24, 2018
 |title=How does an SWR meter really work?
 |website=ham.stackexchange.com
 |url=https://ham.stackexchange.com/questions/9778/how-does-an-swr-meter-really-work/9779#9779
 |access-date=March 18, 2018
}}
</ref>
The common type of SWR / power meter used in amateur operation may contain a dual directional coupler. Other types use a single coupler which can be rotated 180 degrees to sample power flowing in either direction. Unidirectional couplers of this type are available for many frequency ranges  and  power levels and with appropriate coupling values for the analog meter used.
[[File:Bird 43 RF power meter.jpg|300px|thumb|A directional wattmeter using a rotatable directional coupler element.]]
The forward and reflected power measured by directional couplers can be used to calculate SWR. The computations can be done mathematically in analog or digital form or by using graphical methods built into the meter as an additional scale or by reading from the crossing point between two needles on the same meter. 
The above measuring instruments can be used "in line" that is, the full power of the transmitter can pass through the measuring device so as to allow continuous monitoring of SWR.  Other instruments, such as network analyzers, low power directional couplers and antenna bridges use low power for the measurement and must be connected in place of the transmitter.  Bridge circuits can be used to directly measure the real and imaginary parts of a load impedance and to use those values to derive SWR. These methods can provide more information than just SWR or forward and reflected power.<ref>{{cite press release
 |title=Nautel adds two models to NX series
 |date=March 11, 2015
 |website=[[Nautel]]
 |url=http://www.nautel.com/press-releases/nautel-adds-two-models-to-nx-series/
 |access-date=July 6, 2017
 |archive-date=August 18, 2016
 |archive-url=https://web.archive.org/web/20160818224451/http://www.nautel.com/press-releases/nautel-adds-two-models-to-nx-series/
 |url-status=dead
 }}</ref>
Stand alone antenna analyzers use various measuring methods and can display SWR and other parameters plotted against frequency.  By using directional couplers and a bridge in combination, it is possible to make an in line instrument that reads directly in complex impedance or in SWR.<ref>
{{cite web
 |title=Model OIB-1 and OIB-3
 |publisher=Delta Electronics, Inc. 
 |website=www.deltaelectronics.com
 |url=http://www.deltaelectronics.com/data/oib1&3.htm
}}
</ref>
Stand alone [[antenna analyzer]]s also are available that measure multiple parameters.