Editing Standing wave ratio (section)

==Power standing wave ratio==

The term ''power standing wave ratio'' (PSWR) is sometimes referred to, and defined as, the square of the voltage standing wave ratio. The term is widely cited as "misleading".<ref>
{{cite web
 |first=Christian |last=Wolff
 |title=Standing wave ratio
 |website=radartutorial.eu
 |url=http://www.radartutorial.eu/03.linetheory/tl06.en.html
}}
</ref>
{{quote|
The expression "power standing-wave ratio", which may sometimes be encountered, is even more misleading, for the power distribution along a loss-free line is constant.&nbsp;...
|J.H. Gridley (2014)<ref name=Gridley-2014>
{{cite book
 |first=J.H. |last=Gridley
 |year=2014
 |title=Principles of Electrical Transmission Lines in Power and Communication  
 |page=265
 |publisher=Elsevier
 |isbn=978-1483186030
 |url=https://books.google.com/books?id=geNbAwAAQBAJ&pg=PA265
 |via=Google Books
}}
</ref>
}}

However it does correspond to one type of measurement of SWR using what was formerly a standard measuring instrument at microwave frequencies, the [[slotted line]]. The slotted line is a [[waveguide]] (or air-filled coaxial line) in which a small sensing antenna which is part of a ''crystal detector'' or ''detector'' is placed in the electric field in the line. The voltage induced in the antenna is rectified by either a [[diode#Semiconductor diodes|point contact diode (crystal rectifier)]] or a [[Schottky barrier diode]] that is incorporated in the detector.  These detectors have a square law output for low levels of input.  Readings therefore corresponded to the square of the electric field along the slot, ''E''<sup>2</sup>(''x''), with maximum and minimum readings of ''E''<sup>2</sup><sub>max</sub> and  ''E''<sup>2</sup><sub>min</sub> found  as the probe is moved along the slot. The ratio of these yields the ''square'' of the SWR, the so-called PSWR.<ref>
{{cite book
 |first=Bernard Vincent |last=Rollin
 |year=1964
 |title=An Introduction to Electronics
 |page=209
 |publisher=Clarendon Press
 |oclc=1148924
}}
</ref>

This technique of rationalization of terms is fraught with problems.{{clarify|date=January 2019}} The [[square-law detector|square law]] behavior of the detector diode is exhibited only when the voltage across the diode is below the knee of the diode. Once the detected voltage exceeds the knee, the response of the diode becomes nearly linear. In this mode the diode and its associated filtering capacitor produce a voltage that is proportional to the peak of the sampled voltage. The operator of such a detector would not have a ready indication as to the mode in which the detector diode is operating and therefore differentiating the results between SWR or so called PSWR is not practical. Perhaps even worse, is the common case where the minimum detected voltage is below the knee and the maximum voltage is above the knee. In this case, the computed results are largely meaningless. Thus the terms PSWR and Power Standing Wave Ratio are deprecated and should be considered only from a legacy measurement perspective.