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VSWR (Voltage Standing Wave Ratio)

16. December 2020 09:06 by Christian in EMC/EMI, Test Equipment, Calibrations
For a radio (transmitter or receiver) to deliver power to an antenna, the impedance of the radi

For a radio (transmitter or receiver) to deliver power to an antenna, the impedance of the radio and transmission line must be well matched to the antenna's impedance.

The parameter VSWR is a measure that numerically describes how well the antenna impedance is matched to the radio or transmission line it is connected to.

The voltage component of a standing wave in a uniform transmission line consists of the

  • forward wave (with complex amplitude V_{f}) superimposed on the
  • reflected wave (with complex amplitude V_{r}).

A wave is partly reflected when a transmission line is terminated with other than an impedance equal to its characteristic impedance.

The reflection coefficient \Gamma  can be defined as:

\Gamma  = Vr / Vf

\Gamma  is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases with \Gamma  measured at the load are:

  •    {\displaystyle \Gamma =-1}   complete negative reflection, when the line is short-circuited,
  •    \Gamma =0       no reflection,                              when the line is perfectly matched,
  •    {\displaystyle \Gamma =+1}   complete positive reflection,  when the line is open-circuited.

The voltage standing wave ratio is then:

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