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 ) superimposed on the
- reflected wave (with complex amplitude ).
A wave is partly reflected when a transmission line is terminated with other than an impedance equal to its characteristic impedance.
The reflection coefficient can be defined as:
= Vr / Vf
is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases with measured at the load are:
- complete negative reflection, when the line is short-circuited,
- no reflection, when the line is perfectly matched,
- complete positive reflection, when the line is open-circuited.
The voltage standing wave ratio is then: