Travelling wave, a wave motion in which the surface of equal phases (phase waves, fronts) moves with a finite velocity. A travelling wave whose group velocity differs from zero carries energy, momentum or other characteristics. Within the range of validity of the superposition principle (linear systems), two identical periodic travelling waves propagating in opposite directions form the so-called standing wave. When their amplitudes differ, a partially travelling wave arises, characterised by the travelling-wave ratio (TWR), or the standing-wave ratio (SWR), or the reflection coefficient Г, equal to the ratio of the amplitudes of the counter-propagating waves, with
SWR = 1/TWR = (1 + |Г|2)/(1 -|Г|2)
For optimistic transmission of energy the transmission lines must be matched (a travelling-wave regime is obtained inside the line, with SWR = 1, Г=0). For direct-current electrical circuits this regime corresponds to the internal resistance of the source being equal to the resistance of the load.
SWR = 1/TWR = (1 + |Г|2)/(1 -|Г|2)
For optimistic transmission of energy the transmission lines must be matched (a travelling-wave regime is obtained inside the line, with SWR = 1, Г=0). For direct-current electrical circuits this regime corresponds to the internal resistance of the source being equal to the resistance of the load.