Glossary:VSWR
If you connect a transmitter to an aerial via a long cable, at any discontinuity in the cable, some of the energy will be reflected back towards the transmitter instead of reaching the aerial. At some points between the transmitter and the discontinuity, the phases of the reflected and outgoing waves will reinforce and at other points they will partially cancel, creating "standing waves". The Voltage Standing Wave Ratio or VSWR is the ratio of the highest to the lowest voltages in the pattern of standing waves. Ideally, this ratio is 1:1, meaning that all the energy reaches the destination. The same principles apply to light encountering an imperfect splice or a poor termination in a fibre optic cable.