VSWR..

When a transmission line (cable) is terminated by an impedance that does not match the characteristic impedance of the transmission line, not all of the power is absorbed by the termination. Part of the power is reflected back down the transmission line. The forward (or incident) signal mixes with the reverse (or reflected) signal to cause a voltage standing wave pattern on the transmission line. The ratio of the maximum to minimum voltage is known as VSWR, or Voltage Standing Wave Ratio.

A VSWR of 1:1 means that there is no power being reflected back to the source. This is an ideal situation that rarely, if ever, is seen. In the real world, a VSWR of 1.2:1 (or simply 1.2) is considered excellent in most cases. At a VSWR of 2.0, approximately 10% of the power is reflected back to the source. Not only does a high VSWR mean that power is being wasted, the reflected power can cause problems such as heating cables or causing amplifiers to fold-back.
There are various ways of measuring and/or calculating VSWR.

Where Emax = maximum measured voltage
Emin = minimum measured voltage
Ei = incident wave amplitude, volts
Er = reflected wave amplitude, volts
Where Prev = reverse power
Pfwd = forward power

VSWR can also be represented in terms of reflection coefficient:

Where 'p' is the reflection coefficient.