the voltage standing-wave ratio (VSWR)
/ VEE-ess-double-you-arr, or 'vis-war' /
When a wave going down a line meets its reflected echo coming back, the two overlap. At some spots along the line they always add up and the voltage is large; at other spots they always cancel and the voltage is small. The pattern stands still in space even though the waves are moving, which is why it is called a standing wave. The voltage standing-wave ratio, VSWR, is simply the biggest voltage on the line divided by the smallest. It is the everyday number RF engineers use to say how well a load is matched.
VSWR connects directly to the reflection coefficient gamma: VSWR = (1 + |gamma|) / (1 - |gamma|). A perfect match has gamma = 0 and VSWR = 1:1, the peaks and dips are equal because there is no reflected wave to interfere. A total mismatch (open or short) has |gamma| = 1 and VSWR is infinite. A modest mismatch like our earlier 50 ohm line into 75 ohms had gamma = 0.2, giving VSWR = 1.2/0.8 = 1.5:1. As a feel for the numbers, VSWR 1.5:1 reflects about 4 percent of the power, VSWR 2:1 reflects about 11 percent, and VSWR 3:1 reflects about 25 percent.
VSWR is the field meter of matching: cheap SWR meters in series with an antenna feed read it directly, and a ham radio operator tunes an antenna by minimising it. A high VSWR warns that power is being reflected, that the transmitter may overheat, and that range will suffer. Honest caveats: VSWR alone does not tell you which way the load is mismatched (too high or too low, too inductive or too capacitive), so it cannot guide a fix the way a Smith chart can. And a 1:1 reading at one frequency says nothing about the rest of the band, real antennas are only well matched over a limited range.
A ham operator adjusts an antenna and watches the SWR meter fall from 3:1 to 1.2:1. At 3:1 about a quarter of the transmit power was reflecting back down the coax; at 1.2:1 only about 1 percent is, so almost all the power now reaches the antenna and radiates.
Lower VSWR means less reflected power and a better-matched antenna.
VSWR tells you how much is reflected but not in which direction the mismatch lies, so it cannot by itself guide a correction. A perfect 1:1 at one frequency says nothing about the rest of the band.