SWR is an abbreviation for the standing wave ratio, a commonly used measure of the relative mismatch between the load and the transmission line to which it is attached.

Given a transmission line with characteristic impedance Zo and a load impedance Zl, the reflection coefficient Γ at the load is given by

Γ = (Zl - Zo)/(Zl + Zo)

Note that if the load is perfectly matched to the line, i.e. Zl = Zo, the reflection coefficient is zero and all power will be delivered to the load.

Given a certain Γ, the SWR is given by

SWR = (1 + |Γ|)/(1 - Γ)

Given a perfect match, the SWR is 1:1. Given the ultimate mismatch, i.e. Zl = 0 (short circuit) or Zl = ∞ (open circuit), the SWR goes to ∞.

Graphically, the SWR for any load impedance and transmission line can also be found by plotting the normalized impedance circle on a Smith Chart, and finding where the circle intersects the horizontal axis for values greater than or equal to unity.

The SWR is an important measure when dealing with transmitters connected to an antenna system through a feed line. The output amplifiers in such transmitters are generally impedance matched to a certain type of transmission line, for example a 50 Ohm coaxial cable. If the transmitting antenna is mismatched to the transmission line, a high SWR can result. Much of the reflected energy must then be absorbed by the feed line (which is by design low-loss) or the transmitter. A high SWR can lead to burning up or damaging the power transistors or power tube in the amplifier.

Impedance matching the antenna to the feed line is often of utmost importance, and is subject to another writeup. Achieving a 2:1 SWR or less if often a good rule of thumb in achieving a decent match and protecting your transmitting equipment.