When a feed line's load is not equal to its characteristic impedance, part of the wave bounces back toward the transmitter. The forward and reflected waves add up to a standing wave, and the standing wave ratio (SWR) measures how lumpy it is. SWR of 1:1 means no reflection; the bigger the number, the bigger the mismatch.
How it works
The farther the antenna is from the line's 50 Ω, the bigger the reflected wave.
SWR4 : 1
Power reflected36%
The reflection coefficient Γ is the size of the reflected wave relative to the forward one. A 100 Ω antenna on 50 Ω line gives Γ = (100 − 50) ÷ (100 + 50) = 1/3, an SWR of 2:1 and 11% of the power reflected. A 25 Ω load gives exactly the same numbers, because only the ratio matters. A short or an open circuit reflects everything: Γ = 1 and the SWR is infinite.
The voltage on a mismatched line swings between a maximum and a minimum every quarter wavelength. SWR is that maximum divided by that minimum.
Reflection coefficient |Γ|0.333
SWR2 : 1
Power reflected11.1 %
The reflected wave travels back and meets the forward wave, so the two add in some places and subtract in others. Where they add, the voltage is high; a quarter wavelength away they subtract. SWR is literally Vmax ÷ Vmin along the line. The pattern repeats every half wavelength.
The numbers that matter
SWR
Γ
Return loss
Power reflected
Mismatch loss
1.0:1
0
infinite
0%
0 dB
1.5:1
0.20
14 dB
4%
0.18 dB
2:1
0.33
9.5 dB
11%
0.51 dB
3:1
0.50
6 dB
25%
1.2 dB
5:1
0.67
3.5 dB
44%
2.6 dB
10:1
0.82
1.7 dB
67%
4.8 dB
Mismatch loss is how much less power the load absorbs than the forward power: −10 log10(1 − Γ²). At 2:1 that is half a dB, far below what anyone can hear. Even 3:1 costs about a dB.
Misconceptions
SWR is not efficiency. A dummy load is a perfect 1:1 and radiates nothing; a poor, lossy antenna can show 1:1. SWR says how well the load matches the line, not how well it radiates.
Reflected power is not simply "lost". It goes back toward the transmitter. If the source re-reflects it, it returns to the load, and the real cost is extra loss in the line, which grows quickly with a lossy cable. Many solid-state transmitters instead reduce their output when SWR is high, to protect the output stage.
A low reading at the radio can hide a high SWR at the antenna. The reflected wave must travel the lossy line back as well as forward, so a lossy line makes the SWR look better than it is. See Feed line loss and velocity factor.
Tuning for 1:1 at the radio is not the same as fixing the antenna. A tuner transforms what the radio sees; it does not remove the mismatch on the line. See Antenna tuners.
In practice
Measure SWR with a meter in the line, an antenna analyzer, or a vector network analyzer. Measure at each frequency you use, since an antenna's match changes with frequency.
Below about 2:1 is a good working target for most stations, but a lower figure is not worth chasing at the cost of efficiency elsewhere. Amplifiers and high power care most about SWR; short coax runs at low power tolerate quite a lot.
A sudden change in SWR often means a failing connector, water in the line or a broken antenna element.
The SWR does not change as the line gets longer or shorter (apart from loss), but the impedance seen at the radio does. For that, see The Smith chart.