What SWR actually tells you
Standing wave ratio measures how well your antenna system matches the 50 ohms your radio expects. When the match is perfect, all the power travels up the coax and nothing comes back, giving an SWR of 1:1. When it isn't, some power is reflected at the antenna and travels back down the cable, and the forward and reflected waves combine into a pattern of voltage peaks and dips along the line. SWR is the ratio between those peaks and dips.
A high SWR doesn't mean the reflected power is lost forever. Most of it is reflected again at the radio or tuner and goes back up to the antenna. What you actually lose is the extra cable loss on each trip, plus the power your radio holds back to protect its output stage. That's why a 2:1 SWR on a short run of good coax costs you almost nothing, while the same SWR through 30 m of thin cable on VHF can hurt.
The formulas
SWR = (1 + |Γ|) ÷ (1 − |Γ|)
Return loss = −20 × log₁₀ |Γ|
Mismatch loss = −10 × log₁₀ (1 − |Γ|²)
Return loss is how analysers and NanoVNAs usually report a match. They often show it as S11 with a minus sign, so an S11 of −20 dB is 20 dB of return loss, which is an SWR of about 1.22:1.
Reference table
| SWR | Return loss | |Γ| | Power reflected | Mismatch loss |
|---|
What's a good SWR?
- Below 1.5:1 is excellent. There's nothing to gain by chasing it lower.
- 1.5:1 to 2:1 is fine for almost every station. Most modern radios give full power up to about 2:1.
- 2:1 to 3:1 works, but many radios start cutting power. Use a tuner, or trim the antenna.
- Above 3:1 means something needs fixing, so check connectors, the feedpoint and the antenna length before transmitting at full power.
To bring SWR down on a wire antenna, measure where the lowest SWR falls. If it's below the frequency you want, the antenna is too long, so shorten it. If it's above, the antenna is too short. The dipole calculator shows how much to cut.
Questions
How do I calculate SWR from forward and reflected power?
Divide the reflected power by the forward power and take the square root to get the reflection coefficient. Then SWR = (1 + that number) ÷ (1 − that number). For 100 W forward and 4 W reflected, the coefficient is 0.2 and the SWR is 1.5:1.
Is VSWR the same as SWR?
Yes, in practice. VSWR is the voltage standing wave ratio, and it's what nearly every SWR meter shows. Amateurs simply call it SWR.
Does SWR change along the length of coax?
The true SWR on the line is the same everywhere apart from cable loss, which makes it look lower at the radio end. Changing the coax length can change what your meter shows if the meter or the coax itself is part of the problem, for example from common-mode current on the outside of the braid.
What return loss equals an SWR of 2:1?
About 9.5 dB. An SWR of 1.5:1 is about 14 dB, and 1.2:1 is about 20.8 dB.