SWR Calculator

What SWR measures

When a feed-line is connected to a load that does not match its characteristic impedance, part of the signal is reflected. The reflected and forward waves combine into a standing wave along the line, and the standing wave ratio, SWR, is the ratio of its maximum to minimum voltage. The reflection coefficient Γ is the ratio of reflected to forward voltage, and it can be found from a power meter: |Γ| = √(reflected ÷ forward). SWR = (1 + |Γ|) ÷ (1 − |Γ|). With 100 W forward and 4 W reflected, |Γ| = 0.2 and the SWR is 1.5:1. A 75 Ω load on a 50 Ω line gives the same 1.5:1, since Γ = (75 − 50) ÷ (75 + 50) = 0.2.

Return loss and mismatch loss

Return loss expresses the same mismatch in decibels: −20 log₁₀|Γ|. An SWR of 1.5:1 is a return loss of 13.98 dB, 2:1 is 9.54 dB and 3:1 is 6.02 dB; a larger return loss means a better match. Mismatch loss is the share of forward power that the load does not accept, −10 log₁₀(1 − |Γ|²): 0.18 dB at 1.5:1, 0.51 dB at 2:1, where 11% of the power is reflected, and 1.25 dB at 3:1, where 25% is. Antenna analysers and network analysers often show return loss; transceivers and wattmeters show SWR.

Reflected power is not wasted power

The most persistent misconception in amateur radio is that reflected power is lost. In a lossless line, the reflected wave travels back to the transmitter end, where the transmitter output network or an antenna tuner reflects it forward again; in the steady state, all the power except line loss reaches the antenna. A 2:1 SWR on a short run of good coax costs a fraction of a decibel, a change no one could hear on the other end. What high SWR does do is raise line losses, especially on long or lossy cable at VHF, and make many solid-state transmitters reduce power to protect themselves, typically above 2:1 or 3:1. Use the coax loss calculator to see the real cost.

Complex loads

A load with reactance, such as an antenna away from resonance, also causes reflection. The reflection coefficient is (Z − Z₀) ÷ (Z + Z₀) with complex numbers, and its magnitude decides the SWR. A load of 50 + j50 Ω on a 50 Ω line gives an SWR of 2.62:1, the same as 25 + j25 Ω. That is why two very different impedances can show the same SWR, and why an antenna analyser that shows R and X tells you more than an SWR meter about which way to adjust.

How to use it

1. Choose whether you have forward and reflected power, an SWR reading or a load impedance.
2. Enter the values, with the line impedance for an impedance calculation, normally 50 Ω.
3. Read the SWR, reflection coefficient, return loss and mismatch loss.
4. Use the coax loss calculator to see what the SWR costs on your feed-line.