How do reflection coefficient, VSWR and return loss relate?
The reflection coefficient is Γ = (ZL − Z0)/(ZL + Z0). From its magnitude: VSWR = (1 + |Γ|)/(1 − |Γ|), return loss RL = −20·log10|Γ| dB, and mismatch loss ML = −10·log10(1 − |Γ|²) dB. A VSWR of 2:1 means |Γ| = 0.333, RL = 9.5 dB and only 0.51 dB mismatch loss.[1]
What does each quantity mean?
- Γ (complex): ratio of reflected to incident voltage wave at a reference plane; equals S11 of a one-port.
- VSWR: ratio of maximum to minimum voltage of the standing wave on a lossless line.
- Return loss: how far below the incident power the reflected power is, a positive number in dB. (S11 in dB is its negative.)
- Mismatch loss: reduction in delivered power relative to a matched load.
What is the conversion table?
| VSWR | |Γ| | Return loss (dB) | Reflected power | Mismatch loss (dB) |
|---|---|---|---|---|
| 1.05 | 0.024 | 32.3 | 0.06% | 0.003 |
| 1.2 | 0.091 | 20.8 | 0.8% | 0.036 |
| 1.5 | 0.200 | 14.0 | 4.0% | 0.18 |
| 2.0 | 0.333 | 9.5 | 11.1% | 0.51 |
| 3.0 | 0.500 | 6.0 | 25% | 1.25 |
| 5.0 | 0.667 | 3.5 | 44% | 2.55 |
| ∞ | 1 | 0 | 100% | ∞ |
Why does a small mismatch matter at all?
Mismatch loss is small, but reflections also cause ripple in cascaded systems, uncertainty in power measurement (mismatch uncertainty between two imperfect ports is bounded by 1 ± |Γs||ΓL|), potential PA instability or damage, and impedance-dependent behavior in filters.[2]
How does cable loss affect measured return loss?
Reflected power passes through the cable twice, so a line with loss A dB improves apparent return loss by 2A dB. An antenna measured through 3 dB of cable can look 6 dB better matched than it is.
Frequently asked questions
What return loss is considered good?
Depends on application; 10 dB (VSWR ~1.9) is a common antenna threshold, 20 dB for test interfaces.
Is S11 the same as return loss?
Return loss is −|S11| in dB; S11 is negative in dB, return loss positive.
Can VSWR be less than 1?
No; 1:1 is a perfect match.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “Fundamentals of RF and Microwave Power Measurements (Application Note 1449 series)”. Keysight Technologies. application note.