VSWR and Return Loss: Practical Reference Values and Conversions

VSWR and return loss describe the same thing in two ways: how much of a signal bounces back where impedances do not match. A datasheet usually quotes only one of them, so you often need to convert. This guide gives the formulas, a conversion table, a worked example and the reference values seen in practice.

1. Definitions and formulas

• Reflection coefficient Γ = (ZL − Z0) / (ZL + Z0), where Z0 is the reference impedance (usually 50 Ω) and ZL is the load.
• VSWR = (1 + |Γ|) / (1 − |Γ|), and conversely |Γ| = (VSWR − 1) / (VSWR + 1).
• Return loss RL = −20·log₁₀|Γ| (dB). It is written as a positive number, and larger means less reflection. S11 (dB) = −RL.
• Reflected power fraction = |Γ|². Mismatch loss = −10·log₁₀(1 − |Γ|²) (dB).

2. Conversion table

(Calculator: VSWR ↔ return loss ↔ reflection coefficient)

3. Worked example: a 75 Ω load on a 50 Ω system

Γ = (75 − 50) / (75 + 50) = 0.2. VSWR = 1.2 / 0.8 = 1.5 and RL = −20·log₁₀(0.2) = 13.98 dB, about 14 dB. The reflected power is 0.2² = 4% and the mismatch loss is −10·log₁₀(1 − 0.04) = 0.18 dB.

In terms of power: a 10 W transmitter feeding an antenna with a VSWR of 2:1 reflects 10 × 0.111 = 1.11 W, and 8.89 W reaches the antenna. The loss is 10·log₁₀(8.89 / 10) = −0.51 dB, the same as the mismatch loss in the table.

4. Reference values used in practice

• VSWR 2:1 (RL about 9.5 dB) is often quoted as the acceptable limit in antenna specifications. About 11% of the power is reflected and the loss is about 0.5 dB.
• VSWR 1.5:1 (RL about 14 dB) is common in tighter specifications.
• Below a VSWR of 1.22 (RL 20 dB) only about 1% is reflected, so further improvement reduces the loss by less than 0.05 dB. The returns diminish.
• These values are design conventions, not limits set by an EMC standard such as CISPR. Follow the requirement in your product or antenna specification and your test plan.

5. Cable loss hides a mismatch

The return loss measured at the cable input is the antenna’s own RL plus twice the cable loss (the signal travels there and back). If the antenna has an RL of 6 dB (VSWR 3.0) and you measure through a 3 dB cable, you see 6 + 2×3 = 12 dB, a VSWR of about 1.67. Measure at the antenna terminal or correct for the cable loss before judging.

Mismatch also appears in measurement uncertainty. If the reflection coefficients on the two sides are |Γ1| and |Γ2|, the mismatch error lies within 20·log₁₀(1 ± |Γ1|·|Γ2|) dB. For example, joining a VSWR of 1.5 (|Γ| = 0.2) to a VSWR of 1.22 (|Γ| = 0.1) gives a product of 0.02, so the error is between +0.17 dB and −0.18 dB. How to enter this in an uncertainty budget follows the relevant standard and the lab’s method. (Calculator: combined uncertainty)

6. Common mistakes

• Sign: RL is normally written positive (10 dB) and S11 negative (−10 dB). The value is the same and only the sign differs.
• Reading mismatch loss as return loss: a return loss of 10 dB corresponds to a mismatch loss of only 0.46 dB.
• Looking at one frequency: the specification must hold across the whole operating band, so check the worst value in the band.
• VSWR cannot be below 1. If you get a value under 1, check the input or the units.

7. What to check when you receive a value

• Confirm whether the figure is VSWR or RL, and which sign convention it uses.
• Confirm whether the reference plane is the antenna terminal or the end of a cable.
• Use the table or the calculator to get the reflected power and mismatch loss, and decide whether to include them in the link budget.
• Compare against the specification using the worst value across the whole operating band.