VSWR, Return Loss & Reflection Coefficient Calculator

Understanding the VSWR, return loss and reflection coefficient calculator — concept and principle

When a load such as an antenna or amplifier input does not match the impedance of the transmission line feeding it, part of the signal bounces back. This calculator expresses that mismatch in the five ways used on datasheets and analyser screens: reflection coefficient, VSWR, return loss, mismatch loss and percentage of reflected power.

A 75 Ω load on 50 Ω line reflects |Γ| = 0.2, which is just 4% of the power.

How it is calculated

Reflection coefficient: |Γ| = |(Z_L − Z₀) / (Z_L + Z₀)|, from 0 (perfect match) to 1 (total reflection).
VSWR: (1 + |Γ|) / (1 − |Γ|), from 1 upwards.
Return loss: RL = −20 log₁₀|Γ| dB; larger is better.
Mismatch loss: ML = −10 log₁₀(1 − |Γ|²) dB, the drop in delivered power.
Reflected power: |Γ|² × 100%.

Z_L is the load impedance and Z₀ the line impedance in ohms. This calculator takes real (resistive) values only.

The principle

On a line, voltage and current must keep the ratio Z₀ for a travelling wave, but at the load they must satisfy Ohm's law for Z_L. If the two differ, the only way to meet both conditions is for a reflected wave to appear, with amplitude Γ times the incident wave. The incident and reflected waves add in some places and cancel in others, forming a standing wave; VSWR is the ratio of the maximum to minimum voltage along the line. Power goes as voltage squared, which is why reflected power is |Γ|² and why return loss uses 20 log for a voltage ratio.

Worked example

Γ = (75 − 50) / (75 + 50) = 25 / 125 = 0.2.
VSWR = 1.2 / 0.8 = 1.5.
Return loss = −20 log₁₀(0.2) ≈ 13.98 dB.
Reflected power = 0.2² = 0.04 = 4%.
Mismatch loss = −10 log₁₀(1 − 0.04) ≈ 0.177 dB.

Each point is the calculator re-run with only “Line impedance (Ω)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Line impedance (Ω)	VSWR
12.5	6
25	3
37.5	2
50	1.5
62.5	1.2
75	1
100	1.333333333

How to read the result

Where does your result fall?

VSWR 1.5 (RL about 14 dB) is a good match for most work. VSWR 2.0 means |Γ| = 0.333, RL 9.54 dB, about 11% reflected power and 0.51 dB mismatch loss; it is a common acceptance limit for antennas. Above about 3:1 many transmitters reduce power to protect themselves. Note that the same VSWR comes from a load that is too high or too low: 25 Ω and 100 Ω both give 2:1 on a 50 Ω line.

Common mistakes

Reading return loss as a negative number; by convention it is positive, while S11 is the same figure with a minus sign.
Thinking 4% reflected means 4% of power is lost as heat; with a lossless line, reflected power returns to the source.
Ignoring reactance; a complex load needs complex Γ, which this real-only form does not handle.
Measuring at the transmitter end of a lossy cable, where losses make the antenna look better than it is.

Keep in mind: Results assume a lossless line and purely resistive impedances. Real antennas, cables and connectors should be checked with a vector network analyser or an antenna analyser.

Frequently asked questions

What VSWR is acceptable for an antenna?

Below 1.5 is very good, below 2.0 is generally acceptable, and above 3.0 usually needs correction.

How do I convert return loss to VSWR?

First |Γ| = 10^(−RL/20), then VSWR = (1 + |Γ|) / (1 − |Γ|). For example 20 dB gives |Γ| = 0.1 and VSWR 1.22.

Can a tuner fix a high VSWR?

It matches the transmitter to the line, but the mismatch at the antenna and the extra cable loss remain.

Sources: Pozar, Microwave Engineering, chapter on transmission line theory · The ARRL Handbook for Radio Communications, transmission lines chapter

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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