VSWR, Return Loss and Mismatch Loss Calculator

How to use the calculator

Pick the quantity you already know from the "I know" list, type its value, and read everything else from the results. The same number can arrive in several forms: a datasheet may quote VSWR, a vector network analyzer (VNA) plots S11 in dB, and a test plan may ask for return loss. The calculator converts between all of them and also reports how much power is reflected and how much reaches the load.

If you only know the impedance, choose "Impedance R + jX", enter the resistance and reactance of the load and the reference impedance, and the calculator returns the reflection coefficient with its angle. For example, 50 + j50 Ω in a 50 Ω system gives |Γ| = 0.447 at 63.4°, a VSWR of 2.62 and a return loss of 6.99 dB.

What each quantity means

When a wave travelling along a line meets a load that does not equal the line impedance, part of it is reflected, like an echo.

The reflection coefficient Γ is the ratio of the reflected wave to the incident wave. For a load Z on a line of impedance Z0:

Γ is a complex number. Its magnitude |Γ| runs from 0 (perfect match) to 1 (total reflection), and its angle tells you whether the load looks inductive or capacitive. The other quantities are different ways of expressing |Γ|:

The VSWR (voltage standing wave ratio) is the ratio of the largest to the smallest voltage along the line. It is 1 for a perfect match and grows without limit as the reflection approaches 1. Return loss says how many decibels below the incident wave the reflected wave is, so a bigger number is better. Mismatch loss is the power that never reaches the load because it was reflected. The fraction of power reflected is |Γ|², and the fraction delivered is 1 − |Γ|².

Reference table: return loss

Return loss (dB)	|Γ|	VSWR	Mismatch loss (dB)	Reflected power (%)
3	0.708	5.85	3	50.1
6	0.501	3.01	1.3	25.1
10	0.316	1.92	0.46	10
14	0.2	1.5	0.18	3.98
15	0.178	1.43	0.14	3.16
20	0.1	1.22	0.044	1
26	0.0501	1.11	0.011	0.251
30	0.0316	1.07	0.0043	0.1

Reference table: VSWR

VSWR	|Γ|	Return loss (dB)	Mismatch loss (dB)	Reflected power (%)
1.1	0.0476	26.44	0.0099	0.227
1.2	0.0909	20.83	0.036	0.826
1.5	0.2	13.98	0.18	4
2	0.333	9.542	0.51	11.1
3	0.5	6.021	1.2	25

The two columns are the same information. Plotted against each other, they make the shape of the relationship clear: VSWR changes quickly at low return loss and flattens out once the match is good.

Two rules of thumb fall out of these tables. A VSWR of 2:1 reflects 11% of the power, which costs about half a decibel. And improving the match from 10 dB to 20 dB return loss recovers only about 0.4 dB of delivered power, so very good return loss is mostly about system behaviour (ripple, pulling of oscillators, stability of amplifiers) rather than raw power.

How good is good enough?

There is no universal number. A few points help you decide:

• Antennas (see antenna S11 and VSWR) are often specified to a return loss of about 10 dB (VSWR below 1.92) over the operating band. The mismatch loss at that limit is 0.46 dB, small compared with other losses in a radio.
• Cables, connectors, attenuators and test fixtures are usually expected to be much better, commonly 20 dB or more, because they should be transparent to the circuit under test.
• Amplifier inputs and outputs are often matched for gain, noise or efficiency rather than for the lowest reflection, so a modest return loss on a power amplifier output can be intentional.

Always read the requirement in the specification you are working to. The calculator tells you what a number means; it cannot tell you what number you need.

Common mistakes

Mixing up signs. Return loss is quoted as a positive number and S11 as a negative number for the same match. "Return loss of −10 dB" and "S11 of 10 dB" are both mistakes that appear in real documents. If a tool reports S11 above 0 dB for a passive device, check the sign or the measurement.

Using 10·log instead of 20·log. Reflection coefficient is a voltage-like ratio, so the decibel conversion uses 20·log10. Power ratios use 10·log10, which is why mismatch loss uses |Γ|² inside a 10·log10.

Expecting the connector to show its own return loss through a lossy line. If you measure a mismatched load at the end of a cable, the wave travels down the cable and back, so the cable loss counts twice. A cable with 3 dB of loss in front of an open circuit shows only 6 dB return loss, and an attenuator in front of a poor load makes the input look better than the load really is. This is useful for improving a match with a pad, and misleading when you want to characterize the load itself. Calibrate the VNA to the reference plane you care about, or de-embed the cable.

Forgetting the reference impedance. A 75 Ω load has a VSWR of 1.5 in a 50 Ω system, and a perfect match in a 75 Ω system. Make sure the calculator, the instrument and the specification all use the same reference.

Treating VSWR as a single number across a band. VSWR and return loss vary with frequency. A device may meet the limit at the center and fail at the band edge, so check the whole band on a plot rather than one marker.

In plain words · for students and beginners

Return loss and VSWR in plain words

Think of an echo

A signal travels from a source to a load, such as an antenna or a filter. If the load does not accept all of it, part of the signal bounces back, like an echo from a wall. Return loss tells you how quiet the echo is compared with the original shout. A bigger number is better: a quieter echo. VSWR describes the same echo in another way. A VSWR of 1 means no echo at all, and a bigger VSWR means a louder echo.

Why the numbers are in decibels

Decibels write ratios with small numbers. Every 10 dB is a factor of ten in power. So a return loss of 10 dB means one tenth of the power comes back, and 20 dB means only one hundredth (1%) comes back.

A worked example

An antenna with a return loss of 10 dB sends about 10% of the power back and accepts 90%. The lost 10% costs only 0.46 dB, which is hardly noticeable. That is why many antenna specifications accept 10 dB, and why a "bad-looking" number is not always a problem.

Where the picture stops being true

A real echo has only a loudness. In RF, a reflection also has a timing (a phase), and that phase is what the Smith chart draws. A lossy cable also hides the echo: the signal is weakened on the way out and again on the way back, so a poor load behind a long cable can look better than it is.

Mistakes beginners make

• Mixing up the signs: return loss is quoted as a positive number and S11 as a negative number for the same match.
• Thinking a high VSWR means the part is broken. It means the part does not match the system impedance.
• Forgetting the reference impedance. A 75 ohm load is a perfect match in a 75 ohm system and has a VSWR of 1.5 in a 50 ohm system.

Remember

• Return loss: bigger is better. VSWR: closer to 1 is better.
• 10 dB is 10% reflected. 20 dB is 1% reflected.
• Mismatch loss is small at moderate values, so judge the number against what the system needs.

Related pages on RF View

Guides

• How to measure return loss
• How to compute VSWR from S11
• How to verify an impedance match

Glossary and FAQ

• VSWR and S11 and return loss
• Return loss versus VSWR table
• Mismatch loss
• Antenna S11 and VSWR
• What is the difference between S11 and return loss?
• What causes high VSWR?
• VSWR versus SWR

Calculators

The microstrip impedance calculator helps you build a line that does not reflect, and the L-network matching calculator designs the parts that cancel a mismatch between two resistances. RF View plots S11 from Touchstone files as return loss, VSWR or on a Smith chart, with markers that read the values directly.