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SOLT Calibration Step by Step: Short, Open, Load and Thru

By John Shin · Updated · 6 min read

A SOLT calibration measures an ideal short (reflection coefficient -1), an open (+1) and a load (0), then a thru between the ports. The result holds only for the frequency range, IF bandwidth, power level and cable setup used.

For practitionersNew to this? Plain-language version

For practitioners

A vector network analyzer (VNA) measures more than the device. It also measures its own cables, connectors and internal imperfections. Calibration measures known standards, works out those errors and removes them from every later measurement. SOLT is the most common method for coaxial work. It uses four standards: Short, Open, Load and Thru.

What the calibration solves

At each port the raw reflection reading mixes the true reflection with three unknowns. A common one-port error model writes the measured value as:

Γm=e00+e01e10 Γ1−e11 Γ\Gamma_m = e_{00} + \frac{e_{01}e_{10}\,\Gamma}{1 - e_{11}\,\Gamma}

Here Γ is the true reflection coefficient of whatever is connected. The three error terms are directivity (e00), source match (e11) and reflection tracking (e01e10). Three unknowns need three known reflections, and that is why a one-port SOLT calibration measures the short, open and load at the port.

SourceLoadantenna, filterabsorbs 90 %incident wave: 100 %reflected wave (the echo): 10 %transmission line (between the two arrows)example: return loss 10 dB, VSWR 1.92
A mismatch sends part of the signal back like an echo. Concept drawing.

The echo picture matters here. Every imperfection in the test port and cable sends a small echo back to the receiver. Calibration measures those echoes and subtracts them mathematically.

What each standard contributes

Each standard has a known reflection coefficient, and each exposes a different error term.

  • Load (matched termination): ideal Γ = 0. Nothing should come back, so whatever the VNA still sees is its directivity error, e00.
  • Short: ideal Γ = −1. All incident power is reflected with a 180° phase flip.
  • Open: ideal Γ = +1. All incident power is reflected with no phase flip.
  • Thru: connects port 1 to port 2. It is used to determine the transmission tracking and load match terms.

The short and open have a magnitude of 1. The return loss is:

RL=−20log⁡10∣Γ∣RL = -20\log_{10}|\Gamma|

So the ideal short and open give RL = −20·log10(1) = 0 dB. Both reflect all incident power. The load gives |Γ| = 0, where return loss grows without limit.

Worked example. Put the ideal values into the error model.

  1. Load, Γ = 0: the formula gives Γm = e00. Directivity is read directly.
  2. Short, Γ = −1: Γm = e00 − e01e10 / (1 + e11).
  3. Open, Γ = +1: Γm = e00 + e01e10 / (1 − e11).

After step 1, e00 is known. Steps 2 and 3 are then two equations in the two remaining unknowns, e11 and e01e10, so the system is solved. The short and open sit at opposite ends of a diameter of the Smith chart, 2 apart in reflection coefficient. That wide spacing is what makes the solution well conditioned.

Real standards are not ideal. An open has some fringing capacitance and a short has some inductance. The cal kit definition tells the VNA how far each real standard departs from the ideal values. If the definition is wrong, the error terms are wrong.

Step by step at the bench

  1. Set the measurement first. Choose the frequency range, IF bandwidth and power level you will use for the actual measurement. The calibration is valid only for these settings.
  2. Connect the cables. Fit the test cables you will use and leave them in place. If cables are moved or replaced in the measurement path, the calibration must be redone.
  3. Select the cal kit. Pick the definition that matches the kit in your hand, including connector type and gender.
  4. Measure the reflection standards at port 1. Connect the short, open and load in turn. Tighten each connection the same way every time.
  5. Repeat at port 2 for the same three standards.
  6. Measure the thru. Connect port 1 to port 2 directly. This gives the transmission tracking and load match terms.
  7. Apply and save the calibration, then run the check below.
Devicefilter, amplifier, cablePort 1Port 2S21: what gets through, port 1 to port 2S12: what gets through the other wayS11 (echo)S22 (echo)First number = where the wave comes out, second = where it went in.
A two-port device and its four S-parameters. Concept drawing.

The thru is what extends the one-port terms to a full two-port correction. Without it the VNA can correct S11 and S22, but not S21 and S12.

Common mistakes

  • Wrong cal kit definition. The kit model must match the physical kit. The error terms are computed from the assumed standards, so a mismatched definition gives wrong terms with no warning.
  • Worn or damaged load. The load is assumed to give Γ = 0. If the termination is damaged or dirty, the directivity term absorbs its error, and every later reflection measurement carries it.
  • Dirty or loose connectors. A poor connection changes the reflection from standard to standard. Clean the interfaces and use consistent tightening, ideally with a torque wrench.
  • Changing settings after calibration. A different frequency range, IF bandwidth or power level means the calibration no longer applies.
  • Moving the cables. Bending or replacing a cable changes the path that was calibrated.
  • Mixing up short and open. The two are 180° apart, so swapping them produces a plausible but wrong result.
Always calibrate with the cables you will measure with, and do not touch them afterwards. Cable movement is a frequent cause of a good calibration going bad.

Check the result with a known device

Do not trust the calibration just because the procedure finished. Test it with a device whose response you know and that you did not use as a standard.

  1. Open and short as devices. With the calibration applied, connect the short and then the open. Both should show |Γ| ≈ 1, a return loss near 0 dB, with phases about 180° apart.
  2. Load. It should show a very high return loss. A visibly poor result points to a damaged load or a wrong definition.
  3. Thru. S21 should sit near 0 dB and S11 should be low. A ripple here suggests a connector or cable problem.
  4. Known device. Measure a device with a documented response, such as a fixed attenuator with a datasheet value, and compare S11 and S21. Differences much larger than the stated tolerance mean the calibration or the standards need attention.

For converting between reflection coefficient and return loss during these checks, see the VSWR, Return Loss and Mismatch Loss Calculator.

In plain words · for students and beginners

SOLT calibration in plain words

The idea in everyday terms

Think of weighing yourself with a bathroom scale. Before you step on, you check it reads zero with nothing on it. Then you place a known weight and see whether it reads correctly. If the scale is off, you remember the offset and subtract it.

A VNA does the same thing with signals. It measures things whose answer you already know, learns how wrong it is, and corrects every later reading. The analogy breaks down in one place: a scale has one error, while a VNA has several at once, so it needs several known things, not just one.

The four standards

  • Short: a plug that sends all the signal back, flipped upside down (reflection -1).
  • Open: an empty connector that sends all the signal back the right way up (+1).
  • Load: a plug that swallows the signal and returns none (0).
  • Thru: a direct link between the two ports, to learn how the signal travels across.
Return loss, and the share of the signal that bounces back6 dB25 % comes back10 dB10 % comes back14 dB4.0 % comes back20 dB1.0 % comes back30 dB0.1 % comes back
How much of the signal comes back for a given return loss. A bigger return loss means a quieter echo.

The picture shows that a bigger return loss means a quieter echo. The short and open give a return loss of 0 dB, the loudest echo possible. A good load gives a very quiet echo.

The same example in simple steps

  1. Connect the load. It should return nothing, so anything the VNA sees is its own built-in error. That error is now known.
  2. Connect the short. We know the true answer is -1, so any difference tells us a second error.
  3. Connect the open. The true answer is +1, which gives the third error.
  4. Connect the thru between the ports so the VNA learns how the signal crosses.

Mistakes beginners make

  • Choosing the wrong kit in the menu, so the VNA believes the wrong "known" values.
  • Using a load that is damaged or dirty, then trusting the result.
  • Moving a cable after calibrating, or changing the frequency range, bandwidth or power level.
  • Skipping the final check with a known device.

Remember

  • Short, open and load fix the reflection errors, and the thru fixes the transmission errors.
  • Pick the right cal kit definition and keep the load in good condition.
  • Do not move the cables or change the settings after calibrating, and always check with a known device.

Frequently asked questions

What does SOLT stand for in VNA calibration?

SOLT stands for Short, Open, Load and Thru, the four standards used in the calibration. Short, open and load correct each port's reflection measurement, and the thru corrects the transmission path between the ports.

Why do the short and open both have a return loss of 0 dB?

An ideal short has a reflection coefficient of -1 and an ideal open has +1. Both have a magnitude of 1, which is 0 dB return loss, so all the incident power comes back. They differ only in phase.

Do I need to recalibrate if I move the cables?

Yes. The calibration is valid only for the cable configuration used, so moving or replacing cables in the measurement path means you must redo it. The same applies if you change the frequency range, IF bandwidth or power level.

How can I check that a calibration worked?

Measure a known device that was not used as a standard and compare the result with its known response. Also look at the thru and the load for obvious problems.

References and further reading

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