SOLT Calibration Standards

SOLT (Short, Open, Load, Thru) is the most common VNA calibration method. Each standard provides known reflection or transmission values that the firmware uses to calculate error correction terms. Understanding what makes a good standard helps you achieve accurate measurements.

The Four Standards

Short

A short circuit reflects all incident power with a 180-degree phase shift:

S11_ideal = -1 + j0  (magnitude 1, phase 180 degrees)

Physical implementation: A metal cap that connects the center conductor directly to the shield.

Frequency	Ideal Short	Real Short
< 100 MHz	Perfect -1	Nearly ideal
100-500 MHz	Perfect -1	Small inductance from cap
> 500 MHz	Perfect -1	Inductance becomes significant

Short Inductance

At high frequencies, even a small metal cap has inductance. The included calibration standards are characterized, but improvised shorts (like touching wires together) have unknown inductance.

Open

An open circuit reflects all incident power with zero phase shift:

S11_ideal = +1 + j0  (magnitude 1, phase 0 degrees)

Physical implementation: Simply leaving the connector open (nothing connected).

Frequency	Ideal Open	Real Open
< 100 MHz	Perfect +1	Nearly ideal
100-500 MHz	Perfect +1	Fringe capacitance from connector
> 500 MHz	Perfect +1	Capacitance becomes significant

Open Capacitance

An “open” SMA connector has 20-50 fF of fringe capacitance. At 1 GHz, this is about -j160 ohms, causing measurable phase shift. Good calibration kits include characterized open standards.

Load

A matched load absorbs all incident power with no reflection:

S11_ideal = 0 + j0  (magnitude 0, phase undefined)

Physical implementation: A precision 50 ohm resistor terminating the line.

Frequency	Ideal Load	Real Load
< 100 MHz	S11 = 0	Return loss > 40 dB
100-500 MHz	S11 = 0	Return loss 30-40 dB
> 500 MHz	S11 = 0	Return loss 20-30 dB

Load Quality

The load standard is often the accuracy-limiting factor. A good 50 ohm load provides > 40 dB return loss at HF and > 30 dB at VHF. The NanoVNA calibration kit load is typically good to > 35 dB.

Thru (Through)

A thru connection passes all power from Port 1 to Port 2:

S21_ideal = +1 + j0  (0 dB insertion loss)
S11_ideal = 0 + j0   (perfect match at both ports)

Physical implementation: A short coaxial adapter or cable connecting both ports.

Zero-Length Thru

Direct adapter connection:

• Lowest loss
• No additional phase delay
• Requires compatible port connectors
• Best for bench calibration

Known-Length Thru

Short cable of known length:

• Some loss (must be characterized or ignored)
• Known electrical delay
• Allows different connector types
• Used with test fixtures

Accuracy Considerations

Standard Quality Hierarchy

From most to least accurate:

1. Characterized standards: Measured on a reference VNA, correction applied
2. Precision standards: Machined to tight tolerances, assumed ideal
3. Uncorrected standards: SMA connectors, general-purpose loads
4. Improvised standards: Wire shorts, resistors to ground

What Limits Your Accuracy?

Factor	Impact	Mitigation
Load return loss	Sets noise floor for S11 measurements	Use quality 50 ohm termination
Short inductance	Phase error at high frequency	Use characterized or precision short
Open capacitance	Phase error at high frequency	Use characterized or precision open
Connector repeatability	Random error each connection	Clean connectors, consistent torque
Temperature	Standards drift with temperature	Calibrate at operating temperature

Typical Accuracy Expectations

With the included NanoVNA calibration standards:

Frequency Range	S11 Accuracy	S21 Accuracy
50 kHz - 30 MHz	Excellent (< 0.5 dB)	Excellent
30 MHz - 300 MHz	Very Good (< 1 dB)	Very Good
300 MHz - 900 MHz	Good (1-2 dB)	Good
900 MHz - 1.5 GHz	Moderate (2-3 dB)	Moderate
> 1.5 GHz	Limited (harmonics)	Limited

The Calibration Process

1. Set frequency range

   Choose the start, stop, and number of points for your measurement. Calibration is only valid for this exact range.

2. Connect LOAD to Port 1

   Attach the 50 ohm load standard. The firmware measures ED (directivity error).

3. Connect OPEN to Port 1

   Remove the load and leave Port 1 open. This measurement helps calculate ES and ER.

4. Connect SHORT to Port 1

   Attach the short standard. Combined with the open, this completes the SOL calculation.

5. Connect THRU (optional)

   For S21 measurements, connect Port 1 to Port 2 with the thru adapter. Measures ET.

6. Measure ISOLATION (optional)

   With both ports terminated in loads (or disconnected), measure crosstalk. Determines EX.

7. Apply calibration

   The firmware calculates error terms and enables correction. Save to a slot for later use.

Port Extension

When you cannot calibrate at the device under test (DUT), you can use port extension to account for additional cable length:

Electrical Delay = Physical Length * Velocity Factor / Speed of Light

Port extension rotates the calibration plane along the transmission line without requiring a new full calibration.

Limitation

Port extension only corrects for phase rotation, not loss. If your test cables have significant loss, you need to calibrate at the cable ends.

When to Recalibrate

Recalibrate when:

• You change the frequency range (different start, stop, or points)
• You change or move cables
• Temperature changes significantly (> 10 degrees C)
• You swap connectors or adapters
• Measurements seem wrong or noisy
• More than a few hours have passed for critical measurements

Save Multiple Calibrations

The NanoVNA-H can store multiple calibration sets in different slots. Save different calibrations for different frequency ranges (HF, VHF, UHF) so you can quickly switch without recalibrating.

Improving Standard Quality

Homemade Standards

For casual HF use, you can make adequate standards:

Short: Solder the center pin of an SMA plug directly to its shell.

Open: An SMA plug with nothing connected (avoid touching the center pin).

Load: A 49.9 ohm 0805 SMD resistor soldered from center to shell of an SMA plug.

Professional Standards

For best accuracy, use commercial calibration kits designed for your connector type and frequency range. These include:

• Characterized standards with S-parameter data files
• Precision machined connectors
• Traceable calibration certificates
• Protective storage cases

Summary

Standard	Ideal Value	Provides	Key Limitation
Short	S11 = -1	Reference for phase, helps calculate ES/ER	Inductance at high f
Open	S11 = +1	Reference for phase, helps calculate ES/ER	Capacitance at high f
Load	S11 = 0	ED (directivity error)	Return loss floor
Thru	S21 = 1	ET (transmission tracking)	Loss and mismatch
Isolation	S21 = 0	EX (crosstalk)	Rarely needed

Next Steps

Learn how the firmware handles measurements at frequencies between calibration points in Calibration Interpolation.