What calibration actually does
A NanoVNA compares a signal it sends with signals reflected by, or transmitted through, your device under test. Connectors and test cables also affect those signals. Calibration measures known standards so the instrument can correct systematic errors up to a chosen reference plane. It does not remove poor connections, noise or changes made to the setup afterwards.
Choose the reference plane first
For a component on a short test cable, place the standards at the end where the component will connect. That cable is then part of the calibrated setup. Calibrating directly at the instrument instead means that a cable added afterwards becomes part of what you measure. Keep any adapters needed to mate with the device in place during calibration.
Calibrate S11 step by step
- Let the instrument settle, inspect the connectors and set the frequency range you intend to measure. Choose a sweep with enough points to resolve the feature of interest.
- Connect your test cable to the source/reflection port, usually labelled CH0 or PORT1. Start a fresh calibration using the instructions for your model.
- At the free cable end, attach the OPEN standard and record OPEN. Wait for the measurement to complete before changing standards.
- Replace it with SHORT and record SHORT, then fit the 50-ohm LOAD and record LOAD. Use the proper standards rather than improvised open wires or a breadboard resistor.
- Finish the calibration, enable correction and save the setup if your firmware supports it. Record which cable, adapters and frequency span belong to that saved calibration.
Menu names differ between NanoVNA models. For a reflection-only measurement, open, short and load are the relevant standards. For S21, follow your model's full procedure including a through connection; some models also offer an isolation step.
Check before trusting the trace
Reconnect a known 50-ohm termination at the reference plane. It should appear near the centre of the Smith chart and show SWR near 1 across the intended band. An independent check load is more informative than reusing the calibration load: it can reveal a problem that the calibration itself absorbed. Open and short should appear near their expected positions, with increasing departures possible at higher frequencies because real standards are not ideal.
If the trace jumps when a connector is touched, fix the connection before repeating calibration. Avoid bending or moving the test cable after calibration. Recalibrate after changing cables, adapters or the reference plane, and after substantial temperature changes. A fresh calibration over a new frequency span is preferable to relying blindly on interpolated correction.
Your first useful measurement
Replace the check load with a passive antenna or component. Select S11 and the display format you need: SWR for matching, or Smith for impedance. Keep the instrument disconnected from transmitters and powered RF outputs. Then follow the antenna SWR guide or learn how to read the Smith chart.
Model references: original NanoVNA calibration instructions and NanoVNA V2 calibration steps.