A map of impedance
The Smith chart displays the complex reflection coefficient, with a grid that lets you read impedance. Select S11 and the SMITH format, then use a frequency marker. The impedance is Z = R + jX: R is resistance and X is reactance. In the usual 50-ohm NanoVNA setup, the centre represents 50 + j0 ohms.
Recognise five useful locations
- Centre: a matched 50-ohm load with zero reactance.
- Left-hand end: an ideal short circuit, Z = 0.
- Right-hand end: an ideal open circuit, with impedance tending to infinity.
- Upper half: positive reactance, or inductive behaviour.
- Lower half: negative reactance, or capacitive behaviour.
The horizontal diameter represents zero reactance. Points to the left of the centre have resistance below 50 ohms; points to the right have resistance above 50 ohms. Away from that diameter, follow the constant-resistance circles and constant-reactance arcs, or simply read R and X at the marker. Grid values may be normalised: multiply them by 50 ohms when the reference impedance is 50 ohms.
Try three known loads
After calibrating at the fixture, compare suitable RF resistive loads. An ideal 50-ohm load sits at the centre. An ideal 25-ohm load sits to the left and an ideal 100-ohm load to the right. The latter two both have SWR 2:1, although their impedances are different. This is why SWR alone cannot tell you how to design a matching network.
For another example, a marker reading 50 + j25 ohms lies in the upper half. Its normalised impedance is 1 + j0.5. A reading of 50 − j25 ohms lies in the lower half. Neither is at the centre, so neither is a perfect match despite having a resistance of 50 ohms.
Frequency makes the point move
A trace connects measurements across a frequency sweep; each point is a different frequency. An ideal inductor has X = 2πfL, while an ideal capacitor has X = −1/(2πfC). Real parts also have resistance and parasitic capacitance or inductance, so a component can change from inductive to capacitive behaviour around its self-resonant frequency. Start with a narrow enough span to follow the motion clearly.
Why a cable rotates the trace
A length of ideal, lossless 50-ohm cable between the calibrated plane and a mismatched load changes reflection phase and rotates the point about the chart centre. It preserves the magnitude of reflection and therefore SWR, but changes the impedance seen at the input. A lossy cable also reduces reflection magnitude, bringing the point towards the centre.
To read the load itself, calibrate at its connection point. Electrical delay can compensate phase delay in suitable situations, but is not a replacement for a full calibration or a general correction for cable loss. The reflection demo makes the moving measurement plane visible.
For the display controls, consult the original NanoVNA trace-format documentation.