S-parameters (scattering parameters) describe how a circuit or component interacts with signals at different frequencies. They're what a NanoVNA actually measures.
For a two-port device (input and output):
- S11 — how much of the input signal reflects back (return loss). A dip in S11 means the device is absorbing energy at that frequency — which is how you find resonance.
- S21 — how much signal passes from input to output (insertion loss/gain). Useful for measuring filters and transformers.
- S12 — reverse transmission (output to input).
- S22 — output port reflection.
For the single-port measurements in most Open Energy experiments (where you connect a coil to CH0 and measure its properties), only S11 matters. The NanoVNA derives impedance, Q factor, and SRF from the S11 data.
Reading S11 on a NanoVNA
- S11 magnitude (dB): A deep dip (e.g., -20 dB) at a specific frequency = strong resonance at that frequency
- S11 Smith chart: Shows the complex impedance as a point on a circular chart. Real component on the horizontal axis, reactive on the vertical. Useful for seeing whether a component is inductive (upper half) or capacitive (lower half) at each frequency.
Most beginners find the impedance magnitude display more intuitive than the Smith chart. Both show the same information differently.