The NanoVNA is a two-port vector network analyzer that measures S-parameters from 50 kHz to 900 MHz (extended to 1.5 GHz on newer versions). It costs under $60. Understanding what it measures requires understanding S-parameters and the Smith chart — both of which have straightforward physical interpretations that are often obscured by how they are introduced.
S-Parameters: What They Actually Mean
S11 (return loss): The fraction of power reflected back from port 1. An S11 of 0 dB means all power is reflected; -10 dB means 10% of power is reflected (90% transmitted or absorbed). At -20 dB, only 1% is reflected. In antenna terms, this is the measure of how well the antenna accepts power from the feedline.
S21 (insertion loss or gain): The fraction of power transmitted from port 1 to port 2. For a cable, this measures loss. For an amplifier, a positive S21 in dB means gain. For a filter, S21 vs. frequency is the frequency response.
VSWR (Voltage Standing Wave Ratio): Defined as (1 + |S11|) / (1 - |S11|). It describes the ratio of the maximum to minimum voltage on a transmission line with reflections. A VSWR of 1:1 is perfect matching; 2:1 means roughly 11% reflected power. Maximum power transfer from source to load requires matched impedances — this is the maximum power transfer theorem, and it is why the target is always 50 ohms in RF systems.
The Smith Chart
The Smith chart is a conformal mapping of the complex impedance plane onto a unit circle. The transformation is: reflection coefficient Gamma = (Z - Z0) / (Z + Z0), where Z is the load impedance and Z0 is the reference (typically 50 ohms). Because |Gamma| is always between 0 and 1 for passive loads, every possible passive impedance maps to a point inside or on the unit circle.
The chart has two families of circles. The resistance circles (constant real part of Z/Z0) shrink toward the right side of the chart; pure resistance plots on the horizontal axis. The reactance arcs (constant imaginary part) are perpendicular to the resistance circles. The upper half of the chart is inductive (positive reactance); the lower half is capacitive (negative reactance).
Reading a NanoVNA measurement on the Smith chart: if the marker sits in the upper half and to the right, the antenna is inductive and resistive — add a series capacitor or shorten the element to move toward the center (50+0j ohms). If it sits in the lower-left, the impedance is capacitive and low-resistance — lengthen the element.
The Smith chart is used for matching network design: a series inductor moves the point clockwise along a resistance circle; a shunt capacitor moves it along a conductance circle. L-networks, T-networks, and pi-networks are designed by tracing these paths to reach the center.
The NanoVNA puts this capability in a pocket-sized instrument. Used systematically, it eliminates the guesswork from antenna trimming, filter characterization, and RF amplifier stability analysis.