Field Guide · hardware

Also known as: VNA, network analyzer, network analyser

A vector network analyzer (VNA) measures how an RF device or network responds to signals across frequency, reporting both the magnitude and phase of the reflected and transmitted waves as S-parameters.1 Because it captures phase, not just amplitude, a VNA fully characterizes an antenna, filter, cable, amplifier, or matching network — its input match, insertion loss, delay, and complex impedance — where a scalar instrument would see only power.

Source Device under test Port 1 Port 2 S11 Receiver S21
A VNA drives a known signal into the device, samples the incident, reflected, and transmitted waves, and computes S-parameters after calibration removes the fixture's own errors.

Overview

Internally a VNA contains a swept signal source, directional couplers that separate the incident from the reflected wave, and phase-coherent receivers on each port. At every frequency it forms ratios: S11 (input reflection, from which return loss, SWR, and the complex reflection coefficient follow) and, on a two-port instrument, S21 (forward transmission — insertion loss or gain and phase/delay). A one-port VNA measures only reflection; two ports add transmission; larger instruments add more ports for multi-port and differential devices.

Calibration

A VNA’s accuracy comes almost entirely from calibration, which mathematically moves the measurement reference plane to the ends of the test cables and removes the fixture’s own losses, mismatches, and delays. The classic two-port procedure is SOLT — measure known Short, Open, Load, and Through standards — which solves for a 12-term error model that is then applied to every subsequent measurement. Variants such as TRL and SOLR trade different standards for accuracy at higher frequencies or on non-coaxial fixtures. Without calibration a VNA reading is meaningless: the raw response is dominated by cable loss and connector reflections, not the device under test.

In practice

  • Antenna tuning — sweep S11 to find resonance, minimize SWR, and check the match across a band.
  • Filter and cable characterization — S21 gives passband loss, stopband rejection, and group delay; S11 confirms the input match.
  • Impedance measurement — the complex reflection coefficient maps directly onto a Smith chart, giving the device’s impedance at each frequency.
  • Amplifier gain and match — a two-port sweep captures gain, input/output return loss, and stability inputs.

Relevance to SDR

For SDR and scanner work a VNA is the instrument you reach for when building or tuning the front-end plumbing: verifying that an antenna is resonant on the target band, that a bandpass or notch filter sits where you need it, and that coax and connectors are low-loss and well-matched. Affordable pocket instruments such as the NanoVNA have put S-parameter measurement within reach of every hobbyist. GopherTrunk is a receive-only decoder and does not interface with a VNA; the VNA is a bench aid for getting the antenna and filtering right before the signal ever reaches the SDR.

Sources

  1. Network analyzer (electrical) — Wikipedia, on vector network analyzer operation, S-parameters, and calibration. ↩

See also