Smith Chart Matching Tool — with Automatic L-Network

A GTK4 app for visualizing impedance matching: enter a source impedance, add series/shunt R/L/C elements, and watch each one trace its arc across the Smith chart. Live readouts for Z, Gamma, VSWR, and return loss. Includes an analytic auto-match solver, a frequency sweep (with support for importing a real measured antenna sweep from a file), and PNG/PDF/CSV export.

Files

• engine.py — core impedance math (no GUI deps).

  • MatchingNetwork holds a source Z and an ordered list of series/shunt R/L/C elements, and computes the impedance after each step, plus Gamma/VSWR/return loss.
  • solve_l_match() analytically computes a 1- or 2-element L-network that exactly matches a given source Z to Z0.
  • sweep() evaluates a network across a list of frequencies, with either a fixed source impedance or a per-frequency callable (for a real antenna whose impedance varies across the band).
  • load_impedance_csv() / load_touchstone_1port() / interpolate_impedance() — read a measured impedance-vs-frequency dataset and interpolate it at an arbitrary frequency.

  This module is fully unit-testable on its own — see tests/.

• chart.py — draws the Smith chart grid: constant-R circles and constant-X arcs (impedance), plus a fainter overlay of the mirrored constant-G/constant-B admittance grid, both labeled with actual ohm values scaled to Z0. Maps Z -> Gamma for plotting points and paths. Also draws the VSWR-vs-frequency sweep plot.

• schematic.py — draws the matching network as a ladder-network schematic (IEC 60617-style symbols: resistor box, inductor coil, capacitor plates), series elements inline and shunt elements branching to ground. Used for the schematic/report exports, not shown live.

• app.py — the GTK4 windows: the main window (source Z0/R/X/frequency entries, an "Add element" list where each row is (series/shunt, R/L/C, value, unit), the Smith chart, and result readouts) plus a separate "Frequency Sweep" window (VSWR-vs-frequency plot and results table, side by side). All the major panes are separated by draggable dividers.

Setup (Debian/Ubuntu)

sudo apt install python3-gi gir1.2-gtk-4.0 python3-matplotlib python3-numpy

Run

python3 app.py

Using it

1. Source impedance

Set Z0 (system impedance, usually 50), Freq (the single frequency the Smith chart and element arcs are drawn at), and Source R + jX — the impedance you're trying to match, e.g. from an antenna feedpoint measurement.

2. Auto-Match to Z0

Click Auto-Match to Z0… to have the app solve for you instead of dialing in values by hand. It analytically computes the L-network (1 or 2 reactive components) that matches the current source exactly:

• If a component's resistance already equals Z0, only one series element (to cancel the reactance) is needed.
• Otherwise there are up to two distinct exact solutions (e.g. series L + shunt C vs. series L + shunt L) — a dialog lets you pick which one to use, then fills in the element list for you.
• If the source is already matched, or its resistance is zero or negative (not physically matchable with a lossless L-network), you get an explanatory message instead.

3. Importing a measured impedance sweep

A real antenna's impedance isn't constant across the band — if you've measured it with a VNA (NanoVNA, miniVNA, etc.), File → Import Impedance Sweep (CSV/Touchstone)… lets the app use your actual measured data instead of a single fixed source impedance:

1. Pick a .csv or .s1p file (formats below). The app plots the whole imported range as the initial frequency sweep and reports how many points it found.
2. Tick Use imported impedance sweep to switch both the single-frequency view and the sweep over to the measured data — the manual Source R/X fields become disabled (they're not used while this is on), and the status line shows the impedance actually being used, interpolated at the current Freq.
3. Tick Sweep at exact imported frequencies to make the sweep evaluate precisely at your file's own frequency points instead of an evenly-spaced Start/Stop/Steps grid — this also disables those three fields, since they no longer apply.

Between measured points, R and X are linearly interpolated separately; outside the measured range, the nearest end value is held constant (no extrapolation) rather than guessed.

Supported file formats

• CSV — three columns: Freq (MHz), R (ohm), X (ohm). A header row is fine, it's simply skipped as unparsable. See tests/data/sample_antenna_sweep.csv for an example.

• Touchstone .s1p — the standard one-port VNA export format. Supported:

  • Frequency units: Hz, kHz, MHz, GHz (from the # option line; defaults to GHz if omitted, per the Touchstone spec).
  • Parameter types: S (reflection coefficient — by far the most common for antenna analyzers), Y, and Z.
  • Data formats: RI (real/imaginary), MA (magnitude/angle in degrees), DB (dB magnitude/angle in degrees).
  • An explicit reference impedance (R <n> on the option line; defaults to 50 Ω).

  See tests/data/sample_antenna_sweep.s1p for an example — it's the same synthetic antenna as the sample CSV above, so you can compare the two loaders against each other.

4. Matching elements

Click + Add element to add a matching component by hand. Choose:

• series or shunt (topology)
• R, L, or C (component type)
• the value and a unit — Ω/kΩ/MΩ for R, mH/µH/nH/pH for L, mF/µF/nF/pF for C

5. Reading the chart

• Each element appears as a colored arc, moving the impedance point along a constant-resistance circle (series) or constant-conductance circle (shunt) — the same way you'd reason through a match by hand on paper.
• The final point (green star) and the readouts on the left show how close you are to Z0 — aim for VSWR near 1.0 / |Gamma| near 0.
• The grid shows both the impedance circles (solid) and a fainter overlay of the mirrored admittance circles, each labeled with actual ohm values scaled to Z0 (not the normalized 0.2/0.5/1/2/5 you'd see on a printed chart).
• Hover the mouse over the chart to read off the impedance and VSWR at the pointer (e.g. Pointer: Z = 12.34-56.78j Ω VSWR = 3.42) below the canvas — handy for arbitrary points on the grid, not just the plotted ones.
• View → Show Sweep Points on Chart overlays the swept frequency points (dots joined by straight lines) directly on the Smith chart.

6. Frequency sweep

The Frequency sweep section (Start/Stop MHz, Steps) evaluates the same source and elements across a frequency range instead of just the single Freq value, so you can check a match's bandwidth (or see how a real antenna's impedance moves across the band, if you've imported one — see above). Results show up live in the separate "Frequency Sweep" window: a VSWR-vs-frequency plot and a results table, side by side, both with a draggable divider. View → Show Sweep Window shows/hides it — closing it from its own titlebar does the same thing, and that checkbox always reflects whether it's currently open.

7. Resizable layout

Every major pane — the controls column vs. the chart, the sweep plot vs. its table — is split by a draggable divider, so you can resize things to taste.

File menu

• Open… / Save / Save As… (Ctrl+O / Ctrl+S / Ctrl+Shift+S) — save the source settings (Z0, frequency, source R/X), the frequency sweep range (Start/Stop/Steps), and the full element list to a JSON file, or reload one later. Handy for keeping a matching network per antenna/band around instead of re-entering values by hand. (This does not include an imported impedance sweep file — re-import that separately if needed.)
• Import Impedance Sweep (CSV/Touchstone)… — see "Importing a measured impedance sweep" above.
• Export Chart to PNG… (Ctrl+E) — save the current Smith chart (grid, arcs, and points as currently drawn) as a PNG image.
• Export Chart to PDF… — the same chart as a single-page PDF, sized to the paper size chosen in the View menu.
• Export Schematic to PNG… — save the matching network's schematic diagram as a PNG image.
• Export Report to PDF (Chart + Schematic + Sweep)… — a multi-page PDF: the Smith chart (portrait), the schematic (landscape), the VSWR-vs-frequency sweep plot (landscape), and the sweep results table (portrait, paginated if it's long).
• Export Sweep Table to CSV… (Ctrl+Shift+E) — save the frequency sweep results (Freq/VSWR/Return Loss) as a CSV file.
• Quit (Ctrl+Q).

View menu

• Light / Dark — built-in color presets for the chart (background and grid/boundary/marker color).
• Custom Colors… — pick your own background and chart color live.
• Whichever theme you land on (a preset or custom colors) is remembered across restarts, saved to ~/.config/lsmith/config.json.
• Show Sweep Points on Chart — overlay the swept frequency points (dots joined by straight lines) on the Smith chart itself.
• Show Sweep Window — show/hide the separate "Frequency Sweep" window; closing it from its own titlebar does the same thing, and this checkbox always reflects whether it's currently open.
• PDF Paper Size — A4 / Letter / Legal for the PDF exports above. Defaults to your system's configured paper size (via GTK's print settings) the first time you run the app, and is remembered after that.

Testing

python3 -m pytest tests/ -v

tests/ covers the GUI-independent modules (engine.py, chart.py, schematic.py) — impedance math, solve_l_match() solutions verified to land exactly on Z0, the CSV/Touchstone loaders and interpolation, and headless (Agg-backend) smoke tests for the drawing functions. tests/data/ has example sample_antenna_sweep.csv and .s1p files (the same synthetic antenna in both formats) — handy both as test fixtures and as something to try the import feature on.