LinSmith Smith Chart Tool — Ham Radio Guide
LinSmith is a free, open-source Smith chart tool that runs on Linux, Windows, and macOS. It lets you plot impedance trajectories, design L-networks and stub matches, analyse transmission line transformations, and visualise NanoVNA data on a proper Smith chart — all without the cost of commercial RF design software. This guide teaches you to read the Smith chart, use LinSmith for practical antenna matching problems, and apply the same techniques to real VNA measurements.
The Smith chart is a graphical tool for solving transmission line and impedance matching problems without complex arithmetic. It maps the entire complex impedance plane — from zero to infinity ohms of resistance and any amount of positive or negative reactance — onto a bounded circle. Every point on the chart corresponds to a specific complex impedance Z = R + jX, expressed as a normalised value z = Z/Z0 where Z0 is the system reference impedance (usually 50 Ω).
Centre of the chart
The centre point of the Smith chart represents Z = Z0 = 50 Ω — a perfect match. SWR = 1.0:1. This is the target for every matching network design. Any impedance plotted at the chart centre requires no matching.
Left and right edges
The leftmost point is a short circuit (Z = 0). The rightmost point is an open circuit (Z = infinity). Pure resistances (no reactance) lie on the horizontal diameter of the chart — the real axis.
Constant resistance circles
The curved circles running through the right edge of the chart are constant-resistance circles. Moving along one of these circles changes reactance while keeping resistance constant — adding a series inductor or capacitor moves you along a constant-resistance circle.
Constant reactance arcs
The arcs running from the right edge toward the centre are constant-reactance arcs. Moving along one keeps reactance constant while changing resistance — adding a shunt (parallel) component moves you along a constant-reactance arc.
Constant SWR circles
Concentric circles centred on the chart centre represent constant SWR loci. A point on the circle labelled SWR 2:1 has a 2:1 SWR with the 50 Ω system. Moving along this circle (via transmission line length) changes the apparent impedance without changing the SWR magnitude.
Transmission line rotation
Adding transmission line length rotates the impedance point clockwise around the chart centre — one full rotation = λ/2 of line length. This is how transmission line matching works: rotate the impedance point to a useful location then add a matching element.
LinSmith is available in the repositories of most major Linux distributions. On Ubuntu/Debian: sudo apt-get install linsmith. On Fedora/RHEL: sudo dnf install linsmith. Launch from the application menu or with the command linsmith. The interface opens with a blank Smith chart and a component entry panel on the left side.
Download the Windows binary from jcoppens.com/soft/linsmith/down.en.php. The installer places LinSmith in Program Files and creates a desktop shortcut. LinSmith on Windows requires the GTK+ runtime library — the installer typically includes this. If LinSmith fails to launch on first run, download and install the GTK+ runtime separately from the GTK project website.
LinSmith opens with the Smith chart on the right and a Network panel on the left. The Network panel lists circuit elements — transmission lines, series components, and shunt components — that you add to build the matching network. Each element you add moves the impedance point on the chart. The source impedance (starting point) is set in the Source box; the load impedance (what you are matching) is entered first; the chart shows the impedance trajectory as matching elements are added.
A 3-element Yagi's driven element presents approximately 22 + j5 Ω at its feed point. We need to transform this to 50 Ω for direct coax connection. The gamma match does this with a series inductor (the gamma rod) and a series capacitor. This is a classic LinSmith workflow:
In LinSmith's Load box, enter R = 22, X = 5 (the driven element feed point impedance). The impedance point plots on the Smith chart in the lower-left region — below the real axis (slightly inductive) and to the left of centre (resistance below 50 Ω). This is the starting point for the matching network design.
The gamma match adds inductance in series with the driven element. Click Add Element → Series Inductor and increase the inductance value in nH. Watch the impedance point move along the constant-resistance circle (resistance stays near 22 Ω, reactance increases). Continue increasing inductance until the point crosses the constant-resistance circle that passes through 50 Ω on the real axis — this is the "target circle" for the series inductance step.
After the series inductor step, the impedance point is on the 50 Ω resistance circle but not at the chart centre — it has residual inductive reactance. Add a Series Capacitor and increase its value until the impedance point moves to the chart centre (50 + j0 Ω). The capacitor cancels exactly the excess inductance. LinSmith shows the final point at the centre confirming a perfect match. Read off the required inductance (gamma rod) and capacitance (series capacitor) from the component values shown.
LinSmith can print or save the Smith chart as a PDF or PNG showing the complete impedance trajectory with each matching step labelled. This provides the documentation for building the physical gamma match: the inductance value determines gamma rod length and spacing, and the capacitance value determines the variable capacitor range needed.
Smith Chart Position Calculator
Enter an impedance to find its normalised position and SWR on a 50 Ω Smith chart.
One of the most powerful Smith chart applications is analysing how a transmission line transforms impedance. Adding a transmission line section in LinSmith rotates the impedance point clockwise around the chart centre — the rotation angle is 2 × (electrical length in degrees) = 720° × (physical length / wavelength). This allows you to answer questions like: "My antenna presents 35 + j20 Ω at the feed point — what will my antenna analyser read at the shack end of a 15 m coax run at 14.175 MHz?"
Set Load Z = 35 + j20 Ω. The point plots on the Smith chart in the inductive (lower-right) region — above 50 Ω resistance, positive reactance.
Click Add Element → Transmission Line. Enter: Characteristic impedance = 50 Ω (for RG-213), Velocity factor = 0.659, Physical length = 15 m, Frequency = 14.175 MHz. LinSmith calculates the electrical length (15 × 14.175 / (300 × 0.659) = 1.074 wavelengths = 386.6°) and rotates the impedance point clockwise by 386.6° around the chart centre. The resulting point is the impedance your analyser will read at the shack end of the cable.
LinSmith shows the new impedance value numerically in the status bar and graphically as the rotated point on the chart. This is the impedance your transmitter sees. Compare this against what your NanoVNA actually reads — agreement within 10% of R and X confirms your cable velocity factor and length are correctly modelled.
| Operation | Movement on Smith chart | LinSmith element |
|---|---|---|
| Add series inductor | Clockwise along constant-R circle | Series Inductor |
| Add series capacitor | Counter-clockwise along constant-R circle | Series Capacitor |
| Add shunt (parallel) inductor | Clockwise along constant-G (conductance) circle | Shunt Inductor |
| Add shunt capacitor | Counter-clockwise along constant-G circle | Shunt Capacitor |
| Add transmission line length | Clockwise rotation around chart centre | Transmission Line |
| Add lossy transmission line | Clockwise rotation spiralling inward toward centre | Lossy Trans. Line |
| Add series resistor | Move along real axis toward centre | Series Resistor |
| Add shunt resistor | Move along conductance circle toward centre | Shunt Resistor |
LinSmith can import S-parameter data in Touchstone (.s1p) format — the standard format that NanoVNA-Saver and most VNA software export. This allows you to plot your real antenna's measured impedance on a Smith chart and design matching networks directly from measured data rather than estimated values.
In NanoVNA-Saver, after performing a calibrated sweep of your antenna, go to File → Save → Save S1P. This creates a Touchstone format file containing the S11 data (reflection coefficient vs. frequency) from your measurement. The .s1p file is a plain text file with one data point per frequency.
In LinSmith, go to File → Open and select the .s1p file. LinSmith plots the impedance trajectory across frequency as a curve on the Smith chart — the spiral trace shows how the antenna's impedance changes from the start to the end of the sweep frequency range. Each point on the trace corresponds to the impedance at one frequency.
Click on the trace at the design frequency to read the impedance at that frequency from the status bar. This is the actual measured feed point impedance — use this as the Load impedance for subsequent matching network design. The advantage over using NEC2 predicted values is that the measurement includes all real-world effects: nearby structures, actual ground conditions, connector losses, and manufacturing tolerances.
| Tool | Platform | Cost | NanoVNA import | Best for |
|---|---|---|---|---|
| LinSmith | Linux/Win/Mac | Free | Yes (.s1p) | General matching network design |
| Smith V3 (Fritz Dellsperger) | Windows | Free | Yes | Interactive graphical Smith chart |
| SimSmith | Java (all platforms) | Free | Yes | Circuit simulation with Smith chart |
| NanoVNA-Saver | Win/Linux/Mac | Free | Native | VNA data display, basic Smith chart |
| AWR Microwave Office | Windows | Commercial | Yes | Professional RF design |
Do I need to understand complex mathematics to use LinSmith?
No — LinSmith handles all the complex arithmetic internally. You enter component values (inductance in nH, capacitance in pF, line length in metres) and watch the impedance point move on the chart. Understanding which direction the point moves for each component type is sufficient to design practical matching networks. The mathematical background helps you understand why things move as they do, but is not required to use the tool productively.
What is the difference between LinSmith and a NanoVNA Smith chart display?
The NanoVNA displays measured S11 data on a Smith chart in real time — it shows you where your antenna is. LinSmith is a design tool — it shows you how to move from where your antenna is to where you want it to be (the chart centre) by adding matching elements. Use the NanoVNA to measure, use LinSmith to design the match, then verify the result with the NanoVNA again.
Can LinSmith design a balun transformer?
LinSmith can model the impedance transformation effect of a transformer — enter it as a transformer element with the turns ratio. A 4:1 balun transforms 200 Ω to 50 Ω (or 50 Ω to 12.5 Ω). LinSmith plots the impedance before and after the transformation, showing whether the balun brings the impedance close enough to 50 Ω for a direct coax match or whether additional matching is needed.
Why does adding transmission line length not change SWR?
Transmission line length rotates the impedance point around the chart centre — it moves along a constant-SWR circle. The SWR magnitude is determined by how far the impedance point is from the chart centre (which represents |Gamma|), and rotation does not change this distance. SWR only changes when power is dissipated (lossy line, which spirals inward) or when a matching element is added that moves the point toward the centre.
How do I use LinSmith to check if my coax length matters?
Enter your antenna's feed point impedance as the Load, then add a Transmission Line element with the coax parameters and vary the physical length. Watch the impedance point rotate around the chart. If it stays within the SWR 1.5:1 circle regardless of length, the coax length does not matter practically. If it swings through SWR 3:1 or higher at some lengths, the antenna has a significant mismatch that the coax length is making visible at the transmitter end.
What is SimSmith and how does it differ from LinSmith?
SimSmith (by Ward Harriman AE6TY) is a Java-based Smith chart circuit simulator that includes circuit simulation features alongside the Smith chart — it can compute frequency responses across a band, not just at a single frequency. LinSmith is simpler and better suited to single-frequency matching design. SimSmith is more powerful for broadband matching network design and wideband analysis. Both are free; SimSmith has a steeper learning curve but is the more capable tool for advanced users.