linSmith on Linux — Complete Smith Chart Tool Setup and Operating Guide
linSmith is a free interactive Smith chart tool for Linux used for antenna matching, transmission line analysis, and RF circuit design. This complete guide covers installation on Ubuntu and Debian, understanding the Smith chart, using linSmith for antenna tuner design, impedance matching networks, coax cable analysis, stub matching, and interpreting the results for practical ham radio antenna work.
- What is a Smith chart and why use linSmith
- Installation on Ubuntu and Debian
- Understanding the Smith chart
- Understanding the linSmith interface
- Basic operations — entering impedances
- Transmission line analysis
- Impedance matching networks
- Stub matching
- Practical antenna matching with linSmith
- Operating guide — step-by-step workflows
- Saving and exporting results
- Key RF concepts for ham radio use
- Troubleshooting common problems
- Frequently asked questions
What is a Smith chart and why use linSmith
A Smith chart is a graphical tool for solving problems in transmission line theory and RF circuit matching. Developed by Philip Smith at Bell Labs in 1939, it plots complex impedance in a way that makes transmission line transformations, matching network design, and reflection coefficient calculations visual and intuitive rather than requiring complex arithmetic with imaginary numbers.
For amateur radio operators the Smith chart is useful for a specific class of problems that arise regularly in antenna work: understanding what your antenna analyser is telling you about a feed point impedance, designing an L-network or pi-network to match a non-resonant antenna to your transmitter, calculating the impedance transformation of a length of coax, and designing stub filters to eliminate unwanted resonances.
linSmith is a free GTK-based Smith chart tool for Linux written by John Coppens ON6JC/LW3HAZ. It provides an interactive graphical Smith chart where you can plot impedance points, add transmission line segments, lumped component elements, and matching network components, and see how the impedance transforms as you add each element.
Who linSmith is useful for
- Operators building or experimenting with HF, VHF, or microwave antennas who want to understand feed point impedances
- Homebrewers designing matching networks for amplifiers, filters, or baluns
- Anyone who uses an antenna analyser and wants to understand what the complex impedance readings mean
- Students learning RF transmission line theory who want a visual tool to verify hand calculations
Installation on Ubuntu and Debian
Ubuntu 22.04, 24.04 and Debian 11, 12
sudo apt update
sudo apt install linsmith
linSmith is in the standard repositories for all current Ubuntu and Debian releases. The installation is small — the entire package is under 1 MB.
Build from source
sudo apt install git build-essential libgtk2.0-dev \
libglib2.0-dev autoconf automake
git clone https://github.com/mdejong/linsmith.git
cd linsmith
autoreconf -i
./configure
make
sudo make install
Verify installation
linsmith
linSmith opens its main window with an empty Smith chart. If it fails to launch check the terminal for error messages regarding missing GTK libraries.
Understanding the Smith chart
Before using linSmith it helps to understand what the Smith chart represents and how to read it. The chart looks complex at first but follows a logical structure.
The coordinate system
The Smith chart is a plot of the complex reflection coefficient (Γ) in polar form. Every point on the chart represents a specific complex impedance (R + jX) normalised to the system reference impedance — typically 50 ohms for amateur radio work.
- Center of the chart — represents exactly 50 ohms resistive (1.0 normalised) — the perfectly matched condition with SWR = 1:1
- Right edge of the chart — infinite impedance (open circuit)
- Left edge of the chart — zero impedance (short circuit)
- Upper half of the chart — inductive reactance (positive imaginary component)
- Lower half of the chart — capacitive reactance (negative imaginary component)
- Horizontal centerline — purely resistive impedances (no reactance)
The curved grid lines
The Smith chart has two families of circles:
- Resistance circles — circles centred on the horizontal axis. All points on a circle have the same normalised resistance value. The r=1 circle passes through the center of the chart.
- Reactance arcs — arcs that start from the right edge of the chart. Points on an arc have the same normalised reactance value. The x=0 arc is the horizontal centerline.
SWR circles
Circles centred on the middle of the Smith chart represent constant SWR. A point on the 2:1 SWR circle has a 2:1 SWR regardless of where it is on that circle. The goal of any matching network is to move the impedance point to the center of the chart (SWR = 1:1) or at least inside the desired SWR circle.
What transmission lines do on the Smith chart
This is the key insight that makes the Smith chart powerful for antenna work: adding a length of transmission line rotates the impedance point clockwise around a constant SWR circle. One full rotation equals half a wavelength of transmission line. This means you can use a piece of coax to transform an impedance — and you can see exactly how much rotation you need to achieve a desired impedance by working backwards on the chart.
Understanding the linSmith interface
linSmith's main window has the Smith chart dominating the center, with input panels and result displays around it.
Main interface elements
- Smith chart canvas (center) — the main display. Points and circuit elements are plotted here.
- Circuit elements panel (left) — where you add and configure circuit elements: load impedance, transmission lines, series and shunt components.
- Results panel (right) — shows calculated impedance, SWR, reflection coefficient, and other values at each step.
- Frequency control — sets the operating frequency used for all calculations.
- Normalisation resistance — sets the reference impedance (default 50 ohms).
Circuit element types
| Element type | Description | Ham radio use |
|---|---|---|
| Load | Starting impedance point (your antenna or source) | Antenna feed point impedance from analyser |
| Series R | Resistance in series with the circuit | Resistive losses, balun resistance |
| Series L | Inductance in series | Loading coil, series matching inductor |
| Series C | Capacitance in series | Series matching capacitor, blocking capacitor |
| Shunt R | Resistance in parallel | Parallel damping resistance |
| Shunt L | Inductance in parallel (to ground) | Shunt matching inductor |
| Shunt C | Capacitance in parallel (to ground) | Shunt matching capacitor |
| Transmission line | Length of coax or other transmission line | Matching section, delay line, stub |
| Open stub | Open-circuited transmission line stub | Stub filter, matching stub |
| Short stub | Short-circuited transmission line stub | Matching stub |
Basic operations — entering impedances
Setting the frequency
Always set the operating frequency first — linSmith uses it for all transmission line calculations. Enter the frequency in the frequency field in Hz, kHz, or MHz depending on the unit selector.
Entering a load impedance
The load impedance is your starting point — usually the antenna feed point impedance measured with an analyser:
- Select Load from the element type dropdown
- Enter the resistance (R) in ohms — the real part
- Enter the reactance (X) in ohms — the imaginary part (positive = inductive, negative = capacitive)
- Click Add — the impedance point appears on the Smith chart
Example — typical HF antenna impedances
| Antenna situation | Typical impedance | Smith chart location |
|---|---|---|
| Resonant dipole at feed | 70–75 + j0 Ω | Slightly right of center on horizontal axis |
| Short dipole (below resonance) | 25 - j150 Ω | Lower left area — capacitive |
| Long dipole (above resonance) | 100 + j200 Ω | Upper right area — inductive |
| Vertical at resonance | 35–40 + j0 Ω | Left of center on horizontal axis |
| End-fed wire (non-resonant) | 1000 + j500 Ω | Near right edge — high impedance |
| Open circuit (no antenna) | ∞ + j0 Ω | Right edge of chart |
| Short circuit | 0 + j0 Ω | Left edge of chart |
Transmission line analysis
Adding a transmission line segment in linSmith rotates the impedance point clockwise around a constant SWR circle. This is the fundamental operation for coax analysis and quarter-wave transformer design.
Analysing an existing coax feed line
To see what impedance your transmitter sees when connected to a non-resonant antenna via a specific length of coax:
- Enter the antenna feed point impedance as the Load
- Add a Transmission Line element
- Enter the coax velocity factor (0.66 for standard coax, 0.82 for foam dielectric)
- Enter the cable length in electrical degrees or metres
- Enter the characteristic impedance of the coax (50 or 75 ohms)
- Click Add — the point rotates to its new position showing the impedance at the transmitter end
Quarter-wave transformer design
A quarter-wave section of transmission line transforms impedance according to Z_in = Z0² / Z_load. This is a standard technique for matching a non-50-ohm antenna to 50-ohm coax:
- Enter the antenna impedance as the Load — assume purely resistive for a resonant antenna
- Calculate the required transformer impedance: Z0 = √(50 × Z_load)
- Add a Transmission Line of 90 electrical degrees (quarter wavelength)
- Set Z0 to your calculated value
- The result should be close to 50 ohms — confirm by checking the Results panel
Impedance matching networks
The most common ham radio use of the Smith chart is designing L-networks and pi-networks to match a non-50-ohm antenna to a 50-ohm transmitter. linSmith makes this graphical — you can see exactly what each component does to the impedance.
L-network matching — step by step
An L-network consists of two reactive elements — one series and one shunt. There are four configurations depending on whether the source impedance is higher or lower than the load impedance. The goal is to move the impedance point to the center of the Smith chart (50 + j0 ohms).
- Enter the antenna impedance as the Load
- Your goal: reach the center of the chart (50 + j0 Ω)
- Add a Shunt element (parallel to the antenna) — adjust the value until the point moves onto the r=1 resistance circle
- Add a Series element — adjust the value until the point reaches the center of the chart
- Read the component values from the linSmith interface
Reading component values from linSmith
When you add a reactive element linSmith shows its reactance value in ohms in the Results panel. Convert reactance to component value using:
Pi-network and T-network matching
More complex matching networks with higher Q factors can be designed by adding more elements. A pi-network (two shunt elements and one series element) is common in amplifier output matching and antenna tuners. Build it in linSmith by adding elements one at a time and adjusting each to progress toward the center of the chart.
Stub matching
A stub is a short length of transmission line connected in parallel with a feed line, used to cancel unwanted reactance. Stubs are common in antenna arrays, phased verticals, and VHF/UHF matching networks.
Open stub vs short stub
- Open stub — the far end is left open. An open stub acts as a capacitor at lengths shorter than a quarter wavelength and as an inductor at lengths between a quarter and half wavelength.
- Short stub — the far end is connected to ground. A short stub acts as an inductor at lengths shorter than a quarter wavelength.
Designing a single stub match
- Enter the antenna impedance as the Load
- Add a Transmission Line of varying length — adjust until the real part of the impedance equals 50 ohms (the point is on the r=1 circle)
- Note the reactance at this point — you need to cancel it with a stub
- Add an Open Stub or Short Stub with the appropriate reactance to bring the point to the center
- Read the stub length from linSmith's results
Practical antenna matching with linSmith
Matching a shortened dipole
A dipole cut shorter than resonance has a capacitive reactance component — it falls below the horizontal centerline on the Smith chart. The fix is to add inductive reactance equal and opposite to the capacitive reactance, moving the point to the resistance circle.
- Measure the antenna with an analyser — note R and X at the desired frequency
- Enter this as the Load in linSmith
- If X is negative (capacitive) add a Series L — adjust until X = 0
- The remaining resistance mismatch (if R ≠ 50 ohms) requires a transformer or L-network
- Add the appropriate matching network elements to reach the center
Analysing a loading coil
Mobile HF antennas often use a loading coil to reduce physical length. linSmith lets you visualise what the coil does to the antenna impedance and calculate the ideal coil value for resonance:
- Enter the unloaded antenna impedance (very low R, large negative X for a very short antenna)
- Add a Series L element — increase inductance until the point reaches the horizontal axis (X = 0)
- Read the inductance value needed from the Results panel
- Note that resonating the antenna does not change the resistance — further matching may be needed to reach 50 ohms
Coax as a phasing line
In phased vertical arrays coax sections are used as phasing lines to achieve the correct current phase at each element. linSmith calculates the impedance transformation of these phasing lines and helps verify the design before building it.
Balun design verification
A 1:4 balun or other impedance-transforming balun changes the impedance by the turns ratio squared. Verify a balun design by entering the source impedance, adding the transformation, and confirming the resulting impedance is what you expect. This helps catch winding errors before cutting coax or winding toroids.
Operating guide — step-by-step workflows
Workflow 1 — understanding an antenna analyser reading
Your antenna analyser says 35 - j80 at 14.074 MHz. What does this mean and how do you fix it?
- Set frequency to 14.074 MHz in linSmith
- Enter Load: R = 35, X = -80 — the point plots in the lower half of the chart (capacitive, below center line)
- The SWR circle it falls on tells you the current SWR — read it from the chart scale or the Results panel
- The antenna is short of resonance (capacitive reactance) and low resistance — likely a shortened or low dipole
- Add a Series L to resonate: add Series L, increase value until the point reaches the horizontal axis. Read the inductance required.
- The remaining R = 35 ohms needs a matching network — add a quarter-wave transformer or L-network to reach 50 ohms
Workflow 2 — designing a 40m antenna tuner L-network
You want to match a 200 + j100 ohm load to 50 ohms at 7.074 MHz:
- Set frequency to 7.074 MHz
- Enter Load: R = 200, X = 100
- Add a Shunt C — adjust until the impedance point moves onto the r=1 resistance circle (watch the Results panel for R to reach 1.0 normalised)
- Note the reactance remaining — it will be inductive (positive X)
- Add a Series C to cancel the remaining inductive reactance — adjust until X = 0
- You are now at 50 + j0 ohms — the center of the chart
- Read the component values from the Results panel and convert to μH or pF using the formulas above
Workflow 3 — calculating the effect of a coax run
Your antenna measures 30 + j0 ohms at the feed point. You have 15 metres of RG-58 (velocity factor 0.66). What does your transmitter see?
- Set frequency to your operating frequency
- Enter Load: R = 30, X = 0
- Add a Transmission Line: Z0 = 50, velocity factor = 0.66, length = 15 metres
- linSmith converts the physical length to electrical degrees and rotates the impedance point
- Read the new impedance at the transmitter end from the Results panel
- The SWR remains the same as at the antenna — the coax transforms impedance but not SWR (in a lossless line)
Saving and exporting results
Saving a linSmith session
Go to File → Save to save the current circuit as a linSmith file (.lsc format). These files store all your circuit elements and settings and can be reopened later to continue working on the same design.
Printing the Smith chart
Go to File → Print to print the Smith chart with your plotted circuit. This gives a paper record of your antenna analysis or matching network design — useful for noting in an antenna build log.
Exporting as PostScript or PDF
Screenshots for documentation
The most practical way to document a linSmith result is a screenshot. The chart clearly shows the impedance transformation path and is useful for inclusion in antenna build documentation, club presentations, or QRZ page technical notes.
Key RF concepts for ham radio use
A brief reference for the RF concepts that appear throughout linSmith and Smith chart work.
Impedance (Z)
Impedance is the total opposition to AC current flow, expressed as a complex number Z = R + jX. R is resistance (real, always positive), X is reactance (imaginary, positive for inductive, negative for capacitive). The magnitude |Z| = √(R² + X²) and the phase angle θ = arctan(X/R).
Reactance
- Inductive reactance — XL = 2π × f × L — increases with frequency. Inductors oppose changes in current.
- Capacitive reactance — XC = 1 / (2π × f × C) — decreases with frequency. Capacitors oppose changes in voltage.
- At resonance XL = XC and they cancel — the circuit is purely resistive.
SWR — Standing Wave Ratio
SWR is a measure of impedance mismatch. SWR = 1:1 means perfect match (all power transferred). SWR = 2:1 means about 11% of power is reflected. SWR is calculated from the reflection coefficient Γ = (Z_load - Z0) / (Z_load + Z0). On the Smith chart SWR is the radius of the circle centred on the chart center that passes through the impedance point.
Reflection coefficient (Γ)
The reflection coefficient is the ratio of reflected wave to incident wave at a discontinuity. It is a complex number with magnitude 0 (no reflection = perfect match) to 1 (total reflection = open or short circuit). The center of the Smith chart is Γ = 0, the edge of the chart is |Γ| = 1.
Transmission line characteristic impedance (Z0)
The characteristic impedance of a transmission line is the impedance it presents when it is infinitely long — determined by the geometry and dielectric of the line, not its length. Standard coax is 50 ohms (most ham radio) or 75 ohms (TV/cable). Twisted pair telephone wire is typically 100–120 ohms. Open wire ladder line is typically 300–600 ohms.
Velocity factor
Signals travel slower in a transmission line than in free space due to the dielectric. The velocity factor is the ratio of signal speed in the line to the speed of light in vacuum. A velocity factor of 0.66 means the signal travels at 66% of the speed of light in that cable. This affects the physical length needed for a given electrical length — a quarter-wave section of RG-58 at 14 MHz is shorter than a free-space quarter wavelength by a factor of 0.66.
Troubleshooting common problems
linSmith won't launch — GTK error
- Check for missing GTK2 libraries:
ldd $(which linsmith) | grep "not found" - Install GTK2:
sudo apt install libgtk2.0-0 - Run from terminal to see the full error:
linsmith
Impedance point not appearing on chart
- Verify the impedance values are reasonable — very high impedances (thousands of ohms) plot near the right edge of the chart and may be hard to see
- Check that you clicked Add after entering the values
- Verify the normalisation resistance matches your system — it should be 50 ohms for most ham radio work
Transmission line calculation gives unexpected results
- Verify the velocity factor is correct for your cable type
- Check the frequency is set correctly — transmission line electrical length is frequency-dependent
- Verify the cable length units — linSmith may expect electrical degrees rather than physical metres, or vice versa
- Remember that half-wavelength lines (180 electrical degrees) return the same impedance — a 180-degree rotation is a full circle back to the starting point
Matching network values seem unreasonably large or small
- Check that the frequency is set correctly — component values are very frequency-sensitive
- Verify the load impedance is entered correctly — a sign error on the reactance (+ vs -) can give completely wrong results
- Extremely high or low impedances require component values that may be impractical — consider a different matching topology
Frequently asked questions
Do I need to understand complex numbers to use linSmith?
Basic familiarity with the concept of R + jX impedance notation helps significantly. You need to be able to read your antenna analyser's output (which gives R and X separately or as magnitude and phase) and enter those values into linSmith. You do not need to do complex number arithmetic by hand — linSmith handles all the calculations. The Key RF Concepts section in this guide covers the minimum you need to interpret the results.
What antenna analyser data do I need for linSmith?
You need the complex impedance at your operating frequency — the resistance R and reactance X values. Most modern antenna analysers (NanoVNA, RigExpert, MFJ-259 etc) display R and X directly. If your analyser only shows SWR and impedance magnitude you need to know the phase angle to separate R and X. The NanoVNA in particular displays R + jX directly, making it ideal for linSmith work.
Is linSmith useful for VHF and UHF antenna work?
Yes — the Smith chart and linSmith work at any frequency. VHF and UHF matching networks are often more critical than HF because small impedance errors have larger performance impacts at higher frequencies. Stub matching and quarter-wave transformers are particularly common in VHF/UHF work and linSmith handles both. The only difference at higher frequencies is that component values are smaller — inductors are measured in nH rather than μH.
Can linSmith design a Gamma match for a Yagi?
Yes — a Gamma match is an L-network implemented with a parallel conductor to the driven element. Model it in linSmith as a shunt capacitor (the Gamma capacitor) and series transmission line (the Gamma rod). Adjust the values to match the driven element impedance (typically 12–25 ohms for a Yagi driven element) to 50 ohms. The linSmith display visually shows you how close you are to a matched condition.
What is the difference between linSmith and a full antenna simulation program like NEC?
NEC (Numerical Electromagnetics Code) and its derivatives (4NEC2, xnec2c) model the antenna itself — they calculate the current distribution on the antenna structure and derive the feed point impedance and radiation pattern from first principles. linSmith takes a measured or calculated impedance as input and analyses what happens to that impedance in a circuit — transmission lines, matching components, stubs. They are complementary tools: use NEC to model the antenna design and get the feed impedance, then use linSmith to design the matching network.
My antenna SWR is 3:1. How do I use linSmith to fix it?
Start by measuring the actual complex impedance with an antenna analyser — SWR alone does not tell you enough to design a matching network (a 3:1 SWR could be caused by many different R + jX combinations). Once you have R and X enter them in linSmith, look at where the point falls on the chart, and then add matching elements to move it to the center. The Smith chart immediately shows you which direction to move — whether you need inductive or capacitive reactance to cancel the existing reactance and what resistance transformation is required.