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Last modified: Sun, 29 Oct 2017


What is AlterPCB Transmission Line Simulator?

AlterPCB Transmission Line Simulator (a.k.a. TLineSim) is a tool that can be used to design transmission lines for PCBs. When working with RF frequencies, good transmission line design is critical. There are many free tools available for this purpose, both online and offline. Unfortunately, these tools almost always rely on approximate formulas which are often highly inaccurate, or are only accurate for a narrow range of input parameters. Usually they are limited to simple transmission line types for which approximate formulas are known, and can't be adapted to more complex transmission lines that are often necessary (such as differential coplanar waveguides).

TLineSim is different: instead of hard-coded approximate formulas, it uses a field solver to determine the properties of arbitrary transmission lines. TLineSim is essentially solving a slightly simplified version of Maxwell's equations using the finite element method. This method is far more accurate and not restricted to specific transmission line types.

TLineSim will be integrated with AlterPCB once it is ready, but since the tool is quite useful by itself, I decided to release it as a stand-alone program as well. Like AlterPCB itself, TLineSim is completely free, open-source and cross-platform.

Image: alterpcb-tlinesim-screenshot1-small.png

The TLineSim user interface. In this example, TLineSim is used to automatically tune the track width of a differential coplanar waveguide in order to obtain a differential impedance of 100 ohm.


  • Calculates transmission line properties such as characteristic impedance, propagation velocity, wavelength, loss, capacitance, inductance, ...

  • Uses an accurate quasi-TEM field solver rather than approximate formulas. As a result it can simulate arbitrary cross sections.

  • Includes models for uncommon transmission line types such as differential coplanar waveguides.

  • Models optionally include solder mask (which can have a small impact on characteristic impedance and loss).

  • Simulates both resistive and dielectric losses (including skin effect and proximity effect).

  • Supports anisotropic materials (most PCB substrates such as FR4 are in fact anisotropic).

  • Supports single frequency analysis as well as frequency and parameter sweeps.

  • Supports automatic parameter tuning (e.g. to determine the track width that will result in an impedance of 50 ohm).

  • Straightforward graphical user interface (Qt-based).

  • Open-source and cross-platform.



Download AlterPCB-TLineSim 0.0.0 (32-bit)
Download AlterPCB-TLineSim 0.0.0 (64-bit)


Right now there are no precompiled packages for Linux, but I will try to create these soon. For now, you should compile from source.


In theory Mac is supported if you compile from source, but this has not been tested yet.

Source code:

You can find the source code on GitHub:
If you're not familiar with git, you can also download the ZIP file (for Windows) or the TAR.GZ file (for Linux and Mac).


Image: gpl-v3.png

AlterPCB-TLineSim is distributed under the terms of the GNU General Public License.

Feedback, bugs, feature requests ...

You can report bugs and submit feature requests on GitHub:

If you don't want to create a GitHub account, you can also send an email to:

Image: aaaa.png

If you don't get a reply within a week, your message was probably eaten by the spam filter. If that happens, try sending the message again from a different address.



Comment #1: Tue, 30 Apr 2019, 14:05 (GMT+1, DST)


The transmission line simulator is a really nice tool!
The only one that's free and capable of calculating coplanar differential waveguides.
I hope that it will be possible someday to add entries to the substrate material within the GUI.

Last modified: Tue, 30 Apr 2019, 14:08 (GMT+1, DST)

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