Transmission Line Tuning

In order to connect an SMA connector to a RN2483 module, we want the feedline to have the right impedance (e.g. 50 Ω @ 868 MHz).

Drawing proper traces

For that, we need to adjust the length and the width of the trace, as well as the spacing between the trace and the groundplane.

Adjusting the width can already be done using the trace drawing tool in LibrePCB. However, the spacing to the groundplane seems to be constant, right? Or is there a way to adjust spacing?

Also, what about length tuning? Ideally there would be a tool to adjust the length of a trace, but even if there were a tool that would just display (in real time) the total length of the currently selected trace, that would already be sufficient.

Calculating proper lengths and spacings

KiCad has a few calculator tools, among them the “TransLine” calculator:

It’s quite convenient, but I think we could do better than a collection of calculator tools. Maybe something like this could be integrated into LibrePCB? For example, a user might be able to add constraints to a line or net in the schematics, and indicate that the impedance of the trace should be 50 Ω @ 868 MHz. The material constants could then be set up in the PCB properties. With such a system, the PCB editor could provide tools and warnings to ensure proper matching.

Maybe there are some more ideas around?

Currently the clearance is a property of the plane, which can be adjusted of course.

Length tuning would be pretty complicated, but measuring the length should be possible without too much effort.

Of course that would be nice, but that’s really non-trivial. Especially because nets are often not just point-to-point connections, and the schematic doesn’t know anything about the routing of traces, e.g. if a trace goes from R1 to R2 and then to R3, or from R2 to R1 and then to R3 etc.