I have been using the old QUCS for some time but found the newest QUCSstudio update with the built in optimizer (not asco) and several other features to be extremely useful and considerably faster. One issue I am running into is that I need to model transmission lines with complex characteristic impedance in the frequency domain in order accurately account for loss. The old QUCS has this RLCG model (Qucs-core: rlcg.cpp Source File (sourceforge.net)) which after testing I think is essentially exact in terms of loss when terminated into a mismatched load (“Additional Loss Due to SWR” (ac6la.com)). This model does not work in the time domain but it seems to work for AC and Sparameters which is my primary use case.
Is it possible to get a RLCG model in QucsStudio? If not should I try to use the equation defined frequency domain part and derive the parameters in equations? Are there any other suggestions to correctly account for the loss into a mismatch?
Update: I was able to follow the code from QUCS to implement the complex impedance transmission line via Y parameters in the frequency domain device and it seems to work and give the expected loss. Schematic attached. I guess the RLCG model in QUCS is just doing this. The loss should be wiggly vs length because there are standing waves on the line and certain regions have more I^2R/unit length than others. The normal Tline does not know about this apparently and it seems to over or under estimates the loss.