@Leonardo Gomes I think you need to create a symbol for a transmission line, with an input port and an output port. Internally the microstrip equivalent circuit can be described by R,L, C components, ngspice has (section 31.4) also specific models for transmission lines. Once you know the netlist format you can create a symbol for it. I think you can also model by equations by using some bsource.sym (the ngspice bsource device) which allow arbitrary equations for voltage or current.

I'll try to fiddle with the netlist and models. I'll report back what I find. Thanks :)

@Leonardo Gomes Did you check chapter 6 in ngspice documentation? http://ngspice.sourceforge.net/docs/ngspice-manual.pdf

I read it, but I confess I got lost at the number of different models and how to declare them. I managed to get the LTRA model running in xschem, but that's all.

Great @Leonardo Gomes Could you share what you have done?

Sure :) I created a schematic containing the necessary i/o terminals and a netlist (see figure) At the top cell, where I simulated the line response, I added device_model=".MODEL Ustrip_50ohm LTRA (R=0.00762 L=3.082e-13 G=0 C=1.289e-16 Len=500)" to each symbol. Like so we have a model of a 500um-long, 50ohm transmission line.

Great really nice trick