@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. Thread in Slack Conversation