The Equivalent Circuit of a Balun
A balun can be implicitly modeled in AN-SOF using an equivalent generator, as the figure below shows. Since voltage sources admit an internal impedance, we can adjust its resistance value to match the antenna input resistance. Of course, we must previously cancel out the antenna reactance using an opposite lumped reactance.
To calculate the true generator side VSWR, go to Setup tab > Settings panel > VSWR Ref. Impedance and set the same value as the resistance of the equivalent circuit of the balun.
For example, if you have a 4:1 balun (K = 4 being the impedance transformation factor), put a voltage source at the antenna terminals having 2 x the voltage of the true source and an internal resistance of 4 x the true internal resistance. Also, set “4 x the true internal resistance” as the reference impedance for VSWR in the Settings panel.