Impedance reference and Frequency sweep with FEKO

I need to plot S11 parameter , for a rectangular waveguide, on the smith chart.
To calculate S11, feko uses a fixed impedance reference ( 50ohm), but in a rectangulare waveguide,this impedance is frequency dependent. For TE, Z0=(w*u)/kz.
how can i plot S11 with this impedance calculated on the X band ?