CST modeling of rectangular waveguide using Freq solver

Hello everybody,

Im trying to model a rectangular waveguide at Xband using the FREQ solver.
I put the background as PEC, a void of VAC and two ports. boundary are Et=0 in all directions
After completion of the run the S12 and S21 parameters differ one from the other (i found a similar difference between S11 and S22).

Can anyone help me settle this discrepancies?

Thanks

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