CST SRR simulation problem

Hello everybody
I'm new in using CST
I'm facing a problem of a frequency shift in the results of S11 and S21 of the simulation of rectangular SRR there is always about 250 Mhz shift
actually i always facing a problem in defining the dimensions of the vacuum space around the model and the dimension of the waveguide port.
could any body help me with that ?
any contribution would be appreciated
thank you in advance

I didnt exactly get your question. Probably, you doing a waveguide test for short-range-radar. As per port dimension rectangular waveguide, you can pick the inner edge and a point and simply define waveguide port using pick. Also, you can go through the help index of CST, just pressing F1.

I would recommend you to see the port signal, while simulation. As you want only the fundamental mode i.e TE10.