cylindrical cavity in HFSS

I am making a cylindrical cavity in HFSS. The cavity is air-filled. The internal excitation would be provided across teflon through the probe. The air-filled cavity is enclosed inside a copper enclosure. The centre frequency is to be 2.83 GHz. However firstly i am not getting proper S(1,1) results. Secondly i am not getting them at the desired frequency. Another problem is that the equipment i.e. network analyser available in our lab can provide frequencies only upto 3 GHz. And the 2.83 GHz has been obtained through mathematical calculations. Can anyone here guide me properly through this or tell me if i am making any mistakes.