hfss fullbook
Nikoleta
Firstly, for FSS simulation of S-parameters, the best choice of Ansoft's solutions is the Ansoft Designer. You can carried out a thorough FSS simulation in about 20 minutes (inclusive of project setup time and simulation duration).
If you have only the HFSS in your hand, then this gets a little more exhausting, but it can be done (I am sure you know already, but just HOW?)
asdfaaa is right, you don't use waveguide port. In fact, it is definitely the plane wave source. This is how you would do it in a the laboratory where the FSS under test is placed between a pair of horn antennas are used to lauch and receive a continuous plane wave propagation. Never in one's mind should a waveguide port be employed.
Secondly, mustn't keep thinking about S-parameters. It is actually the power levels you are concern about. There are three power-levels you must modelled. They are Total Radiated Power, Total Reflected Power and Total Transmitted Power for a sweep of frequencies. You need to define three cut planes in your air box corresponding to the three items I just mentioned.
Finally, this is a more advanced task. I only know this in version 8. The Field Calculator is used to integrate E-field on each cut-planes to obtained the corresponding power level. Sadly this is too much of an effort to explain it to you. However, the HFSS technical support will have instructions and tutorials on how this is done. In version 9 and later, I believe the field calculator is no longer available and I think one needs to write a vbscript program to do all of that. A very challenging task!
These are the advice I can offer you in regards to FSS simulation in HFSS. Hope this has shredded some sense to you. As I said in the beginning, depending on the level of experience you have with HFSS, a easy solution would be to use Designer.
Kind regards, Sassyboy