I don't know my friend

S parameter cannot be obtained from plane wave excitation in CST. Rather 2D and 3D electric field and other field distributions can be found out

S-parameters are a means for describing the behaviour of electromagnetic circuits (e.g. antennas, microwave components, etc.) A plane wave excitation is mostly used to get Radar Cross Sections (RCS) which is a measure of the target's ability to reflect electromagnetic energy towards a receiver. As the source is often assumed to be in the Far Field of the target a plane wave excitation is a good assumption. If your source is not in the Far Field then this method of excitation might not be suitable, but this is obviously problem dependent. 

If it is a periodic meta-material you are trying to look at then perhaps this is the approach you need?

http://www.pro-physik.de/SpringboardWebApp/userfiles/prophy/file/PhysikJournal/PB_2014_30_32.pdf

Thank you, my friends.

You're welcome.

You simulate the structure with the unit cell boundary condition in frequency domain and in z-axis ( direction of propagation ) you will have z(max) and z(min) port . These are the equivalent to port-1 and port-2. So S21 means Z(max)(1) Z(min)(1) with same modes. and Z(max)Z(max) (1) means S11.

Hi, @malcolm white

I am interested to look into the link you shared in your answer but it says that the page is not found. Could you reshare the paper? I would really appreciate it!

I'm sorry, that was 4 years ago and I can't remember what the article was.