periodic boundary conditions!

hi!
I am trying to use the frequency domain solver in CST. For my structure, which is very small respect to the wavelength, I want to have as source excitation a plane wave. More or less, choosing a port and periodic boundary conditions I can create a wave plane. My question would be: if I choose all the boundary conditions: periodic and the port as "full plane" do I have a plane wave?
If not can someone help me how to choose this periodic boundaries? What about the phase shift, constant angle?
thanks.