Measurement of Transm. coef. in FSS using HFSSv.10

Dear collegues,

I am currently simulating an FSS structure using Ansoft's HFSS v.10.
The structure consists of two series of dielectric rods placed one above the other separated by vacum and forming a kind of a parallel plate waveguide. I use a plane wave to excite the structure. The excitation point lies on the level parallel and in the middle of the structure. I want to measure the transmission coefficient (S21), as a function of the angle of incidence of the plane wave, from the upper radiation boundary.
When the excitation is placed outside the structure this is easy but in my case that measurement seems impossible because the FSS radiation boundary provided by HFSSv.10 can not be placed inside the structure.
Any kind of help would be thankfully appreciated.

Of course the Radiation cannot be placed inside the structure. It is a BOUNDARY condition.

Exciting with a plane wave, you do not select any starting point of the excitation, only a phase zero. The plane wave is everywhere everytime.

Do you use periodic boundary condition?
In which direction do you want to measure transmission?

Dear cd79,

Thanks for the quick reply. My experience is limited as I am currently studying at the university. The problem goes like this:
I use master/slave boundaries to create periodicity but only in one direction. I mean that the periodicity creates an infinite structure of rods as far as their number but their length is not infinite. So I use master/slave boundaries in that direction and simple rad. boundaries in all other directions. I want to examine the frequency bandgap of the structure as a function of the angle of incidence of the plane wave. This will be done by measuring transmission from the upper (or the lower) level of rods but I can not find the way of setting the plane wave excitation as the reference and the upper boundary as the level where I want to compute the transmission. Could you help me ?