Simulating asymmetric FSS (calculate S21, how? )

Hello everyone,

I'm trying to simulate a single asymmetric cell of FSS structure. I'm using HFSS 10.0. It has no ground plane, so I need to look for the transmission S21 afterwardes.

1-I tried to use simple E-H planes with two wave ports. The results are not so bad, ut they are very sensitive to delta (even with 0.001 !) and the size of the vacum (even with 7 times the substrate thickness).

Am I doing something wrong?

I'm looking for very small effect when I have small change in the structure, so I need very accurate results.

2-I heard that it is possible to use periodic bouandries and having incidence-wave port instead of wave port. I have all the fields calculated at the frequencies of interest.

Is it possible to calculate the transmission from that ? and how ?

Waiting your hints...