Hi D. Marathe , 

Thanks for your reply. Using the floquet/lattice setup I am able to simulate unit cells with no problem. However, it doesnt seem to work when I try and simulate a larger structure like a Frequency Selective Surface for example. I think the problem I am facing is that the global material variable doesnt allow me to inject a plane wave from a floquet port assuming 1 material and then measure the far fields assuming another material. 

For example in the image above, if I set the global variable to water the plane wave enters the bottom water box as expected. It then refracts (I am assuming) correctly into the air box , but then once it leaves the air box into the "infinite" environment, the far fields are calculated using the global variable as water where I want it to be air. 

Is there a way to adjust this?