how to set the boundaries of an infinite pec coated by dielectric medium in cst?

how to set the boundaries of an infinite pec board coated by dielectric medium ?
period boundaries or unit cell? I want get the reflection coefficient changed with the incident angle .I read the example in the cst web site (cst.com/Content/Applications/Article/Article.aspx?id=355) but i can not figure it out .
Thanks very much!