vivaldi cst

Hi,
I am trying to create a vivaldi antenna that is usable for (at least) 3GHz to 8GHz. That shouldn't be a problem because these antennas are for broadband applications. I entered a model that is similar to the test antenna at the CST homepage. But my wont work as good. Does anybody has that file from these vivaldi antenna or can give me some hints?

Thanks in advance!!