Antipodal vivaldi ultrawideband antenna design using CST

Hi,
I have made a base design for an antipodal vivaldi antenna for ultra wide band application.
I'd like to know what changes to be made to improve gain or reduce the lower band frequency.
I am using the software CST for designing

Hi,
Start with specifications (like every design should start)
Then show what (and how) you tried to solve it.
Klaus