Ansoft HFSS is a design program which can be used to create your own antenna designs. The application has options that can be used to design even an trapezoidal antenna. This can be done simply by choosing the trapezoidal form from the form toolbar or by combining other elements to reach the desired form. The Internet is full of tutorials on creating antenna models using Ansoft HFSS. I suggest that you use either Google or Youtube to find an antenna modeling tutorial, even if it's not for a trapezoidal one.
