CST Microwave Studio - how to simulate characteristic impedance

Hi,
I?m using CST2009 Microwave Studio to simulate transmission line characteristic impedance. The scenario is to have a pair of copper wires that are about 1 inch above the earth ground plate. Additionally, I?d like to see the characteristic impedance of a shielded twist pair (STP) with the shield connected to earth ground. Currently I?m using the ?Waveguide Ports? on simple 2 wires without problem. How do I add an earth-grounded shield? That is, I can draw a metallic shield, but how do I specify it to be earth grounded?

Thanks for answering my question.
B. Riverton