CST Microwave Studio modeling - Shielded wires

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 a distance 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 (one source and one return) 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

I think you should use a discrete port for excitation.

you can specify a ground using the boundary conditions. The condition E=0 gives you a perfekt ground.

That sounds great. I'll give it a try. Thank you.