The simplest and most accurate way to set up the source for a coplanar waveguide is to assign a wave port to the end of the center conductor. In the following image, a coplanar line has been constructed out of three metal boxes on a dielectric plane over a PEC ground plane. All other boundaries are approximate open.
The port attribute only needs to be applied to the center conductor. As explained in “Wave Ports on Metal Traces” , RF3p attempts to bring every non-port conductor in the port plane to a common ground. Thus, the return current comes back on the side conductors and the ground plane, if present, resulting in the expected behavior for a coplanar waveguide. The following field annotation shows the real part of the electric field on the port plane.
Using zero-thickness conductors in place of very thin conductors can simplify the mesh with no loss of accuracy in many cases. This is achieved by using rectangles instead of boxes (or by setting the box height to zero). The following is the field for a port solution in the case where all three conductors have zero thickness.