EM.Cube sets the internal resistance of each of the two coupled lumped sources equal to twice the specified port (or line) impedance since the CPW port is a parallel connection of the two individual lumped sources. If you want to model a CPW with laterally infinite substrate and ground planes, make sure to set the domain offsets in the ±X and ±Y directions equal to zero. In the -Z direction, you need a nonzero offset to push the bottom CPML boundary down away outside the dielectric layer of finite thickness. Alternatively, you can terminate the bottom boundary by PEC and set its domain offset equal to zero to represent a CPW with a bottom PEC ground.
[[Image:FDTD51.png|800px]]
Figure 1: A coplanar waveguide (CPW) port on a dielectric substrate.