Re: sonnet lite
The ports above and below the line of symmetry should be the same. In your case, the CPW port on the signal line should be 1 and the two ports on either ground plane should be -1, indicating that "Port 1" is a push-pull port, or that the port is fed 180 degrees out of phase with respect to the ground planes. Labeling both ground plane ports "-1" ties them together as far as Sonnet is concerned; there's no need for bridges or additional metalization to connect these two ground planes. The same goes for "Port 2." If you're having trouble just don't draw anything below the symmetry line/plane; Sonnet ignores it in the analysis anyway. You're getting an error, or warning, message because Sonnet is detecting that your model is not symmetrical.
Good Luck!