Hi,
back metallization of so called GCPW implies some advantages in terms of surface waves suppression respect to what happens at the the lower air-dielectric interface of conventional CPW, as well as suppressing back-radiation effects in CPW-fed slot and folded slot antennas, resulting in higher directivities. Propagating modes are highly affected by the presence of back-side ground plane. In general, if substrate height is comparable or less with respect to the sum of CPW gaps' and signal line widths, typical microstrip mode starts to overcome the CPW mode and the structure behaves like a microstrip. As substrate height increases and gets greater than 2*CPWgapWidth+CPWlineWidth, the structure modes tend to typical CPW modes.
I.