coplanar waveguide 50 ohm
There are CPW without ground plane, as described above, and also, there are CPW with ground plane underneath. The main difference between microstrip line and coplanar waveguide is indeed the presence of ground lines on the same layer as signal trace, spaced at equal distances in one side and the other. CPW has some advantages over microstrip. The impedance of a microstrip line can be controlled only by changing its width (assuming a given substrate). The impedance of a CPW can be controlled by changing the signal trace width and can also be controlled by changing the width between signal trace and nearby grounds. Also, CPW radiates less RF energy into surrounding traces, due to presence of ground. Another transmission line calculator is AppCAD from Agilent, you can download it free from their site.
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