Microstrip and Grounded coplanar

Hi,

Can anyone tell me what is the difference between Microstrip and Grounded co-planar?

If you take a grounded co-planar waveguide and separate the two ground conductors (near the center trace) to infinity! , why wont you get a microstrip line?

as I understand, microstrip is a trace where the ground is located below the trace.
Coplanar has a center conductor, 2 ground trace distanced at a GAP and a ground plane below.

why taking the gap to infinity doesnt degenerate the coplanar to microstrip?

Regards,
Jim

the key point is you can get a small size of transmission line for GCPW

usually the strip is too wide for 50 ohm Microstrip

When you take the gap to infinity, in a practical sense it does degenerate to microstrip. When the top side grounds are moved a very long distance away, nearly all the return current flows in the bottom ground (current seeking the shortest path/path of least resistance).

As dingjing pointed out, because of the higher capacitance per unit length you can make a given transmission line smaller to realize a certain characteristic impedance. However, the trick with GCPW is to keep the top and bottom ground planes at the same potential; usually a high number of vias are needed between them to keep them both at the same potential. So GCPW can become somewhat more expensive or difficult to manufacture.

--Max