microstrip line ground plane

I saw some Power amplifier design use microstip line with ground plane on the top, which look like a coplanar waveguide.

I am wonder how to difine the miminum distance between ground plane and line to make sure it is a microstrip line?

Thanks in advance

Whether a trace on a PCB is a microstrip or a coplanar waveguide depends on where the return current is flowing for the signal on the trace. If it is really a microstrip, the ground flood on the same layer as the trace would have to be far enough away from the trace to be completely decoupled. The coupling would have to be between the trace and the undelying ground plane for the microstrip. For a microstrip, the return current should be flowing entirely in the ground plane under the strip.

To answer your question, you would have to use a 3D field solver to look at the coupling between the trace and each of the planes.

Unfortunately, many designers use copper flooding because they think it looks good and that it is a good thing to do. They don't really understand what they are doing with regard to signal coupling and trace impedance. In many cases, copper flooding degrades the performance of high frequency boards by increasing capacitance to ground, destroying impedance control, increasing signal cross coupling, and by providing paths for ground current loops.

A good (free) tool to use for designing microstrips is appcad provided by Agilent.

I've compared its results with the microstip calculator in Agilents Genesys product at they are very close.

http://www.hp.woodshot.com/

Cheers!
Andrew