Mar 21, 2012 · Why should we consider using coplanar waveguide over ground (CPWG)? It allows us to narrow the trace almost arbitrarily for a given layer thickness while maintaining a 50 or 75 ohm impedance. CPWG also allows less current into the air than a microstrip structure, because the topside ground copper shunts the field current locally to the topside ground. By Steve Hageman.