Compact and Broadband Transition of Microstrip Line to Finite-Ground Coplanar Waveguide

In this paper, a compact and broadband transition of microstrip line to finite-ground coplanar waveguide (FGCPW) is proposed. Design rules are presented on the basis of simulation and measurement results. Both electric and magnetic fields are gradually transformed from microstrip line to FGCPW by tapering the signal conductor. The microstrip ground plane on the bottom side of the substrate is directly connected to the grounds of the FGCPW on the top side with metallic vias, providing an RF short-circuit for suppressing parasitic modes. Measured return loss is lower than -10 dB for a back-to-back structure from 0.04 GHz to 24 GHz with a low insertion loss of about 1.5 dB at 20 GHz. This kind of transition can find applications in highly integrated circuits and systems using both FGCPW and microstrip line techniques, and also offer easy and accurate measurements of FGCPW circuits with the help of simple coaxial adapters.