Miniaturised quadrature hybrid coupler using composite planar transmission lines

In this Letter, a novel miniaturisation technique for a 3-dB quadrature hybrid coupler is described. The revolutionary structure is composed of folded microstrip lines and folded coplanar waveguides that are printed on the top- and bottom-surfaces of a dielectric substrate, respectively. The two different planar lines are connected by copper vias to form the double-path composite microstrip-coplanar waveguide (CMCPW) line. To demonstrate the effectiveness of the technique, a simple yet miniaturised hybrid coupler is devised by introducing folded S-shaped CMCPW-lines, wherein their lengths are gradually changed to address the design goals. The simulated and measured results of the proposed coupler validate its good performance metrics in the amplitude responses of power splitting, return- and insertion-loss. Moreover, its isolation and quadrature phase between outputs have been examined. A very small footprint size of 0.0667λ₀ at 0.5 GHz has been realised.