Realization of Ultra-Compact Planar Microstrip Branch-Line Couplers With High-Impedance Open Stubs

The ultra-compact microstrip branch-line couplers are proposed in this paper. Using the provided criteria to shorten the lengths of four quarter-wavelength transmission lines, the sizes of branch-line couplers can be greatly miniaturized. In this study, the reduction rate of the proposed couplers´ sizes are over 88.9% and 76.8% using approaches with the pi-model and the T-model, respectively. Moreover, the measured frequency and phase responses show good agreement with the electromagnetic simulated results.