I. Introduction
Directional couplers are useful microwave components that combine and divide signals with quadrature phase splitting, and that can be employed to implement various microwave circuits such as image rejection mixers, balanced amplifiers, modulators, and antenna beam-forming networks [1], [2]. the parallel-coupled microstrip line structure is the most frequently adopted structure in realizing microwave directional couplers, filters, and other components. However, a coupler with a tight coupling coefficient larger than 6 dB is difficult to realize using the parallel-coupled lines because the required gap between the coupled microstrips is hard to achieve by using commercial printed circuit board (PCB) fabrication technology.