Coupled-Line Directional Coupler with Arbitrary Coupling Level and Adjustable Operating Band

A directional coupler with arbitrary coupling level and adjustable operating band is proposed based on the distinct phase velocities of the odd mode and even mode of spoof surface plasmon polaritons (SSPP). Two SSPP transmission lines are employed to design directional couplers due to the open and confined field distribution of the SSPP. Arbitrary coupling level can be achieved by tuning interval width between the two lines. Besides, phase constants of even and odd SSPP modes are different, and the operating band could therefore be adjusted by both the phase constant difference and length of the coupling section. Two prototype couplers, one with adjustable coupling level in wide band and another with adjustable operating bands, are designed to verify the design principle. This design has capability in wideband application and potential in frequency division duplexing systems.