A Filtering Quadrature Coupler with Ultra-Wide Stopband

This paper presents a 90° filtering quadrature coupler with ultra-wide upper stopband. It consists of four parallel coupled feeding lines with enhanced coupling strength and a circular patch. The filtering response is achieved by the coupled feeding lines. To improve the out-of-band suppression level, two types of open stubs are utilized at the end of the coupled-line section. A slot is introduced at the center of the circular patch to realize equal power division. For demonstration, a filtering coupler operating at 4 GHz is designed, fabricated and measured. A 18 dB out-of-band suppression level ranged from 5 to 19 GHz is realized.