Design of Compact Branch-Line Coupler with Coupled Resonators

A compact branch-line coupler is proposed in this paper. The quarter-wavelength transmission lines are equivalent to the couplings between resonators. Thus, the conventional branch-line coupler can be transformed into a coupled-resonator network. With the proposed miniaturized resonators and coupling structure, the circuit size can be reduced by 50% and a 4% fractional bandwidth can be achieved.