Novel compact branch-line coupler using non-uniform transmission line

A new method of compact branch-line coupler is presented in this paper. The method utilizes nonuniform transmission lines (NTLs) instead of uniform transmission lines (UTLs) in each arm of branch-line. The normalized width w(z) /h function of the NTLs which is expanded in a truncated Fourier series is used for optimization. A 2-GHz branch-line coupler is designed, fabricated and measured. The measurement results are in good agreement with the simulation results. This new structure exhibits 50% size reduction (in circuit area) as compared with the conventional branch-line coupler.