Derivation of Generalized Design Formulas for Modified Branch-Line Coupler with the Second Harmonic Suppression Characteristics

We propose a modified branch-line coupler composed of the open stubs and stepped impedance lines. This structure allows the second harmonic suppression and size reduction. We present the steps to transform from a conventional branch-line coupler to the proposed structure using equivalent circuits and derive the generalized design formulas. From the results analysis of the design sample, we demonstrate the validity of the derived formulas. The fabricated branch-line coupler provides more than 30 dB the second harmonic suppression and about 30 % size reduction compared to a conventional branch-line coupler.