Balanced quad‐band diplexer with wide common‐mode suppression and high differential‐mode isolation

A balanced quad‐band diplexer is proposed with wide common‐mode (CM) suppression and high differential‐mode (DM) isolation in this study. The six‐port balanced diplexer was formed by combining two dual‐band bandpass filters operating at 1.92/5.25 and 2.45/5.8 GHz through two stepped‐impedance resonator (SIR) inserted T ‐junctions. By carefully designing the section length of the T ‐junction and the SIR length from its open‐end to the joint‐point with the T ‐junction, high isolation between diplexer channels can be attained. A prototype of the proposed balanced quad‐band diplexer was implemented and measured with a DM isolation of higher than 51.8 dB and a CM suppression of larger than 47.9 dB in the four DM passbands. Good agreement was observed between the simulated and the measured results.