Design of a compact diplexer using microstrip and slotline dual-mode resonators

A novel compact diplexer with hybrid resonant structure is proposed in this paper. The hybrid structure includes one microstrip stub-loaded dual-mode resonator and one slotline stub-loaded dual-mode resonator. These two dual-mode resonators both with two controllable resonant modes and one transmission zero are used to construct the desired passbands of the proposed diplexer. Meanwhile, the inherent transmission zeros are designed to locate in the stopbands and thereby improve the isolation between the two passbands. Finally a diplexer with the lower and upper passbands centered at 2.5 GHz and 3.5 GHz is designed to verify the proposed method. Without using any extra matching network, the proposed diplexer shows a isolation larger than 28 dB.