Compact Mode‐Composite Bandpass Filter and Diplexer With Flexible Transmission Zeros Control Based on Mixed Electro‐Magnetic and Source‐Load Coupling Mechanism

ABSTRACT This paper presents a series of mode‐composite bandpass filters (BPFs) and diplexer, which integrate both electro‐magnetic (EM) and source‐load (SL) coupling within the hybrid eighth‐mode substrate‐integrated waveguide (EMSIW) and microstrip line (MSL) resonators. The mixed EM coupling can achieve a distinct distribution of transmission zeros (TZs), and SL coupling can further enhance the selectivity near the operating passband. Case I presents a mode‐composite BPF operating at 3.5 GHz. The proposed BPF employs SL coupling and EM coupling, with electric coupling (EC) being predominant. Consequently, this configuration yields three TZs adjacent to the passband: one to the left and two to the right. Case II presents a similar BPF design operating at 5.6 GHz, but with magnetic coupling (MC) dominating the mixed EM coupling. This configuration also results in three TZs near the passband: one to the right and two to the left. To demonstrate and validate the novel property, both types of mode‐composite BPFs and the relevant diplexer have been designed, fabricated, and measured. The simulation and measurement have reached to a satisfactory agreement.