Inline Waveguide Filter With Compact Frequency-Dependent Coupling Producing Two Additional Poles and Three Transmission Zeros

This work reports a compact frequency-dependent coupling (FDC) structure introduced in a rectangular waveguide, which allows to generate two additional in-band transmission poles and three transmission zeros (TZs). This serves to increase the order/selectivity of the waveguide bandpass filter in a compact inline topology, thus without the need for any additional space/volume or cross coupling. The proposed FDC consists of a partial-height post and a T-shaped post (P&T) that are placed side by side along the width of the waveguide. An equivalent circuit of the P&T FDC is also provided for the filter synthesis process. The TZs can be flexibly placed at either passband side. Electromagnetic-(EM)-simulated results for a fourth-order inline generalized-Chebyshev bandpass filter shaped by two cavity resonators that are coupled through a P&T FDC are presented to validate the RF operational principle of the conceived FDC.