Mode composite waveguide filter with dual-mode operation

In this work, a mode composite waveguide (MCW) based dual-mode filter is proposed and studied. This filter uses the inner waveguide of MCW as the input and output feedings, and the outer waveguide of MCW as the dual-mode resonator. The two degenerate modes in the outer waveguide resonator are used for the dual-mode filter operation, which generates two transmission poles and one finite transmission zero. The two transmission poles are used to control the filter bandwidth, and the finite transmission zero is used to improve the out of band selectivity. The generated finite transmission zero can be placed at either the left or right vicinity of the passband by adjusting the input and output feeding parameters. Two types of dual-mode filter are designed at 10 GHz with 2% fractional bandwidth. Type I filter exhibits a transmission zero at the left vicinity of the passband while type II filter exhibits one at the right. The type I dual-mode filter is fabricated and its measurement results agree with its simulations.