Waveguide evanescent mode filters using high-Q bypassed cavities for extreme close-in rejection

An original solution for the realization of pseudoelliptic evanescent mode filters with extreme close-in rejection is presented in this paper. The basic idea consists of using high-Q bypassed TE₂₀₁ mode cavities embedded within two or more evanescent mode resonators. Such a combination of resonators allows the generation of extremely close transmission zeros while realizing at the same time relatively wide passbands and stopbands, the latter being a typical feature of the evanescent mode filter class. In contrast with other pseudoelliptic evanescent mode filters using conventional approaches, the insertion loss degradation at the filter cut-off frequencies is minimized thanks to the high-Q factor of the bypassed TE₂₀₁ mode cavities, whose corresponding poles are located right at the edges of the passband. The experimental results of a waveguide evanescent mode filter having 9.950-11.000 GHz passband and providing more than 45 dB rejection between 11.040 and 11.050 GHz validate the proposed solution.