A new technique to realize quasi-elliptic bandpass filters with extreme close-in rejection is presented in this paper. The basic idea consists of embedding non-resonating mode waveguide cavities within the structure of an evanescent-mode filter. Such a combination of cavities and resonators allows the generation of very close transmission zeros while realizing at the same time relatively wide passbands and stopbands. Both H-plane TE201 mode and E-plane TM110 mode configurations are used as non-resonating mode waveguide cavities. In contrast with the other elliptic 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 waveguide cavities, whose corresponding poles are located right at the edges of the passband. The experimental results of an 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.
E-plane waveguide bandpass filters with wide stopband