[Legacy Report] Nonresonating Mode Waveguide Filters

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Abstract — The multimode approach is a well-established technique for the design of microwave filters. The idea underlying a multimode cavity filter is that a single physical cavity can be designed so as to handle more than one resonant mode. In terms of filtering function this means that each cavity is able to generate multiple poles, thus decreasing the number of physical cavities which are necessary to implement a certain filter order. As a result, one of the most successful applications of this technique is the realization of reduced-size waveguide cavity filters for satellites.

Recently, the classic multimode concept has been extended to also involve a different type of modes, namely the nonresonating modes. What is meant by nonresonating mode? The term nonresonating indicates a mode that resonates far away from the filter passband. Such a mode can be propagating or evanescent at the operative frequencies, but not resonating. In this new multimode approach the nonresonating modes can be used in combination with either a single or a pair of resonant modes. The essential characteristic of the nonresonating modes is that they can be exploited to create additional paths for the electromagnetic energy flow that by-passes the resonant modes. By properly exploiting the interference between the different paths, pseudoelliptic filtering functions with finite frequency transmission zeroes can be obtained. In contrast with the classic multimode approach in which only resonant modes are employed, the selectivity provided by a single cavity is dramatically enhanced by generating transmission zeroes rather than by increasing the number of poles. Aside from the compactness, a major advantage of nonresonating mode filters is the possibility of generating a maximum number of transmission zeroes that equals the number of poles.

This talk will present an overview of the main contributions that recently introduced the use of nonresonating modes for the realization of pseudoelliptic waveguide filters. The attention will be especially focused on how the multimode operation in those structures can be easily related to well-known filter topologies, thus enabling the application of standard synthesis techniques as well as the understanding of otherwise rather complicated electromagnetic phenomena.

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