Ultra-compact filters using TM dual-mode dielectric-loaded cavities with asymmetric transmission zeros

This paper presents ultra-compact dual-mode dielectric-loaded waveguide bandpass filters with N^th-order pseudoelliptic filtering function and asymmetric positioning of N transmission zeros. A waveguide cavity is loaded with a very high-permittivity rod (εr>30). Two degenerate resonant TM modes are employed along with nonresonating modes. In contrast to previous works, that are limited to structures with a symmetric response, in the proposed solution both modes are directly coupled to the input and output interconnections. Nonresonating modes are exploited to produce a direct source-load coupling. In this way, a single cavity generates two transmission poles and two transmission zeros located either below or above the two poles, thus realising a basic building block for N-pole and N-zero filters with asymmetric response. The dimensions of a 3% FBW 4^th-order filter with 4 zeros at 4.35 GHz are 40×40×12 mm³ (using εr=34). A preliminary prototype has been fabricated and measured showing an unloaded Q of 1750 and good agreement with HFSS^® simulations.