This paper proposes a highly selective dual-band bandpass filter (BPF) utilizing hemispherical resonators and an in-band transmission zeros (TZs) method. For obtaining high isolation between two passbands, a compound resonator topology is realized by arranging six hemispherical cavity resonators (HCRs) in a two-by-two vertical configuration, and then three TZs between the passbands are introduced. The filter selectivity is further optimized through elaborately placing three metal pillars in the cavity, and then one TZ in the lower stopband and two in the upper stopband are generated. Notably, the size of the HCR is reduced by about 50% compared to the conventional spherical resonator. To validate the design, a dual-band filter is 3D printed, which operates at 9.1 and 9.77 GHz with bandwidths of 210 and 200 MHz, respectively. The measured results show good agreement with the simulated ones.

In this paper, the influence of the cutting plane as well as the orientation of the cavities in cross-coupled W-band waveguide filters are investigated. When waveguide filters are manufactured with the commonly known CNC (computer numerical control) milling technique, at least one cutting plane is required. The position of this cutting plane has an impact on the composition of the cavities, the manufacturing accuracy, and on the maximal number of transmission zeros (TZs) introduced by a direct source to load (SL) cross-coupling. Similar filter set-ups therefore may show different performances depending on the position of this cutting plane. To examine all these effects, three similar fourth-order W-band filter set-ups are realized with distinct cutting planes and different oriented cavities. The filters are compared in terms of the sensitivity to manufacturing tolerances, the maximal number of TZs introduced by a direct SL cross-coupling as well as their spurious mode performance.

This paper presents the first experimental results of a novel MEMS-based waveguide filter for mobile back-hauling at K-band with adjustable center frequency. The tuning concept employs commercial packaged RF MEMS switches, wire-bonded within a rectangular cavity. The switches are used to perturb the current distribution of the TE101 mode so as to tune the resonant frequency of the cavity. A tuning range up to 2% with unloaded Qs of the order of 1000 in K-band can be obtained. A second-order filter prototype employing commercially available MEMS on silicon substrate has been tested. Measurements show a 160 MHz frequency shift (0.7%) and Qs up to 1000 using only one MEMS per resonator. New higher-order prototypes using low-loss substrate MEMS (e.g. quartz) will be fabricated in order to get higher Qs. The concept here present leads to a very simple structure. The number of MEMS per resonator is, in fact, minimized (we use only one MEMS) and the design is simple. On the other hand, the achievable tuning range is lower (<2%) than the other concepts. Higher tuning ranges are achievable using more MEMS, but at the expense of lower Qs of the resonators.