This study develops a compact 28 GHz bandpass filter on a low-temperature co-fired ceramic substrate for applications in LMDS (Local Multipoint Distribution Service) bands. The filter comprises two pairs of verticallystacked cross-coupled open loops with vertical interconnection structures, achieving compactness, high integration, and superior frequency selectivity. Attaining selective response with two transmission zeros requires adjusting the couplings of adjacent resonators and external quality factor. The open loops are fed by using the three-via vertical interconnections to prevent any electrical effect on the filter. Measurements correlate closely with the simulation results: this study achieved a bandwidth of 2.1 GHz (27.6-29.7 GHz) with two zeros located at 25.8 GHz and 31.1 GHz, and a compact size of 2.69 x 2.66 x 0.4 mm3.
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