In this paper, we present a new class of miniaturized microstrip bandpass filters with low-insertion loss, sharp-rejection and narrow-band performance. The proposed filters are composed of two spiral-shaped resonators and rectangle window feed structures. Both back-to-back and interdigital combinations of the resonators are adopted to obtain the miniaturized filter size. Compared to the traditional square loop bandpass filter, the sizes are reduced by 82% and 80%. It is also found that there is a pair of transmission zeros located on each side of the passbands, resulting in high selectivity. To validate the proposed idea, two demonstration filters with back-to-back and interdigital spiral-shaped resonators are implemented. The measured results exhibit good agreement with the full-wave simulation results.
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