Compact dual-band bandpass filters (BPFs) are proposed using the novel embedded spiral resonators (ESRs) in this letter. Each ESR consists of two sets of spiral-lines embedded in a microstrip rectangular open-loop (MROP). The resonator has two functions in the filter: 1) dual-resonance operation mainly provided by the spiral-lines is employed to generate two passbands simultaneously. 2) MROP acting as the dual-feeding structure is introduced to achieve good input/output (I/O) coupling for both passbands. With proper tapping scheme, additional transmission zeros can be obtained to improve the passband selectivity. To verify the principle above, two dual-band BPFs are implemented. These filters with passbands operated at 2.4/5.2 GHz and 2.45/5.7 GHz can finely meet the wireless local area network (WLAN) bands requirements, respectively.
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Luo et al. (2010) studied this question.
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