Compact Dual-Band Bandpass Filter Using Overlapped Helical Resonators

In this paper, we propose a compact dual-band bandpass filter using overlapped helical resonators. One helical resonator is placed inside of the other one to generate the two resonant frequencies for the two passbands. Thus, the circuit size of the two overlapped helical resonators is most the same as one resonator, resulting in significant size reduction. The filter topologies at the two operating frequencies are analyzed to obtain the desired filtering responses of the two passbands. Since the two overlapped helical resonators have no influence on the resonant frequencies to each other, the center frequencies of the two passbands can be controlled independently. To validate the proposed ideas, a dual-band bandpass filter is designed and the results are presented.