A novel dual-band bandpass filter using co-directional split ring resonators with closely spaced passbands and wide upper stopband

In this article, interdigital capacitor loaded co-directional split ring resonators (CDSRRs) and their dual-band bandpass filter applications are proposed. The proposed resonator is formed by nested open loop resonators having open ends at the same place unlike conventional split ring resonators (SRRs). In addition, the inner open loop resonator has interdigital capacitor located between the open ends. The proposed resonator exhibits dual resonance behavior with a small center frequency ratio. Both of resonance frequencies can be controlled due to the changes in the interdigital capacitor and the electrical length of the outer resonator. A dual-band microstrip bandpass filter is designed by using the proposed CDSRR. Two CDSRRs are used to obtain two poles in each passband. Overall electrical length of the designed filter is 0.23 (g) x 0.14 (g) (0.0329 (2)(g)), where (g) is the guided wavelength for the used substrate at the lowest passband center frequency of 1.8 GHz. A small center frequency is obtained by adjusting the second passband at 2.27 GHz. A very wide upper stopband, closely spaced passbands, low insertion losses and high selectivity at both passbands can be obtained by means of the proposed structure. The designed filter was also fabricated and tested. The measured results show a very good agreement with the predicted results.