In this paper, a tunable dual-band bandpass filter using asymmetrical varactor-loaded half-wavelength resonators (HWR) and defected ground structure (DGS) is proposed. First, varactors are added at the end of the conventional HWRs to change their resonant frequencies. And to determine the bandwidth of the passbands, the coupling coefficients are calculated to the requirements based on the theoretical analyses. However, the isolation will deteriorate if the passbands are tuned close to each

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... . Then in order to solve this problem, a pair of circular DGS is introduced at the ground plane of the filter. This structure can prevent electromagnetic energy from propagating at some certain frequency point and if it is designed between the passbands, the isolation will be improved. In our design, the proposed dual-band bandpass filter using varactor-loaded HWRs and DGS is simulated and fabricated. The simulated results and measured results are in good agreement and the proposed filter can be applied in microwave circuits and communication systems. (a) (b) Fig. 7. Tuning of the second passband (a) S 21 and (b) S 11