Novel compact tunable bandpass filter using capacitively loaded H-shaped resonator

In this paper, a new compact bandpass filter with continued large tunable frequency band is proposed and developed for reconfigurable communication applications and mostly for filtenna designs in cognitive radio (CR) systems. The symmetrical H-shaped resonator consists of two connected rectangular microstrip resonators. The technique is based on centrally loading the H-shaped resonator with only a single varactor diode. Tunability is achieved by electrically tuning the operating frequency of the filter by adjusting the varactor capacitance using DC voltage. Increasing the varactor capacitance results in decreasing the resonant even-mode of the H-shaped resonator without affecting the dominant odd-mode. Furthermore, the simple biasing of the varactor diode has no effect on the filter performance. Measurement results show that the proposed filter is capable of tuning over a wide operating band of 2.32 GHz covering continuous bands from 3.68 to 6 GHz. Moreover, the scattering parameters of the proposed filter are successfully optimized with numerical experimentation techniques using 3D full-wave electromagnetic simulator. The proposed design is fabricated and measured for verification purposes. Good agreement is found between the simulated and the measured data.