Design of tunable microstrip dual-mode bandpass filter having reconfigurable filtering characteristics for mobile applications

Design of a microstrip narrowband bandpass filter with bandwidth tunability and reconfigurability is presented in this paper. For this purpose, patch reference elements and a varactor diode as tuning element are used to excite the degenerate modes of a dual-mode square loop resonator. Even/odd mode resonance conditions of the proposed filter are expressed to define the locations of mode frequencies. According to the capacitance of the varactor diode, bandwidth tuning for linear phase/quasi elliptical filtering characteristics and band rejection can be achieved. A microstrip bandpass filter having tunable bandwidth and reconfigurable filtering characteristics is fabricated and measured for the experimental verification of the proposed method.