A varactor-tuned microstrip bandpass filter (BPF) with independently tunable center frequency and bandwidth is proposed in this paper. The proposed BPF with a simple configuration is composed of a half-wavelength transmission line with both ends short-ended and a T-shaped transmission line. Meanwhile, two varactors are inserted symmetrically in the middle section of the half-wavelength transmission line to adjust the resonant frequency. The T-shaped transmission line is connected to the half-wavelength transmission line by a lumped capacitor. In addition, two inductors loaded symmetrically in the feed line are employed to control the coupling coefficient. It is convenient to adjust the frequency and bandwidth of the filter independently by using only three varactors, which simplifies the circuit structure greatly. The predicted results on S parameters are compared with the measured ones, and a reasonable agreement is achieved.
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