Tune-all microstrip bandpass filter

A compact, varactor-incorporated microstrip bandpass filter of controllable center frequency and bandwidth, as well as reconfigurable response is presented. The quasi-lumped dual-mode resonator is adopted for filter compactness, and the incorporated varactors are employed for resonance perturbation and mode coupling control pertinent to frequency agility. Compared to the authors' previous design, the electrical size is significantly reduced (4 times smaller), and the largest to smallest bandwidth ratio (6:1) is greatly extended.