Microstrip Tunable Bandpass Filters with Combined Resonators

The paper presents a varicap-tunable fourth-order tunable microstrip bandpass filter with combined resonators: a U-shaped loop resonator and a resonator short-circuited at one end, which improved the symmetry of the frequency response while maintaining a wide rejection band inherent in comb filters with resonators short-circuited at one end. For the first time, the resonance equation for odd resonance frequencies of a U-shaped loop resonator with variable capacitance is obtained, which makes it possible to control the ratio of the first parasitic and the fundamental resonance frequencies of this resonator, f(1)/f(0), on which the rejection band of the filter depends. An experimental sample of a filter, tunable in the frequency range of 225-400 MHz, is presented.