This work presents a new technique that uses floating plate overlays to realize the open-loop resonator bandpass filter with characteristics of compact size and having four controllable transmission zeros to achieve the multispurious suppression. Three floating plate overlays are used to cover parts of the open-loop resonator filter to increase the coupling between resonators and move the transmission zeros to desired frequencies to enhance harmonic suppression. A design procedure that developed based on an equivalent circuit model of such a new type of filter is proposed. Two experimental prototypes are designed and fabricated to verify the proposed design method. The measured results agree well with the simulations. The measured insertion losses of the prototypes in passband are all less than 2 dB. One prototype is designed to suppress third, fourth, and fifth harmonics with the suppression greater than 30 dBc. Another prototype can suppress second, third, and fourth order harmonics below 20 dBc. In addition, prototype circuit areas are only about 50 percent of the conventional open-loop filter.
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