A dual‐mode open‐loop slotline resonator with a centre‐loaded stub is proposed. The loaded stub is used to excite non‐degenerate modes, that is, the even and odd modes. Distinct characteristics of this dual‐mode resonator are investigated by using even–odd mode analysis. Also, equivalent circuit models for non‐degenerate modes are given to determine the resonance conditions and the location of transmission zero. Then, two novel approaches for designing dual‐mode dual‐band bandpass filters are presented. One approach consists of simple microstrip feedlines and two embedded open‐loop slotline resonators for single substrate configuration. The other is composed by using dual‐mode open‐loop resonators in the form of microstrip and slotline structures on a single substrate. A microstrip invert L‐shape feeding structure is employed to simultaneously feed the microstrip and slotline resonator on different layers for broadside coupling. Both of them provide sufficient degrees of freedom to control the centre frequencies and fractional bandwidths of both passbands independently. The two filters are designed, fabricated and measured. A good agreement between the simulation and measurement verifies the validity of this design methodology.

This letter presents a compact dual-mode tri-band bandpass filter by using a short-circuited stub-loaded stepped-impedance resonator (SIR) and a short-circuited stub-loaded uniform impedance resonator. Also, a hairpin SIR geometry is introduced to miniature the size of this filter while maintaining excellent performance. The use of a short-circuited stub at the central point of the hairpin SIR can generate two resonant modes in two passbands. Its equivalent circuit structure is analyzed by using the even-odd mode theory. For demonstration purposes, a tri-band filter for the applications of the Global Positioning System at 1.57 GHz, Worldwide Interoperability for Microwave Access at 3.5 GHz, and wireless local area networks at 5.2 GHz is designed, fabricated, and measured.

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In this letter, a compact dual-band hightemperature superconducting (HTS) hairpin-resonator bandpass filter (BPF) based on a multimode resonator is proposed. This multimode resonator consists of two hairpin resonators loaded with a common T-shaped stub. The T-shaped stub excites one pair of non-degenerate modes for the inner hairpin resonator while the combination of T-shaped stub and the inner hairpin resonator produces another pair of non-degenerate modes for the outer hairpin resonator. Moreover, the former generates the higher passband and the latter produces the lower passband which are controlled independently by the corresponding hairpin resonator. Finally, the proposed dual-band HTS filter was fabricated and measured. Good agreement between simulation and measurement verifies the validity of this design methodology.  Index Terms-superconductor, dual-band, bandpass filter, split-ring resonator

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