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A novel four-band microstrip bandpass filter using double-layered structure is proposed. The basic components used to build the proposed filter are four pairs of stepped impedance resonators on the top metal layer and middle metal layer. The four pairs of resonators have different fundamental frequencies. The first and second fundamental frequencies are designed to operate at 3 and 5 GHz on the middle layer; the third and fourth fundamental frequencies are designed to operate at 7 and 9 GHz on the top layer. Due to cross-coupling effect, attenuation poles at both sides of the passbands can be created to improve the filter selectivity. The full-wave simulator IE3D is used to design the four-band bandpass filter and to calculate the coupling coefficients of the resonators.

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