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Novel microstrip dual-mode bandpass filter (BPF) based on the triangular loop resonator with coplanar-waveguide (CPW) feeding structure is presented. The novelty of this design is to use two forms of perturbation, the isosceles triangle shape of microstrip loop resonator and the open stub element, both of which contribute to the coupling between two degenerate modes. By making a various combinations of these two parameters, it is possible to design both narrow and wide band filters and to control the attenuation pole frequency simultaneously. Moreover, the issue of harmonic suppression using dumb-bell-shaped defected ground structure (DGS) cells integrated at each input/output port is also investigated. Experimental results show a good agreement with the simulated ones, which validates the proposed design.