Dual-mode dual-band microstrip bandpass filter with high selection performance

In this letter, a new dual-mode microstrip bandpass filter (BPF) which is mainly composed of a dual-mode resonator, i.e., meandered loop resonator, with high selection performance is proposed. Compared with the traditional square loop resonator (SLR), meander loop resonator has more advantages in miniaturization. Besides, a Minkowski-like pre-fractal geometry with 1st order iteration embedded at the center of the resonator is utilized to improve the BPF capability. Both input and output ports are spatially separated 90° from each other. A small patch located in the orthogonal corner of the proposed resonator where is 135° offset from input and output ports is introduced to excite two degenerate modes. By tuning the perturbation, the two degenerate modes are excited. The influence of the variation of the perturbation size on the resonant frequencies and the coupling coefficient of the two degenerate modes is investigated. By observing the distribution of the surface current, it can be more intuitive to understand which part of the resonator has played a role when operating in different frequency bands.