Novel T-Shaped Non-orthogonal Dual-Mode Band-Pass Filters with Two Differently Structured Resonators

Novel dual-mode filters with two differently structured resonators were designed on Al₂O₃ ceramic substrates to achieve pattern minimization and better filter properties. The input/output microstrip lines were arranged in a straight line, making the designed filter easy to connect with other RF circuits. To promote a coupling effect between the input/output microstrip lines and the resonator, two additional coupling stubs were used to form modified T-shaped microstrip coupling lines. In a dual-mode filter, adding a perturbation stub could generate two degenerate modes and increased the bandwidth in the desired passband. To achieve this, the type-A dual-mode filter was designed on a square-ring resonator with two perturbation stubs. For the type-B filter, two open stubs, one perturbation patch, and one meandering loop ring resonator were investigated to achieve the same purpose but with a smaller pattern size than type-A. According to simulated results, the proposed filters with optimized parameters exhibited the wide passbands.