A modified star-shaped dual-mode microstrip loop resonator with a triangular patch is proposed to design compact narrow band bandpass filters with transmission zeros. The triangular patch is used as perturbation element to couple degenerated modes. The position of transmission zeros can be tuned by varying the size and location of the perturbation element. The coupling mechanism (inductive or capacitive) and the type of input/output feed (orthogonal or non-orthogonal) determine the type of filter response like symmetric or asymmetric. Three highly selective two-pole narrow band bandpass filters are designed using a modified star-shaped resonator with tunable transmission zeros. First, two filters with non-orthogonal feed lines have an asymmetric response with transmission zeros on one side of the passband, whereas the third filter with orthogonal feed lines exhibits symmetric frequency response with transmission zeros on both sides of the passband. Even and odd mode analysis is applied to the dual mode filters to calculate the position of transmission zeros. Filters are realized using a low loss dielectric substrate, and measured results are in good agreement with the theoretical and simulated ones.
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