In order to realize bandpass filter with Chebyshev and elliptic function responses using a single resonator only, a dual-mode square resonant structure is proposed. Four small square loops are added into a large loop. One of small loops is defined a perturbation element, other small ones are references. By changing the thickness of the perturbation element with respect to that of a reference element, the resonator can produce a capacitive or inductive coupling between two degenerate modes. The capacitive coupling can acquire an elliptic function characteristic since it can create the transmission zeros on both the lower and upper sides of the passband, while the inductive coupling cancels them which exhibits a Chebyshev characteristic. Therefore, both Chebyshev and elliptic function responses can be obtained in a filter. The proposed bandpass filter is with a wider stopband and more compact in size. Experiment results have been verified this design.
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