Asymmetric Dual-Mode Microstrip Filters with Adjustable Transmission Zero

A new type of dual-mode resonator that produces asymmetrical frequency responses is proposed. The effect of nonorthogonal input/output feed lines located along a straight line on filter response is investigated for the dual-mode filter. The coupling between degenerate modes of the proposed dual-mode microstrip resonator is discussed depending on the perturbation size. The two samples of asymmetric dual-mode microstrip filter were designed, fabricated and measured to demonstrate the design of dual-mode microstrip filters exhibiting transmission zero shifting property. The one of them produces a filter characteristic with two TZs in the upper stopband the other has two TZs in the lower and upper stopbands. The shifting of the one of the TZs from one side of the passband to the other is realized by only changing the perturbation size.