Implementation of second-order open-loop resonator filter with skew-symmetric input/output coupling structure

Second-order open-loop resonator filters are implemented with a skew-symmetric or so-called zero-degree input/output feed structure. The two resonators are coupled through their two end sections, and two transmission zeros are created on both sides of the passband. One more zero is tuned by changing the length of the coupled end section. Transmission line theory is applied to predicting these transmission zeros. At 2.46 GHz, an experimental circuit with a fractional bandwidth of 4% is fabricated and measured. Measured S-parameter responses show good agreement with simulation data.