A miniaturized bandpass filter using two-layers of microstrip open-loop resonators

This paper proposes a new microstrip bandpass filter using multilayer four-pole cross-coupled open-loop stepped impedance resonators, resulting to size reduction. The filter structure consists of four resonators placed on two stacked microstrip layers, where the coupling between the resonators on the upper layer and the lower layer is obtained by introducing two slots in a common ground plane. The full-wave simulator IE3D has been employed to design the open-loop stepped impedance resonators and to calculate the coupling coefficients of the filter structure. Finally, the filter has been optimized, resulting to low passband insertion loss (less than 3 dB) and high return loss (higher than 25 dB) at the center frequency. The results from measurement agree very well with simulation expectation.