A new compact microstrip two-layer bandpass filter using aperture-coupled SIR-hairpin resonators with transmission zeros

A new design of a compact two-layer bandpass filter using aperture-coupled microstrip stepped-impedance hairpin resonators is proposed and investigated in this paper. The proposed filter is composed of four stepped-impedance hairpin resonators located on two stacked microstrip layers, and the couplings between the resonators on the upper layer and those on the lower one are obtained by using three coupling apertures etched on a common ground plane placed between the two layers. In addition, the use of tapped input/output couplings allow this bandpass filter to generate two independent transmission zeros. A full-wave simulator is used to design the proposed structure and calculate the aperture coupling coefficients. With this two-layer configuration, the proposed filter becomes very compact and occupies a small space. To demonstrate the proposed design, a four-order bandpass-filter prototype was designed, fabricated, and tested at 2.14 GHz. Furthermore, a single-layer filter prototype was also designed and fabricated at the same frequency band and then compared to the first one. As a result, a significant reduction of approximately 50% was achieved. These features make the proposed structure suitable for compact and high-performance circuit component designs in microwave circuits.