Experimental investigation of a compact aperture-coupled multilayer bandpass filter for wireless systems

A new microstrip coupled miniaturized hairpin resonator filter with multilayer configuration is proposed. This structure consists of six resonators placed on two stacked layers, where the coupling between the resonators on the upper layer and those on the lower one is obtained by introducing two slots in a common ground plane. An electromagnetic simulator is used to design the proposed structure and calculate the coupling coefficients of the three basic coupling structures. To validate this approach, a six-pole bandpass filter prototype was designed, fabricated and measured. The simulated and experimental results are presented and discussed, and the comparison between them indicates a fairly good agreement. Compared to conventional filters, the proposed structure provides a filter size reduction of 50%.