Improving the Coupling Characteristics of Bandpass Filters by Using Multilayer Structure and Defect Ground Units

This paper proposes a novel multi-layer microstrip 2.4 GHz bandpass filter fabricated on two bonded substrates. The multi-layer filter has a symmetrically-bent, open-loop, square- ring stepped impedance resonator (SIR) structure on the top surface of the upper substrate, a similar SIR pattern rotated through 90deg on the lower surface of the upper substrate, and a pair of L-shaped symmetrical defected ground units on the bottom surface of the lower substrate. The simulation results demonstrate that the multi-layer structure improves the electromagnetic coupling characteristics of the bandpass filter and increases both the return loss by more than 30 dB at the center frequency compared to that achieved using a conventional two-layer (SIR-ground) structure, hi addition, the defected ground units etched onto the metallic ground plane beneath the feed lines yield a significant reduction in the spurious harmonics at a frequency of 5.2 GHz. Experimental results are presented to show the performance of the proposed filters.