A design of a miniaturized 2-pole bandpass filter by using a slot and a hair-pin line

A miniaturized 2-pole band pass filter (BPF) is proposed and it has an overlapped hairpin and slot structure on the upper and bottom plane of microstrip lines. The equivalent circuit, predicted performances, and the measured results of the proposed BPF are also presented. Due to the overlapped structure of a hairpin and a slot, the size of the proposed BPF is reduced to 30% comparing with the conventional λ/4 coupled BPF. The attenuation is improved by controlling the attenuation poles properly using the proposed structure. In addition, the attenuation characteristic of the presented 2-pole BPF is comparative to that of 3-pole BPFs. The proposed 2-pole BPF is fabricated with a narrow bandwidth (70 MHz) at center frequency, 1.9 GHz, which is difficult to make that narrow bandwidth in the conventional coupled lines due to impractical spacing. The design method in this paper can be applied to the design of other microwave circuits to reduce the size and improve the performances.