Design of the UWB bandpass filter by coupled microstrip lines with U-shaped defected ground structure

A novel ultra-wideband (UWB) bandpass filter (BPF) is proposed by using coupled microstrip lines and U-shaped defected ground structure (DGS). The input and output feeding lines are connected to the ends in the same side of the coupled microstrip lines on one side of a substrate, and the U-shaped DGS is introduced behind the coupled lines on the other side of the substrate. The coupling of this kind of structure, if without DGS, is very weak. However, the U-shaped DGS generates a strong coupling, and improves the passband property of the coupled lines. The filter is simulated by 3D EM commercial software, and the results demonstrate good UWB performances. The passband defined by |S₁₁| < -10 dB is about 3.44-10.6 GHz, and the insertion loss is about 0.11 dB at central frequency of 7 GHz. The group delay variation, which is an important factor to UWB system, is less than 0.5 ns within the operating band.