Multi-spurious suppression for microstrip dual-mode bandpass filter using triple U-shaped defected ground structure

A novel triple U-shaped defected ground structure (DGS) unit is proposed. In contrasts to the conventional single bandgap DGS, the proposed DGS unit provides three controllable bandgaps at different frequencies but still retaining the size as compact as that of the single-bandgap DGS. Cascading this new DGS element to the conventional dual-mode bandpass filter, multi-spurious suppression of the dual-mode filter can be achieved. An example square-loop dual-mode bandpass filter centered at 2.04 GHz and 4.4% fractional bandwidth with the proposed DGS element embedded to the input and output feedlines, obtains better than 30-dB suppression on its second, third and fourth harmonics, resulting in a wide stopband upto 9 GHz.