A novel wideband bandpass filter using coupled lines and T-shaped transmission lines with wide stopband on low-cost substrate

Abstract

This paper presents the design, simulation, fabrication and measurement of a wideband bandpass filter with wide stopband performance operating at 3.5 GHz. The proposed filter consists of two parallel coupled lines (T-PCL) centered by T-inverted shape. The location of transmission zeros can be adjusted by varying the physical lengths of T-inverted shape to improve the filter selectivity. The wide bandwidth is achieved through enhanced coupling between the input and the parallel coupled lines. Due to the transmission zeros in the lower and upper stopbands, the filter exhibits good performance including an extremely wide stopband and sharp attenuations near the passband together with low insertion and good return losses in the passband. The filter performance is investigated numerically by using CST-MWS. Finally, the microstrip wideband BPF with minimum insertion losses 0.3 dB, centered at 3.5 GHz with a 3-dB fraction bandwidth of 70 % and four transmission zeros is implemented and verified experimentally. In addition, good agreement between the simulated and measured results is achieved.