Three-line coupled-line narrowband bandstop filter with wide upper bandpass bandwidth

A three-line coupled-line narrowband bandstop filter is proposed to obtain wide upper bandpass bandwidth and avoid the tightly-coupled limitation. This structure of the proposed bandstop filter has two three-line coupled-line sections, four open-circuit stubs and one through transmission line. The main advantages of this filter include very narrow band rejection, wide upper bandpass bandwidth, high selectivity with sharp roll-off skirt, small and flat group delay performances. The measured results indicate that the stop bandwidth with 10-dB suppression at the center operating frequency of 0.97 GHz is about 4.12% and the highest frequency of the upper passband with 15-dB return loss is 6.44 GHz (the frequency ratio h is almost 6.64). The circuit size is 0.027 lambda(2)(g). In addition, this proposed bandstop filter is very suitable for low-cost single-layer microstrip implementations with small size.