Planar dual-band coupled-line absorptive bandstop filter with an input lossy step-impedance stub

In this article, a novel planar dual-band coupled-line absorptive bandstop filter (ABSF) is presented. The proposed structure consists of four pairs of couple-lines, a grounded step-impedance stub and a resistor R. The return loss is absorbed by resistor R effectively, while the step-impedance stub reduces the insertion loss in passbands caused by resistor R. This proposed bandstop filter has planar layout, free design parameters, high-selectivity dual-frequency bandstop function, and good absorptive character. A typical example is designed, simulated, and measured to verify the proposed structure. The measured results demonstrate that -10 dB rejection bandwidth is 30.42 % (from 0.53 GHz to 0.71 GHz) in the first stopband and 13.87 % (1.26 GHz to 1.44 GHz) in the second stopband, respectively. Stopband rejection is -40.45 dB at 1.33 GHz, and return loss S11 is under -12.95 dB in whole working band. The Min. insertion loss reaches to 0.15 dB in passbands. The circuit size is 0.24 lambda 0 x 0.17 lambda 0 (72.5 mm x 51.0 mm). The proposed filter can be concurrently applied to select frequency and suppress return loss interference in wireless systems.