A new microstrip coupling system for realization of a differential dual-band bandpass filter

Abstract In this paper, a new Differential Dual-Band Wide-Bandpass Filter (DDWBF) using a new coupling system is presented. The adjustable coupling system composed of Two Coupled U-Shaped Structures (TCUSs) which create and control two initial passbands. Four TCUSs are then placed in four ports and connected through a defected fractal-based low impedance transmission line. Third order of Sierpinski fractal is applied on the transmission line to improve the out of band rejection of the differential mode along with common mode suppression. An analytical analysis is also presented to calculate the transmission zeros and poles of the DDWBF which verifies the authenticity of the design procedure. A prototype of the filter has been fabricated and tested which shows a very good agreement with the predicted results. According to the measured results, two wide passbands with fractional bandwidths of 45% and 13.5% at central frequencies of 4.32 and 8.61 GHz, respectively, have been achieved. The differential mode return loss is higher than 15 dB in both bands and the common mode suppression level is better than 14.85 dB.