Dual-band quadrature coupler with high frequency ratio using cascaded coupled-line structures

This paper presents a dual-band quadrature coupler with a cascaded coupled-line structure. Conventionally, the parallel coupled lines with an electrical length of 90° only operate at a single frequency. The proposed design achieves dual-band operation by cascading three coupled-line sections with appropriate electrical lengths, which has the advantage of high frequency ratio. The design strategy was validated by designing an experimental dual-band quadrature coupler. The measured results show excellent dual-band performance of -10.6 and -10.8 dB coupling level at 0.84 and 5.17 GHz with bandwidths of 47.6% and 9.1%, respectively. The measured return loss and isolation were better than -18 and -22 dB, respectively, and a phase difference of 90.5° and -88.5° at the dual center frequencies between the two output ports.