Large Frequency-Ratio Dual-Band and Broad Dual-Band Parallel-Line Couplers

In this paper, a symmetric stepped-impedance coupled line (SSICL) is proposed to realize dual-band quarterwave coupled line (DQCL), giving the same equivalent even and odd characteristic impedances at two different frequencies. The DQCL can be directly used to design dual-band parallel-line coupler (DBPC) with arbitrary coupling coefficient and large frequency ratio, resulting in even shorter total length when compared with traditional quarter-wave parallel-line coupler working at lower frequency f
₁
. In addition, broad DBPC (BDPC) is constructed by cascading multiple sections of SSICLs, as the SSICL is quite similar with quarter-wave coupled line in circuit. For demonstrating the design methods, in the first step, two edge-coupled 20-dB DBPCs and one broadside coupled 6-dB DBPC, working at 1/5.2, 1/7.5, and 1/5.2 GHz, respectively, are designed, fabricated and measured, exhibiting good features like large frequency ratio, high isolations, and compact dimensions. Then, for broadening bandwidths, three-section BDPC working at 1 GHz/5.2 GHz is designed and measured, resulting in working bandwidths larger than 1.2 GHz. These good features have proved the feasibility of the proposed circuits and the corresponding design methods.