Dual-Band Branch-Line Coupler Based on Crossed Lines for Arbitrary Power-Split Ratios

Dual-band branch-line couplers with arbitrary power-split ratios are presented. The use of crossed lines at the center of the dual-band coupler enables it to independently provide different power-split ratios to the two bands. Additionally, open stubs are utilized to enhance the stopband responses. The complete design procedure with example design curves is provided. For experimental verification, three dual-band couplers with power-split ratio combinations of +3 dB ($S_{21}:S_{31}=2:1$) and −3 dB ($S_{21}:S_{31}=1:2$), −3 dB and +3 dB, and 0 dB ($S_{21}:S_{31}=1:1$) and +13 dB ($S_{21}:S_{31}=20:1$) at 1 GHz and 2.5 GHz were designed and fabricated. The measured results are in excellent agreement with the ideal and full-wave simulated results. The measured difference of −13.3 dB between the power-split ratios of the two bands is the largest reported for a dual-band branch-line coupler.