Chang, Chih-Chun; Chin, Kuo-Sheng; Zheng, Yu-Feng: 'Design of dual-band −3 dB couplers with a wide range of dual-band frequency ratios', Electronics Letters, 2016, 52, (14), p. 1231-1233, DOI: 10.1049/el.2016.1340
IET Digital Library, https://digital-library.theiet.org/;jsessionid=3m3t73h53mqm5.x-iet-live-01content/journals/10.1049/el.2016.1340