1. Levy, R. and L. F. Lind, "Synthesis of symmetrical branch-guide directional couplers," IEEE Trans. Microw. Theory Techn., Vol. 16, No. 2, 80-89, Feb. 1968.
doi:10.1109/TMTT.1968.1126612
2. Zhang, H. and K. J. Chen, "A stub tapped branch-line coupler for dual-band operations," IEEE Microw. Wireless Compon. Lett., Vol. 17, No. 2, 106-108, Feb. 2007.
doi:10.1109/LMWC.2006.890330
3. Maktoomi, M. A., M. S. Hashmi, and F. M. Ghannouchi, "Systematic design technique for dualband branch-line coupler using T- and Pi-networks and their application in novel wideband-ratio crossover," IEEE Trans. Compon. Packag. Technol., Vol. 6, No. 5, 784-795, May 2016.
4. Chi, P. and K. Ho, "Design of dual-band coupler with arbitrary power division ratios and phase differences," IEEE Trans. Microw. Theory Techn., Vol. 62, No. 12, 2965-2974, Dec. 2014.
doi:10.1109/TMTT.2014.2364218
5. Cheng, Y., L. Wang, J. Wu, and Y. Fan, "Directional coupler with good restraint outside the passband and its frequency agile application," Progress In Electromagnetics Research, Vol. 135, 759-771, 2013.
doi:10.2528/PIER12121009
6. Gai, C., Y. Jiao, and Y. Zhao, "Compact dual-band branch-line coupler with dual transmission lines," IEEE Microw. Wireless Compon. Lett., Vol. 26, No. 5, 325-327, May 2016.
doi:10.1109/LMWC.2016.2549099
7. Wu, Y., S. Y. Zheng, S. Leung, Y. Liu, and Q. Xue, "An analytical design method for a novel dual-band unequal coupler with four arbitrary trminated resistances," IEEE Trans. Ind. Electron., Vol. 61, No. 10, 5509-5516, Oct. 2014.
8. Wong, Y. S., S. Y. Zheng, and W. S. Chan, "Multifolded bandwidth banch line couplerwith filtering characteristic using coupled port feeding," Progress In Electromagnetics Research, Vol. 118, 17-35, 2011.
doi:10.2528/PIER11041401
9. Chaudhary, G. and Y. Jeong, "Arbitrary power division ratio rat-race coupler with negative group delay characteristics," IEEE Microw. Wireless Compon. Lett., Vol. 26, No. 8, 565-567, Aug. 2016.
doi:10.1109/LMWC.2016.2585561
10. Ahn, H. and M. M. Tentzeris, "Arbitrary power-division branch-line hybrids for high-performance, wideband, and selective harmonic suppressions from 2f0," IEEE Trans. Microw. Theory Techn., Vol. 67, No. 3, 978-987, Mar. 2019.
doi:10.1109/TMTT.2019.2892444
11. Yeung, L. K., "A compact dual-band 90 coupler with coupled-line sections," IEEE Trans. Microw. Theory Techn., Vol. 59, No. 9, 2227-2232, Sep. 2011.
doi:10.1109/TMTT.2011.2160199
12. Wang, X., W. Yin, and K. Wu, "A dual-band coupled-line coupler with an arbitrary coupling coefficient," IEEE Trans. Microw. Theory Techn., Vol. 60, No. 4, 945-951, Apr. 2012.
doi:10.1109/TMTT.2012.2185949
13. Chang, C., K. Chin, and Y. Chiang, "Dual-band coupled-line couplers with wide separation between bands," IEEE Trans. Microw. Theory Techn., Vol. 65, No. 8, Aug. 2017.
doi:10.1109/TMTT.2017.2693153
14. Corrales, E., A. Baldomero, and P. Paco, "A dual-band 180-degree hybrid coupler based on coupledline sections," IEEE Microw. Wireless Compon. Lett., Vol. 25, No. 4, 211-213, Apr. 2015.
doi:10.1109/LMWC.2015.2400922
15. Chang, C., K. Chin, and Y. Zheng, "Design of dual-band −3 dB couplers with a wide range of dual-band frequency ratios," Electron. Lett., Vol. 52, No. 14, 1231-1233, Jul. 2016.
doi:10.1049/el.2016.1340
16. Wu, Y., J. Shen, Y. Liu, S. Leung, and Q. Xue, "Miniaturized arbitrary phase-difference couplers for arbitrary coupling coefficients," IEEE Trans. Microw. Theory Techn., Vol. 61, No. 6, 2317-2324, Jun. 2013.
doi:10.1109/TMTT.2013.2259501
17. Reshma, S. and M. K. Mandal, "Miniaturization of a 90◦ hybrid coupler with improved bandwidth performance," IEEE Microw. Wireless Compon. Lett., Vol. 26, No. 11, 891-893, Nov. 2016.
doi:10.1109/LMWC.2016.2614977