1. Levy, R. and L. F. Lind, "Synthesis of symmetrical branch-guide directional couplers," IEEE Trans. Microw. Theory Tech., Vol. 16, No. 2, 80-89, Feb. 1968.
doi:10.1109/TMTT.1968.1126612
2. Yoon, H.-J. and B.-W. Min, "Two section wideband 90◦ hybrid coupler using parallel-coupled three-line," IEEE Microw. Wireless Compon. Lett., Vol. 27, No. 6, 548-550, Jun. 2017.
doi:10.1109/LMWC.2017.2701304
3. Pon, C. Y., "Hybrid-ring directional coupler for arbitrary power divisions," IRE Trans. Microw. Theory Tech., Vol. 9, No. 6, 529-535, Nov. 1961.
doi:10.1109/TMTT.1961.1125385
4. Wilkinson, E. J., "An N-way hybrid power divider," IRE Trans. Microw. Theory Tech., Vol. 8, No. 1, 116-118, Jan. 1960.
doi:10.1109/TMTT.1960.1124668
5. Lange, J., "Interdigitated strip-line quadrature hybrid," IEEE Trans. Microw. Theory Tech., Vol. 17, 1150-1151, Dec. 1969.
doi:10.1109/TMTT.1969.1127115
6. Han, L., K. Wu, and X. Chen, "Accurate synthesis of four-line interdigitated coupler," IEEE Trans. Microw. Theory Tech., Vol. 57, No. 10, 2444-2455, Oct. 2009.
7. Chang, C., J. Chiu, H. Chiu, and Y. Wang, "A 3-dB quadrature coupler using broadside-coupled coplanar waveguides," IEEE Microw. Wireless Compon. Lett., Vol. 18, No. 3, 191-193, Mar. 2008.
doi:10.1109/LMWC.2008.916805
8. Cho, J.-H., H.-Y. Hwang, and S.-W. Yun, "A design of wideband 3-dB coupler with N-section microstrip tandem structure," IEEE Microw. Wireless Compon. Lett., Vol. 15, No. 2, 113-115, Feb. 2005.
doi:10.1109/LMWC.2004.842850
9. Tang, C., C. Tseng, and K. Hsu, "Design of the modified planar tandem couplers with a wide passband," IEEE Trans. Microw. Theory Tech., Vol. 61, No. 1, 48-54, Jan. 2013.
doi:10.1109/TMTT.2012.2226745
10. Hayes, R. E., J. W. Gipprich, and M. G. Hershfeld, "A stripline re-entrant coupler network for cofired multilayer microwave circuits," IEEE S Int. Microw. Symp. Dig., 801-804, 1996.