1. Sandrin, W. A. (1974). The Butler matrix transponder. Comsat Technical Review, 4(2), 319–345.
2. Pozar, D. (2005). Microwave engineering (3rd ed.). New York: Wiley.
3. Mongia, R., Bahl, I., & Bhartia, P. (1999). RF and microwave coupled-line circuits. Norwood, MA: Artech House.
4. Engen, G. F. (1977). An improved circuit for implementing the six-port technique of microwave measurements. IEEE Transactions on Microwave Theory and Techniques, MTT-25(12), 1080–1083.
5. Bialkowski, M. E., & Dimitrios, A. P. (1993). A step-frequency six-port network analyser with a real-time display. AEU International Journal of Electronics and Communication, 47(3), 193–197.