1. Brit. Patent Specification No. 692 468 (1952)
2. K. Wakino, T. Nishikawa, S. Tamura and Y. Ishikawa, Proc. 1975 IEEE MTT Symposium p. 63
3. K. Wakino, T. Nishikawa, H. Matsumoto and Y. Ishikawa, Proc. 1978 IEEE MTT Symposium p. 230
4. K. Wakino, T. Nishikawa, H. Matsumoto and Y. Ishikawa, Proc. 1979 IEEE MTT Symposium p. 278