Selected References

1. S. B. Cohn, "Characteristic Impedance of the Shielded-Strip Transmission Line", IRE Transactions on Microwave Theory and Techniques, Vol MTT-2, No 2, July 1954, pp. 52-57.
2. H. A. Wheeler, "Formulas for the skin effect", Proc. IRE, Vol. 30, No. 9, September 1942, pp. 412-4124
3. Stanislaw Rosloniec, Algorithms For Computer-Aided Design of Linear Microwave Circuits, Archtech House, 1990