Capacitances of a Shielded Balanced-Pair Transmission Line

The shielded balanced pair consists of two straight cylindrical conductors immersed in a homogeneous dielectric, surrounded by an electrically thick tubular shield. Figure 1 is a cross section of the shieldedpair structure. Dimensional restrictions are imposed which serve to keep the three conductors of the structure from touching. These restrictions are S > d and D > S + d. In terms of the traditional dimensional ratios u and V, u 0.5 and V 1/(1 + 2u). Thus, the variables u and V are contained in the shaded area in Fig. 2. The exact capacitance expression for a pair in free space (unshielded) is derived in most texts. Cm = Treo/cosh"1 (0.5/w) 2 _i 12 (1)