he inner conductor and inner layer of the outer conductor. The or Z screen cable. field is then contained between the two conductors of a pair and Typical characteristics of a new pair cable for 2Mbit/s systems are mutual interference between cables is greatly reduced. given in CCITT Rec. G.613, from which Table 15.6 is extracted. The effective depth of penetration of an electric current flowing FEXT (far end crosstalk) is measured with the detector at the end of over a plane surface is given by Equation 15.11. 15/6 Telecommunication cables se(e)" (15.11) Where p conductor resistivity ohm.cm E is the dielectric constant (= 1 in air) o is the depth at which the current at the surface has reduced to D is the inside diameter of outer conductor Io/e. (At a penetration of 48 the current will only be 0.02IO d is the diameter of the inner conductor p is the electrical resistivity in ohm cm. f is the frequency u is the magnetic permeability in Henry cm. tan 8 is the power factor of the dielectric f is the frequency in Hz. It will be seen that only the first term of the expression for From this equation it will be seen that 8 increases with resistivity attenuation is dependent on the ratio of conductor diameters as in and decreases with frequency. Table 15.7 gives the value of o for Equation 15.20. copper. The greater depth of penetration in a higher resistivity material, (15.20) tends to offset the effect of its greater resistivity, as in Equation 15.12 H.F. resistance (15.12) where 4 - DI Then for a given attenuation Equation 15.21 can be obtained. Thus, although the relative d.c. resistance of aluminium to copper is 1.64, the relative HF resistivity is 1.28. Thus skin effect is the (15.21) most important factor determining the functioning of a coaxial cable because it controls the resistance losses and screening efficiency at high frequencies. By partial differentiation, D will be a minimum when Equation 15.3.1 Design for minimum loss 15.22 holds. The primary coefficients of a coaxial cable are related to the second- (