s, being specially chosen to minimise crosstalk. The pairs are then assembled into basic units of either 15.2.1 Analysis of balanced pair cables twenty-five, fifty or one hundred pair units. Each unit is identified by a lapping of coloured tape. The characteristics of a cable can be defined in terms of its primary To prevent the cables filling with water in the event of damage to coefficients, where the distributed characteristics are represented by the cable sheath, or due to a faulty joint sheath closure, the cable can circuit elements lumped at unit lengths apart. be fully filled with petroleum jelly during manufacture. The cable The primary coefficients of a uniform transmission line are: sheath is of polyethylene with an internal Glover barrier. This vapour barrier comprises an aluminium polyethylene laminate tape R = Resistance in Ohms per unit length which is applied longitudinally with a small overlap, and with the G = Leakance in Siemens per unit length polyethylene surface facing outwards. As the polyethylene sheath is L = Inductance in Henries per unit length extruded around the finished cable it bonds to the laminate. This C = Capacitance in Farads per unit length The secondary coefficients are the propagation coefficient P and Table 15.1 Typical characteristics of unit twin cable the characteristic impedance Z given by Equations 15.1 and 15.2. P=(R+ jwL)(G+jwc)]" (15.1) Conductor Maximum average Maximum average diameter (mm) resistance mutual (15.2) (ohms/km @ 20°C) capacitance (nF/km) The propagation coefficient P is complex, as in Equation 15.3. P=a+jB (15.3) 15/4 Telecommunication cables Non-loaded 44mH Ca 1830m 88mH @ 1830m Frequency (kHz) Figure 15.1 Effect of loading. 0.63mm conductors of mutual capacitance 45nF/km But, at high frequencies skin effect causes R to increase in propor- The real part a of the propagation coefficient thus gives the tion to the square root of the frequency, and SO at high frequencies attenuation of the line (in nepers per unit length) and is the attenua- the line att