 initial saving has to be balanced against the extra complication of installation, and maintenance operations on the layer pairs. The complete cable is sheathed in polyethylene preceded by a Glover barrier of 0.2mm aluminium foil. Steel tape or wire armouring can be added if the cable is to be directly buried. 15.3.3 Coaxial cable parameters 15.3.3.1 Impedance In a transmission system the source, cable and !oad impedances are all matched. An impedance mismatch causes a reflected backward travelling wave which serves to reduce the power of the forward travelling wave, causing an apparent increase in transmission loss. Manufactured cable lengths can be selected to be of the nominal impedance at repeater points, and of matching impedances at other joints. However, this practice is not viable on cables carrying a large number of pairs, and is not necessary if the impedance is controlled in manufacture to within 10hm at 1MHz. The biggest source of reflections arise at the cable/repeater interface and these accumulate along a repeatered line to steep attenuation frequency rolls which are difficult to equalise. To limit these effects the CCITT recom- mend that the cable and repeater impedances satisfy Equation 15.24. (G332.5). ZL+ZR (15.24) N = 2A + 2010811 where N = 55dB minimum for a 12MHz line system and 65dB minimum for a 60MHz line system A = section loss at frequency f Z repeater output impedance at frequency f Figure 15.2 Stages in the construction of the STC moulded shell insulation ZL cable impedance at frequency f Z repeater input impedance at frequency f in the construction of the STC moulded shell insulation are as follows: 15.3.3.2 Impedance irregularities Stage 1. Continuously extruded strip of high density polyethylene copolymer. Impedance irregularities within a manufactured length of cable can Stage 2. Two half-insulations moulded into strip by embossed arise, for example, from variations in the concentricity of the inner rollers. conductor, imperfect circularity of the inner and outer, and variation St