At high frequencies w is large, giving Equations 15.15 to 15.19 (15.23) (15.15) Most air spaced cables are 750hm impedance and flexible polye- thylene insulated cables of 50ohm impedance. Cables can, of (15.16) course, be proportioned for different optimisations and complexities of materials (Dummer, 1961). 15.3.2 Coaxial cable construction B = w( LC ) " (15.17) For general telecommunication use, coaxial cables are manufac- (15.18) tured with a nominal impedance of 75ohms. By using a minimum amount of dielectric material, e.g. widely spaced polyethylene discs, they are effectively air spaced. The inner conductor is a solid (15.19) copper wire and the outer usually formed from a folded copper tape with a longitudinal seam. One commonly used manufacturing process, developed and +9.08x10~/(etano) licensed world-wide by STC plc, incorporates the following fea- tures. The insulation between the centre and the outer conductors consists of an air filled moulding made from high density polye- Table 15.7 Effect of frequency on d in copper thylene copolymer which is continuously applied to the centre conductor of 1.2mm diameter. The insulation is formed by an on line moulding process in which two half mouldings are consolidated Frequency (MHz) 8 (mm) approximately together around the centre conductor as shown in Figure 15.2. The effective dielectric constant of this composite insulator is 1.15. The outer conductor is formed from 0.180mm copper tape into a tube 0.009 closely fitting around the centre moulding. The tape edges are 0.003 corrugated in order to prevent overlap at the tube seam. The stages Coaxial pair cables 15/7 Coaxial pairs, or tubes of the type described are laid up into cables of 4, 6, 8, 12, 18 and even 40 coaxial pairs. Balanced pairs are sometimes provided in the interstitial spaces between the coaxials for the control and supervision of the main transmission equipment. Some operating companies combine the coaxial pairs with com- plete layers of balanced pairs to be used for local circuits. However, the