Type L Coaxial Cable Carrier Telephone Systems

As previously mentioned, much of the terminal equipment for the types J, K, and L carrier systems is the same. Thus, a complete discussion of the type L system for coaxials is not necessary. The method⁵¹ used in arranging 480 telephone conversations for transmission over a coaxial system is as follows: The 480 incoming subscriber telephone lines are separated into groups of 12 and are impressed on carrier equipment such as is discussed in the preceding pages. Each group of 12 channels emerges from the modulators in the frequency band of 60 to 108 kilocycles. Five of these 60- to 108-kilocycle bands (each of which contains 12 conversations) are separated into groups and modulated with "group carriers" so that the 60 telephone conversations they contain lie in the 312- to 552-kilocycle band. Each of these bands (with the exception of one, see Fig. 32) is then modulated with "super-group carriers" and placed in the assigned region in the frequency spectrum. A comparable process is used at the receiving end.

Figure 32. In the type L1 carrier system used on coaxial telephone cables there are three amplitude-modulation steps used to translate, or move, the 480 (or more) telephone conversations to their respective assigned frequency bands. (Reference 51.)

One coaxial tube is used for transmission in one direction, and another tube for transmission in the opposite direction. As many as eight of these tubes are combined into a single sheath, providing four coaxial systems.⁵² The early coaxial cables used a 13-gauge copper center conductor and a 0.27-inch copper outer conductor. For 480 telephone channels repeaters were necessary at approximately 5.4-mile intervals. A later coaxial cable has an outer diameter of 0.375 inch and repeaters every 7.9 miles. The repeaters use vacuum-tube amplifiers⁵⁰ specially designed to pass the entire frequency band transmitted.