The Evolution of Inductive Loading for Bell System Telephone Facilities Part III: Loading For Exchange Area Cables

The Evolution of Inductive Loading for Bell System Telephone Facilities By THOMAS SHAW {Continued from January 1951 issue) P A R T I I I . L O A D I N G FOR E X C H A N G E A R E A CABLES T H I S portion of the present review is primarily concerned with nonphantom type of loading on non-repeatered non-quadded cables, since the evolution of exchange area loading has been almost entirely in terms of these facilities. Phantom working has not been extensively practiced because in general it is not economical on exchange cables. In the occasional long cables where phantoming is economical, the phantom group loading makes use of loading apparatus developed for short-haul, two-wire type toll cable facilities. T h e very wide range of impedance characteristics of the m a n y different types of exchange cables (with and without loading, and as influenced by the terminal impedances provided by the many different kinds of subscriber loops and station sets) is such that telephone repeaters are necessarily limited to low gains when used at exchange area switching points. Moreover, it has not been generally feasible to use intermediate repeaters in the lines. Furthermore, the conventional two-wire type of telephone repeater used 447 448 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1 9 5 1 in the toll plant is quite expensive in relation to the feasible gains in the exchange plant. Consequently, there has not been an extensive use of repeaters in the exchange plant. Looking towards the future, however, the use of a low-cost telephone repeater of an entirely new design (Type E l ) is expected to result in a much more extensive use of repeaters, and in consequence some considerable reduction in the demand for the heaviest weights of exchange area loading.