384 BELL SYSTEM TECHNICAL JOURNAL 



impedance of the toll entrance cable, which is usually about 125 ohms. 

 Adjustment of this impedance over the necessary range to match 

 impedances of particular cable pairs is provided by means of taps on 

 the transformer. At the office another transformer similarly tapped is 

 employed to match the toll entrance cable pair impedance to that of the 

 office wiring. 



Fig. 12 shows the losses of the commonly used 19-, 16- and 13-gauge 

 paper-insulated toll entrance cable, a new 10-gauge low capacity cable, 

 and the new disc-insulated cable. Because of the high losses of the 

 smaller gauge pairs, it is sometimes economical to place new 10-gauge 

 cable to save repeater costs. 



For the office wiring of the J system a rubber-covered shielded pair 

 is used to provide the desired flexibility and freedom from capacitance 

 variation due to humidity changes. Its impedance at 140 kilocycles 

 is approximately 125 ohms. The repeater and terminal high frequency 

 impedances are designed to match this impedance very closely. 



Fig. 13 illustrates the arrangement of the toll entrance equipment 

 involved in matching the line impedance to that of the equipment with 

 a minimum of reflection. The terminal is illustrated to the left. The 

 high frequency line passes to the line filter set which is here shown as 

 located in a filter hut. There it is joined by the type C and lower 

 frequency circuits and passes through the lead-in cable and protective 

 arrangements on the terminal pole. 



Proceeding toward the right in the figure, the arrangement at an 

 auxiliary repeater station is shown. In this case the type J frequencies 

 are amplified in the repeater, but the type C and lower frequencies are 

 by-passed through filters which suppress longitudinal and metallic 

 transmission above 30 kilocycles. At the right is shown a combined 

 type J and type C main repeater office. 



Satisfactory crosstalk between pairs in entrance and intermediate 

 cables carrying J systems is efifected through special selection methods 

 and the application of balancing condensers. 



Reflection Coefficients 



The success of the various measures taken to insure good impedance 

 matching is shown by the curves of Fig. 14, which are of reflection 

 coefficients measured at an auxiliary repeater station. Curve A, the 

 solid line, gives the coefficient between the open-wire pair and the lead- 

 in cable at the terminal pole. The smaller variations are due partly to 

 irregularities of the open-wire line and, at the lower frequencies, partly 

 to the test terminations at the distant end. The contribution of the 

 cable loading and office equipment is indicated by the dash-line curve 



