TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 377 



It was also seen to be necessary to block the paths provided by 

 the wires of the telephone line itself. For this purpose, crosstalk 

 suppression filters were designed and built to be installed in all of the 

 non-J circuits on the line. These give losses of the order of 70 db at 

 140 kilocycles not only in the metallic transmission circuits but also in 

 other circuits, made up of various combinations of the line wires, which 

 may conduct crosstalk currents through the stations. 



In addition to the crosstalk suppression filters and in order to 

 provide an extra margin of safety against interaction crosstalk currents 

 which might find their way through the repeater station by stray paths, 

 longitudinal choke coils have been connected at the pole heads between 

 the open wires and the lead-in cables. These coils do not disturb 

 ordinary transmission but add high impedance in the longitudinal 

 circuits. 



These measures for controlling interaction crosstalk have been 

 found to be adequate so far as the telephone line is concerned. At an 

 occasional J repeater station, however, located on a right-of-way 

 occupied by several pole lines, there is found another pole line paral- 

 leling the telephone line with a separation sometimes as little as 2 to 5 

 feet between the nearest wires of the two lines. Such wires provide 

 other interaction crosstalk paths past the repeater station and impair 

 the effectiveness of suppression measures installed in the line on which 

 the J system is operated. The by-passing effects of such a foreign line 

 can be controlled by crosstalk suppression devices similar to those used 

 in the telephone line wires. 



Figure 10 shows a comparison of the interaction crosstalk measured 

 at a J repeater station before any suppression measures were installed, 

 the other wires of the line being continuous at the station location, 

 with the corresponding interaction crosstalk when the line was run 

 through the suppression devices in the station. The values shown 

 would be amplified by the gain of the J repeater on the disturbed 

 circuit before they reached the listener. The efTect of the by-passing 

 foreign line is illustrated by the difference between the middle and 

 bottom curves, the bottom curve showing the measured crosstalk 

 when the by-passing line was cut to simulate the effect of suppression 

 measures in it. 



Staggered Systems 



It would not be possible with the open-wire line configurations now 

 in use to design transposition arrangements that would permit the 

 operation of identical J systems on all pairs. For this reason four 

 types of J systems with difTerent channel carrier frequency allocations 



