TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 383 



characteristic impedance of bridle wire is roughly one-fifth of that of 

 the open-wire circuit and it has been necessary to avoid the use of even 

 several feet of it between the open-wire and the cable terminal or 

 protectors. To accomplish this, separate terminals for each disc- 

 insulated unit are mounted on the crossarm near the open-wire pairs to 

 which they connect. Four insulated wires from each terminal go by 

 the shortest feasible route to the longitudinal choke coils and protectors 

 and thence to the open-wire pairs. 



Toll Entrance Arrangements 



The new disc-insulated cable used for intermediate cables was also 

 suited for lead-in or toll entrance cables. 



When an auxiliary station is established at a point along an open- 

 wire line where there has not previously been an office, it is usually 

 located close to the line so that the lengths of lead-in cable required are 

 comparatively short. Lengths of this cable up to about 175 feet can be 

 loaded to open-wire impedances with adjustable loading units in the 

 repeater station. For longer lead-in cables up to 300 feet, supple- 

 mentary loading may be mounted directly on the pole at the cable 

 terminals. 



When an auxiliary repeater station is not close to the open-wire line, 

 or at main repeater stations which are frequently in towns and sepa- 

 rated from the open-wire line by greater lengths of toll entrance cable, 

 it is still possible to use the loaded disc-insulated cable. Because of the 

 cost of this cable and its loading, however, it has sometimes been found 

 more economical to build a hut near the open-wire terminal pole and to 

 separate the type J from the type C and lower frequency facilities at 

 that point by means of filters. The connection from the open-wire 

 line to the hut is provided by what is usually a short length of loaded 

 disc-insulated cable. From that point, the type J frequencies are led 

 into the toll office over non-loaded paper-insulated pairs while the C 

 and lower frequency facilities are brought in over the existing pairs, 

 usually loaded. By thus limiting the frequencies transmitted over the 

 non-loaded cable pairs to the J range, it becomes practical to design 

 transformers for suitable impedance matching. 



The line filter sets located in the hut are designed for a nominal 

 impedance of 560 ohms which is a compromise for the range of 

 impedances normally found with diflterent wire sizes and spacings. An 

 accurate match with the line is obtained with a building-out network 

 which is adjusted at the time of installation to fit the particular open- 

 wire pair involved. On the office side of this line filter set a trans- 

 former provides for stepping down the impedance from 560 ohms to the 



