TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 365 



systems the lower frequency limit of the J system was set at 36 

 kilocycles; the necessary frequency space for 12 channels in each 

 direction set the upper limit at about 140 kilocycles. This range is 

 split into two parts, one used for transmission in one direction and the 

 other for the opposite direction. Figure 1 illustrates the relation of the 

 frequency bands occupied by the type J and type C systems and the 

 voice-frequency channel. Different "staggered" locations of the 

 frequency bands are to be employed in order to simplify crosstalk 

 problems. 



Filters are used for separation of the type J from the type C and 

 lower frequency facilities on the same pair of wjres. This separation is 

 done by means of a combination of high and low pass filters which 

 split apart the frequency ranges above and below the band between 30 

 and 36 kilocycles. To simplify the design of these filters, the low 

 frequency group of the type J system is transmitted in the same 

 direction as the high frequency group of the type C system. This 

 arrangement of transmitting certain frequencies in a particular direc- 

 tion is generally used throughout the telephone plant in order to avoid 

 serious crosstalk difficulties. Accordingly, with few exceptions, west to 

 east transmission or south to north transmission takes place in the 

 same frequency bands throughout the country and similarly, east to 

 west or north to south transmission employs the same frequency bands. 

 These are indicated in Fig. 1. 



Line Attenuation 



An open-wire pair affords the lowest loss transmission medium of any 

 conductor employed in the telephone plant. It is, however, peculiarly 

 subject to the effect of weather, which may cause large and often rapid 

 changes in the attenuation. In consequence, some form of gain 

 regulation is required. 



Even for carrier systems operating up to 30 kilocycles, manual 

 regulation is inadequate for the longer systems and automatic devices 

 have been provided for most systems over 500 miles in length. The 

 attenuation changes caused by changes in resistance of the wire with 

 temperature or by changes in the shunt losses when insulators become 

 wet are much larger at the higher frequencies of the J system, and 

 therefore, an automatic regulating scheme is required. Tests were 

 made on open-wire circuits to determine more precisely the charac- 

 teristics needed for such a regulator. During sleet storms, when wires 

 are covered with ice, the increases in attenuation are far beyond any 

 caused by rain. Figure 2 shows increases which may be caused by ice 

 as compared with the normal dry and wet weather values. 



