588 



BELL SYSTEM TECHNICAL JOURNAL 



mitting terminal. A transmission level diagram will serve to explain 

 this limitation. 



Let it be assumed that it is desired to effect carrier transmission 

 using a type C-N system between points A and B, 240 miles apart on 

 165-mil conductors. The highest frequency channel is normally 

 considered, which in this case would be 26 kilocycles. The total 

 attenuation of the line at this frequency, as determined from the 

 line attenuation data already presented, would be 35 TU for wet 

 weather conditions of operation. A level diagram would accordingly 

 picture the situation as noted in Figure 13. x\t point A sufficient 



^' 



Carrier 

 ApP 



i I I I I I 



Carrier 

 App 



=S=tl 



40 80 120 160 200 240 280 



MILE5 



Figure 13 — Transmission level diagram 



transmitting gain would be provided by the equipment to bring 

 the sending level to + 20 TU. The line attenuation in connection 

 with transmission over the 240-mile circuit at point B would bring 

 the level to — 15 TU. In order to obtain an overall talking circuit of, 

 say, 10 TU., it would be necessary to operate with a receiving gain 

 of 5 TU. It will be noted that in this particular layout the minimum 

 line level is well above the limit set above. In fact, computations 

 would indicate that the line circuit might be extended to the total 

 length of about 300 miles, before the level limits would be exceeded. 

 On longer lines, however, involving may repeater sections, the level 

 limits are raised because of the cumulative effect of noise entering 

 the circuit from a greater number of sources. 



The line circuit illustrated is of the simplest type and in a practical 

 case involving sections of intermediate and terminal cable construction 

 the attenuation would be considerably greater and the effective 

 geographical distance covered for a particular type of apparatus would, 

 therefore, be less. 



