CARRIER SYSTEMS ON TELEPHONE LINES 



585 



work which has made possible these improved insulators will be 

 made available at a later date. 



While the circuits employed for the transmission of carrier telephone 

 systems as noted above are largely of open-wire construction, where 

 these circuits pass through the more populated districts of the country 

 it is frequently necessary to insert sections of cable. The smaller 

 closely spaced wires of cables make the problem of attenuation at 

 high frequencies more serious, even where the cables are relatively 

 short, say a mile or so in length. Typical attenuation curves of non- 

 loaded cable pairs are shown in Figure 12. 



4.0 



3.0 



Q. 



z 

 O 2 



z 

 o 



lO 



to 



<0 



z 

 < 



a: 



1 .0 



10 



40 



20 30 



FREQUENCY -KILOCYCLES 



Figure 12 — Attenuation of non-loaded cable circuits 



50 



This situation has led to the development of a special type of cable 

 loading which permits making a substantial reduction in the attenu- 

 ation for the higher frequencies and which also makes the characteristic 

 impedance of the cable circuit more closely simulate that of the open- 

 wire circuit so that the reflection effects discussed in detail later are 

 thus greatly reduced. This is important, for, whereas the open-wire 

 circuit characteristic impedance varies from 600 to 700 ohms, the non- 

 loaded cable impedance is of the order of 130 to 150 ohms and the 

 reflection losses and also certain resultant crosstalk effects as discussed 

 later are, therefore, very substantial for even short lengths of non- 



