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BELL SYSTEM TECHNICAL JOURNAL 



attenuation with frequency. These factors are not important on the 

 shorter circuits for which the system has primarily been designed. 



Overall Transmission Characteristics 



The circuit provided by a Type H system without a repeater has a 

 band width of about 2750 cycles, extending from about 250 to 3000 

 cycles. This is somewhat wider than that for the Type D system. 

 The introduction of repeaters will tend to narrow the band somewhat. 

 Representative frequency characteristics are shown in Fig. 11. One 



i 



+ 15 

 + 10 

 + 5 





 _i -5 



LU 



m 



[iJ+IS 

 o 



+10 



1000 1500 2000 2500 



FREQUENCY IN CYCLES PER SECOND 



Fig. 1 1 — Representative overall circuit net loss characteristic of system. 



set of characteristics is for a typical circuit without a repeater, and the 

 other for a circuit including two repeaters. It is assumed that the line 

 conditions are such that no large reflection effects are present. 



The band width is limited principally by the characteristics of the 

 band filters. The small differences in the characteristics for the two 

 directions of transmission are due partly to differences in the filters 

 and partly to the fact that the attenuation of the line increases with 

 frequency and is greater at the 3000-cycle point in the east-to-west 

 channel than for the 3000-cycle point of the west-to-east channel. 

 As the circuit is increased in length this difference tends to increase. 



Variations in overall circuit net loss are due largely to variations in 

 the loss of the high-frequency line. For a circuit 200 miles long these 

 may amount to ± 3 db. The key controlled pads which are included 

 at each terminal and repeater are provided for making adjustments to 



