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BELL SYSTEM TECHNICAL JOURNAL 



spaced 6,000 feet apart (approximately 1.8 kilometers); the extra 

 light loading employs coils having an inductance of about 0.044 henry 

 for the side circuits with the same spacing. The capacity per load- 

 ing section for the side circuits is approximately 0.074 mf. 



The medium heavy loaded side circuits have a characteristic impe- 

 dance of about 1600 ohms, and a cutoff frequency of about 2800 cycles. 

 The extra light loaded side circuits have an impedance of about 800 

 ohms and a cutoff frequency of about 5600 cycles. 



<3 5 



500 1000 1500 2000 2500 J000 350C 4000 4500 5000 



Frequency m Cycles per Second 



Fig. 2 — Attenuation-frequency characteristics of loaded cable side circuits. 



Figure 2 shows the attenuation-frequency characteristics of No. 

 19 and No. 16 gauge side circuits with the two types of loading. It 

 will be observed that the M.H.L. circuits have lower attenuation 

 for frequencies below about 2500 cycles, as should be expected from 

 the fact that the inductance per mile introduced by the loading coils 

 is greater. However, the attenuation is more nearly equal at dif- 

 ferent frequencies in the case of the X.L.L. circuits, this being par- 

 ticularly true at the higher voice frequencies. 



Another important characteristic of loaded circuits when repeaters 

 are involved is their velocity of propagation. Since the inductance 

 per mile of X.L.L. circuits is only X of that for M.H.L. circuits, the 

 velocity of propagation is twice as great for the X.L.L. circuits as 

 indicated by the well-known approximate formula — - 



v = (L cy 



