LOADED TELEGRAPH CABLES 



389 



for typical loaded and non-loaded cables is shown in Fig. i, the signal 

 frequencies at which they are designed to operate being as indicated. 

 In the case of the non-loaded cable the resistance and capacity are 

 practically constant over the frequency range and the attenuation 



30 40 50 CO 



FREQUENCY (CYCLES PER SEC) 



Fig. 1 — A — non-loaded cable; B — actual loaded cable; C — ideal loaded cable 



curve is approximately a parabola as indicated by formula (3). The 

 curve for the loaded cable for small values of frequency is similar to 

 the curve for the non-loaded cable, since for such frequencies the 

 loading inductance has very little effect upon transmission. As soon 

 as 27r/L becomes appreciable compared with R the beneficial effect 

 of the inductance becomes apparent and the attenuation constant 

 increases at a less rapid rate. If the cable parameters were constant 

 throughout the frequency range, as in the case of the ideal cable, 

 the attenuation constant would, at a value of frequency considerably 

 below the signal frequency, attain a constant value, as represented 

 by the dotted curve. On account of the fact, however, that R and G 

 increase rather rapidly with frequency, the attenuation-frequency 

 characteristic of an actual cable merely inflects, then increases, and 

 at some frequency will actually cross the attenuation curve of the non- 

 loaded cable. 



To insure that legible signals will be obtained at the desired signal 

 frequency the amplitude of the extraneous interference must be 



