SMOOTH LINES AND SIMULATING NETWORKS 5 



where the leakance is approximately proportional to the frequency 

 and approximately independent of the frequency, corresponding to 

 consideration (E). 



While the classification of smooth lines into open-wire lines and 

 cables is convenient, there is, of course, no very sharp distinction 

 between the open-wire type of lines and the cable type of lines, since 

 the distinction depends on the line parameters and on the frequency 

 range involved, rather than on the physical form of the line; for, any 

 line at sufficiently high frequencies has the open-wire type of char- 

 acteristics, and at sufficiently low frequencies the cable type of char- 

 acteristics. With regard to the relative importance of the funda- 

 mental line constants R, G, L, C when the frequency range is that of 

 the voice, it may be said that for the open-wire type of lines L and C 

 are of about equal importance, R of secondary, and G of tertiary im- 

 portance; while, on the other hand, for the cable type, R and C are 

 of about equal importance, L of secondary, and G of tertiary import- 

 ance. In illustration of the above remarks it may be noted that 

 smoothly loaded cables (unless loaded very lightly) have the open- 

 wire type of characteristics; as have also periodically loaded cables at 

 low frequencies. 



Before proceeding to the separate treatments of open-wire lines 

 and cables, it seems desirable to indicate the general nature of the 

 effect produced on the impedance by leakance. 



The General Effect of Leakance 



The amount of leakance that is normally allowable as regards its 

 attenuating effects is so small as to produce only very slight effects on 

 the characteristic impedance of either type of line (except at very low 

 frequencies). 



In ordinary telephone cables the leakance is so small that, except at 

 very low frequencies, the impedance of such cables is very closely the 

 same as in the limiting case of no leakance; whence that limiting case 

 may be taken as being a good approximation to the actual case. In 

 open-wire lines leakance may be much larger than in cables, yet 

 normally it is small enough so that its effects on the impedance are 

 slight, except at very low frequencies, so that usually the limiting 

 value of zero leakance is still a good approximation when calculating 

 the characteristic impedance. However, during wet weather and 

 in particularly humid climates and locations the leakance in open- 

 wire lines becomes large enough to affect the impedance quite ap- 

 preciably, even within the voice frequency range, while enormously 

 affecting it at very low frequencies. 



