260 ni-1-1- SYSTEM TFCHXICAL JOURNAL 



ron-espoiuliiiij, smooth line is closely similar to the non-loaded open 

 wire line. The second and more complicated part of the problem is 

 to determine the coil spacing. This usually involves some degree of 

 compromise, l)ecause of the dependence of the impedance of a loaded 

 cable upon the loading termination. 



The hrst general requirement is that the ratio of inductance to 

 capacitance to resistance per unit length in the loaded cable should be 

 the same as the corresponding ratio in the non-loaded open wire line. 

 Ordinarily, the loading coil resistance does not play an important 

 part in the determination of the optimum resistance for the loaded 

 cable, the choice of conductor gage being far more important. From 

 this point of view% No. 13 A.w.g. is practically the best gage of con- 

 ductor for entrance cable circuits connecting with 165-mil open wire 

 lines. For the optimum impedance match on cables connecting with 

 104-mil open ware lines, it is necessary to use much higher resistance 

 conductors, the choice between Nos. 16 and 19 A.w.g. conductors 

 depending upon a number of factors which space limits do not allow- 

 to be discussed. 



As noted in the discussion under "Theory" in the first part of the 

 paper, the characteristic impedance of a uniform line is substantially 

 a pure resistance, having the value Vl/C over the frequency range 

 throughout which the inductive reactance per unit length is large 

 with reference to the resistance. On the other hand, the character- 

 istic impedance of a coil loaded cable varies over a wide range with 

 frequency, depending upon the particular loading termination used. 



Typical impedance-frequency curves for mid-coil and mid-section 

 terminations are illustrated in Fig. 3. As will be seen from this dia- 

 gram, the rising slope of the mid-section termination and the drooping 

 slope of the mid-coil termination do not deviate greatly from a straight 

 line relation for frequencies below approximately 0.5 of the cut-olif 

 frequency. The higher the cut-off frequency is, the more closely 

 will the impedance-frequency characteristic of the loaded cable 

 approach the fiat characteristic of the non-loaded open wire line over 

 the range of frequencies in\-olved in speech transmission. In this 

 connection, it is to be noted that the repeaters now used on open wire 

 lines are designed to transmit frequencies between approximately 

 200 and 2,600 cycles. Of course, the higher the cut-off frequency, the 

 more expensive will be the loading. Practical reasons make it desir- 

 able to space the loading coils on the cable circuits connecting with 

 non-loaded open wire lines at the same intervals as the coils which 

 are used on the cable circuits connected with the loaded open wire 

 lines. This consideration in combination with the nominal impedance 



