OPEN-WIRE TELEPHONE LINES 



751 



Impedance : 

 The characteristic impedance is defined by the well-known formula: 



Zo = 



R + jLco 



G + jG 



ohms. 



CjO 



(8) 



It is doubtless unnecessary to explain why this impedance must be 

 matched in the apparatus. 



When R is small compared to Leo and G is small compared to Coj, the 

 value of Zo evidently becomes 



Zo = 



(9) 



This is known as the nominal characteristic impedance. It will be 

 noted that this impedance is a pure resistance. In the carrier range 

 the actual impedance of an open-wire pair is substantially equal to the 

 nominal characteristic impedance. 



750 



500 



tn 



^ 250 



X 



o 



-250 



10 20 30 40 



FREQUENCY — KILOCYCLES 



50 



Fig. 15 — Carrier-frequency impedance of 12-in. open-wire pairs. 



Values of the characteristic impedance in dry weather of open-wire 

 pairs with 12-inch wire spacing are presented in Fig. 15. These values 

 have been derived from the standard values of inductance, capaci- 

 tance, and leakage conductance and from resistance values at 20 deg. 

 cent. The basis for the impedance value of 600 ohms resistance, 



