TRANSMISSION LINES FOR RADIO SYSTEMS 



433 



from which the resistance R per unit lengtli may be obtained with a 

 degree of accuracy depending chiefly on how accurately the character- 

 istic impedance Zo is known. 



The current distribution along the line is most conveniently ob- 

 tained by means of a portable indicating device. Three designs which 

 have been found useful are shown in the following figures. The indica- 

 tor to the left of Fig. 15 is used for measurements upon open-wire lines. 

 The manner in which it operates is evident from the figure. Note that 



pjg 15 — Three types of portable indicating devices for observing the current 

 distribution on transmission lines. The device to the left is sensitive chiefly to 

 the current flowing in one conductor. The device in the center responds chiefly to 

 balanced currents and is not sensitive to unbalanced currents. The device to the 

 right is suitable for observing the voltage amplitudes on concentric tube lines. 



except for closely spaced wires the device is sensitive only to the cur- 

 rent in one side of the line. When this device is used the currents in both 

 sides should be measured to assure that there are no large current un- 

 balances and for the purpose of averaging out any small unbalances. 

 The device in the center of Fig. 15 is coupled to both sides of the line 

 and is not sensitive to currents which employ the two-line conductors 



