TRANSMISSION LINKS FOR RADIO SYSTEMS 431 



means of short line devices.''' A complex impedance at the proper posi- 

 tion along a partially terminated line is selected such that a shunt 

 reactance at this position transforms the real part of the impedance to 

 the surge impedance of the line at essentially unity power factor. The 

 shunt could, of course, be a lumped reactance. It is found convenient 

 to employ a short section of line for this reactance. A position along 

 the line for the shunt reactance of either leading or lagging power factor 

 may be chosen. In the former case the shunt reactance must be 

 inductive and in the latter case capacitive. Computed shunt im- 

 pedance positions for the two cases and the values of the shunt im- 

 pedance in terms of line length for 600-ohm lines and various standing 

 wave amplitudes on the unterminated section appear in Fig. 14. 

 Actual settings correspond very well with the calculated settings. 



V. Resistance and Attenuation Measurements on 

 Transmission Lines 



The following is a description of some of the measurement methods 

 which have been found useful in the study of transmission lines. The 

 schemes may not be applicable to every phase of the transmission line 

 roblem. However, it is hoped that they may suggest precautions top 

 be observed in performing transmission line studies. 



One scheme, very commonly employed, is to measure the attenuation 

 along a transmission line by actual current measurements. This 

 method is particularly suited to measurements upon a long line ter- 

 minated in its characteristic impedance. It has been found desirable 

 to measure the current amplitudes at close intervals for at least a one- 

 half wave-length section at the near end and the far end of the line. In 

 this manner an average result which reduces observational errors and 

 errors arising from standing waves of small amplitudes is obtained. 

 From the ratio of the average sending end current Is and the average 

 receiving end current Ir and the average distance / between the two 

 sections of line the attenuation per unit length is obtained from the 

 definition: 



db = 20 logio f* , (9) 



and since 



^ = e«'/2zo , (10) 



J- r 



^-^ = 4.343 f, (11) 



l Zo 



^^ Disclosed to the writers by P. H. Smith, Bell Telephone Laboratories, Inc., 

 New York, N. Y. 



