Transmission Lines for Short- Wave Radio Systems * 



By E. J. STERBA AND C. B. FELDMAN 



The requirements imposed on transmission lines by short-wave radio 

 systems are discussed, and the difference in the requirements for transmitting 

 and receiving purposes is emphasized. Various line types are discussed, 

 particular attention being given to concentric tube lines and balanced two- 

 wire lines. The concentric tube line is particularly valuable in receiving 

 stations where great directional discrimination is involved and low noise and 

 static pick-up is required. 



Excellent agreement between calculations and measurements is found for 

 the high-frequency resistance of concentric lines, using the asymptotic skin 

 elTect formula of Russell. Other losses in correctly designed concentric 

 tube lines are found to be negligible. Measured losses in two-wire lines are 

 found to be greater than losses predicted by the asymptotic skin effect 

 formula owing, in part, to losses brought about by unbalanced currents. 



Practical aspects of line construction such as joints, insulation, and 

 pro\ision for expansion with increasing temperature are discussed. 



Some difficulties encountered in transmission line practice, such as losses 

 due to radiation, reflections from irregularities, effects of weather, and 

 spurious couplings between antenna and line are discussed. 



I. General Requirements 



THE transmission line systems employed for the purpose of trans- 

 ferring energy between radio units and antennas are funda- 

 mentally no different from line systems used in power or telephone 

 work. Owing, however, to the high frequencies employed in radio 

 transmission an operating technique differing from that found econom- 

 ical in low-frequency practice is necessary. An important considera- 

 tion in 60-cycle power practice is that the voltage at the far end of the 

 line be maintained constant irrespective of load variations. At radio 

 frequencies a transmission line may be many wave-lengths long and 

 the reflections from a load other than one equal to the characteristic 

 impedance of the line produce standing waves. Transmission losses in 

 radio-frequency lines are appreciably augmented when the currents 

 and voltages on the line appear in the form of standing waves. The 

 operation of the radio unit connected to the line is sometimes affected 

 by the presence of standing waves. 



Induction and cross-talk problems familiar to every telephone 

 engineer are increasingly important as line operation approaches radio 

 frequencies. Owing to the high sensitivity of radio receiving equip- 

 ment as compared with that of telephone equipment the difficulties 



* Presented at I. R. E. Convention Pittsburgh, Pa., April 7-9, 1932. Published 

 in Proc. I. R. E., July, 1932. 



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