Transmission Lines for Short-Wave Radio Systems

HE transmission line systems employed for the purpose of transferring energy between radio units and antennas are fundamentally 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 economical in low-frequency practice is necessary. An important consideration 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 equipment as compared with that of telephone equipment the difficulties * Presented at I. R. E. Convention Pittsburgh, Pa., April 7-9, 1932. in Proc. I. R. E., July, 1932. 411 Published 412 BELL SYSTEM TECHNICAL JOURNAL arising from static and noise pick-up are more marked at radio frequencies. Spurious radiations from lines operated at radio frequencies may completely destroy the directional characteristics of an antenna and in addition may cause interference to other radio stations.