TRANSMISSION LINES FOR RADIO SYSTEMS 413 



equal to that radiated by the antenna to which it is connected.' In 

 fact, single-wire lines, particularly when terminated, are for certain 

 services desirable radiating elements. Diamond-shaped arrays of such 

 elements are employed in some of the radio facilities of the Bell 

 System. 2 



It is generally appreciated that the power losses due to radiation 

 may be reduced by employing two conductors in a go-and-return cir- 

 cuit, the wires being separated a small fraction of a wave-length. A 

 necessary requirement is that the two wires carry equal currents ex- 

 actly opposite in phase. Otherwise, there will appear current com- 

 ponents which employ the two conductors in parallel. In the latter 

 event the radiation losses ascribed to single-wire conductors occur. 



Although there is a very great reduction in radiated power in bal- 

 anced two-wire lines as compared with single-wire lines there are many 

 practical cases where the radiation from two-wire lines produces cross- 

 talk and loss of signal discrimination. Multiple-wire lines comprising 

 several pairs of conductors in go-and-return circuits may be employed 

 to reduce the undesired radiation couplings. As in the two-wire case, 

 care must be exercised in maintaining the required current amplitudes 

 and phases since otherwise the radiation losses ascribed to single-wire 

 lines may destroy the utility of the multiple-wire system. Multiple- 

 wire lines, of course, reduce static and noise interference. 



From the standpoint of isolation an ideal electrical connection be- 

 tween antennas and radio apparatus is approached when one conductor 

 completely encloses the other conductor. A concentric-tube line com- 

 prising an outer sheath and an inner conductor is the practical form of 

 this construction. Long transmission lines often pick up a large 

 amount of static and other electrical disturbances. Spurious couplings 

 may introduce these disturbances into the radio circuit. Electrical 

 disturbances so introduced are greatly reduced when the outer sheath 

 of a line may be grounded at frequent intervals. In fact, concentric- 

 tube lines may be buried in the ground. 



The effect of weather is a factor which in some instances may de- 

 termine the type of construction to be employed in radio-frequency 

 lines. It is generally appreciated that rain and sleet storms may 

 materially lower the insulation of a line. The velocity of propagation 

 and characteristic impedance also are affected by a coating of water or 

 sleet upon the wires. Concentric-tube lines may be constructed so as 

 to be weather proof. 



This paper will be confined entirely to concentric-tube lines, to 



^ See calculations in the appendix. 



2 E. Bruce, Proc. I. R. E., p. 1406, August, 1931. 



