412 BELL SYSTEM TECHNICAL JOURNAL 



arising from static and noise pick-up are more marked at radio fre- 

 quencies. 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. Under 

 certain conditions the spurious power radiated by a transmission line 

 may be an appreciable fraction of that radiated by the antenna con- 

 nected to the line. 



It follows that although the primary purpose of a transmission line 

 in a radio station is to provide a means for transferring energy between 

 an antenna and the radio unit, a consideration of great importance is 

 the degree of isolation from its associated antenna, from other antennas 

 and lines, and from extraneous sources of signals. This is particularly 

 true in receiving stations where discrimination against undesired sig- 

 nals is oftentimes of greater importance than the over-all sensitivity 

 to the desired signal. Extraneous pick-up on a line to the receiving 

 unit may not only destroy the directional pattern of the antenna but 

 it may also introduce a noise level into the receiver output comparable 

 to the desired signal and so destroy the utility of the apparatus. As 

 compared with transmitters, receivers are generally small units. It is 

 economical to house several units in one building. The lines not only 

 to one receiver but those to other receivers must of necessity be in 

 close proximity. Thus the possibilities for cross-talk with ensuing 

 increase in noise levels, loss of circuit gain, and loss of discrimination 

 are greatly augmented. 



Of course, cross-talk possibilities between lines of adjacent trans- 

 mitters cannot be ignored. Transmitters, however, usually occupy 

 sufficient space so that a desirable degree of line separation is obtained. 

 With the exception of local lightning storms static pick-up is of no 

 great importance. High voltage surges due to lightning may be 

 drained by means of properly placed horn gaps and grounds. 



Insulation for high voltages at radio frequencies is an important 

 consideration for the case of lines connected to transmitters. Insu- 

 lators for balanced open-wire construction may be selected from mate- 

 rials designed for high voltage power transmission. However, insu- 

 lators for concentric tube lines capable of transmitting several kilowatts 

 of modulated radio-frequency power require special consideration. 



The lines commonly employed in radio stations may be divided into 

 four classes: single-wire lines, balanced open-wire lines, multiple-wire 

 lines, and concentric-tube lines. 



Single-wire lines are of limited utility owing to the low efficiencies 

 arising from the marked radiation characteristics of such wires. The 

 power radiated by a smgle-wire line several wave-lengths long may be 



