TRANSMISSION LINES FOR RADIO SYSTEMS 439 



and more rugged small diameter sheath without an appreciable dif- 

 ference in copper expenditure. Other factors which involve the 

 remainder of the radio plant often determine the size of the outer 

 sheath. We have found that for outer sheaths a diameter of 2.5 inches 

 and a radial thickness of 0.0875 to 0.10 inch provides lines which are 

 sufficiently rugged for transmitting 15 kw of modulated power at 16 

 meters wave-length. 



Careful consideration needs to be given to the problem of protecting 

 concentric lines from voltage overloads which may be brought about 

 by accidental open or short circuits or by flashovers. Voltages of 

 the order of 30,000 to 90,000 volts may easily be built up in this man- 

 ner at the shorter wave-lengths. Horn gaps are useful if located in the 

 proper way. It is fortunate that conventional line input circuits are 

 apt to be detuned in the event of an accidental open or short circuit 

 on the line and that very little power may then be transmitted to the 

 line. 



Beads of high grade porcelain in diameters up to one inch are satis- 

 factory insulators for low power and receiving lines. However, such 

 simple insulators are not suitable for high power work. Owing to the 

 volume of dielectric in large annular insulators sufficient heating may 

 occur at the higher voltages to destroy the insulator. Insulators such 

 as those described for line B Fig. 1 have been found suitable at the 

 higher voltages. 



The air film between the insulator and the inner conductor lies in a 

 region of steep voltage gradient. Even under w^hat is considered 

 normal operating voltage there may be enough corona in this region to 

 produce heating of the insulator. It may be of interest to mention 

 that a line approximately as described in B Fig. 1 has been found 

 satisfactory for normal operation at 16 meters for a carrier power of 

 15 kw. The line breaks down in the region of the insulator at 9000 

 r.m.s. volts. 



For transmitting purposes it has been found desirable to employ 

 glazed insulators in concentric tube lines because dirt, soldering fluxes, 

 etc. acquired in assembly operations are more readily removed from 

 glazed insulators. 



There are a number of simple ways in which insulators may be held 

 in place in concentric tube lines. For low power work and receiving 

 purposes wire clips, rivets or even extruded metal ears upon the inner 

 conductor, are satisfactory. As a rule these do not prove satisfactory 

 at higher powers owing to high potential gradients at points and 

 sharp edges. Small rings riveted or soldered upon either side of the 

 insulator have proven satisfactory. Lines with soldered rings are more 



