WIDE-BAND TRANSMISSION OVER COAXIAL LINES 657 



tionable interference. Fortunately, recent advances in repeater tech- 

 nique have made this result possible, as will be appreciated from the 

 subsequent description. 



An interesting characteristic of this type of system is the way in 

 which the width of the transmitted band is controlled -by the repeater 

 spacing and line size, as follows: 



1. For a given size of conductor and given length of line, the band 



width increases nearly as the square of the number of the re- 

 peater points. Thus, for a coaxial circuit with about .3-inch 

 inner diameter of outer conductor, a 20-mile repeater spacing 

 will enable a band up to about 250,000 cycles to be transmitted, 

 a 10-mile spacing will increase the band to about 1,000,000 

 cycles, and a 5-mile spacing to about 4,000,000 cycles. 



2. For a given repeater spacing, the band width increases approxi- 



mately as the square of the conductor diameter. Thus, whereas 

 a tube of .3-inch inner diameter will transmit a band of about 

 1,000,000 cycles, .6-inch diameter will transmit about 4,000,000 

 cycles, while a diameter corresponding to a full-sized telephone 

 cable might transmit something of the order of 50,000,000 

 cycles, depending upon the dielectric employed and upon the 

 ability to provide suitable repeaters. 



Earlier Work 



It may be of interest to note that as a structure, the coaxial form of 

 line is old — in fact, classical. During the latter half of the last century 

 it was the object of theoretical study, in respect to skin effect and other 

 problems, by some of the most prominent mathematical physicists of 

 the time. Reference to some of this work is made in a paper by 

 Schelkunoff, dealing with the theory of the coaxial circuit.*^ 



On the practical side, it is found on looking back over the art that the 

 coaxial form of line structure has been used in two rather widely differ- 

 ent applications: first, as a long line for the transmission of low fre- 

 quencies, examples of which are usage for submarine cables,^- ^ and for 

 power distribution purposes, and second as a short-distance, high- 

 frequency line serving as an antenna lead-in.^' ^^ 



The coaxial conductor system herein described may be regarded as 

 an extension of these earlier applications to the long-distance trans- 

 mission of a very wide range of frequencies suitable for multiplex 

 telephony or television. ^^ Although dealing with radio frequencies, 

 this system represents an extreme departure from radio systems in that 

 a relatively broad band of waves is transmitted, this band being con- 



