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BELL SYSTEM TECHNICAL JOURNAL 



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FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 9 — Transfer impedance of coaxial circuit. 



down to a level determined largely by the noise due to thermal agita- 

 tion of electricity in the conductors and tube noise in the associated 

 amplifiers. It appears uneconomical to make the outer conductor 

 sufficiently thick to provide adequate shielding for the very low fre- 

 quencies. Also it seems impractical to design the repeaters to trans- 

 mit very low frequencies. Hence the best system design appears to 

 be one in which the lowest five or ten per cent of the frequency range 

 is not used for signal transmission. The coaxial circuit is, however, 

 well suited to the transmission of 60-cycle current for operating re- 

 peaters, a matter which will be referred to later. 



Broad-Band Amplifiers 



In order to realize the full advantage of broad-band transmission, 

 the repeater for this type of system should be capable of amplifying 

 the entire frequency band en bloc. Furthermore, it should be so stable 

 and free from distortion that a large number of repeaters may be op- 

 erated in tandem. Although high-gain radio frequency amplifiers are 

 in everyday use, these are generally arranged to amplify at any one 

 time only a relatively narrow band of frequencies, a variable tuning 

 device being provided so that the amplification may be obtained at 



