656 



BELL SYSTEM TECHNICAL JOURNAL 



TERMINAL 



MULTIPLEXING 



APPARATUS 



TERMINAL 



MULTIPLEXING 



APPARATUS 



LINES AND REPEATERS 



"-^ 



L^-- 



Fig. 1 — Diagram of coaxial system. 



subsequently. Such a coaxial line can be constructed to have the 

 same degree of mechanical flexibility as the familiar telephone cable. 

 While this line has a relatively high loss at high frequencies, the trans- 

 mission path is particularly well adapted to the frequent application 

 of repeaters, since the shielding permits the transmission currents to 

 fall to low power levels at the high frequencies. 



Of no little importance also is the fact that the attenuation-fre- 

 quency characteristic is smooth throughout the entire band and obeys 

 a simple law of change withjtemperature. (This is due to the fact that 

 the dielectric is largely gaseous and that insulation material of good 

 dielectric properties is employed.) This smooth relation is extremely 



Fig. 2 — Small flexible coaxial structure. 



helpful in the provision of means in the repeaters for automatically 

 compensating for the variations which occur in the line attenuation 

 with changes of temperature. This type of system is featured by 

 large transmission losses which are offset by large amplification, and it 

 is necessary that the two effects match each other accurately at all 

 times throughout the frequency range. 



It will be evident that the repeater is of outstanding importance in 

 this type of system, for it must not only transmit the wide band of 

 frequencies with a transmission characteristic inverse to that of the 

 line, with automatic regulation to care for temperature changes, but 

 must also have sufficient freedom from inter-modulation effects to 

 permit the use of large numbers of repeaters in tandem without objec- 



