754 



BELL SYSTEM TECHNICAL JOURNAL 



rise to what is known as phase distortion, and it may be necessary to 

 correct this distortion by suitable networks. ^^' ^^ 



200,000 



a 



z 

 o 

 o 



^ 180,000 



a 



UJ 



5 160,000 



z 

 o 



140,000 



< 



o 

 o 

 a. 

 o 

 a: 

 a. 



O 



> 120,000 



t- 



o 



s 



UJ 



> 



100,000 



100 



500 



1000 

 FREQUENCY- 



5000 10,000 

 -CYCLES PER SECOND 



50,000 100,000 



Fig. 17 — Velocity of propagation for open-wire pairs. 



Characteristics of Phantom Circuits 



Phantom circuits, which are derived from two pairs or side circuits 

 by transmitting over the wires of one pair in parallel and using the 

 wires of the other pair in parallel as a return, have been employed in the 

 telephone plant for voice-frequency transmission for a number of years. 

 Their use for carrier transmission is limited chiefly by the difficulty of 

 reducing the cross induction with other circuits at high frequencies. 



Phantom circuits are generally derived either from horizontally 

 adjacent non-pole pairs or from vertically adjacent pole pairs. Thus 

 wires 1 and 2 are "phantomed" with wires 3 and 4, wires 5 and 6 with 

 wires 15 and 16, etc. In some of the newer transposition arrangements 

 the non-pole pairs are not phantomed, since it has been found that the 

 omission of the phantom permits the operation of a larger number of 

 carrier systems on one line without excessive mutual interference. 



The resistance of a phantom circuit is evidently half of the corre- 

 sponding value for the side circuit. 



^^ See "Distortion Correction in Electrical Circuits with Constant Resistance 

 Recurrent Networks," by O. J. Zobel, Bell System Tech. Jl., July, 1928, pp. 438-534. 



1' See "Wire Transmission System for Television," by D. K. Gannett and E. I. 

 Green, A. I.E. E. Trans., Vol. 46, 1927, pp. 946-953 {Bell System Tech. JL, October, 

 1927, pp. 616-632). 



