254 BELL SYSTEM TECHNICAL JOURNAL 



attenuation constant at 1,000 cycles as the associated H-174-S load- 

 ing. This resulted in the reduction of the phantom loading coil 

 inductance to 63 milhenrys. On the l)asis of equal attenuation losses 

 in the phantom circuit and its side circuits, the continued use of a 

 higher grade coil in the phantom circuit was no longer justified from 

 a cost standpoint. Accordingly, the new 63-milhenry phantom coil 

 (Code No. 587, Table IX) was designed to have approximately the 

 same d-c. resistance as the earlier standard 106-milhenry coil (Code 

 No. 583), since this permitted a substantial reduction in the size of the 

 loading coil and a consequent reduction in cost, without increasing the 

 over-all losses in the associated side circuits. The design finally chosen 

 resulted in the phantom coil having approximately the same over-all 

 dimensions as the associated side circuit coils. This permitted the 

 phantom coils to be mounted on the same spindles with the associated 

 side circuit loading coils as phantom groups, thus reducing the amount 

 of inside cabling. This gave impro\-ed electrical results, besides 

 reducing the potting costs. The use of the smaller size phantom 

 coil, in combination with a larger size case, made it practicable to pot a 

 total of 45 phantom group combinations (135 coils) in a single case. 

 Using the same size case for potting phantom group combinations 

 involving the older large size phantom coils, the limit on the number 

 of coils was 108 (36 phantom groups). 



The reduction of the phantom coil Inductance from 106 to 63 

 milhenrys made a substantial increase in the cut-off frequency and 

 in the velocity of transmission, as noted in Table X. These improved 

 characteristics made the H-63-P circuit much superior to the H-106-P 

 circuit from the standpoint of echoes and velocity distortion character- 

 istics. On this basis the H-63-P circuit is intermediate in transmission 

 excellence between H-174-S and H-44-25 circuits. 



It was found inadvisable to make a similar change in the H-44-25 

 loading system owing to cross-talk reactions following from the 

 necessary use of higher repeater gains in the phantom circuit. These 

 undesirable reactions, though present to a lesser degree in the case 

 of the H-174-63 system were offset by the factors already descril)ed. 

 The size of the H-25-P coil was, however, reduced to coniform to the 

 potting method adopted for H-174-63 loading. 



From the standpoint of repeater circuits the H-174-63 system is 

 inherently better than the H-245-155 system because of its higher 

 velocity and higher cut-off, with resulting higher quality ol trans- 

 mission. Furthermore, as far as non-repeatered circuits are concerned, 

 there is a negligibly small difference between the transmission per- 

 formances, considering frecjuency distortion effects as well as \olunie 



