INDUCTIVE LOADING FOR TELEPHONE FACILITIES 745 



used on toll cable circuits along the same routes. Studies of these coordination 

 possibilities resulted in the standardization of H28-16 entrance cable loading 

 during 1927. Referring to Table XV, it will be seen that the change from 

 5575-ft. spacing to 6000-ft. spacing, using the same loading inductance 

 values, resulted in a small drop (about 4%) in nominal impedance and 

 theoretical cut-off frequency. A contemporary allied development made 

 available new types of balancing networks which simulated the iterative 

 impedances of the H28-16 loaded cables. The use of these new networks with 

 repeaters at the office ends of long H28-16 loaded cables gave considerably 

 better repeater balances than those obtained with open-wire balancing net- 

 works at the office ends of long E28-16 loaded cables. Up to this time, 

 balancing networks which simulated the impedances of the associated open- 

 wire lines had been used with the open-wire repeaters. This early practice 

 was continued on open-wire lines having short entrance cables with H28-16 

 loading. 



H31-18 Loading 



General: It was known prior to the standardization of H28-16 loading that 

 the 28-16 mh loading inductances were not optimum from the impedance- 

 matching standpoint for use at 60(X)-ft. spacing. However, it was appreciated 

 that the concurrent development work on the compressed permalloy-powder 

 core-material previously described (Section 9.1) was approaching completion 

 and that a general size-reduction redesign of all toll cable and toll entrance 

 loading coils would soon be undertaken. These considerations made it un- 

 desirable to develop for the H-spaced 28-18 loading new iron-dust core 

 loading coils which would in all probability be superseded in a short time by 

 permalloy-core coils. Thus it happened that the development work for the 

 improved H31-18 loading system was coordinated with that on smaller-size, 

 permalloy-core, loading coils having the necessary new inductance values 

 for use in that system. 



The H31-18 loading was designed to have a slightly higher nominal 

 impedance than E28-16 loading, to make it more suitable for use on inci- 

 dental cables in 104-mil open-wire lines, which were expected to be its 

 principal field of use, since the more expensive, larger conductors (128 and 

 165-mil) were destined for use principally on a carrier basis and would 

 require carrier loading on their incidental cables. 



Improved Loading Terminations: It was also considered desirable to pro- 

 vide better impedance-matching characteristics at high voice-frequencies to 

 assist in obtaining more satisfactory repeater operation on long-haul, multi- 

 repeater, voice-frequency circuits which were becoming more common and 

 more important in the rapid expansion of the open-wire plant. This require- 



