INDUCTIVE LOADING FOR TELEPHONE FACILITIES 749 



A 2. 7 and 3.0 Loading: During the late 1920's the rapid expansion of 

 carrier working led to extensive studies of the practicability of obtaining a 

 larger number of telephone channels in long-haul carrier systems. These 

 studies indicated a good prospect of using a wider frequency-band extending 

 up to a top frequency of about 50 kc. In order to secure a much better con- 

 trol of intersystem crosstalk over the wider frequency-band, plans were 

 made for spacing the wires of individual pairs much closer together, and for 

 spacing adjacent pairs at greater distances apart. Also, improved trans- 

 position systems were designed for these new open-wire arrangements. In 

 the period of interest, the open-wire plant was expanding very rapidly, and 

 as a part of this expansion several entirely new pole lines were required for 

 important long-haul service. These Unes incorporated the improved con- 

 struction features above mentioned. Even though the proposed new broader- 

 band carrier systems were still in the "discussion stage" of development, it 

 seemed desirable that a new type of broader-band loading should be installed 

 on the incidental cables in the new pole lines, in order to avoid the larger 

 expense of eventually replacing the 30-kc Type C loading, if it should be 

 used initially. These considerations resulted in the rush development of the 

 Types A2.7 and A3 carrier loading systems specifically to meet the im- 

 pedance-matching requirements over the proposed 50-kc band. This loading 

 was duly installed according to plan, but fate decreed that it should never 

 be used for its originally intended purpose. Type C carrier telephone systems 

 were immediately installed on the new lines, in the expectation of removal 

 when broader-band systems became available, and the Type A loading was 

 actually used only for 30-kc transmission. 



The explanation for this turn of events was that before the final develop- 

 ment requirements could be established for the proposed new 4 or 5-channel 

 systems, some entirely new factors^''^ entered the continuing studies and 

 eventually resulted in a decision to develop a 12-channel system.**^ This was 

 designed for placement above a Type C system on the same open-wire pair, 

 making a total of 15 carrier channels above the voice-frequency circuit. The 

 new broad-band carrier telephone system was coded in the "J" series. Its 

 top working-frequency was about 143 kc. 



Type J Loading: In due course, the development of new carrier loading 

 was coordinated with the work on the new carrier telephone system. Three 

 loading systems, designated J-0.72, J-0.85, and J-0.94, became available 

 during 1937-1938 and are still in good standing, although they are not 

 extensively used. 



In the following pages, the general transmission characteristics of the 

 Type C, B, and J loading systems are described, and some general informa- 



(^^^ Including high-gain, high-stability, negative-feedback repeaters, and crystal filters. 



