754 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



The low-capacitance construction, above described, also results in a much 

 higher ratio of distributed inductance to distributed capacitance than that 

 of paper insulated cables. This makes it necessary to build out the series 

 inductance in a proper ratio to shunt capacitance, when geographical spacing- 

 deviations require the use of corrective building-out adjustments. These 

 capacitance-inductance adjustment devices are considerably more expensive 

 than the relatively simple condensers used in the adjustments on Type C 

 and B loaded circuits. Closer coil-spacing also makes the Type J loading 

 more expensive. All in all, the total cost of the Type J loaded cable pairs 

 is very high relative to that of the Type C loading. Furthermore, the attenu- 

 ation-reduction feature of the loading, although it is substantial in magnitude 

 per unit length, does not have a large economic value in reducing the number 

 and cost of repeaters required in a complete carrier system. These con- 

 siderations have limited the use of Type J loading to short cables, seldom 

 more than 0.5 mile long. 



In entrance-cable installations of greater length it is common practice to 

 use line filters at the outer end of the cable to separate the ''J" frequencies 

 from the lower frequencies. The "J" frequencies are then transmitted to the 

 office over non-loaded pairs terminated at each end in impedance modifying 

 transformers. Separate cable pairs having Type C loading transmit the "C" 

 carrier channels and the voice frequencies. 



In such installations a special type of adjustable loading is used on the 

 short ''lead-in" cables from the bare open wire to the line filters, when they 

 are installed in "filter huts" at the outer end of the cable. At the filter hut, 

 this loading uses a continuously variable air-core inductance coil of the 

 solenoidal, inductometer type, with which adjustable condensers are associ- 

 ated, one on each side of the coil. This provides a variable impedance loading 

 which is adjustable for a predetermined range of impedances and for a pre- 

 determined range of lengths of lead-in cable. Long lead-in cables also require 

 a (non-adjustable) loading unit at their open-wire end. The adjustments for 

 optimum impedance-matching are made in terms of return-losses measured 

 at the open-wire end of the lead-in cable. 



Carrier Loading Apparatus 



General: The initial, experimental designs used large-size, toroidal-shaped, 

 non-magnetic cores, and finely stranded copper conductors. These coils were 

 nearly as large as the biggest coil shown in the headpiece, (page 149). Their 

 construction made it possible to secure lower effective resistances at the high 

 carrier-frequencies than could be obtained for the same total cost using the 

 best magnetic materials then available. An additional advantage was that 

 their non-magnetic cores could not cause non-linear distortion. This particu- 



