LOADING FOR TELEPHONE CIRCUITS 263 



loading is designed for oi-dinar\- speech transniission. In order to 

 transmit the high frequency carrier currents over the side circuits, 

 it is necessary to have the side circuit lo^iding coils spaced much 

 more closely than for the ordinary voice frequency loading coils in 

 the phantom circuit. On this account the theoretically best loading 

 points for the carrier circuits frequently occur at places where it is 

 inconvenient to locate the loading coils. The actual loading sections 

 in such cases are made shorter than the theoretical lengths, and the 

 deficiencies in loading section capacitance are remedied by addmg 

 lumped capacitances in the form of "building-out condensers." 

 Recently, special types of stub cable designed specially for building 

 out purposes have come into use as substitutes for building-out con- 

 densers. 



Loading Coils. The design of the coils used in the E-28-16 and 

 M-44-25 loading systems is generally similar to the toll cable loading 

 coils having 35-permeability compressed powdered iron cores already 

 described. The loading coils used in the E-248-154 loading system 

 are larger coils of the air-gap type 65-permeability ware core con- 

 struction listed in Table VII. 



As regards the carrier loading system, CF:-4. 1-12.8, since this in- 

 \olves the transmission through the loading coils of frequencies up 

 to 30,000 cycles or somewhat higher, special coil designs are required. 

 The coil which loads the audio frequency phantom circuit, aside from 

 being specially balanced for association with the side circuit coils, 

 is generally similar in construction to the compressed powdered iron 

 core phantom coil for toll cables. 



The side circuit coil, however, is used for loading the high fre- 

 quency circuit, and more severe requirements are, therefore, imposed 

 on it owing to the multi-frequency transmission. Ordinarily the cir- 

 cuits are equipped to provide three or four carrier telephone channels 

 or 10 carrier telegraph channels over a pair of wires, in addition to the 

 ordinary audio frequency telephone and grounded telegraph channels. 

 The primary added requirements as regards the loading coils are 

 freedom from intermodulation between channels, and low energy 

 losses at carrier frequencies. The most satisfactory solution as regards 

 freedom from magnetic modulation is the avoidance of the use of 

 ferro-magnetic core materials. The side circuit loading coils were, 

 accordingly, designed as toroidal wood core coils, with finely stranded 

 copper windings in order to limit the edd>-current losses. Data 

 regarding resistance-frequency characteristics are included in Table 

 XIII. 



