rNDUCTIVE LOADING FOR TELEPHONE FACILITIES 743 



5575 ft. (0.062 mf/mi cable) was adopted so as to have loading section 

 capacitances close to those of the open-wire loading sections. This cable 

 loading system originally known as "extra-heavy" loading, and later desig- 

 nated E248-154, was used on coarse-gauge cable conductors and had a 

 slightly lower attenuation loss than that of the then standard ''heavy" 

 loading for coarse-gauge toll cables. (In the loading designation, E is the 

 symbol for 5575-ft. spacing.) 



The loading coils used 65-permeability iron-wire cores with two short, 

 series air-gaps, to secure good magnetic stability. 



The long obsolescence of open-wire loading makes further description of 

 the E248-154 cable loading unimportant. 



Loading for Cables in Non-Loaded Open-Wire Lines 



When open-wire repeaters first came into general use, it was a common 

 situation for entrance and intermediate cables to have one group of circuits 

 associated with loaded open-wire pairs, and another group connected to 

 non-loaded pairs. In such situations, it was obviously very desirable that the 

 different types of cable loading associated with the loaded and the non- 

 loaded lines should be installed at the same cable loading points. 



Early Standard Loading Systems 



E28-16 Loading: It was found that a satisfactory, low-impedance type of 

 impedance-matching loading could be obtained by using 28 mh side circuit 

 coils and 16 mh phantom coils at the spacing used for E248-154 loading. 

 Some quantitative data regarding this low-impedance loading, designated 

 E28-16, are included in Table XV (page 746) along with corresponding data 

 on other voice-frequency loading systems subsequently standardized for 

 cables in non-loaded lines. 



M44-25 Loading: In many situations where the impedance-matching 

 requirements were not so severe, and where loaded open-wire hnes were not 

 involved in the incidental cables along with the non-loaded lines, a some- 

 what cheaper type of low-impedance loading using a longer coil-spacing was 

 utilized. Data regarding this loading, designated M44-25, are included in 

 Table XV. (It is of interest that this type of loading had been used on a 

 small scale prior to the extensive utilization of telephone repeaters.) 



From Table XV it will be noted that the two loading systems had the 

 same nominal impedance and that the better system, E28-16, had much 

 higher cut-off frequencies. A brief digression regarding the important part 

 which the cut-off frequency plays in the impedance-matching problem in the 

 upper speech-frequency band is appropriate at this piont. 



