378 BELL SYSTEM TECHNICAL JOURNAL 



there by the factor 



1 - e-2T^ 

 2yl ' 



where 1 = 73 feet and L = 5 miles. This is the crosstalk which 

 would exist in five miles in the absence of a sheath and quads. 



The lower two curves were measured at opposite ends of the cable 

 and the difference of about 10 db between these curves and the upper 

 curve is due to the tertiary circuit effects. As might be expected from 

 the discussion of equation (21), this reduction is considerably less than 

 in the case of far-end crosstalk. 



Interaction Crosstalk Between Sections 



The methods of measuring the various types of interaction crosstalk 

 between two sections have already been discussed in reference to Fig. 2. 

 Besides showing the influence of interaction crosstalk in the summation 

 of crosstalk within a repeater section the results presented below are 

 indicative of the importance of interaction crosstalk which takes place 

 between repeater sections, that is, around repeaters, when all or only a 

 part of the tertiary is continuous at repeater points. 



Values of near-end near-end interaction crosstalk, Fnn, were meas- 

 ured between various section lengths from 73 to 12,000 feet. It was 

 found that the results are roughly independent of the section lengths 

 above 1500 feet, and curve F„„ of Fig. 11 for the crosstalk measured 

 between two 12,000 foot sections is typical. This independence of 

 length is because of the high attenuation of the tertiary circuits which 

 annihilates the effects of crosstalk in the more remote portions of the 

 sections as may be seen from equation (8) if 74 is made large. The 

 relatively unimportant contribution of this type of interaction crosstalk 

 to the summation of far-end crosstalk within a repeater section has been 

 discussed. 



Similarly, measured values of far-end far-end and near-end far-end 

 interaction crosstalk between various sections lengths were found to be 

 practically independent of length above 1500 feet. Curves F/f and 

 Nnf of Fig. 11 for the crosstalk between 12,000-foot sections are typical. 

 The far-end far-end component of interaction crosstalk has an im- 

 portant influence on the summation of far-end crosstalk within a 

 repeater section as already mentioned in the section on far-end cross- 

 talk summation. The influence of near-end far-end interaction cross- 

 talk Nnf, on the summation of near-end crosstalk within a repeater 

 section has not been very thoroughly investigated here but it is respon- 



