44 



BELL SYSTEM TECHNICAL JOURNAL 



expressed in crosstalk units are current ratios times a million, their 

 product nacficb is a current ratio times a million squared. The crosstalk 

 measured would be this current ratio times a million or WacWc!,10"^. 



For small values of d, riac and Ucb vary directly as the frequency and 

 as the length d. Therefore: 



neb = NcbKd, 



WacWcf-lO-^ = NacNcbKHnQi-\ 



where Nac and Neb are the near-end crosstalk coefficients, K is the 

 frequency in kilocycles and d is expressed in miles. The measured 



TO TERMINATIONS 

 REPRESENTING 

 LONG CIRCUITS 



TO TERMINATIONS 

 REPRESENTING 

 LONG CIRCUITS 



DETECTOR 



Fig. 12 — Theoretical method of measuring interaction crosstalk coefficient. 



crosstalk WacWcblQ-^ divided by KW gives the quantity iVaciVcblO"'' 

 which may be designated as /«& and called the interaction crosstalk 

 coefficient. Values of lab determined from crosstalk measurements on 

 multi-wire lines would include the efifect of numerous tertiary circuits 

 instead of that of a single tertiary circuit as indicated by Fig. 12. 



While the interaction crosstalk coefficient hb could theoretically be 

 measured as outlined above, it is simpler to deduce an approximate 

 value from the measured value of the far-end crosstalk coefficient Fab- 

 The indirect component of Fab is due to the tertiary circuits and must, 

 therefore, be related to lab which is also due to these circuits. As 

 discussed in detail in Appendix A: 



lab — — 



K 



approximately. 



