180 BELL SYSTEM TECHNICAL JOURNAL 



of the proximity effect on crosstalk between long non-loaded cable 

 circuits which are being studied in connection with the development of 

 high frequency carrier systems suitable for toll telephone cables. 

 More specifically, the paper covers tests made to determine the in- 

 fluence of the proximity effect on the mutual inductance between 

 circuits ; data are given both for the case of two isolated non-twisted 

 pairs and for the case of pairs in a quadded 19-gauge cable. 



In cable carrier systems it is not practicable to operate like fre- 

 quency bands in opposite directions on different pairs in the same 

 cable without heavy shields between the pairs. The relatively large 

 level differences that may exist between pairs transmitting in opposite 

 directions would result in excessive crosstalk of the near-end type. 

 Like carrier frequency bands are, therefore, transmitted in the same 

 direction in a cable and the crosstalk between pairs used for carrier 

 systems is of the far-end type. 



It has been shown ^ that far-end crosstalk at carrier frequencies 

 between long non-loaded cable pairs can be considerably reduced by 

 the use of simple networks connected between the two pairs at one 

 point in their length. The crosstalk balanced out by such networks is 

 of the "transverse"^ type. Crosstalk of the interaction type varies 

 in a complicated way with frequency, and cannot, therefore, be annulled 

 by a simple network. For any two similar circuits all the elements of 

 transverse crosstalk, due to the unbalances occurring at various 

 points along the line, arrive at the same time at the far end of the line. 

 The crosstalk currents due to unbalances of the same type such as 

 capacitance unbalances arrive in the same or opposite phase (if the 

 circuits are perfectly smooth). It will be seen, therefore, that a 

 properly designed network connected at one point in the line may be 

 used to practically annul the far-end transverse crosstalk. In order 

 to design the most effective type of network for balancing transverse 

 crosstalk it is necessary to know the manner in which the crosstalk 

 coupling in any elementary length varies with frequency. 



The crosstalk coupling between two pairs in an elementary length 

 may be represented by a mutual admittance and a mutual impedance. 

 It can be considered that the voltage between the two wires of the 

 disturbing circuit drives crosstalk currents into the disturbed circuit 

 through the mutual admittance. The currents in the disturbing 

 circuit acting through the mutual impedance also cause crosstalk 



* As discussed in the Clark-Kendall paper on "Carrier in Cable" in the Bell 

 System Technical Journal of July, 1933. 



^ The various types of crosstalk are discussed in the paper on "Open-Wire Cross- 

 talk" by A. G. Chapman in the Bell System Technical Journal of January and 

 April, 1934. 



