OPEN-WIRE CROSSTALK 221 



In Fig. 29 the output-to-output far-end crosstalk in each repeater 

 section is indicated by /i to /e- The transmission path through any 

 one of these crosstalk couplings is (for like circuits) a loss 10 db greater 

 than the value of the coupling expressed as a db loss. With the 

 repeater arrangement of the figure, the far-end crosstalk paths are 

 attenuated by 10 db while the near-end crosstalk paths are not 

 attenuated. Furthermore, the far-end crosstalk paths ordinarily 

 introduce greater losses than the near-end paths. With greater 

 spacing between repeaters, the near-end crosstalk is amplified but the 

 far-end crosstalk (for like circuits) is still attenuated by the net loss 

 of the circuits. At a given frequency the near-end crosstalk between 

 such "two-wire" circuits is, therefore, much greater than the far-end 

 crosstalk. 



Review 



Evidently the problem of keeping crosstalk between open-wire 

 circuits within tolerable bounds is by no means a simple one. As we 

 have seen, the work begins with consideration of complete circuits 

 (telephone, program transmission or carrier telegraph) which may be 

 hundreds or even thousands of miles long. The total crosstalk 

 allowance for such long circuits must first be broken down into allow- 

 ances for the various sections of line between repeaters and then into 

 allowances for the individual transposition section, these individual 

 sections ranging from less than 1/4 to about 6 miles in length. 



Then bearing in mind that irregularities in pole spacing and in wire 

 configuration set limits to crosstalk reduction which it is not practicable 

 to overcome by transpositions, the crosstalk designer determines by 

 computation whether, when considering these irregularity effects 

 alone, the crosstalk requirements for the individual transposition 

 sections can be met. If these requirements can not be met he must 

 either have the general circuit layout altered so that, for example, 

 the repeater gains will be more favorably disposed from the standpoint 

 of crosstalk, or he must alter the pole head configuration so that the 

 electrical separation between the circuits will be increased. 



Having obtained an overall circuit layout and a configuration of the 

 wires which makes it possible to attain the desired overall crosstalk 

 results, the design of the transpositions proper is undertaken. In 

 this work the transposition designer makes every effort to keep the 

 number of transpositions at a minimum. He does this partly to save 

 money but more particularly because he recognizes that more than 

 enough transpositions do harm rather than good by increasing the 

 number of pole spacing irregularities. 



