OPEN-WIRE CROSSTALK 57 



unimportant as tertiary circuits, that is, two pairs having small 

 crosstalk between them usually contribute but little to the crosstalk 

 between one of these pairs and any third pair. In some cases, however, 

 this effect is important. On some lines certain pairs may be transposed 

 for carrier operation and other circuits on the line for voice frequencies 

 only. A combination of the two kinds of circuits may have large 

 crosstalk between them at carrier frequencies and rnay contribute 

 appreciably to the carrier frequency crosstalk between the pair 

 transposed for carrier operation and some other pair also so transposed. 

 Far-end type unbalances which take account of transpositions in a 

 tertiary circuit must, therefore, be calculated. This can be done by 

 following the same general method discussed in connection with Fig. 17. 

 From the discussion of coefficients it follows that the far-end coefficient 

 for use in computing such a t^'pe unbalance will be: 



^jf^E^ io-«, 



where Nac and Neb are the near-end crosstalk coefficients for the 

 combination of disturbing circuit and tertiary circuit and the combi- 

 nation of tertiary circuit and disturbed circuit. Since these circuit 

 combinations involve recognized transmission circuits, their near-end 

 coefficients will be available since they must be measured or computed 

 in order to compute the near-end crosstalk. 



If a parallel between two circuits is divided into a large number of 

 segments by transposition poles there is a wide variety of transposition 

 arrangements which may be installed at these poles. It is, therefore, 

 a complicated problem to devise charts and tables in reasonable 

 numbers which will cover all the possible type unbalance values for 

 the various transposition arrangements over a wide range of fre- 

 quencies. This is particularly true in the case of far-end type un- 

 balances since the type unbalance is altered by transposing both 

 circuits at the same points and it is necessary to work out a type 

 unbalance for each combination of transposition arrangements which 

 may be used in two circuits. In the case of near-end crosstalk a 

 number of different transposition arrangements will have the same 

 type unbalance since only the relative transpositions need be con- 

 sidered. 



The circuit capacity of a line may be increased by the use of phantom 

 circuits (generally when carrier-frequency systems are not involved) 

 which must, of course, be transposed to avoid noise and crosstalk. 

 The crosstalk between phantom circuits may be calculated in a manner 

 similar to that for pairs. The calculation of crosstalk between side 



