IMPROVEMENTS IN COMMUNICATION TRANSFORMERS 147 



been found in practice that this symmetry is realized most readily by 

 close coupling between the various parts of the windings. By im- 

 provements in design, the crosstalk between phantom and side circuits 

 has been reduced to values in the order of 20 milHonths in current ratio, 

 compared to values 5 times as large, formerly tolerated. 



Reduction in Impedance Distortion 



As a further consequence of the extension of carrier systems, it has 

 become necessary to match the impedance presented by transformers, 

 when terminated in the succeeding circuits, to particular values over 

 the frequency range. For example, the transposition schemes used on 

 open wire lines are such as to minimize crosstalk primarily for carrier 

 signals propagated in one direction in any line. If the transformer 

 terminating such a line does not present an impedance under load 

 equal to the characteristic impedance of the line, a portion of the wave 

 is propagated in the reverse direction, that is reflected, causing cross- 

 talk into adjacent circuits. This reflection efifect increases with the 

 vector difiference in the impedances of the transformer and the line, 

 the latter impedance approaching a pure resistance as the frequency 

 increases. 



The impedance of transformers has become increasingly important 

 where such transformers terminate filters that require a nearly con- 

 stant resistance termination to maintain proper attenuation charac- 

 teristics. Another example is in transformers terminating screen grid 

 tubes where the plate impedances are relatively very high. Here the 

 energy abstracted from the plate circuit and transmitted by the trans- 

 former is directly dependent upon the resistance component of the 

 impedance of the transformer when terminated in its load. 



Better impedance characteristics of transformers for these various 

 applications have been obtained by increasing the mutual impedance 

 and decreasing the leakage and capacitance efTects. This procedure 

 is made difficult by the necessity for meeting at the same time other 

 and newer requirements, as, for example, modulation limits. Correct- 

 ing elements consisting of capacitances and inductances usually are 

 added and are proportioned with the transformer elements in accord- 

 ance with network theory. Typical impedance characteristics of such 

 transformers are shown in Figs. 11 and 12. 



Input transformers operating into the grid circuits of vacuum tubes 

 inherently have impedances that depart widely from the nearly pure 

 resistances usually desired, because of the reactive termination pro- 

 vided by the grid circuit. This makes it necessary to add resistances 

 to serve in place of the usual load resistance. The required dissipation 



