1070 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1954 



is, in fact, what is done with the E23 combination repeater. Fig. 10 shows 

 how this is accompHshed by using a center tapped Hne coil for the E2 

 repeater with the E3 connected as a shunt element to the midpoints of 

 the coil. If the coil is considered to be ideal this arrangement is equiva- 

 lent to the configuration shown in Fig. 1 1 in which the coil provides the 

 basic bridge structure: the E2 repeater is the series arm; and the E3, 

 the shunt arm. Incidentally this arrangement of E2 and E3 repeaters 

 is similar to G. Crisson's twin 21-type repeater. 



For a bridged-T structure, the image impedance equals the square 

 root of the product of the series and shunt arms and the attenuation 

 (in db) is as indicated on Fig. 11. 



If a network is to be inserted in an electrically long line without in- 

 troducing an irregularity, its image impedance must match the char- 

 acteristic impedance of the line. This would be the case for the bridged T 

 network if Za were set equal to iVZo and Zb were set equal to Zo divided 

 by N . Then the square root of the product of Za and Zb would be Zo . 



A network make up of negative impedances is designed to match a 

 line in the same way and Fig. 12 is a representation of such a structure. 

 Here, as a matter of convenience, the shunt arm is shown as an impedance 



E2 



CONVERTER 



^-vOMy-" 



LINE TRANSFORM 



)RMER I 



I 



E3 



CONVERTER 



Fig. 10 — The bridged T repeater E23. 



