NEGATIVE IMPEDANCES AND THE TWIN 21 -TYPE REPEATER 507 



If the incoming wave arrives over the Hne E, the action is the same 

 except that the direction of the current los due to the series 21 -type 

 circuit is reversed with respect to the current Ion which causes the 

 amplified wave to travel toward the W end of the line. 



Fig. 15 — The ideal twin 21-type repeater. 



If the internal connection of both 21-type circuits is changed the 

 direction of both output currents will be reversed. The action of the 

 twin 21-type circuit will not be altered except that the phase of the 

 amplified wave will be reversed. Changing only one of the 21-type 

 circuits, however, will cause the amplified wave to travel toward the 

 speaker as a powerful echo and will prevent transmission toward the 

 listener. 



Effect of Phase Shift 



In the foregoing description the discussion has been simplified by 

 assuming ideal transformers and 21-type circuits. It is sufficient, 



