NEGATIVE IMPEDANCES AND THE TWIN 21-TYPE REPEATER 499 

 and the return loss ^ due to the irregularity is: 



5= 20 1ogio(l + ~) • (22) 



If, now, Zs is made a negative impedance of the series type smaller 

 in magnitude than 2Zo, the potential difference between its terminals 

 reverses in sign, the current at the point of insertion increases, the loss 

 becomes a gain and the reflected wave reverses in sign. As Zg ap- 

 proaches — 2Zo, the transmitted and reflected waves increase until 

 singing occurs; but the reflected wave is always smaller than the 

 transmitted though they approach each other as the gain increases. 

 Such a booster, therefore, causes a smaller returned wave or echo than 

 an ideal 21 -type repeater circuit working between ideal line impedances 

 which always returns a wave toward the source which is equal to that 

 transmitted toward the receiver. 



The series booster would also operate if Z^ were made a shunt type 

 negative impedance greater in magnitude than 2Zo, but in this case the 

 current at the booster and the wa\-e traveling toward the receiver 

 would be re\ersed in phase and the reflected wave or echo would be 

 greater than the wave traveling toward the receiver. This arrange- 

 ment would, therefore, give greater echoes for a given gain than a 21- 

 type repeater. The curves of Fig. 12 show the relation between the 

 return loss and transmission gain for these boosters in comparison with 

 a 21 -type repeater. 



The echoes referred to above are, of course, those inherent in the 

 operation of the devices described and would not occur if a 22-type 

 repeater were used with perfect lines. Echoes due to line irregularities 

 would be amplified to the same extent by boosters as by any other 

 type of two-way repeater giving the same gain. 



Shunt Booster 



Fig. 11 show^s an impedance Zb bridged across the line. The effect 

 of this impedance is to reduce the wave tra\eling toward the receiver, 

 causing a transmission loss, 



L= 20 1og:o(l+^J, (23) 



and causing a reflected wave to return to the source with a return loss, 



5= 20 1ogio(l+^) • (24) 



^ When a wave is partially reflected at an irregularity the relation between the 

 reflected part and the original wave, expressed in decibels, is called the return loss. 



