NEGATIVE IMPEDANCES AND THE TWIN 21-TYPE REPEATER 503 



from the booster. For similar reasons, condensers must be connected 

 in series with the shunt booster when it is bridged on the line. These 

 devices, particularly the filters, shift the phase of the amplified waves, 

 and modify the negative impedances so that the gain varies with 

 frequency in the useful range to a greater extent than is the case with 

 the 21 and 22-type repeaters and the echoes are increased. This 

 variation of gain is due to the fact that the booster, in effect, super- 

 imposes an amplified wave upon the wave that would exist if the 



30 



20 



< 



o 



o 

 z 

 o 



S 10 

 in 



Fig. 13 — Singing gains of boosters and repeaters. 



booster were removed. The received wave, being the resultant of 

 these two waves, varies with the phase angle between them. 



It should also be noted that boosters do not avoid the problem of 

 matching line impedances or the difficulties due to impedance irregu- 

 larities in the line. To obtain a gain that is constant over a wide 

 range of frequencies, the negative impedance must be fitted to the 

 line impedance over this range and there must be no large irregularities. 

 It will be shown below that most of the difficulties described above may 

 be avoided by using a series and a shunt booster in combination. 



