20 BELL SYSTEM TECHNICAL JOURSAL 



lines of out- n-iriinilar \\\k-. Ii li.is just Uvn shown that rcllection 

 takt-s pla(X' if a st-rit-s or a i)riilv;i-ti inipetlamo is inserted in a line. 

 This rolkttion will take place whether the impedance is inserted 

 al some interme«liaie |)«>int in a line or atljacent to a repeater. In- 

 serting such an im|K-<lance adjacent to a re|)eatcr would, on account 

 of this reflection, sc-riousK limit the gain which could be produced 

 by the repeater. Now insiTling an irregularity adjacent to a repeater 

 amounts to the s;»me thing as substituting a line having a different 

 im|Miiance for the line with which Uie repeater is designed to function. 

 Since any change in the impe<iance of a line connected to a repeater 

 away from the imix-dance with which the repeater is designed to 

 work is equivalent to inserting an irregularity adjacent to the repeater, 

 it is evident that it is impossible to construct a repeater system whose 

 amplification u-ill be constant in both directions and whose ^ain will 

 not be limited by irrev,ularities in the lines and by any departure of the 

 line impedance from that for which the repeater is designed. 



SiccKssi-ti. Tvi'i;s OF Ri:pi:.\ti;rs 



Two forms of rejx'ater circuit, the well known 21 and 22 t\'pc 

 circuits, have In-en develoix-d to the |M)int where they have become 

 highly important and successful parts of the telephone plant. These 

 have lieen m) completeK" descrilK-d in a pafier entitled, "Telephone 

 Refx-aters" by Messrs. Ciherardi and Jewett,' that no further de- 

 Mription will be attempti-d here. It is suflicient to point out that 

 in the case of the 22 ty|X' re|X'ater the necessiiry impedance require- 

 ments arc met b\' providing netwurks which imitate closely the 

 characteristic im|x-<lances of the two associated lines. Any de- 

 parture of the line imjKxIance from the \'aluc for which the network 

 was designi-*! or any irregularities in the line or terminal equip- 

 ment im|)ose limits on the obtainable gain in the manner described 

 alwtve. In the case of the 21 type circuit the impedance require- 

 ments are met by putting the repeater between two similar lines 

 whose im|K-dances i>.dance each other. 



.\nother ty|x- of re|Kater circuit, called the booster circuit, was 

 mi'niione<l in the pa|X'r just referretl to. This circuit does not de- 

 ixnd ufxin im|H-<lance b.ilance in the simc way as the 21 and 22 

 tyjx- < ircuits and it is capable of giving two-way amplification but 

 its iN-rformaiUT is even nmre seriously affected by impedance devia- 

 tions in the lines than the latter circuits. The booster form of re- 

 |xMtor <irriiit has niit yet pro\e<l useful in a commercial way. 



' rtoccttlinic* o( the .Vnicricin Inslitiiti: of ticclrical linKiiiccrs, 1919, page 1255. 



