384 BELL SYSTEM TECHNICAL JOURNAL 



The 22-circuit also has important practical service flexibility advantages 

 in that the lines between which it works may be of radically different types, 

 provided each line has associated with it an artificial balancing line having 

 closely similar impedance-frequency characteristics. This flexibility fea- 

 ture also permits the 22-repeater to be used as a terminal repeater. Since 

 in such service the terminal impedances (switching trunks and loops) vary 

 over a wide range, operating flexibility requires the use of a compromise 

 impedance balancing network instead of one that simulates the line. In 

 consequence, the terminal repeater gains are restricted to values much 

 smaller than those obtainable with intermediate repeaters. 



In considering Campbell's proposal to use the 22-circuit mstead of the 

 21-circuit, it initially appeared that the artificial balancing lines for use with 

 loaded circuits would have to be complicated multi-section loaded lines 

 which themselves would tend to possess appreciable irregularities in their 

 own impedance-frequency characteristics. Since the technical difficulties 

 involved were critical handicaps, and because of adverse cost factors, the 

 question arose as to whether a simpler form of balancing network could be 

 devised. The study of this problem resulted in the development of a simple 

 3-element 2-terminal network, to balance a regularly loaded line terminated 

 at about 0.2 fractional section. To provide flexibility in use, a simple pro- 

 cedure was devised for buildmg out this "basic network" to match the 

 actual line termination when different from approximately 0.2 section 

 termination. For example, if the loaded line should be terminated at mid- 

 coil, i.e., with a half -weight loading coil, the basic network would be built 

 out to full-section using a shunt condenser of proper capacitance and then a 

 series inductance equivalent to the half-coil would be inserted in tandem. 

 An alternative procedure would be to build out the line at the repeater 

 station. 



The simple basic network above mentioned consists of a fixed resistance 

 equal to the nominal impedance of the loaded line, in series with an induc- 

 tance shunted by a capacitance, these elements being proportioned to shape 

 properly the reactance component of the impedance-frequency characteris- 

 tic. The special virtue of the line termination chosen for the basic network 

 design is that at 0.2-section termination the resistance component of the 

 characteristic impedance of a regularly loaded line is approximately con- 

 stant over the most important part of the working-frequency band. 



It was this possibility of constructing a simple balancing network instead 

 of a complicated loaded artificial line which made practicable the use of the 

 22-repeater circuit. 



The simple t\^e of basic network above mentioned was first used in 22- 

 repeater circuit trials on loaded circuits in the Boston-Washington cable. 

 Different proportioning of the elements was of course required for the later 



