NEGATIVE IMPEDANCES AND THE TWIN 21 -TYPE REPEATER 511 



The even numbered harmonics from the two 21-type circuits are 

 not annihilated, however, but combine to form an echo which travels 

 toward the speaker, and this echo must not be permitted to become 

 too strong. It would be possible to eliminate such echoes by using 

 the push-pull connection in the amplifier of each 21-type circuit, but 

 this, of course, would double the number of tubes required. 



The Twin 21 -Type Phantom Group of Repeaters 

 Three twin 21-type circuits may be connected with the wires of a 

 phantom group so that voice-frequency waves traveling over either 

 side circuit or the phantom may be amplified and low-frequency signals 

 may be passed through the apparatus. This arrangement does not 

 break up the phantom group and requires no phantom repeating coils 

 or compositing apparatus. It cannot be used, of course, at points 

 where it is necessary to separate the side and phantom circuits. A 

 simple diagram of the phantom group of repeaters is shown in Fig. 16. 

 Each 21-type circuit with its own hybrid coil, amplifier, network, etc., 

 is indicated by a small square. A twin 21-type circuit is connected 

 in tandem with each side circuit. The repeater in the side ^i com- 

 prises a line hybrid coil SiHy, a series 21-type circuit SiS and a bridged 

 21-type circuit SiB as indicated. The side circuit 6'2 is similarly 

 equipped. Repeating coils Ri and Ro are shown between the bridged 

 21-type circuits and the bridge terminals of the line hybrid coils in the 

 side circuits. Taps are provided at the mid-points of the line windings 

 of these coils by which the 21-type circuit PB is bridged across the 

 phantom circuit. While separate transformers are shown in the 

 diagram to provide the connections for the phantom bridged 21-type 

 circuit, they might be omitted if the side circuit bridged 21-type cir- 

 cuits SiB and 52^ are each so arranged as to provide a tap which is 

 symmetrical with respect to the line wires. This arrangement, how- 

 ever, introduces additional possibilities of unbalance with the resulting 

 noise and crosstalk which are avoided by the use of the coils Ri and i?2- 

 The phantom series 21-type circuit PS is coupled effectively in 

 series with the phantom circuit by means of the phantom line hybrid 

 coil PHy. This is a special transformer having eight carefully bal- 

 anced sections in the line winding and a drop winding to which the 

 21-type circuit is connected. Two of the line winding sections are 

 connected in series aiding with each line conductor, one on each side 

 of the side circuit line hybrid coil. The sections in series with the 

 several line wires are so poled that they are non-inductive to waves 

 traversing the side circuit, but they are inductive to waves traversing 

 the phantom, thus producing the desired coupling. 



