340 BELL SYSTEM TECHNICAL JOURNAL 



described in the preceding paragraph are shown in Fig. 11. Antennas 

 A and C form one lateral array; antennas B and D form the second. 

 Since antennas C and D are further removed from the station than 

 A and B, phase correctors are inserted in the circuits from A and B 

 to compensate for the phase change in the transmission lines from C 

 and D, so that the desired signals are combined in phase. The com- 

 bination of the 2 TU fixed pads and the variable attenuators makes 

 it possible to correct for the attenuation in the transmission lines to 

 the more distant antennas. These several output currents are actually 

 combined in hybrid coils, since this method of combination prevents 

 the antennas from reacting one upon another through the combining 

 system. 



After the antennas are combined in pairs to form two lateral arrays, 

 the lateral arrays are combined in the longitudinal array. 



The change of phase of space waves between one antenna and the 

 next in an array is a linear function of frequency, and that on the 

 metallic transmission lines practically so. By using phase correctors 

 which have a phase change linear with frequency, ^^ the outputs of the 

 antennas in the array may then be combined to produce a null point 

 or a reduction in receptivity, as a result of the array, which retains 

 the same position in the directional diagram for every frequency 

 within a finite band. The longitudinal array at Houlton is designed 

 and combined to produce such an invariable null point in the direction 

 161.4 degrees relative to the axis of the wave-antenna array. At this 

 angle of incidence, it is evident that the space waves a i rive at the 

 lateral array of antennas A and C before arriving at the lateral array 

 of antennas B and D. To bring these undesired signals in phase, 

 therefore, phase shift must be introduced into the output of the first 

 of these arrays. Part of this phase shift is supplied by the metallic 

 transmission lines and part by the phase correctors in the combining 

 equipment. At this point, the undesired signals remain in phase as the 

 frequency is varied, so that a turn-over (reversal) inserted in the circuit 

 to the lateral array of antennas A and C before the array is combined 

 produces the null point which is invariable with variation of the 

 frequency. Under these conditions, the phase of combination of the 

 desired signals, incident at zero angle, varies as the frequency of the 

 desired signals varies. To minimize the effect of this change in phase 

 over the desired frequency band, the spacing of the antennas in the 

 longitudinal array must be so chosen that the desired signals combine 

 very nearly in phase at the middle of the frequency band. For that 



1^0. J. Zobel, "Distortion Correction in Electrical Circuits with Constant 

 Resistance Recurrent Networks," Bell System Tech. Jour., 7, 438; July, 1928. 



