A MULTIPLE UNIT STEERABLE ANTENNA 



341 



enough to be substantially isolated from each other. Choosing an- 

 tenna No. 1 for reference and considering a plane wave arriving at an 

 angle 8 with the axis of the arra3^ it is clear that the output of No. 2 

 will add in phase with that of No. 1 if the phase advance cj) is made 

 equal to Iwac/v — lira cos 8, where c = velocity of light and v = the 

 phase velocity of the transmission lines. Similarly, the output of 

 No. 3 will add to that of No. 1 and No. 2 if its phase is advanced 2(f), 

 etc. If the spacing, a\, is sufficient there will be other angles for which 

 the N outputs add in phase; at intermediate angles the outputs 

 interfere with the result that zeros and minor maxima occur. JBy 

 properly designing the unit antenna the undesired maxima may be 

 suppressed. 



TRANSMISSION LINES 



Fig. 1 — A steerable antenna array using variable phase shift.-: 4>, 2<A, 30, etc. 

 The transmission lines indicated by broken lines are assumed to be of zero length; 

 a is the spacing in free space wave-lengths. 



In the Holmdel experimental system the unit antennas are of the 

 rhombic type. An aerial view of the six antennas, which are located on 

 the great circle through England, is shown in Fig. 2. These six an- 

 tennas, combined as in Fig. 1, yield polar directional patterns such as 

 those shown at the top of Fig. 3. The solid line pattern and the dashed 

 line pattern correspond to different values of the phase shift 4>. The 

 multiple phase shifts of Fig. 1 are obtained by gearing the phase shifters 

 to a common shaft which enables the directional pattern to be steered 

 simply by rotating the shaft. 



Thus far we have discussed the problem of sharp steerable direc- 

 tivity from the point of view of a single plane wave, whereas it is well 



