A MULTIPLE UNIT STEERABLE ANTENNA 



347 



shifters would be a rather extensive undertaking. Provided a re- 

 stricted range of steering is permissible, a simpler solution is to employ 

 comparatively few large unit antennas and to let their directional 

 pattern suppress the undesired principal lobes of the array pattern. 

 Useful angles for transatlantic circuits are confined to the range from 

 zero, or some low undetermined limit, to some higher limit. In what 

 follows let 8m represent an angle a little above the useful range so that a 



a=io •0 = 1 



lAl 



lAl 



10 15 20 25 30 



EARTH ANGLE 6, IN DEGREES 



35 



Fig. 5 — Plots of the array factor for a 90-wave-length array; that of Fig. 4 used at 



twice the frequency. 



null may be located at 5,„ without imposing an excessive loss. The 

 array may then be designed so that when the first principal lobe is 

 steered at zero angle the second falls at 8,n or beyond. The question of 

 whether the array design permits the construction of a suitable unit 

 antenna in the length a\ allotted to it is considered in the following 

 paragraph. As a matter of fact, this analysis closely follows the actual 

 steps in the development of the MUSA system. 



