A MULTIPLE UNIT STEERABLE ANTENNA 



349 



5 = 0. It drops to zero at 5 = cos~^ (1 — 1/a). For higher angles 

 nothing but minor maxima occur since "a" is small and n is large. 

 The second factor represents the steerable array pattern of the N unit 

 antennas. With ^j, adjusted for maximum response at zero angle (this 

 makes </>„ = w0/) this system is identical with the array of Nn sub- 

 units. In this case, all principal lobes of the array factor for the N 

 units, excepting the first, coincide exactly with nulls of the array 

 factor for the w subunits, and the familiar tapered distribution of 

 minor maxima associated with the array of Nn subunits results. As 

 0„ is varied to steer for other angles than 5 = 0, the coincidence of nulls 

 and undesired principal lobes no longer occurs. Since, however, the 



N 



2 



O (M — 



Of 4 



2<t)f -f 



[r\-\)<t>f -4 



rO <\l — 



n fvj _ 

 ? ? ? 

 I I I— 



I I 



Of 



2<t)f -♦ I . 1 



^1 I I I 



1 I '- 



20f 



(n-l)(Df ^ I 



(n-l)(t)f -i 



(N-1)0v 



Fig. 6^A steerable array formed by dividing the antennas of Fig. 1 into iV groups of n 

 each. The subscripts "/" and "v" refer to fixed and variable phase shifts, i, 



fixed unit antenna has only minor response beyond its first null, those 

 undesired principal lobes are adequately suppressed, and the array 

 may therefore be steered anywhere within the range from 5 = to 

 5 = cos~^ (1 — l/o), with single lobed response. As the principal 

 lobe is steered away from 5 = the maximum amplitude falls off in 

 comparison with that of the array of Nn subunits. This represents a 

 loss of signal-to-noise ratio and is to be regarded as a penalty for com- 

 promising to the extent of using fixed arrays as unit antennas. The 

 loss is appreciable, however, only if the array is steered near the 

 upper cutoff angle of the unit antenna. It remains but to select "a" 

 so that cos~^ (1 — \ja) represents the upper limit of the range, 5„,. 



