10-7] THE LINEAR ARRAY AS A FAN BEAM ANTENNA 527 



The azimuth pattern of this type of antenna is well controlled and quite 

 predictable. For a given beam shape, the aperture illumination is well 

 known, and this illumination is forced by the many slots in the waveguide 

 wall. With the vertical slots again, for example, the angle of each slot to 

 the normal is adjusted to give the required coupling, taking into account 

 the fact that the power- in the waveguide is becoming progressively less 

 in passing down the array. 



Finally, the beam is shaped in elevation to a modified cosecant-squared 

 pattern by a reflector and /or lens system adjusted to give the most power 

 at the depression angle of maximum range and the least power at the angle 

 of minimum range. For uniform display intensity. Silver states that this 

 pattern determined experimentally is approximated by csc^ 6 X cos d for 

 wavelengths from 1 to 10 cm. Pattern irregularities in this plane need be 

 reduced below the intensity resolution of the system to prevent strips of 

 light and darkness on the recorded picture. A typical system limit between 

 peaks and valleys is 2 db. 



The Loaded Resonator Array as a Fan Beam for AEW Radar. 



Closely related electrically, but differing physically from the slotted 

 waveguide array, are the low-frequency arrays used for long-range AEW 

 radar. Because the AEW radar usually requires less in angular resolution 

 and more in range than the ground surveillance radar, it is usual to go to 

 lower frequencies where much greater power-handling capacity is available 

 and atmospheric attenuation is negligible. 



Here the waveguide array is unrealistic. It is usual to find an array of 

 dipole elements or of cavity elements fed from a branching line which has 

 been termed a corporate feed structure. The term "corporate feed" describes 

 the branching nature of the feed, analogous to charts of line authority in 

 corporation management. By suitably designing this branching line, the 

 power distribution between elements in the array is established to control 

 the radiation pattern, beamwidth, and sidelobe structure — consistent 

 with the number of elements allowed within the array. The general nature 

 of the power division is similar to that described under the ground surveil- 

 lance antenna. The number of elements, however, is substantially smaller, 

 so that it is preferable to analyze the problem from the point of view of an 

 N element array rather than from the point of view of a continuous distri- 

 bution of fields in an aperture. 



The radiating element is frequently a slot, suitably backed up by a cavity 

 to limit radiation to one primary direction. In Fig. 10-8 (a) an array of 

 twenty "bow-tie slots" are shown in an AEW antenna developed by Hughes 

 Aircraft. The feed structure is visible in Fig. 10-8(b). The slots are 

 horizontally polarized. The reasons for narrowing the slots in the center 

 are multiple. This is one dimension in controlling the impedance match 



