10-7] 



THE LINEAR ARRAY AS A FAN BEAM ANTENNA 



525 



Fig. 10-6 A Perspective View Showing Schematically the Radiation Pattern of 

 Side-Looking Fan Beam for Ground Surveillance Radar. 



surface. This type of radiation pattern is shown in Fig. 10-6 for a side- 

 looking ground surveillance system. 



The pattern in azimuth is a fraction of one degree to give the required 

 angular resolution. Targets are resolved in elevation on a time basis. 

 That is, for a given altitude, objects directly beneath the antenna are at 

 minimum range, and those on the horizon are at maximum range. The 

 antenna will be designed for a specific angular coverage in elevation which 

 is less than these two extremes — the actual pattern being so shaped in 

 elevation as to give uniform display brightness within the design range. 

 Mathematically speaking, such an antenna has, in elevation, a power 

 density pattern which varies as the square of the cosecant of the angle 

 between the horizontal and the slant range line-of-sight. Some antennas 

 of this type cover as much as 70° in elevation in this manner. 



The antenna, then, must be swept in azimuth. One usual system — 

 already described in Chapter 1 — is to mount the antenna rigidly to the 

 airframe for side looking and allow the forward motion of the aircraft to 

 sweep the coordinate as the aircraft flies straight and level over the ground 

 to be studied. In this event, a storage or photographic means will be 

 required to give a coherent picture of the slowly unfolding panorama. 



For such a fan-shaped beam as described, it is almost impossible to feed 

 a rectangular parabola as the radiating aperture. In addition, a rectangular 

 aperture does not have the optimum mechanical form for the application. 

 As a result it is usual to approximate a continuous antenna aperture by a 

 discontinuous array of slots in a waveguide wall. 



Along the long aperture, illumination is first chosen based upon the gain 

 and sidelobe requirements for a given aperture length. The type of slot 

 used in the guide wall determines the antenna polarization. Longitudinal 

 slots in the top wall of the waveguide result in a polarization in the plane of 



