10-3] 



PARABOLOIDAL REFLECTOR AS TRACKING ANTENNA 



517 



exactly at the focal point of the paraboloid but rather is slightly displaced 

 laterally and rotated in a circle of constant radius. This provides a motion 

 of the radiated pattern conically about the antenna centerline — hence, 

 the name, conical scan. The results of this are twofold. First, the signal, 

 upon passing through the antenna, is modulated by the spinning feed for 

 all targets except those on the centerline of the antenna. The phase of this 

 modulation gives the direction in which the antenna must move to bring 

 the target onto the centerline. Second, angular resolution is increased so 

 that a target can be located to a small fraction of the actual beamwidth. 

 Adversely, however, the greater the feed offset, the greater the beam 

 asymmetry and sidelobes that accompany the beam offset. For a typical 

 antenna, however, with the beam offset so that extreme beam positions 

 cross one another about 1^ db below the peak, these difficulties are not 

 excessive. A typical antenna of this type is shown in Fig. 10-3. This has a 

 4.0° beam from a 20-inch dish with a H db crossover. 



Fig. 



10-3 A Typical Center-Fed Parabolic Tracking Antenna with a 4° Beam 

 and a l^-db Crossover. (Courtesy, Westinghouse Electric Corporation) 



