515 



10-3] PARABOLOIDAL REFLECTOR AS TRACKING ANTENNA 



Some antennas have cylindrical 

 symmetry resulting in equal beam- 

 widths in azimuth and elevation. 

 Such a pattern is called ?i pencil beam 

 and is used for tracking and search 

 functions. Other antennas have a 

 fan-shaped beam with greatly differ- 

 ent beamwidths and sidelobes in 

 azimuth and elevation. These an- 

 tennas are used for search and 

 ground surveillance, for example. 

 Still others have essentially non- 

 directional patterns and are called 

 omnidirectional antennas. 



The concept of antenna beam- 

 width is useful in narrow-beam radar 

 antennas. Beamwidth is defined as 

 the angular displacement between Side Lobe 

 points at which the received signal 

 power is one-half of its value at the 

 center of the beam. The pattern in 

 Fig. 10-1 has a 4° beamwidth. From 

 the preceding discussion on direc- 

 tivity, beamwidth is in some man- 

 ner inverse to directivity. The exact 

 relation is not simple, however. For 



example, beamwidth becomes undefined for very low directivity where the 

 received signal power may never drop below one-half its peak intensity. An 

 approximate relation between directivity and beamwidth is given in a 

 later discussion on pattern approximations. 



6 4 2 2 4 6 



AZIMUTH ANGLE (deg) 



Fig. 10-1 Typical 4°-Beamwidth An- 

 tenna Pattern Showing the Major Lobe 

 and Two Minor or Sidelobes. 



10-3 THE PARABOLOIDAL REFLECTOR AS A RADAR 

 TRACKING ANTENNA 

 There are several different fire-control systems in common use requiring 

 some difference in antenna design. Common to them all, at least in airborne 

 radar, is a paraboloidal reflector. The chief difference between antennas for 

 a conically scanned, a sequentially lobed, a monopulse, etc. radar is in the 

 means by which energy is coupled to the reflector in order to obtain an error 

 signal for automatic tracking. For brevity, it will be necessary to select 

 one system for discussion such as the conical scan. The other systems have 

 identical aperture (that is, the reflector) problems and different but 

 analogous feed problems. Extension of this discussion and these principles 

 to other antennas can be made. 



