518 ANTENNAS AND RF COMPONENTS 



Quite analogous to the conical scan antenna are the sequentially lobed 

 and the simultaneously lobed tracking antennas. Where the conical scan 

 beam makes a complete circle in deriving error signal, these antennas look 

 only up-down and left-right. With the sequentially lobed tracker, this is 

 accomplished in a programed sequence (as the name implies), while in the 

 simultaneously lobed tracker, multiple receiving channels measure up-down 

 and left-right tracking errors simultaneously. Both of these antennas have 

 four stationary feeds suitably offset from the focal point of the reflector- and 

 derive error information by signal comparison from the several feed points. 

 Monopulse systems have already been discussed in Paragraph 6-3. 



10-4 SYSTEM REQUIREMENTS FOR RADAR ANTENNAS 



Up to this point, antenna characteristics such as gain, directivity, etc. 

 have been discussed as referred principally to a parabolic reflector. These 

 are directly measurable antenna characteristics that describe not only radar 

 antennas but many others as well, regardless of the way in which they are 

 used. This is the vocabulary of the antenna trade. From the point of view 

 of system requirements as derived in Chapter 2, however, it is necessary 

 to start from different characteristics — maximum range, resolution, 

 tracking accuracy, etc. The required antenna parameters are then derived. 



In the search function, the relation between the antenna and radar is 

 straightforward. The antenna gain may be used directly in the radar range 

 equation, (Paragraph 3-2) 



^' - iMv '''-'^ 



where R is the maximum useful range to a target 



Pt is transmitted power 



S,„in is the minimum detectable signal 



G is antenna gain 



X is wavelength 



a is target back-scattering cross section. 



This equation shows the importance of antenna gain in determining the 

 useful range of a radar as compared with transmitted power, for example. 

 A second characteristic of the search function of radar is angular reso- 

 lution. This is defined as the minimum angular separation of two point 

 targets at the same range. As shown in Paragraph 3-6, resolution is a 

 complicated function of beam shape as well as the way the radar circuitry 

 handles the information; but at least to a rough degree, angular resolution 

 is about one beamwidth. 



