## 2 Phased Array Radar (PAR)
### Radiation Pattern Control
#### 2.2.3 Implementation  
Although the use of PAs in radar systems has been consolidated for quite some time, their construction can be rather complex, requiring many elements and connections, depending on the application requirements, such as the main beam pointing accuracy, reducing sidelobes or nulls level of the radiation pattern [35]. Therefore, the electronic control of the radiation pattern has proven to be one of the main challenges in the design of antennas for civil and military radar systems [12,15,48]. The PAs designer analyzes details of the array and constraints imposed by the radar system to try and optimize the design, whereas the analysis of a PA is much more complex than that of a passive antenna  
Figure 5: (**a**) Bearn radar [46]. (**b**) Cobra Dane radar [47].  
Figure 6: Tracking network with phased array radars (PARs) and parabolic reflector radars (PRRs).  
positioned mechanically, due to variation of performance characteristics in the array according to the scanning angle [10]. Also, in order to control radiation pattern, PAs are composed of several elements fed with variable amplitude and phase, or through a time delay in each element excitation [10,35].  
Determining weights of amplitude and phase of PA elements can be made by several offline or online (real time) optimization methods with various kinds of adaptive algorithms for controlling the radiation pattern [25,28,36,44,49]. One must highlight the results of offline calculation of sidelobe suppression from Electronic Sidelobes Steerable Radar (ELRA) [41]. In offline mode, when a pointing angle is specified, there is a previously calculated table by the algorithm with the combination of signals corresponding to that angle [48]. Once in real time, there is no such table and the algorithm is executed online during the tracking. It is common for PAs to possess many elements, such as: Hard Point Demonstration Array Radar (HAPDAR) with 4300 elements [35], Multi-function Electronically Scanned Adaptive Radar (MESAR) with 900 elements [40], and ELRA with 300 elements on the transmitter and another 768 elements on the receiver [41]. Nevertheless, a possible difficulty for PAs is the limited search region, since, as a general rule, the maximum scanning angle of a planar array is 60 \({}^{\circ}\) in broadside, which limits the face scanning of the planar array to 120 \({}^{\circ}\)[9,41].  
Thus, alternatives are necessary for a complete azimuth scan. The first alternative, used on fighter aircrafts, is a 3D array conformed to the fuselage. Its elements are active in all directions at the same time, providing a wider visual field and a greater scanning speed. Such implementation, however, can be quite costly [9]. Another alternative is a planar array on a mechanical rotating base, which would be a cheaper solution, despite inherently bringing the disadvantages of electromechanical devices, such as lower scanning speed and higher possibility of defects [43]. An intermediate alternative is to use four planar arrays, forming the faces of a pyramid, in order to achieve the best balance between cost and scanned area. This solution was built by the United States for air defense, both on ground and ships [42]. In Figure 7, there is an outline of the two last mentioned solutions.