# Mechanically Reconfigurable, Beam-Scanning Reflectarray and Transmitarray Antennas: A Review
## 3 Array Movement
### Risley-Prism-Based Antennas  
Recently, mechanically beam-steerable antennas based on the Risley prism have attracted attention due to their simplicity and good beam scanning performance [20; 21; 22; 23; 24; 25; 26]. In the optical regime, a Risley-prism system typically consists of a pair of rotatable, wedge-shaped prisms used in conjunction with a well-collimated beam of light (e.g., a laser beam) [27; 28]. The beam can be steered when the prisms are rotated with respect to one another. Later, the Risley prism concept was extended to the microwave regime. Different configurations of Risley-prism-based beam-steering systems were proposed and implemented [20; 21; 22; 23; 24; 25; 26]. Figure 5 shows three different configurations of Risley-prism-based beam-steerable antenna systems used at microwave frequencies [20; 21; 22; 23; 24; 26]. Figure 5a shows the architecture of the first reported beam-steerable antenna that employed the Risely prism concept. Here, the two wedge-shaped prisms used in optical systems were replaced by a pair of planar, metasurface and effective prisms that were illuminated by a feed antenna. The spherical wave radiated by the feed antenna is collimated by the lens placed below the two flat prisms. The relative rotational displacement of the two flat prisms, Prism #1 and Prism #2, provides beam steering in both azimuth and elevation in the upper hemisphere of the outgoing aperture. This design configuration has been demonstrated at Ka-band with a \(\pm\)75\({}^{\circ}\) scan range [20]. The structure shown in Figure 5b combines the collimating lens with the bottom flat prism. In this manner, the single structure not only collimates the spherical wave radiated by the feed antenna but also provides the corresponding phase gradient needed from the first prism for beam steering. This architecture of a Risley-prism-based beam steering antenna has been used in [21; 22]. Reference [21] presented a high-power-capable beam steering lens antenna working at X-band with a \(\pm\)20\({}^{\circ}\) beam scan range. In [22], a planar Risley-prism-based system was reported to generate a quasi-nondiffractive beam at Ka-band with a \(\pm\)54\({}^{\circ}\) beam scan range. Due to the use of the horn antenna that must be placed at some distance from the input aperture of the collimating lens/first prism, these approaches still have a large overall profile [20; 21; 22]. Such a high-profile is problematic in applications that require a compact geometry. Therefore, this high overall thickness motivated the development of the low-profile Risley-prism-based beam-steerable antenna architecture shown in Figure 5c. Here, the horn antenna was replaced by a planar antenna or an array to achieve a low-profile beam steering system [23; 24]. In [23], a Fabry-Perot resonator antenna was used to feed the two prisms to steer the main beam with \(\pm\)51\({}^{\circ}\) range. This antenna system provides a peak gain of 19.4 dBi with a corresponding aperture efficiency of 19.2%. In [24], a dual-linear-polarized continuous transverse stub array was implemented to illuminate the two prisms and achieved a \(\pm\)40\({}^{\circ}\) beam scan range with an aperture efficiency of 17.8%.