# Mechanically Reconfigurable, Beam-Scanning Reflectarray and Transmitarray Antennas: A Review
## 4 Feed Movement
### Feed Displacement on Fully Illuminated Apertures
#### 4.1.1 Arrays with Single Focal Point  
In arrays with a single focal point, the phase distribution on the aperture of RA/TA is designed to have a parabolic distribution in order to provide a collimated radiation beam with high gain [38; 59]. It was shown in [39] that the lateral feed displacement scanning range of single focus configurations is restricted to a few beamwidths. This is due to the aberration introduced by the defocused phase center of the feed antenna and is improved by increasing F/D ratios, similar to parabolic dish reflector antennas. Furthermore, larger feed displacements introduce significant phase errors, especially for smaller values of the F/D ratio, since the reflection phases of the unit cells of the antenna array are functions of the angle of incidence. The beam scanning by lateral feed displacement for RAs with extremely short focal distances was investigated in [40] by using metasurface structures named metamirrors. Simulations and experiments demonstrated that the scanning capability of the metamirrors, despite having subwavelength focal distances, is comparable to RAs with focal distances of about several wavelengths. This is due to their deep subwavelength constituent elements which enables them to confine incident energy to extremely small focal distances. However, these RAs have high side lobe levels compared to RAs with larger focal distances. Although RAs with lateral feed displacement are easier to fabricate, displacement along a circular arc or spiral has demonstrated better performance [5]. Reference [58] proposed a metamaterial-based TA with a circular focal arc as a solution to obtain wide scanning coverage. Reference [58] also investigated the effects of the F/D ratio on the power distribution of the lens. A large F/D ratio yields a high scan resolution, low defocusing and a narrow scan coverage. In contrast, an F/D = 0.5 results in a very low aperture efficiency that is caused by low element illumination efficiency and a large taper effect. An FSS-based multibeam lens was developed in [70] by using multiple feeds placed on the  
Figure 8: Feed displacement technique in (**a**) reflectarrays and (**b**) transmitarray.  
focal plane of the lens antenna. This is in contrast to conventional feed movements along the focal arc, offering the possibility to implement a complete planar feed system.  
Single reflectors with parabolic phase distributions were investigated in early works on RAs [41]. In these RAs, the increase in spatial delay for scan angles far from broadside limited the scan range. Many attempts have been made to increase the scan range of RAs with feed displacements [42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53]. A multibeam phase matching method was studied in [42] to design wide-angle beam-scanning RAs. It was shown that optimizing the beam deviation factors and reference phase can minimize the total phase error. The RA unit cells were optimized to match the desired phase shift at multiple beam scanning angles. Measurement results showed that for scan angles from \(-45^{\circ}\) to \(+45^{\circ}\), the gain only drops 1.7 dB, which is a significant improvement in beam scanning performance compared to the conventional single-focal point RAs.