# Mechanically Reconfigurable, Beam-Scanning Reflectarray and Transmitarray Antennas: A Review
###### Abstract
[[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: Applied Sciences  
**2021**, _11_, 6890. [[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: Applied Sciences  
**2021**, _11_, 6890. [[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: Applied Sciences  
**2021**, _11_, 6890. [[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: Applied Sciences  
**2021**, _11_, 6890. [[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: Applied Sciences  
**2021**, _11_, 6890. [[https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890)]([https://doi.org/10.3390/app11156890](https://doi.org/10.3390/app11156890))  
Footnote †: journal: journal: Applied Sciences  
**2021**, _11_, 6890 In order to discuss tunability for beam-scanning in RAs, Figure 1 illustrates the geometry of a RA including an illuminating feed antenna and an aperture of reflecting elements. To point the reflected beam in a given direction, one needs to impose a particular distribution of re-radiated (i.e., reflected) field phases across the array elements. The phase (\(\Phi(x_{i},y_{i})\)) of the re-radiated field from each element consists of two components: (i) the spatial delay due to the electrical distance ( \(k_{0}R_{i}\)) between the phase center of the feed antenna and the position of the element and (ii) the intrinsic reflection phase of the element (\(\Phi_{R}(x_{i},y_{i})\)). These two components are the first and the second terms of the equation below, respectively:  
\[\Phi(x_{i},y_{i})=-k_{0}R_{i}+\Phi_{R}(x_{i},y_{i}), \tag{1}\]  
where \(k_{0}\) represents the free-space wavenumber and \((x_{i},y_{i})\) shows the location of the element in the array. One may obtain a beam-scannable RA by adjusting either or both of the two phase contributors. The same discussion holds for TAs with the difference that the reflection phases of elements are replaced by the transmission phases. The second term in (1) represents the element-level phase control that many reconfigurable RA research studies have attempted to achieve electronically [6; 7; 8]. Electronic adjustment of the constituting elements' reflection phases have been studied using pin diodes, varactor diodes and micro-electromechanical systems (MEMS). These electronically reconfigurable RAs have fast response times but encounter two major challenges. The first challenge is the amplitude loss of the element. This loss, typically ranging between 0.5 and 2 dB, is attributed to the presence of the phase electronic switching/tuning devices in the unit cell design. The other challenge is the phase quantization error, especially in RAs using switching-type elements. For a one-bit RA having two discrete phase states, the quantization error results in about 3 dB gain loss [9]. Alternatively, one can mechanically adjust the phase distribution of a RA. This can be conducted either by changing the relative distance of the feed antenna to the array elements on the aperture or by mechanically repositioning (tuning) the constituent elements. Generally, this mechanical tuning has a slower response time than the electronic reconfiguration due to limits on the speed of mechanical actuators. On the other hand, mechanical tuning often does not disturb unit-cell design. This minimizes a major contributor to element loss that manifests in electronically reconfigurable RAs. Actuators and micromotors are mostly integrated beneath the reflector so as to not block the RA's aperture.