# Mechanically Reconfigurable, Beam-Scanning Reflectarray and Transmitarray Antennas: A Review
## 2 Element Movement
### Reconfigurable Resonant Elements
#### 2.1.2 Rotating Elements  
The concept of phase variation using rotating elements was first demonstrated in a radio telescope where mechanically rotatable spiral antennas were used as feed elements for a parabolic reflector [14]. Building upon this concept, the authors of [15] investigated a linear array of patch antennas and demonstrated the scanning capability by mechanically rotating the elements in the array. The drawback of this approach is its need for rotational symmetry, thus, limiting this technique to circularly polarized (CP) RAs. The interaction between CP elements of a RA and the incident wave and the resulting phase shift is thoroughly discussed in [16]. Simply speaking, the incident and the reflected signals will have the same type of circular polarization if the element can reflect the incident wave with the same phase (reflection coefficient = +1) in one direction and with reversed phase (reflection coefficient = -1) in the orthogonal direction. The reflected wave is phase shifted by \(2\alpha\) if the element of the RA is rotated by \(\alpha\) degrees. Using this technique, one can achieve full phase compensation (\(360^{\circ}\) phase shift) by rotating the CP element \(180^{\circ}\) around its axis. This could be a fruitful area for further research, integrating micromotors beneath the RA to create mechanically reconfigurable RAs with favorable features. Some of the related research are discussed below as illustrative examples.  
Figure 2(a) shows a RA consisting of a circular patch having a center and two diametrically opposed shorting pins [17]. The fundamental \(TM_{11}\) mode is excited beneath the circular patch antenna and phase shifting of the incident CP wave can be achieved by mechanically rotating the element. Reference [16] discusses the phase-shifting capabilities of different resonant elements such as a single rotating dipole, multiple dipoles and rotating stubs. Similar to the works discussed in Section 2.1.1, the RAs consisting of such elements have limited frequency bandwidths as they rely on the resonant features of the elements. The conventional techniques to broaden the frequency bandwidths of the elements, such as stacked patch antennas and tightly coupled elements, can consequently broaden the RA's frequency response. For example, a recently proposed RA, shown in Figure 2(b), uses multiple concentric resonators to broaden the element's frequency bandwidth [4]. This is  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**References**} & \multirow{2}{*}{**Methodology**} & **Operating Frequency \({}^{*}\)** & **3 dB Beam** & \multirow{2}{*}{**Aperture Efficiency**} \\  & & **(GHz)-Bandwidth** & **Scanning Range** & \\ \hline \multirow{2}{*}{[9]} & Resonant & & & \\  & elements—dimensions & 4.7–4.9, (4.7\%) & \(\pm 60^{\circ}\) & 48.6\% \\  & adjustment & & & \\ \hline \multirow{2}{*}{[4]} & Resonant & & & \\  & elements—rotating & 7.5–9, (17\%) & \(\pm 60^{\circ}\) & 51.8\% \\  & elements & & & \\ \hline \multirow{2}{*}{[13]} & Non-resonant elements & 90–110, (20\%) & NA & 50.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparative analysis of mechanically reconfigurable RAs using reconfigurable elements.  
Figure 2: Height-tunable patch antenna unit cell of the RA in [9] with adjustable middle layer.  
also an example of a mechanically reconfigurable RA which includes a micromotor beneath each individual element. Full phase compensation is achieved by rotating the elements.