## 3 Beam Steering/Beam Forming Using Reconfigurable Components
### MEMS Switches  
In the past decade, dynamic progress and development have been seen for reconfigurable devices in antenna and array applications. In radar and communications system applications, reconfigurable components' demand has increased day by day due to the multi-functionality and versatile adaptability of array systems. As a result, emerging technologies (such as MEMS) have been developed, and innovative solutions have recently been introduced. MEMS are an excellent solution to realize beam forming/beam steering characteristics in reconfigurable transmitarrays (RTAs). A detail Summary of beam steering reconfigurable transmitarrays with MEMS in Table 3.  
These are miniature elements of structure created with semiconductor technology. They have remarkable advantages over other reconfigurable devices, such as being small in size, low power usage, and high-level integration. Several MEMS switches reconfigurable transmitarrays have been reported in [98; 99; 100; 101; 102; 103; 104] the literature for beam steering. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ref** & \begin{tabular}{c} **Unit Cell** \\ **Technique** \\ \end{tabular} & \begin{tabular}{c} **Phase** \\ **Control** \\ **Device** \\ \end{tabular} & \begin{tabular}{c} **Frequency** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{c} **Polarization** \\ **Range** \\ \end{tabular} & \begin{tabular}{c} **Phase** \\ **(dBi)** \\ \end{tabular} & \begin{tabular}{c} **Gain** \\ **Efficiency** \\ \end{tabular} & \
\begin{tabular}{c} **Aperture** \\ **Width** \\ \end{tabular} &
\begin{tabular}{c} **Band** \\ **Seam Steering** \\ **Capacity** \\ \end{tabular} \\ \hline
[94] & stacked layers &
\begin{tabular}{c} varactor \\ diode \\ \end{tabular} & 24.6 & LP & 360\({}^{\circ}\) & – & – & 1 GHz & \(\pm\)50\({}^{\circ}\) E and H plane \\
[95] & \
\begin{tabular}{c} compact \\ varactor \\ diode \\ \end{tabular} & 5.6 & LP &
\begin{tabular}{c} 360\({}^{\circ}\), \\ 1-BIT \\ \end{tabular} & 15.7 & 33.3 & 16.7\% & 60\({}^{\circ}\) E and H plane \\
[96] & FSS &
\begin{tabular}{c} varactor \\ diode \\ \end{tabular} & 5.2 & LP & 480\({}^{\circ}\) & 20.2 & – & 13\% & \(\pm\)30\({}^{\circ}\) E and H plane \\
[98] &
\begin{tabular}{c} integrated \\ leaky wave \\ \end{tabular} & 4.8 & LP & 400\({}^{\circ}\) & 15.6 & 34 & 9\% & \(\pm\)45\({}^{\circ}\) E and H plane \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of beam steering reconfigurable transmitarrays with varactor diodes. Figure 8: **(a)** FSS proposed unit cell; **(b)** equivalent circuit diagram (reprinted with permission of [97]. Copyright 2021 [PERSON], et al.). A convenient 1-bit reconfigurable transmitarray (RTA) unit cell for X-band applications was demonstrated in [34]. A four-metal layer unit cell with two identical substrates was used in the proposed unit cell. The O-slot patch acts as a transmitter (Tx), and U-slot patch behaves as a receiver (Rx) etched on RO4003 substrate connected with a metal probe. The electronic phase shift control is obtained by MEMS switches with series bias via two bias lines printed on the substrates. The proposed element retains low insertion losses of 0.8 dB and wide 3 dB bandwidth of 16%. A bandpass filter reconfigurable frequency selective surface (FSS) was reported in [100] using MEMS switches, as shown in Figure 9. A four-legged bridge was mounted with RF-MEMS switches and DC-bias lines to control the ON/OFF mechanism of the switches. There are 909 elements and 3636 RF-MEMS switch bridges integrated to control the phase shift for the beam steering of the array. MEMS switches FSS transmitarray elements gain 3.2 GHz, 3 dB bandwidth at 30 GHz, and 99.9% accuracy is achieved during the experimental test verified for the proposed array. A 2-bit programmable MEMS, antenna filter antenna (AFA), and millimeter-wave frequency reconfigurable lens-array are presented in [104]. A AFA lens-Array integrated with 1021 switches was fabricated on two quartz wafer substrates.