## 5 Reconfigurability  
Since past decades, reconfigurable antennas have gained considerable attention [33; 73; 74]. Unlike wideband and multiband antennas, the reconfigurable antennas have excellent feasibility for customers to select a frequency band on their own [74]. For facing different environments with different complexities, the response of the antenna is a key asset for flawless, smooth, and reliable communication. For several applications such as searching, rescue, and tracking, the antenna needs to have effective switching capabilities as well as performance-boosting technologies. To handle different applications scenarios, different \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**References** & **Number of** & **Total Size** & **Effective Bandwidth** & **Maximum** & **Radiation** & **Efficiency** & **Fabrication** \\
**Antennas** & **Unit Cells** & **Array (X0)** & **(S11 \(\leftarrow\) -10 dB)** & **Gain** & & **(\%)** & **Diffusion** \\ \hline
[65] & \(8\times 8\) & \(150\times 75\times 7\) & \(6.4\times 6.4\) & Ground Slot & 34-40.1 Hz & 3.3–6.0 & 24 dBi & \textgreater{}18 & 41\% & PCB \\
[66] & \(5\times 6\) & \(124\times 74\times 6\) & \(5.07\times 3.82\times 0.1\) & Ground Slot & 33.95–34.86 & 3.3–3.6 & 17.09 dBi & \textgreater{}15 & 22\% & PCB \\  & & & \(3.82\times 0.1\) & GHz & & & & Multi- \\
[67] & \(8\times 8\) & \(150\times 80\times 0.8\) & \(6.12\times 6.8\) & No & 55.4–66.5 & 3.4–3.6 & 26.1 dBi & \textgreater{}17.5 & 70\% & Layer \\  & & & \(\times\) 0.5 & & GHz & & & & PCB \\
[68] & \(4\times 4\) & \(150\times 75\times 0.8\) & \(2\times 2\times 0.1\) & Orthogonal & 25.5–40.2 & 3.3–3.8 & 16.1 dBi & \textgreater{}15 & 83\% & LTCC \\  & & & \(0.12\times 1\) & Polarization & GHz & & & & \\
[69] & \(16\times 16\) & \(150\times 75\times 8\) & \(15.7\times 16\) & Orthogonal & 71–86 GHz & 3.4–3.6 & 32.9 dBi & \textgreater{}17 & 86.60\% & Diffusion \\  & & & \(5.3\times 5.3\) & No & 29.6–30.7 & 3.4–3.6 & 22.4 dBi & \textgreater{}15 & 99\% & Machining \\
[70] & \(4\times 4\) & \(150\times 75\times 7\) & \(3.5\times 3.4\) & No & 28.8–34 GHz & 3.4–3.6 & 21.2 dBi & \textgreater{}12.7 & 70\% & PCB \\  & & & \(11.8\times 1\) & Neutralization & 86.7–10.2 & 3.4–3.6 & 23 dBi & \textgreater{}11.5 & N/A\% & PCB + \\
[72] & \(4\times 8\) & \(150\times 75\times 7\) & \(11.4\times 2.2\) & Line & GHz & & & & \\
[58] & \(8\times 8\) & \(140\times 70\times 1\) & \(5.4\times 5.4\) & No & 26.05–31.15 & 3.4–3.6 & 25 dBi & \textgreater{}11.2 & 85\% & \\  & & & \(\times\) 0.6 & & GHz & & & & \\  & & & & & & & & & \\  & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A Summary of different Antennas Performances with Different Fabrication Techniques. Figure 16: The geometrical configuration of the proposed DRA array. (**a**) 3D view. (**b**) Top view [62]. switching techniques, phased arrays with adaptive nulling characteristics, and multiple beams with low side lobe are required. For achieving such multiple tasks, different kinds of signal processing techniques can excellently provide effective solutions. A high degree of reconfigurable antenna is required for reliable communication. In [33], a low-profile simple reconfigurable antenna is proposed. Two pin diodes are used, as shown in Figure 17. These pin diodes connect the two additional stubs and the triangular monopole. The same techniques are used in Figure 18.  
The antenna has a feasibility in two different bands and when the two switching states are ON, thus the antenna operates in a single band of 3.3 to 4.2 GHz. In the case of only one switch being in the ON position, the antenna then offers the dual band ranging from 3.3 to 4.2 and 5.8 to 7.2. The antenna with low profile and compact size of 0.27\(\lambda\)0 \(\times\) 0.16\(\lambda\)0 \(\times\) 0.017\(\lambda\)0 can be used for several applications, such as for wireless systems in vehicular systems and wireless local area networks (WLAN). A bidirectional beam characteristic is achieved.