# Reconfigurable Antennas: Switching Techniques--A Survey
## 3 Optical Reconfiguration  
This type of reconfiguration technique is based on the use of photoconductive switches, made of a semiconductor material (silicon or gallium arsenide) [44; 45]. In optical reconfiguration, the photoconductive switches obviate the need for metallic wires, and bias lines are used which provide less interference and high isolation compared to electrical switches [46; 47; 48]. In addition, they exhibit extremely fast switching speeds, switching in nanoseconds. Using photoconductive switches allows one to optically control an antenna's operational bandwidths and radiation pattern. In [49], an optically reconfigurable antenna is proposed for cognitive radio applications. The fabricated prototype of the design is shown in Figure 5. Its structure contains two parts including spectrum sensing and communications parts.  
The first part is a U-shaped monopole antenna with ultra-wideband (UWB) characteristics, and the second part is an open-annulus antenna with four switches. Based on the different combinations of ON/OFF states of the four switches, the antenna exhibits four different frequency bands around 6, 7,  
\begin{table}
\begin{tabular}{c c c c} \hline
**Reconfiguration Technique** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{5}{*}{**PIN-Diodes**} & • Very reliable & • High tuning speed \\  & • Extremely low-cost & • High DC bias in ON-state \\  & • Common choice & • High power \\  & for reconfiguration & & handling capacity \\ \hline \multirow{5}{*}{**Varactors**} & • Small current flow & • • Nonlinear \\  & • Continuous tuning & • Low dynamic range \\  & • Ease of integration & • Complex bias circuitry \\ \hline \multirow{5}{*}{**RF MEMS**} & • High isolation and linearity & • High-control voltage \\  & • Wide impedance bandwidth & • Low power losses and low & • Slow switching speed \\ \cline{1-1}  & • noise figure & • Limited life cycle \\ \hline \end{tabular}
\end{table}
Table 1: A comparison of different switch components [39; 40; 41; 42; 43].  
Figure 4: (a) Configuration of the micro-electromechanical system (MEMS) reconfigurable antenna and (b) magnified view of the switch implementation (redrawn from [38]).  
8 and 9 GHz with wide and narrow bandwidths. The UWB antenna has a stable frequency response (S\({}_{11}\)) with UWB performance. Another optically reconfigurable antenna design is presented in [50] for millimeter-wave (mm-wave) 5G applications. The configuration and reconfiguration performance of the antenna are illustrated in Figure 6. The antenna is composed of a slotted-waveguide array design with a pair of photoconductive switches, as shown in Figure 6a. As shown in Figure 6b, by employing a pair of switches across different slots, different configurations of the antenna can be obtained which can increase the flexibility of the antenna's performance. The operation frequency and radiation gain of the design can be reconfigured at 28 and 38 GHz (5G candidate bands), as illustrated in Figure 6c. The radiation pattern of the designed antenna is providing a gain value of 8.0 and 9.0 dBi for 28 and 38 GHz, respectively.  
Figure 5: (**a**) Fabricated Prototype and (**b**) measured/simulated results (redrawn from [49]).  
Figure 6: (**a**) Measurement setup of the fabricated prototype (**b**) different configurations, and (**c**) measured gain results at 28 and 38 GHz, respectively (redrawn from [50]).  
Unlike the electrical reconfiguration, the photoconductive approach does not require the use of bias lines and can lie in the antenna mainboard without adding a complex system to modify the radiating structure [51; 52; 53].