**2.2 Techniques for reconfiguration**

Four major techniques are available to implement reconfigurable antennas, i.e., electrically reconfigurable antennas, optically reconfigurable antennas, physically reconfigurable antennas, and materially reconfigurable antennas as shown in **Figure 1**. Electrically reconfigurable antennas use electrical switches such as RF-MEMS, PIN diodes, or varactors to redistribute the surface current and alter the antenna radiating structure topology and/or radiating edges [17–20]. Optically reconfigurable antennas rely on photoconductive switching elements. Physically reconfigurable antennas can be achieved by altering the structure of the antenna [21, 22]. Finally, reconfigurable antennas can also be implemented through tunable materials such as liquid crystals and graphene as in **Figure 1** [4, 23].
