# Spiral-Resonator-Based Frequency Reconfigurable Antenna Design for Sub-6 GHz Applications
## 1 Introduction  
With the advent of the Internet of Things (IoT), many researchers from different fields have focused on this technology over the past years [1; 2; 3; 4; 5; 6]. The IoT technology offers collaboration between wireless devices or components through the Internet interface. In other words, IoT provides a platform to remotely control devices via wireless protocols. Shortly, it is expected that almost all electronic communication tools are planned to be implemented for device-to-device communications with the help of the IoT [3; 4]. The major requirements of digitalization are reliability, high speed, increment of network capacity, remote monitoring, patient tracking, and low latency. Fifth-generation (5G) communication systems, Internet of Things (IoT), Wi-Fi 6E, WiMAX, and WLAN have been emerged to cope up with these aforementioned requirements [6; 7; 8; 9; 10]. It is also important to note that these advanced technologies are required to reduce the drawbacks of the existing wireless communication technologies. 5G communication systems have been utilized in many countries due to their lower latency, higher capacity, higher reliability, higher data rates, and higher mobility [3; 5]. To date, multiband, broadband, and ultrawide-band antenna structures have generally been utilized for wireless communication systems [11; 12; 13; 14; 15]. However, there are still somedrawbacks to providing high speed and low latency, and inadequacy in bandwidth. With the advancement of wireless communication technology, the need for multiple wireless services in a single device has increased significantly. Therefore, multi-purpose antennas have been attracted by scientists and researchers since the conventional antenna structures have become insufficient to respond to the requirements of wireless communication systems. At this point, designing a compact multipurpose antenna becomes a crucial task for researchers and scientists because of their complexity and restrictions. The reconfigurable antenna has received profound interest in the research of wireless communication and IoT applications. In particular, reconfigurable antennas can be used to operate on different frequency bands on a single platform. The reconfigurable antenna has been patented by [PERSON] et al. [16]. A reconfigurable antenna adjusts its operating frequency [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31], impedance bandwidth, radiation pattern [32; 33; 34], and polarization [35; 36; 37; 38] to provide the operating requirements of the host system. In addition to these adjustments, hybrid reconfigurable antenna structures alter at least two antenna performance parameters [39; 40]. Therefore, the reconfigurability feature of the antenna offers saving in costs, weight, and volume. The reconfigurable antennas can be designed by three different switching mechanisms such as electrical, mechanical, or optical [41]. Especially, the electrical switching method employs discrete and continuous frequency tuning properties. Discrete tuning can be achieved by radio frequency (RF) switches such as PIN diodes, and MEMS. Varactor diodes are utilized so as to accomplish the continuous frequency tuning feature [42]. The mechanical switching technique employs impedance loading through tunable materials such as liquid crystals and metasurfaces to achieve frequency agility. On the other hand, optically controlled frequency reconfigurable antenna is a type of antenna that can change its resonant frequency through optical means. This is achieved by incorporating optical materials or components, such as optical waveguides or photonic crystals, into the antenna structure. By controlling the optical signal, the antenna can be reconfigured to operate at different frequencies, making it more versatile and adaptable for various communication systems and application ns. The advantages of optically controlled frequency reconfigurable antennas are their low loss, high speed, and low power consumption.