# A Compact Sub-GHz Wide Tunable Antenna Design for IoT Applications
## 1 Introduction  
The exponential Internet of things (IoT) growth with the use of small-cell devices results in the massive expansion of the traffic loads. This demand has revolutionized the current technology into 5G, which requires ultra-low latency, a high data rate, and improved capacity. By 2030, multibillion IoT technology users are expected. The growth of sub-GHz low-cost and reliable RF front-end devices is increasing as a requirement of this massive connectivity. Such systems are good candidates as the spectrum below 1 GHz is less crowded and reliable propagation can be ensured in large infrastructures. The Global System for Mobile Communications (GSMA) industry completely described the details of 3 GPP standards for both NB-IoT and LTE-M technologies as part of the developing specifications for 5G. NB-IoT and LTE-M technologies are continuously evolving as a part of 5G as per 3 GPP. Both technologies are suitable for 5G-enabled NB-IoT devices. NB-IoT applications include street lighting, electronic appliances, sensors, actuators, and other machines that can be connected to the internet and with each other through device-to-device (D2D) connectivity. NB-IoT networks are suitable for a large number of connected devices with extended battery life. For such applications, long-range and low-power consumption communications are required to work in the sub-GHz bands. As a result, designing compact structure sub-GHz antenna systems that work over multiple frequency bands lower than 1 GHz has become a necessity. This is attainable using frequency reconfigurable (FR) antennas that have omni-directional radiation characteristics, whichextend the antenna coverage area [1]. The proposed NB-IoT antenna design is suitable for numerous applications, such as healthcare, pet tracking, kid monitoring, smart metering, parking alarms, event detectors, and home appliances. With the fast-growing IoT applications, the need for efficient power management and communication over long distances are challenging requirements. In the literature, several IoT antenna designs that address various IoT application challenges at different frequencies have been reported. The reported solutions include compact and low-profile antenna designs for multi-standard IoT antennas with wide-range frequency coverage in sub-6 GHz bands. These various antenna designs include inverted-F antennas, loop antennas, and monopole antennas. Antenna designs based on monopoles for IoT applications were presented in [2; 3; 4]. Such solutions could be appropriate for wideband antenna operation intended to cover many wireless communication systems standards. In [2], a compact (20 mm \(\times\) 30 mm) multi-band single element antenna is presented. The proposed antenna covers the following frequency bands: 1.79-2.63 GHz, 3.46-3.97 GHz, 4.92-5.85 GHz, and 7.87-8.40 GHz. This is an integrated 4G/5G antenna design to operate at sub-6 GHz and in the mm-wave band. Slots and monopoles were utilized to achieve the desired integrated operation. This is one of the pioneering works in 4G/5G integrated solutions. A compact structure monopole antenna that is resonating over the frequency band from 3 to 12 GHz, the board dimensions are 9.45 mm \(\times\) 18.5 mm was presented in [3]. In this work, a slot antenna based 4G/5G integrated solution was provided using a dual-function planar connected array. The authors in [4], a multi-band antenna was presented for IoT applications. The main antenna features were its suitability for near-field communications in both microwave frequency bands and ultra-high frequency (UHF) bands. In addition, the antenna is suitable for low-frequency bands operating in IoT applications. This work also reported a 4G/5G integrated solution based on a monopole antenna array. Several patch antenna designs for IoT applications were investigated in [5; 6]. A 2.4 GHz high fractional bandwidth compact patch antenna was presented in [5]. The proposed antenna consists of an inverse S-shaped meander line that is connected to the rectangular box with a slot.