# A Compact Sub-GHz Wide Tunable Antenna Design for IoT Applications
## 1 Introduction
The proposed antenna consisted of a rectangular patch along with a filter and rectifier circuitry operating at a frequency band of 2.4 GHz. The authors provided an energy harvesting solution without optimizing the antenna. The authors in [27] present a 60 GHz mm-wave antenna for the IoT. The total bandwidth of this antenna is 9.8 GHz having a peak gain of 9.6 dBi. The performance of the antenna was thoroughly evaluated to determine the radiation coverage. The given antenna design is a well-suited option for body-centric IoT applications because of its compact structure. Very few antennas for sub-GHz were reported, as it is quite challenging to design a compact antenna structure. This is because of the poor input impedance matching with the small antenna size. In [28], a detailed survey based on fractal antennas was presented. The significance of the fractal antennas for IoT application is well studies and provided a thorough literature survey on fractal antennas. Various IoT antennas are reported in the literature [29; 30; 31]. In [29], a transparent 2-element 5G MIMO antenna for sub-6 GHz applications is described. Thus, this work provided a guideline on the use of transparent for IoT applications. Similarly, a low-profile single-band and dual-band antenna were presented in [30] for IoT applications. The slot structure was optimized to operation in single as well as dual band. in [31], the authors proposed an artificial magnetic conductor-backed compact wearable antenna was proposed. Such antennas are good candidates for IoT applications. At the expense of complex antenna geometry. Comparing to other works, the proposed antenna design outperformed in terms of various features are comprehensively discussed in this article. The novelty and distinguishing features of the proposed work are described in detail as follows:  
1. To overcome the challenging requirement for IoT devices to have extended coverage with minimal power consumption, it is highly desirable to design a compact antenna for lower frequency bands. The proposed antenna structure is an attempt to meet the desired performance and standards of narrow-band IoT (NB-IoT) applications for 5G-enabled IoT devices. None of the available literature [2; 3; 4; 5; 6; 15; 2; 6; 16; 17; 18; 19; 20; 21; 22; 23], and [32; 33; 34; 35; 36] can be used for NB-IoT in sub-GHz with wide tuning capability;
2. A compact meandered loop slot-line antenna with frequency agility suitable for NB-IoT applications that operate in sub-GHz bands is proposed. The work cited in [4; 6; 15; 16; 17; 18; 19; 21; 23] covered sub-GHz bands, but the majority of them were not suitable for NB-IoT operations;3. The proposed antenna benefits from the following unique features: simple biasing circuitry, compact planar structure, and it operates over the sub-GHz bands which fit for NB operations, but most of the existing antennas are not suitable for NB operation [2; 3; 6; 16; 17; 18; 19; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36];
4. Miniaturization was achieved using a unique combination of meandered loop slot-line along with a reactively loaded slot antenna. A 173% size reduction in area is obtained using this technique. As per the authors' information, none of the works achieved such a high level of miniaturization;
5. Resonating bands' smooth variation is noticed over a very wide band from 758 to 1034 MHz with antenna dimensions of 60 \(\times\) 27 mm\({}^{2}\);
6. Moreover, a step-by-step antenna design procedure and analysis are provided to give general guidelines to understanding how to scale its design for any desired frequency band;
7. As per 3 GPP Release 13 and 15, the proposed antenna design is suitable to be utilized in the following NB-IoT bands: B-5, B-8, B-13, B-14, B-17, B-18, B-19, B-20, and B-26. None of the reported literature can cover such a large number of sub-1 GHz bands. 8. The proposed NB-IoT antenna is best suited for 5G-enabled NB-IoT devices. Section 2 of this paper discusses the proposed antenna design details and the theoretical analysis.