# A Compact Sub-GHz Wide Tunable Antenna Design for IoT Applications
## 1 Introduction
This technique is utilized to reduce the antenna dimensions. The board dimensions are 40 mm \(\times\) 1.6 mm. Both parasitic patch and capacitive loading were used to achieve a gain of \(-\)0.256 dBi and an overall efficiency of 79%. The work in [6] presented a miniaturized patch antenna design that operates in sub-GHz. A 95% miniaturization was achieved, compared to a conventional patch antenna, over the frequency band 805 to 835 MHz. The use of high folding, slots, and slits with inductive loading, utilizing vias, helped in achieving this excellent miniaturization. The dimensions of the antenna board with the elevated structure are 30 mm \(\times\) 31.84 mm \(\times\) 4.37 mm. Other antenna types for IoT applications are reported in the literature. For example, the following: monopole antennas loaded with inverted-L shaped stubs in [7], 3D-printed antennas [8], loop antennas [9], and glass frame antennas [10]. The solutions provided in [7; 8] are based on monopole antennas and non-planar antenna structures. In [9], a 2.45 GHz transparent conductor antenna for IoT applications was presented. The variations in the realized antenna gain in the presence of metal frames nearby have been discussed in detail. The antenna is compact in size, with dimensions of 36 mm \(\times\) 36 mm \(\times\) 0.5 mm. In this work, a dual-band shared-aperture based antenna is presented with high channel isolation. Some other recent work on antenna designs for IoT applications was also reported in [11; 12; 13; 14]. All these antennas operate above 1 GHz and hence are not suitable for narrow band IoT applications for long-range communication. Most of the antenna designs operating in sub-GHz are elevated printed inverted F-antennas (PIFA), monopole, or dipole antennas [15; 16; 17; 18; 19]. In [15], a folded miniaturized patch antenna was presented for IoT device operating at 805-835 MHz. In [16], digitally tunable capacitor (DTC) was used to tune the antenna from 600-900 MHz. The digital control and the low power consumption make this antenna a good candidate for IoT applications. In [17], a transparent double folded loop antenna was presented for IoT applications. The antenna was operating at 2.4 GHz with a bandwidth of 500 MHz. In [18], a 3D printed antenna-on-package antenna was presented with isotropic patterns for an IoT application operating at 2.4 GHz. Similarly, in [19], an energy harvesting rectenna was presented for IoT applications. The proposed antenna design would utilize an energy harvesting mechanism using rectenna to mitigate the challenges of battery constraint issues. The antenna was operated at a frequency band of 2.4 GHz. Also, other frequency reconfigurable antennas closely resembling the proposed antenna are reported in [20; 21; 22; 23]. The solutions provided are based on either reactive loading or meandering structures. However, neither of them was able to achieve a compact planar structure with sub-GHz operation. Also, the solutions provided in [20; 21; 22; 23] are non-planar structures or non-reconfigurable structures with large antenna dimensions. Thus, the non-compact and non-planar antennae limited the tuning capabilities, affecting their suitability for small IoT applications and terminal devices. Only a few slot-based antenna designs that operate in sub-GHz bands are reported in the literature, such as [24; 25]. The solutions provided in [24; 25] used a simple slot structure to tune the antenna in the desired frequency. Most of the reported frequency reconfigurable antenna designs are operating in frequency bands that are above 2 GHz. The proposed design in [24] covers the frequency bands of 0.9, 1.8, 1.9, and 2.4 GHz. The antenna size is 38 mm \(\times\) 16 mm. In [25], a dual antenna design that covers the 0.860 and 2.45 GHz frequency bands was presented. The antenna is 50 mm \(\times\) 50 mm in size. In [26], an energy harvesting antenna design for IoT applications was presented.