# Compact Multilayer Yagi-Uda Based Antenna for IoT/5G Sensors
## 1 Introduction
In the works of [12; 13], two microstrip patch antennas are presented in a multilayer configuration. In the work of [12], an antenna composed of two stacked patches is proposed as a low-cost solution, operating around 3 GHz. The antenna's bandwidth is 14%, but the information about the radiation pattern and the gain is not provided. On the other hand, in another paper [13], an antenna based on a microstrip patch is presented. It is formed by three parasitic patches working as director elements, operating around 10 GHz. This antenna uses a dielectric foam to ensure the distance between directors, as well as a V-shaped ground plane to improve the reflection coefficient. This antenna has about 190 MHz bandwidth and gain of 11.85 dB, although at the expense of some complexity, and any prototype built is not shown. Additionally, in the work of [14], a structure of eight layers in Low Temperature Co-fired Ceramic (LTCC) technology is presented, also designed for 10 GHz. This antenna consists of a microstrip patch using a proximity coupled feed, presenting a gain of only 3.43 dBi and an S\({}_{11}\) of \(-\)11.52 dB at 10 GHz, in addition to a narrow bandwidth. Moreover, a slot Yagi-like multilayered antenna is proposed by the authors of [15] for 4.2 GHz. This is a vertically stacked structure with  
Figure 1: Example of 5G communications and a massive Internet of Things (IoT) scenario. three patches, which uses slit slots as the parasitic elements. The prototype shows a bandwidth of about 28% and a gain of 12.20 dBi. However, this structure is bulky and complex. Furthermore, a multilayer substrate integrated waveguide horn antenna array is proposed in another paper [16], in a 4 \(\times\) 4 array configuration. This antenna uses multilayer cavities with gradually decreased permittivity and expanded aperture sizes above the slots to increase the bandwidth. The array operates between 22.4 to 29.8 GHz (about 28.4% bandwidth) and presents a gain around 15 dBi; however, its complexity is huge. The authors of [17] designed a Yagi antenna resonant at 5.8 GHz using air gaps, presenting a bandwidth of 14% and a gain of 11 dBi. These air gaps between layers are function of the operation frequency, and this antenna requires a huge spacing between the elements, turning the antenna bulky and likely to vary its physical structure and properties over the time. The authors of [18] implemented a multilayer Yagi-like microstrip patch antenna, without air gaps. A gain of 11 dBi was reached and this antenna presents a reasonable measured bandwidth of nearly 20%. However, the structure uses foam between the directors, with low permittivity, which increases the volume of the antenna. No efficiency values are referred to in this work. This antenna is resonant at 10 GHz, which is still a low frequency for the 5G and the massive IoT referred scenarios. A multilayered antenna for 60 GHz is proposed in the work of [19]. This antenna has a reduced size and presents an individual gain of 11 dBi. However, its bandwidth is just 4.2%, which is a clear disadvantage considering the high traffic data rates that, predictably, will be witnessed. In this paper, two printed antennas are presented for IoT sensors, operating at 24 GHz, and designed according to the theoretical principles of Yagi-Uda structures. A planar printed Yagi-Uda is designed and characterized, and its prototype is shown. This antenna has a gain of 8.9 dBi and a bandwidth of 18%. Then, a compact multilayer stacked antenna is presented. This antenna improves all the main properties of the planar printed Yagi and outperforms the previously referred antennas, showing a gain of 10.9 dBi, an operation bandwidth of 27%, and a total efficiency of 86%, besides its reduced size. This antenna is also an interesting element to be used in larger arrays, and is suited for beamforming applications. This paper is divided into five sections, starting with an introduction regarding some of the possible applications for these prototypes, where our goals are settled.