# Compact Multilayer Yagi-Uda Based Antenna for IoT/5G Sensors
## 1 Introduction  
People's interconnection was improved by 4G, making the communication more efficient, fluent, and natural. Nowadays, 5G intends to continue that demand and take it to a higher level. In this framework, 5G aims to guarantee people's interaction, devices interconnection, as well as the connection among people and devices. A network composed of users, daily gadgets, traffic information, personal health conditions, or the status of all the home electronic devices is only a small example of a possible scheme, shown in Figure 1, within many 5G scenarios that are not yet discovered. For now, it is known that 5G services will be divided in three categories; namely, enhanced mobile broadband, mission-critical control, and massive Internet of Things (IoT) [1]. Probably the most effective method to fulfil some demands for 5G cellular services, which are expected to be commercially available in 2020, is to increase the bandwidth [2]; hence, the migration to higher frequencies (in the mmWaves) is mandatory, mainly to support the required gigabit data rate service [3]. On the other hand, as a disadvantage, the frequency ranges of mmWaves present higher path-loss resulting in fragile link, due to weak diffractions at these frequency bands. To overcome these issues, high gain antennas and highly directive antenna arrays using several elements, as well as the application of beamforming techniques, can be used, in order to transmit the wave in the right direction. Yagi-Uda antennas present several characteristics that make them suitable for application in sensors or in larger antenna arrays. Structurally Yagi-Uda antennas are well known. They are composed of a driven dipole, a reflector element, and a variable number of directors. This scheme guarantees the creation of an end-fire beam [4], appropriate for beamforming applications. This antenna's layout has been widely analyzed, and historically used, mainly because these structures present a reasonable bandwidth and relatively high gains at moderated costs [5]. With the emergence of printed circuits, several Yagi-like antennas have been proposed for different frequencies of operation and a wide number of applications [6; 7]. In Internet of Things (IoT) scenarios, antennas are to be implemented in every small gadget or wearable gadget, mainly to assure the interaction between sensors placed in the most various devices and people. Because of their widespread use, it is highly recommended that antennas are small and compact. Over the years, some effort has been made to find antenna miniaturization techniques that allow reducing antenna size, maintaining or improving its main characteristics. Some of these methods are described in the literature [8], presented in different classes, including techniques such as fractals, metamaterials, high dielectric constant substrates, slow wave structures, or engineered ground planes. A miniaturization technique described in the literature that is widely used because it also allows obtaining broadband characteristics is the use of reactive impedance substrates (RIS) [9]. In the works of [10; 11], two patches are presented, designed on an RIS, which convert them into Ultra-wideband (UWB) antennas. In another work [10], the bandwidth increases over than 100 times, compared with the conventional patch antenna, while in the work of [11], the bandwidth is increased by a factor of 16. These solutions based on RIS are, however, complex to design in millimeter waves, and are used in antenna structures with ground planes. In addition, for IoT and 5G applications, gain is a concern and even though Yagi antennas naturally present higher gains than microstrip patch antennas [4], this work intends to analyze the advantages of implementing a Yagi-Uda antenna in a multilayer configuration to further enhance its most important properties, such as the gain and the bandwidth. Multilayer structures have been tested as an attempt to increase the antenna's gain.