# A Dual-Polarized Omnidirectional Rectenna Array for RF Energy Harvesting Harvesting
## 1 Introduction  
RF energy harvesting (REH) is the technology by which energy is derived from sources in the ambience, and then stored for use by low-power electronics [1]. In recent years, the demand for low-power electronics in smart homes [2], smart healthcare [3], and environmental monitoring [4] has increased substantially. The energy consumption of these devices has attracted widespread attention. REH has become a suitable choice for these low-power electronics [5, 6]. Compared with other types of energy in the ambience, such as solar energy, wind energy, etc., the ubiquitous ambient RF energy has its unique advantages [7]. First, there is a wide range of RF energy sources [8]. Ambient RF energy can be obtained from the ubiquitous wireless transmitters around, including cell phones, handheld radios, mobile base stations, TV/ radio stations, wireless routers, etc. On the other hand, RF energy sources are stable. For example, TV broadcast signals can be collected by energy harvesting systems throughout the day without having to consider energy interruptions. Therefore, the use of this free and inexhaustible ambient RF energy is a promising topic, and a lot of research work [9, 10, 11, 12, 13, 14] is devoted to RF energy harvesting.  
In a REH system, the key device is the rectenna which harvests the incident electromagnetic (EM) wave and converts RF power into DC power. The power conversion efficiency (PCE) is the main concern of the rectenna's performance, which is defined as the ratio of the output DC power to the input RF power. Thus, many research efforts have been dedicated to improving the PCE of rectennas in different application scenarios [15, 16]. In the process of REH, the direction of the incident EM wave in the ambience is unknown, which could change at different moments [17, 18, 19, 20]. Therefore, to collect EM waves fromdifferent directions, an omnidirectional antenna can be used to cover a larger area [21; 22]. In addition, the diverse sources of EM waves in the ambience lead to unknown polarizations of the incident EM waves. If a linearly polarized antenna is used for REH, the polarization mismatch problem may be encountered, which reduces the performance of the system. A dual-polarized antenna can be considered for REH, decomposing the incoming wave of any polarization into two components of vertical and horizontal polarizations [23; 24; 25]. The RF energy of these two polarization components can be received separately and then energy conversion is performed, thus avoiding polarization loss. In addition, to collect as much energy as possible, antenna arrays can be used to receive RF energy with more antenna elements. For rectennas based on antenna arrays, RF power combining or DC power combining can generally be utilized. However, for ambient REH, because the directions and power levels of incoming EM waves are time-varying, merely using RF power combining or merely using DC power combining is not the optimal choice [26; 27]. A combination of the two methods can be considered according to the application scenarios. The above ideas are fully considered and applied to this research work.  
In this paper, a dual-polarized omnidirectional rectenna array with a hybrid power-combining scheme is proposed for REH applications. An omnidirectional antenna array containing horizontally and vertically polarized subarrays is designed for RF power receiving. Then, a hybrid power-combining network is created to evenly distribute all the received RF power into two portions. By merging the antenna array, power combining network, and designed rectifiers, a rectenna array is formed. Measurement results show that the rectenna array achieves good omnidirectional-recycling capability. In addition, it can maintain excellent stability when receiving EM waves at different incidence angles. Therefore, the proposed rectenna array is ready for applications in REH.