# A Quad-Band RF Circuit for Enhancement of Energy Harvesting
## 1 Introduction  
Portable devices are powered by batteries. Hence, bulky cords and complications related to power interconnections can be eliminated [1, 2, 3]. However, the traditional battery systems have defects. These defects include the demand for power maintenance, limited life span, requiring periodic replacements, and the processing of the battery wastes. All these are considered to be weak points for traditional batteries. In applications such as wireless sensor networks (WSNs), wireless sensor nodes must be autonomous since battery replacement is not feasible in such applications. Furthermore, battery replacement in the case of thousands of spread embedded nodes will be a complicated and costly process, especially in harsh environments [4, 5]. As a result, the concept of power harvesting has been introduced as a technique for reaping energies from the external environment using different resources including thermal energy [6, 7, 8], solar energy [9, 10, 11], wind energy [12, 13, 14], electromagnetic radio frequency (RF) waves [15, 16, 17], and vibrations excitation and pressure gradients [18, 19, 20]. Energy harvesting will become a promising technique in the future with tremendous scope instead of replacing small batteries in low-power electrical devices that overcomes the defects related to traditional batteries stated above. Hence, the demand for such energy-harvesting devices is growing. RF energy that belongs to the electromagnetic waves are abundant in space since they are broadcast vastly in the urban ambient environment through RF/microwave frequency bands for instance, the Global System for Mobile communications (GSM 900, GSM 1800) bands [21, 22], and Wireless Fidelity (Wi-Fi 2.45 GHz/5.8 GHz) [23, 24] bands due to the current applications utilized by societies such as smart medical applications [25], intelligent environment monitoring [26], satellite communications [27, 28], mobile telecommunications [29; 30], wireless internet [31; 32], radar systems [33; 34; 35], digital multimedia broadcasting [36; 37; 38], etc. RF energy harvesting holds vast potential for replacing batteries or increasing their lifespans. Applying RF energy harvesting technologies can make micro-devices self-sustaining concerning the energy required for operation, thereby obtaining an unlimited operating lifespan. As a result, the dependency on traditional batteries can be reduced, which keeps the environment healthy. Therefore, waste is not expected with the harnessing of electromagnetic energy. Moreover, some international foundations such as the Federal Communications Commission (FCC), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the Institute of Electrical and Electronics Engineers (IEEE) have recommended regulations for the maximum output power levels. These regulations aim to protect humans against the potential adverse health effects of non-ionizing electromagnetic fields. Hence, these restrictions prevent any health hazards that can result from radiated RF waves. As a result, RF waves are emitted with low power levels in the space, which complies with the maximum output power levels' regulations [39]. RF is not limited by time, i.e., RF energy is available 24 h, seven days a week, during the whole year [40]. This feature promotes research to realize RF energy harvesting technology through the applications of WSNs [41; 42], internet of things (IoT) [43; 44], implantable devices and remote patient monitoring [45; 46], and radio frequency identification (RFID) [47; 48]. Despite the mentioned attractive RF energy source features, it has low power density in the environment. The maximum reported results in the literature for the harvested power of RF energy were in the range of \(\mu\)W to mW, such as the reported in [3; 49; 50; 51; 52; 53; 54], which is considered to be the main challenge of RF energy harvesting to power up any micro-devices efficiently. The rest of this paper is organized as follows, \"Related Work\" is summarized in Section 2.