## 3 Fundamentals of RF Energy Harvesting
### Components of an RF Energy-Harvesting System
#### 3.2.1 Antenna  
The antenna serves as the primary interface for capturing ambient RF signals, making it a crucial component of RFEH systems. The effectiveness of energy harvesting largely depends on the antenna's ability to efficiently collect RF signals from the surrounding environment. Antennas designed for RFEH must exhibit characteristics such as high efficiency, wideband or multi-band operation, and impedance matching with the rectifier to maximize power transfer. Different antenna designs, including dipole, patch, fractal antennas, and planar inverted-F antennas (PIFAs), as shown in Figure 3, have been explored to optimize energy reception. Dipole antennas are among the simplest and most widely used designs due to their omnidirectional radiation pattern and ease of fabrication [8; 15]. They operate efficiently over narrow frequency ranges, making them suitable for harvesting RF energy from known, fixed-frequency sources such as broadcast transmitters and cellular base stations. Microstrip patch antennas are compact, lightweight, and suitable for integration into wearable and embedded systems. They typically operate at a single frequency, but with modifications such as slotted designs or stacked configurations, they can achieve multi-band or broadband operation, allowing them to capture RF signals from multiple sources simultaneously [16; 17]. Fractal antennas are designed using self-replicating geometric patterns, enabling them to operate efficiently over multiple frequency bands while maintaining a compact size. This property makes them particularly useful for RFEH in urban environments, where multiple RF sources exist at varying frequencies. Monopole antennas and PIFAs offer compact designs and are commonly used in mobile and IoT applications. Their ability to be embedded in small devices while maintaining reasonable efficiency makes them attractive for RFEH in wearable electronics and wireless sensor networks [18; 19]. Recent advancements have introduced metamaterial-based and reconfigurable antennas, which utilize artificial electromagnetic structures to enhance signal reception and dynamically adapt to varying RF environments. These antennas can be tuned to optimize performance based on the availability of RF signals, improving the overall efficiency of  
Figure 2: RF energy-harvesting process. energy-harvesting systems [20]. Traditional antennas are often designed for a specific frequency band, limiting their ability to harvest RF energy from diverse sources. Multi-band and broadband antennas address this limitation by capturing energy from multiple frequency bands simultaneously, significantly improving the total harvested power. These antennas are particularly beneficial in urban environments, where RF signals from cellular networks (e.g., 4G, 5G), Wi-Fi, Bluetooth, and broadcast stations coexist across various frequency ranges. The choice of an antenna for RFEH depends on factors such as operating frequency, efficiency, gain, polarization, and impedance matching with the rectifier circuit. Antenna design for RFEH must prioritize high efficiency due to the typically low power levels of ambient RF signals. Multi-band or broadband antennas enhance energy capture by covering various RF sources, while compact and flexible designs are essential for applications like wearable electronics and IoT devices [22]. Directional antennas improve harvesting from specific sources, whereas omnidirectional antennas capture signals from multiple directions. Proper impedance matching with the rectifier is crucial in minimizing reflection losses and optimizing power transfer. Considering these factors ensures higher efficiency, supporting the development of self-sustaining wireless devices for smart cities, IoT networks, and biomedical applications. Table 3 provides a comparison of different antenna designs, highlighting their frequency ranges, circuits, and efficiency for RFEH systems.