## 4 RF Energy-Harvesting Techniques
### Single-Band vs. Multi-Band RF Energy Harvesting  
RFEH systems are classified based on their ability to capture energy from either a single frequency band or multiple frequency bands. Single-band systems are designed to operate at a specific frequency, such as 2.4 GHz or 5.8 GHz, which are commonly used in Wi-Fi, industrial, and ISM (Industrial, Scientific, and Medical) band applications. These systems optimize impedance matching networks and rectifier circuits to maximize efficiency at a single frequency, ensuring minimal losses. Typical antennas used for single-band RREH are microstrip patch antennas, monopole antennas, and dipole antennas. They leverage the principle of resonance, whereby the antenna and matching circuit are finely tuned to a specific frequency, achieving maximum power transfer. The high-Q (quality factor) design enables narrowband operation with minimal signal loss, enhancing the energy harvested per unit of RF power. Studies have demonstrated high conversion efficiencies in single-band designs. A rectifier designed for 2.4 GHz operation, shown in Figure 9a, achieved a conversion efficiency of 62.4% at an input power of \(-10\) dBm and a conversion efficiency of 31.1% at an input power of \(-20\) dBm, incorporating a voltage booster circuit to enhance performance at low power levels [58]. Another optimized rectifying circuit for RF energy scavenging at 2.45 GHz reached a conversion efficiency of 31.16% at an input power of 20 dBm, delivering an output voltage of 10.32 V [59]. These advancements highlight the progress made in enhancing rectifier circuit efficiencies, particularly under conditions with varying input power. However, single-band systems are limited in their applicability as they rely on a single RF source, making them less effective in environments where RF energy levels fluctuate. In addition, their narrow frequency targeting makes them susceptible to detuning due to temperature variations or material inconsistencies, which can degrade performance. These systems are best suited for predictable environments with stable RF sources, where maximum efficiency at one frequency outweighs adaptability. In contrast, multi-band RFEH systems operate across multiple frequency bands, allowing them to harvest energy from diverse RF sources such as GSM at 900 MHz and 1.8 GHz, LTE ranging from 700 MHz to 2.6 GHz, and Wi-Fi at 2.4 GHz and 5 GHz. The typical antennas used for multi-band RF energy harvesting are multi-band microstrip patch antennas, PIFAs, and fractal antennas. The underlying design principle in multi-band  
Figure 9: RF energy-harvesting system. (**a**) Single band [58]. (**b**) Multiple bands [62]. systems involves creating antennas and rectifiers that resonate at multiple frequencies. This can be achieved through multi-resonant structures, nested antenna elements, or fractal geometries that support various harmonic modes. Recent studies have demonstrated high-efficiency multi-band rectennas. [PERSON] et al. (2018) reported that a dual-band rectenna operating at 1.8 GHz and 2.45 GHz achieved measured peak conversion efficiencies of 72% at 1.8 GHz (input power of 4 dbm) and 65% at 2.45 GHz (input power of 3 dbm), as shown in Figure 9(b) [60]. Another study designed a triple-band rectenna operating at 915 MHz, 1.8 GHz, and 2.1 GHz frequency bands. The proposed rectenna achieved efficiencies greater than 52% when harvesting RF energy from outdoor sources. The design incorporates a modified E-shaped patch antenna and an optimized impedance matching network to enhance performance across the targeted frequency bands [61]. While multi-band RF energy harvesters offer greater flexibility, particularly in urban and industrial environments where RF signals vary across multiple frequencies, they also introduce additional design complexity. Efficiency variations across different frequency bands require careful impedance matching and rectifier optimization to ensure stable energy conversion across multiple bands.