## 4 RF Energy-Harvesting Techniques
### Passive vs. Active RF Energy Harvesting  
RFEH systems can be broadly categorized into two types: passive and active. Passive systems rely solely on ambient RF energy present in the environment, capturing and converting it into usable electrical power without the aid of external power sources. In contrast, active systems incorporate external power sources to enhance their energy-harvesting capabilities, often employing amplifiers or other active components to boost performance. Passive RF energy harvesters capture ambient RF signals emitted from various sourcessuch as mobile phones, Wi-Fi routers, and broadcasting stations. These systems typically consist of an antenna to capture RF energy, an impedance matching circuit to maximize power transfer, a rectifier to convert AC signals into DC signals, and an energy storage unit. Typical antennas used for passive RF energy harvesting are dipole, simple patch, and omnidirectional antennas. The energy captured in passive systems is often in the microwatt range, making ultra-low-power circuit design essential. The careful tuning of the impedance matching network is critical to ensure maximum voltage transfer to the rectifier. Energy storage units such as supercapacitors or thin-film batteries are used to accumulate harvested energy for intermittent power delivery. One notable design improvement in passive systems is the incorporation of harmonic harvesting loops, which can enhance rectifying efficiency by over 25% compared to conventional rectifiers. This improvement allows for more efficient energy harvesting from ambient RF signals within the 825 MHz to 2.5 GHz range [57]. These loops operate by capturing and recycling harmonic components of the RF signal, which would otherwise be dissipated, thereby increasing the total harvested energy. They also help stabilize the output voltage by reducing spectral leakage. Active RFEH systems utilize external power sources to enhance their energy-harvesting capabilities. By integrating amplifiers with the antenna feed line, these systems can significantly improve the dynamic range and efficiency of energy harvesting. For instance, active wearable energy-harvesting systems have demonstrated substantial bandwidth and gain improvements, achieving an active antenna gain of 24 \(\pm\) 2.5 dB for frequencies ranging from 200 MHz to 900 MHz. This enhancement is particularly beneficial for wearable sensors in medical and sports applications, where consistent and reliable energy harvesting is crucial. The inclusion of low-noise amplifiers (LNAs) or power amplifiers (PAs) in the signal chain compensates for weak ambient signals and enables operation, even in RF-sparse environments. This also allows active systems to respond dynamically to changes in the RF environment by tuning or steering the antenna pattern. Typical antennas used for active RF energy harvesting are tunable/reconfigurable microstrip, electronically steered antennas. These antennas are capable of frequency agility or beamforming, enabling the system to selectively focus on the strongest RF sources and dynamically adapt to environmental changes. This adaptability extends the operational range and improves link reliability, especially in mobile or obstructed scenarios. The choice between passive and active RF energy-harvesting systems depends on the specific application requirements and environmental conditions. Passive systems are advantageous in scenarios where simplicity, low cost, and maintenance-free operation are prioritized. They are suitable for powering low-energy devices such as wireless sensors and IoT devices in environments with sufficient ambient RF energy. However, their performance is limited by the availability and strength of ambient RF signals. To compensate for low input power levels, passive systems often employ highly efficient rectifier topologies, such as voltage doublers or Schottky diode-based circuits, and require careful PCB layout design to minimize losses.