## 4 RF Energy-Harvesting Techniques
### Narrowband vs. Broadband RF Energy Harvesting  
RFEH systems can be categorized based on their operational bandwidth into narrowband and broadband systems. Narrowband systems are engineered to capture and convert RF energy at specific, predetermined frequencies (a few MHz around the centre frequency), optimizing components such as impedance matching networks and rectifiers to achieve high efficiency at these targeted frequencies. This design approach ensures minimal energy loss and is particularly effective in environments where the RF source operates consistently at a known frequency. High-Q microstrip patch antennas, helical antennas, and loop antennas are the typical antennas used for narrowband RF energy-harvesting systems. The high-Q nature of these antennas means they are resonant over a very narrow frequency band, which leads to increased voltage across the rectifier and thus higher conversion efficiency. This narrow resonance also allows for simplified impedance matching circuits, which are easier to design and fabricate. However, the primary limitation of narrowband systems lies in their restricted frequency range, which renders them less effective in dynamic environments where RF energy sources vary in frequency. In contrast, broadband RFEH systems are designed to operate over a wide range of frequencies (from hundreds of MHz to GHz), enabling them to capture energy from multiple RF sources si multaneously. This capability is particularly advantageous in urban and industrial settings, where numerous RF signals are present across various frequencies. Vivaldi antennas, spiral antennas, log-periodic dipole arrays, bowtie antennas and ultra-wideband patch antennas are the typical antennas used for broadband RFEH systems. These antennas are inherently frequency-independent or support multiple resonant modes, allowing them to respond to a broad spectrum of incoming signals. Their geometrical designs are often based on self-similarity or logarithmic scaling, which enables effective energy reception across wide frequency bands. A notable example of broadband design was presented by the authors in [60] where a quad-band RF energy harvester was developed to operate within the 0.8 GHz to 2.6 GHz frequency range. The proposed system incorporated a self-complementary log-periodic antenna and an improved impedance matching network, achieving a DC rectification efficiency of approximately 35% at an input power of \(-15\) dBm, which went up to 52% at \(-20\) dBm. These results underscore the potential of broadband systems to effectively harness ambient RF energy across multiple frequency bands. The performance gains seen in this system were attributed not only to the antenna design but also to a dynamic impedance matching network capable of adapting to varying signal strengths and frequencies, ensuring a relatively stable DC output even under fluctuating RF conditions. Despite their versatility, broadband systems face challenges related to design complexity. Achieving efficient impedance matching across a wide frequency spectrum is inherently more complex than in narrowband systems. Additionally, the integration of broadband antennas and rectifiers requires meticulous design to maintain consistent performance across all targeted frequencies. This often involves trade-offs such as using wideband matching techniques like multisection transformers or active tuning mechanisms, which can increase circuit size, power consumption, or component count. Maintaining a flat response across the frequency range is difficult, and performance may drop significantly at frequency extremes. A comprehensive review by the authors in [13; 63] highlights various impedance matching network configurations and their impact on the performance of broadband RF energy harvesters, emphasizing the need for advanced design methodologies to address these challenges. The choice between narrowband and broadband RF energy-harvesting systems involves a trade-off between efficiency at specific frequencies and versatility across multiple frequencies.