## 4 Results
#### 4.2.2 Correlation Between Frequency Bands and Performance Metrics
The L and S bands both show a higher mean gain (4 dBi), yet with broader distributions and notable outliers on both extremes. While this variability allows for potentially higher gain values, it also introduces the risk of performance degradation in less favorable configurations. These bands may therefore be appropriate for mobile or identification systems where some degree of gain fluctuation is tolerable. The C band reflects a similar trend, with a mean gain of approximately 3 dBi and a wide spread dominated by low outliers. This suggests high sensitivity to design and environmental factors, which must be carefully managed in applications that depend on consistent directional performance. In contrast, the X band stands out with a substantially higher mean gain (9 dBi) and a very narrow distribution concentrated around elevated values. This indicates low variability and high directional efficiency, making it particularly well suited for precision-driven applications such as radar systems, satellite communications, and high-frequency line-of-sight links where predictable gain is essential. Taken together, these results highlight how gain behavior is closely tied to frequency band selection. While higher frequency bands, such as the X band, offer greater stability and performance for directionally demanding applications, lower bands like UHF and UHF/ISM prioritize simplicity and integration flexibility at the expense of gain consistency. Intermediate bands, including L, S, and C, offer a compromise, with performance contingent on design tolerance and environmental conditions. * Comparative insights Analysis of frequency bands reveals distinct trade-offs in antenna design in terms of area, efficiency, and gain. High-frequency bands such as X and Ku enable more compact designs (X: \(\approx\)1000 mm\({}^{2}\), Ku: \(\approx\)3000 mm\({}^{2}\)), consistently offering high efficiency (X: \(\approx\)75%, Ku: \(\approx\)60%). In the case of the X band, they also deliver the highest and most stable gain (\(\approx\)9 dBi), making them ideal for precision applications. In contrast, low-frequency bands such as UHF and UHF/ISM exhibit larger areas (\(\approx\)2500-3000 mm\({}^{2}\)), while demonstrating notably lower and more variable efficiency and gain (UHF: \(\approx\)45%, \(\approx\)3 dBi; UHF/ISM: \(\approx\)65%, \(\approx\)3 dBi), making them more suitable for short-range and low-cost applications. The L and S bands occupy an intermediate position, with larger areas, moderate efficiencies (both \(\approx\)60%), and relatively higher gains (\(\approx\)4 dBi), albeit with considerable variability. The C band, in turn, maintains a relatively compact area (\(\approx\)1800 mm\({}^{2}\)) and moderate efficiency (\(\approx\)65%), although its gain tends to be low and design sensitive. The HF band stands out for its high efficiency (\(\approx\)85%) and compact design (\(\approx\)2000 mm\({}^{2}\)), making it suitable for long-range, low-frequency applications. RQ3. What Are the Main Rectifier Systems Used in Converting Energy Captured by Antennas in Energy Harvesting Systems, and What Are the Key Characteristics That Influence Their Efficiency and Conversion Capacity? To address the third research question--focused on identifying the main rectifier systems used in RF energy harvesting and key characteristics that influence their performance--this section provides a comprehensive analysis of the rectifier topologies employed in the reviewed literature. Discussion begins with a visual and categorical presentation of the most commonly used rectifier designs, offering a reference framework for classification. A statistical breakdown of rectifier type distribution is then presented, followed by a detailed technical evaluation of each configuration, including parameters such as output voltage or power, conversion efficiency, gain, and matching network used.