## 4 Results
#### 4.2.2 Correlation Between Frequency Bands and Performance Metrics  
Understanding the impact of frequency band selection on the performance of RF energy harvesting systems requires examining how spectral choices relate to key design metrics. In this subsection, we explore the correlation between frequency bands and three fundamental parameters: antenna surface area, radiation efficiency, and gain. Through  
Figure 14: Percentage distribution of reviewed bibliography according to the type of frequency bands. \begin{table}
\begin{tabular}{c c} \hline
**Frequency Bands** & **References** \\ \hline S band & [29; 32; 33; 34; 35; 37; 38; 39; 41; 43; 45; 46; 49; 50; 51; 52; 54; 57; 58; 61; 63; 64; 65; 66; 70; 72  
statistical visualizations and comparative analysis, we aim to uncover patterns, tradeoffs, and frequency-dependent constraints that influence the design and effectiveness of rectenna-based systems. This investigation provides a more comprehensive view of how electromagnetic spectrum allocation affects both the physical and functional behavior of energy harvesting antennas. Area vs. frequency bands  
This subsection examines the relationship between antenna surface area and frequency bands targeted for RF energy harvesting. Since antenna dimensions are inherently linked to the wavelength of operation, lower frequency bands typically require larger physical structures. By analyzing area distribution across different spectral ranges, this section highlights how frequency selection constrains or enables design miniaturization, with implications for integration in space-limited or wearable applications. Figure 15 presents a boxplot analysis of antenna surface area across different frequency bands, revealing notable variations in the spatial requirements associated with each spectral range. The HF band shows a relatively low median area (\(\approx\)2000 mm\({}^{2}\)) with a narrow distribution and a low outlier, indicating low variability and compact designs suitable for long-range, low-frequency applications. The UHF and L bands display higher mean areas (\(\approx\)2500-3000 mm\({}^{2}\)) with broader distributions and several high outliers, suggesting greater variability and the potential for enhanced energy collection, particularly in systems such as mobile networks, radar, or satellite communication. The UHF/ISM and S bands both present median areas around 2000 mm\({}^{2}\) with moderately narrow distributions, although the S band exhibits more pronounced high outliers, pointing to a wider range of design adaptations depending on application demands. In contrast, the C band maintains a slightly lower average area (\(\approx\)1800 mm\({}^{2}\)), with a relatively tight spread and a single high outlier, indicative of compact configurations typically used in wireless communication systems where a balance between size and performance is critical. Higher frequency bands such as X and Ku show narrower distributions centered around smaller median values--approximately 1000 mm\({}^{2}\) and 3000 mm\({}^{2}\), respectively--highlighting their potential for precise, low-variability designs. While the X band empha  
Figure 15: Boxplot showing area distribution of band frequency for energy harvesting. sizes miniaturization and stability, the Ku band, despite its narrow distribution, includes a single high outlier, suggesting its use in specialized high-frequency applications requiring both compactness and the option for increased collection area. Overall, the analysis confirms that frequency selection imposes direct constraints on antenna size, with lower bands generally requiring larger structures, while higher bands enable more compact and consistent designs. This trade-off plays a critical role in determining the suitability of antennas for specific energy harvesting applications, ranging from wearable and IoT systems to large-scale satellite and radar deployments.