## 4 Results
#### 4.1.3 Influence of Substrate Materials on Antenna Behavior  
Based on an evaluation of the literature, the percentage distribution of each substrate type is analyzed, as presented in Figure 9. The data clearly highlight that FR4 substrates are discussed far more frequently than other types, reflecting their widespread use and accessibility in energy harvesting designs. Additionally, Table 13 provides a detailed classification of articles included in this schematic review. This combined analysis underscores the critical role of FR4 and its dominance in the field while also acknowledging the growing interest in alternative materials for specialized applications. Area vs. antenna substrate  
This subsection explores the relationship between antenna surface area and the substrate material type used in fabrication. Substrate selection plays a fundamental role in defining electrical and physical characteristics of an antenna, influencing not only its electromagnetic performance but also its size and integration potential. Through boxplot analysis, distribution of antenna areas across different substrate types is examined, enabling a com  
Figure 8: Boxplot showing gain distribution of different antenna designs for energy harvesting. -parity assessment of how material properties--such as dielectric constant, thickness, and loss tangent--affect spatial requirements in energy harvesting designs. Analysis of antenna surface area by substrate type, as shown in Figure 10, reveals substantial variability linked to the material employed. Textile substrates exhibit the largest average area, approximately 5000 mm\({}^{2}\), alongside a relatively wide distribution and high outliers. This indicates significant variability and the capacity for large-scale implementations. FR4 substrates show a lower mean area (\(\approx\)2000 mm\({}^{2}\)) but retain a broad distribution with high outliers, suggesting moderate variability and the potential for relatively large antennas, albeit with greater structural consistency than textiles. Rogers substrates demonstrate the smallest average area, around 1000 mm\({}^{2}\), and are characterized by a very narrow distribution with occasional high outliers. This reflects generally compact antenna designs with localized exceptions, suitable for high-frequency miniaturized applications. Non-conventional materials, with a mean area near 3000 mm\({}^{2}\), show a moderately wide distribution and a prominent outlier, indicating a flexible performance envelope with applications ranging from moderate to large areas. RT Duroid substrates also exhibit a mean area of approximately 3000 mm\({}^{2}\), with a relatively narrow distribution skewed toward lower values, suggesting a stable tendency toward moderate-sized antenna configurations. Overall, our findings underscore substrate selection's pivotal role in determining a rectenna design's spatial footprint. Textile materials, with their substantial and variable area profiles, are particularly well suited for wearable or conformal applications. FR4 substrates, offering moderate and more stable dimensions, align effectively with portable \begin{table}
\begin{tabular}{c c} \hline \hline
**Substrates** & **References** \\ \hline FR4 & [30–32,34,35,44,46,48,50,53–55,57–61,64,66–73,77,78,80–82,85,87–91,94,95,99,102,103,105,107,108] \\ \hline Not conventional & [33,36–38,42,47,51,62,65,92,98,101,106] \\ \hline Rogers & [29,41,49,63,74,84,86,100,104] \\ \hline RT Duroid & [39,40,52,56,75,76,96] \\ \hline Textile & [43,45,79,83,93,97] \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Article classification by substrate. Figure 9: Percentage distribution of reviewed bibliography according to type of antenna analyzed. and embedded systems. Rogers and RT Duroid substrates, characterized by compact and consistent designs, prove ideal for high-frequency, space-constrained applications. Non-conventional materials, meanwhile, provide considerable versatility, making them appealing for specialized or experimental implementations.