## 4 Results
#### 4.2.2 Correlation Between Frequency Bands and Performance Metrics
* Efficiency vs. frequency bands  
Figure 16 shows the distribution of radiation efficiency across various frequency bands used in RF energy harvesting applications. The HF band stands out with a high mean efficiency of approximately 85% and a very narrow distribution, indicating low variability and strong consistency--making it highly suitable for long-range, stable communication systems. In contrast, the UHF band presents a significantly lower mean efficiency (\(\approx\)45%) and a broader distribution with low-end values, reflecting greater variability and reduced performance reliability, which may limit its effectiveness in scenarios demanding high energy conversion rates. The UHF/ISM band shows a moderate average efficiency (\(\approx\)65%) with a relatively wide spread and several high values, suggesting a good balance between performance and adaptability in diverse environments, particularly for IoT and short-range wireless systems. Similarly, the L and S bands exhibit mean efficiencies near 60%, but with wider distributions and the presence of low outliers, indicating high variability. This suggests that while high efficiency is achievable in these bands, performance may degrade significantly under suboptimal design or environmental conditions. The C band also maintains a mean efficiency of approximately 65%, with most values concentrated at the higher end, although the presence of low outliers reveals the potential for performance degradation in specific implementations. Meanwhile, the X band demonstrates a higher mean efficiency (\(\approx\)75%) and a very narrow distribution, indicating low variability and consistently high performance, which is desirable in high-frequency,  
Figure 16: Boxplot showing efficiency distribution of band frequency for energy harvesting. precision-oriented applications. The Ku band, with a mean efficiency around 60% and a similarly narrow range, shows the potential for elevated performance under controlled conditions, further supporting its use in satellite and radar systems. These findings suggest that higher frequency bands such as X and Ku are more likely to deliver consistent and efficient energy harvesting performance, making them ideal for applications requiring high reliability and spectral precision. In contrast, lower bands such as UHF, L, and S exhibit broader variability and may be better suited for cost-sensitive, less demanding implementations where design constraints permit performance trade-offs. * Gain vs. frequency bands  
This subsection explores the relationship between antenna gain and the operating frequency bands used in RF energy harvesting systems. Gain, as a measure of directional amplification, plays a pivotal role in determining how effectively an antenna can capture ambient electromagnetic energy. Since gain performance can vary significantly across frequency bands due to propagation characteristics, wavelength constraints, and design trade-offs, this analysis aims to identify patterns in gain behavior associated with different spectral ranges. Understanding these correlations supports the optimization of antenna configurations for frequency-specific harvesting scenarios. Figure 17 shows antenna gain distribution across different frequency bands in RF energy harvesting systems. The UHF band presents a mean gain of approximately 3 dBi, characterized by a narrow distribution featuring both high and low outliers. This observation indicates considerable variability and limited reliability for applications where stable gain is paramount. Similarly, the UHF/ISM band exhibits a comparable mean gain of 3 dBi and a similar distribution profile. However, it displays slightly lower median values and more pronounced low outliers, pointing to moderate but less predictable performance. These characteristics collectively render both bands more suitable for short-range, low-power applications where design simplicity and cost outweigh the need for stringent directional stability.