## 4 Results
#### 4.3.2 Correlation Between Rectifiers and Performance Metrics
These configurations offer the potential for high efficiency in favorable scenarios but also show susceptibility to performance drops, underscoring the need for robust system design and impedance matching. Systems without a clearly defined rectifier (either external or unspecified) show a median efficiency of around 55%, with high variability and a tendency toward lower values, again suggesting the importance of considering rectifier integration in overall performance assessment. Microwave rectifiers, in contrast, demonstrate a lower mean efficiency (\(\approx\)50%) but with a narrow distribution, suggesting consistent and predictable performance, albeit at a moderate level. This topology may be best suited for compact systems where space and stability are critical, even if it limits maximum power conversion. Overall, analysis confirms that rectifier selection significantly impacts system efficiency. While topologies such as Schottky and multi-step offer high performance potential, their variability must be managed through careful design. Conversely, configurations like single-step and microwave provide more stable behavior, making them preferable in scenarios where predictability and integration simplicity are prioritized over peak efficiency. Operation frequency vs. rectifier  
This subsection analyzes the relationship between rectifier topology and the operating frequency of RF energy harvesting systems. The rectifier circuit not only affects DC output but also interacts with the antenna's impedance and matching network, influencing the system's resonant behavior. Different rectifier designs may exhibit preferences or limitations across specific frequency ranges due to parasitic effects, diode switching characteristics, and impedance matching constraints. By examining the distribution of operating frequencies associated with each rectifier type, this analysis aims to identify design patterns, compatibility ranges, and potential trade-offs that guide the selection of rectifiers for specific spectral applications. Figure 22 illustrates the distribution of operating frequencies across different rectifier configurations used in RF energy harvesting systems. Systems without an explicitly defined rectifier exhibit the widest variability, with a median operating frequency around 2.4 GHz and several high outliers. This broad range suggests a lack of spectral constraint, making such configurations less suitable for applications that demand frequency stability. Schottky rectifiers show a tighter distribution, with a mean frequency also centered near 2.4 GHz but bounded by lower outliers. This limited variability indicates predictable performance and makes them well suited for applications requiring stable operation at relatively low frequencies. Multi-step rectifiers follow a similar pattern, with a median frequency of approximately 2.5 GHz and a narrow distribution containing both high and low outliers. Their frequency stability, combined with a tendency to operate in lower spectral bands, renders them advantageous in systems where consistent RF-to-DC conversion is critical. Single-step rectifiers exhibit a slightly lower median frequency (\(\approx\)2.0 GHz) with narrow variability and occasional extreme values. This suggests reliability for operation at moderate frequencies, which is particularly beneficial in applications where spectrum coordination and system synchronization are required. Full-wave rectifiers also demonstrate a central frequency around 2.4 GHz, though with a slightly wider spread and upper outliers, indicating moderate variability and potential for use in systems where controlled spectral deviation is tolerable. Half-wave rectifiers show a median near 2.5 GHz with a very narrow range and low outliers, reflecting strong consistency and a tendency to operate in the lower part of spectrum. This makes them ideal for systems prioritizing frequency stability at sub-GHz or low-GHz ranges.