## 4 Results  
This work aims to address the research questions presented above. After extracting relevant information from articles included in our review, we identified several key variables that will allow us to structure the answers in a more organized, coherent, and easy-to-understand manner. These are the key variables in our analysis:  
* Antenna design. * Rectifier systems. * Frequency bands. Each key variable play a critical role in determining the performance of energy harvesting. By thoroughly analyzing these factors, we aim to highlight their individual contributions to area, efficiency, gain, and frequency. Table 3 summarizes selected studies, systematically classified based on the key variables established for analytical purposes. Figure 2: Distribution of included articles by country. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Antenna** & **Rectifier** & **Bands frequency** \\ \hline \hline [66] & Array & Multi-step & S band \\ \hline \hline [60] & Array & Schottky & Ultra high frequency (UHF), S band \\ \hline \hline [82] & Rectangular patch & Schottky & UHF, L band, L band \\ \hline \hline [29] & Fractal & Single-step & S band \\ \hline \hline [30] & Slot insertion & Multi-step & UHF \\ \hline \hline [31] & Rectangular patch & Schottky & UHF \\ \hline \hline [32] & Rectangular patch & Schottky & S band \\ \hline \hline [33] & Other & Schottky & S band \\ \hline \hline [34] & Other & Schottky & UHF/ industrial, scientific, and medical (ISM), S band, UHF \\ \hline \hline [35] & Spiral & Multi-step & S band–C band, UHF \\ \hline \hline [36] & Rectangular patch & Schottky & UHF, UHF \\ \hline \hline [37] & Array & Multi-step & S band \\ \hline \hline [38] & Circular patch & Without & S band \\ \hline \hline [39] & Array & Schottky & S band \\ \hline \hline [40] & Fractal & Single-step & S band \\ \hline \hline [41] & Bow-tie & Single-step & UHF–L band, L band–S band \\ \hline \hline [42] & Spiral & Without & HF \\ \hline \hline [43] & Slot insertion & Schottky & S band, C band \\ \hline \hline [44] & Slot insertion & Schottky & L band, C band \\ \hline \hline [45] & Rectangular patch & Multi-step & UHF, UHF/ISM, L band, S band \\ \hline \hline [46] & Fractal & Single-step & S band \\ \hline \hline [47] & Other & Schottky & UHF \\ \hline \hline [48] & Slot insertion & Schottky & L band, L band \\ \hline \hline [49] & Array & Without & S band \\ \hline \hline [50] & Rectangular patch & Single-step & S band \\ \hline \hline [51] & Circular patch & Schottky & S band \\ \hline \hline [52] & Rectangular patch & Schottky & S band \\ \hline \hline [53] & Array & Multi-step & X band \\ \hline \hline [54] & Rectangular patch & Without & S band, C band \\ \hline \hline [55] & Spiral & Schottky & UHF \\ \hline \hline [56] & Other & Full-wave & UHF \\ \hline \hline [57] & Array & Multi-step & S band, C band \\ \hline \hline [58] & Rectangular patch & Schottky & L band, S band \\ \hline \hline [59] & Rectangular patch & Without & S band \\ \hline \hline [61] & Slot insertion & Schottky & S band \\ \hline \hline [62] & Other & Full-wave & UHF, UHF/ISM, L band \\ \hline \hline [63] & Array & Schottky & L band, L band, S band, S band \\ \hline \hline [64] & Bow-tie & Full-wave & L band, S band \\ \hline \hline [65] & Array & Full-wave & S band, C band \\ \hline \hline [67] & Slot insertion & Schottky & S band \\ \hline \hline [68] & Rectangular patch & Without & S band, C band \\ \hline \hline [69] & Other & Half-wave & L band \\ \hline \hline [70] & Slot insertion & Multi-step & L band, S band, S band \\ \hline \hline [71] & Rectangular patch & Full-wave & UHF/ISM, S band \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Classification of reviewed articles based on key analytical variables related to rectenna-based energy harvesting systems. RQ1. What Are the Different Types of Antenna Designs Commonly Used in Energy Harvesting Applications, and What Are Their Specific Characteristics?