## 4 Results
#### 4.2.1 Frequency Bands Used in RF Energy Collection  
As a foundation for analyzing frequency usage in RF energy harvesting, Table 14 provides a structured classification of the most relevant spectral bands reported in the literature. It summarizes standardized International Telecommunication Union (ITU)/IEEE designations, common application domains, and corresponding energy harvesting methods typically employed in each range. This categorization serves as a conceptual framework for identifying dominant frequency bands, understanding their functional implications, and interpreting design decisions associated with their selection in rectenna-based energy harvesting systems. Building upon the classification framework from Table 14, reviewed articles were categorized by their targeted frequency bands for RF energy harvesting. Figure 14 shows percentage distribution of these studies across the main frequency bands, enabling a quantitative overview of current research trends and spectral preferences within the field. This classification supports identification of dominant bands and offers insights into practical and technical considerations guiding frequency selection in rectenna-based designs. As shown in Figure 14, S band (2-4 GHz) emerges as the most frequently studied frequency range, accounting for 47.4% of the reviewed literature. This predominance reflects the widespread availability of ambient signals in the 2.4 GHz ISM band (e.g., Wi-Fi, Bluetooth, and microwave communications), which are especially prevalent in urban and indoor environments. L band (1-2 GHz) follows with 18.4%, likely due to its relevance in satellite navigation (e.g., Global Positioning System (GPS)) and mobile communication systems. UHF band (300 MHz-1 GHz) and UHF/ISM sub-band around 900 MHz also \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Frequency Range** & **ITU/IEEE Classification** & **Application Area** & **Energy Harvesting Method** \\ \hline
10–30 MHz & HF (high frequency) & \
\begin{tabular}{c} AM radio, maritime and \\ emergency communications \\ \end{tabular} &
\begin{tabular}{c} Long-range radiofrequency energy \\ harvesting antennas \\ \end{tabular} \\ \hline
30–300 MHz & VHF (very high frequency) & \
\begin{tabular}{c} FM radio, analog television, \\ amateur radio \\ \end{tabular} &
\begin{tabular}{c} Energy harvesting from digital radio and \\ broadband communications \\ \end{tabular} \\ \hline
300 MHz–1 GHz & UHF (ultra high frequency) & \
\begin{tabular}{c} Digital television, RFID, 2G/3G \\ mobile communications \\ \end{tabular} &
\begin{tabular}{c} Harvesting of mobile signals and passive \\ sensors \\ \end{tabular} \\ \hline
900 MHz–1 GHz & UHF / ISM band & Cellular telephony, RFID, IoT & Powering wireless sensors and IoT devices \\ \hline
1–2 GHz & L band & \
\begin{tabular}{c} GPS, satellite communications, \\ 3G/4G mobile telephony \\ \end{tabular} &
\begin{tabular}{c} Harvesting of GPS signals and mobile \\ communications for autonomous sensors \\ \end{tabular} \\ \hline
2–4 GHz & S band & \
\begin{tabular}{c} Wi-Fi (2.4 GHz), bluetooth, \\ microwaves \\ \end{tabular} &
\begin{tabular}{c} Energy harvesting from wireless networks \\ in urban environments \\ \end{tabular} \\ \hline
4–8 GHz & C band & \
\begin{tabular}{c} Satellite communications, \\ weather radars \\ \end{tabular} &
\begin{tabular}{c} Antennas for energy harvesting in \\ aerospace applications \\ \end{tabular} \\ \hline
8–12 GHz & X band & Military and satellite radars &
\begin{tabular}{c} Applications in defense and industrial \\ environments \\ \end{tabular} \\ \hline
12–15 GHz & Ku band & \
\begin{tabular}{c} High-frequency satellite \\ communications \\ \end{tabular} &
\begin{tabular}{c} Energy harvesting from satellite signals for \\ telecommunications \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Classification of RF frequency bands relevant to energy harvesting applications, including standard ITU/IEEE designations, typical use cases, and associated energy harvesting methods.