## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
These studies underscore the adaptability of rectangular antennas to meet diverse frequency requirements and their potential in advancing energy harvesting technologies. Rectangular antennas are widely used in energy harvesting applications due to their versatility. [PERSON] et al. [31] propose an E-shaped design with circular polarization and high gain, while [PERSON] et al. [32] present a T-shaped microstrip antenna efficient in two frequency bands. [PERSON] et al. [36] highlight a loop design on an artificial magnetic conductor for dual reception, and [PERSON] et al. [68] develop a stepped structure to optimize \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[82] & 0.863, 1.84, 1.95 & 35, 35, 35 & FR4 & 1.6 \\ \hline
[86] & 0.9 2.45 & 300, 50 & Rogers 3010, RT Duroid 5880 & — \\ \hline
[89] & 2.1, 2.6 & — & FR4 & 1.6 \\ \hline
[59] & 2.45 & — & FR4 & 1.6 \\ \hline
[100] & 0.915, 2.45 & 380, 850 & Rogers 3210 & 0.635 \\ \hline
[32] & 2.4 & \(\approx\)200 & FR4 & 0.8 \\ \hline
[54] & 2.5, 4.5 & — & FR4, not conventional & 1.5, 0.69 \\ \hline
[78] & 2.5, 4.5 & 420, 20 & FR4 & 1.6 \\ \hline
[36] & 0.5 0.875 & 50, 100 & Not conventional & 0.8 \\ \hline
[45] & 0.85, 0.9, 1.8, 2.4 & 1500 & FR4 & 1.6 \\ \hline
[106] & 2.45 & 60 & Not conventional & 1.5 \\ \hline
[92] & 2.45 & \(\approx\)100 & Not conventional & 1 \\ \hline
[58] & 1.8, 2.45 & 650 & FR4 & 1.6 \\ \hline
[31] & 0.434 & 200 & RT Duroid 5880 & 1.57 \\ \hline
[75] & 0.878, 0.937, 1, 2.45 & 50 & RT Duroid 5880 & 0.787 \\ \hline
[52] & 2.45 & 100 & RT Duroid 5880 & 1.57 \\ \hline
[94] & 2.4, 4, 5 & 100, 150 & FR4 & 1.6 \\ \hline
[50] & 2.4 & 25 & FR4 & 1.6 \\ \hline
[81] & 0.868 & 32, 26 & FR4 & 0.8 \\ \hline
[87] & 2.4 & 0.10 & FR4 & 1.6 \\ \hline
[68] & 2.53, 4.46,5.5 & 84, 300, 550 & FR4 & 1.6 \\ \hline
[71] & 0.925, 2.45 & 7, 80 & FR4 & 0.3 \\ \hline
[97] & 0.868, 0.915, 2.4, 0.86–0.96 & 407, 407, 407, 464 & Textil & 0.252, 0.322, 0.372 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Construction characteristics of antennas within the rectangular geometry classification. Resonant frequencies, bandwidth (BW) at each frequency, substrate used, and antenna width are included. RF signal conversion. [PERSON] et al. [81] introduce a compact 3D antenna with high gain and directivity, while [PERSON] et al. [106] evaluate the use of polylactic acid (PLA) as an efficient substrate in rectennas. [PERSON] et al. [97] examine how fabric structures impact efficiency and functionality of printed radio frequency identification--ultra-high-frequency (RFID UHF) antennas, highlighting materials' relevance in energy harvesting applications through the implementation of three antennas with differing thicknesses. The results show that thickness variations affect operating frequencies, which in turn influence the bandwidth achieved by each antenna. Similarly, [PERSON] et al. [59] analyze how the substrate material affects the efficiency of meander antennas, concluding that appropriate selection can optimize RF energy transfer. On the other hand, [PERSON] et al. [75] present a dual-band rectenna designed for UHF and ISM applications, showing high efficiency in converting energy across multiple spectrums. Finally, [PERSON] et al. [92] develop a compact rectangular rectenna that simplifies wireless power transmission, proving effective in converting electromagnetic energy into usable electrical power. On the other hand, [PERSON] et al. [87], [PERSON] et al. [86], and [PERSON] et al. [50] present an innovative low-cost antenna system design for energy harvesting, aimed at implementation in IoT applications. Their focus is on developing an accessible solution to leverage energy from radio signals in this widely utilized frequency range. In [PERSON] et al. [52] and [PERSON] and [PERSON] [54], focus is placed on using microstrip antennas specifically designed for resonant frequencies, such as the 2.45 GHz ISM band.