## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[55] & 0.675 & 10 & FR4 & 1.6 \\ \hline
[95] & 0.85, 1.8, 2.1 & \(>\)80 & FR4 & 1.6 \\ \hline
[35] & 3.1–4.8, 0.886 & 1700, 35 & FR4 & 1.5 \\ \hline
[42] & 0.01356 & 4 & Not conventional & 0.025 \\ \hline
[96] & 2.4 & 100 & Not conventional & 0.625 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Construction characteristics of antennas within the spiral geometry classification. Resonant frequencies, bandwidth at each frequency, substrate used, and antenna width are included. [PERSON] et al. [41] analyze matching network elimination in wideband rectennas, aiming to significantly improve efficiency in wireless transmission and RF energy harvesting through a bow-tie design. On the other hand, [PERSON] and [PERSON] [64] present research on synthesizing a bow-tie antenna specifically designed to power microwatt sensors, leveraging energy from available ambient RF signals. Finally, [PERSON] et al. [77] focus on designing a quad-band rectenna optimized for ambient RF energy harvesting. This design is conceived to operate across four distinct frequency bands, providing high versatility for various RF energy harvesting applications. * Fractal  
Antenna design is a fundamental factor in shaping the performance of energy harvesting systems, and each type of antenna offers distinct advantages depending on the application at hand. For instance, fractal antennas are widely recognized for their remarkable efficiency and ability to operate across multiple frequency bands. Their intricate geometric patterns enable miniaturization while still maintaining performance, making them ideal for devices where space is at a premium, such as in wearable technology, as shown in Table 11.  
[PERSON] et al. [46] present a compact rectenna design that integrates a rectifier circuit and a fractal-based microstrip patch antenna. Incorporating a fractal design for the antenna is fundamental to achieving significant circuit area reduction, making this rectenna highly suitable for energy harvesting and wireless power transfer applications where size is a critical factor. The antenna is optimized to harvest RF energy in the 2.45 GHz ISM band. [PERSON] et al. [29] present a fractal antenna designed for an energy harvesting system that utilizes ambient Wi-Fi signals as a source. This study focuses on developing an efficient system capable of capturing and converting energy from available Wi-Fi networks in the environment, transforming them into a usable power source. The article by [PERSON] et al. [40] presents research focused on creating a hybrid algorithm that combines a bacterial foraging optimization (BFO) algorithm with an artificial neural network (ANN) for designing a compact fractal antenna intended for a rectenna system. [PERSON] et al. [102] present a fractal antenna for the creation of a hybrid energy storage system combining a rectenna and photovoltaic cells to power low-power wireless IoT devices. \begin{table}
\begin{tabular}{c c c c c} \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[64] & 1.9, 2.2 & — & Not conventional & 14.78 \\ \hline
[77] & 0.89, 1.83, 2.19, 2.45 & 2900 & FR4 & 1.6 \\ \hline
[41] & 0.9–1.1, 1.8–2.5 & 200, 700 & Rogers RT6002 & 1.92 \\ \hline \end{tabular}
\end{table} 
Table 10: Construction characteristics of antennas within the bow-tie geometry classification. Resonant frequencies, bandwidth at each frequency, substrate used, and antenna width are included. \begin{table}
\begin{tabular}{c c c c c} \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[46] & 2.45 & \(\approx\)40.9 & FR4 & 2.5 \\ \hline
[40] & 2.45 & 152 & RT Duroid & 3.175 \\ \hline
[102] & 0.9, 1.2, 1.83, 2.4, 2.6, 3 & — & FR4 & 1.6 \\ \hline
[29] & 2.45 & \(\approx\)180 & Rogers RO6010 & 0.5 \\ \hline \end{tabular}
\end{table}