## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
[88] present a fractal loop antenna based on Hilbert geometry, designed for triple-band rectenna applications. Fabricated on an FR4 substrate, this antenna utilizes a coaxial feeder and incorporates fractal elements within a square loop to reduce operating frequency and enhance bandwidth. The fractal structure achieves multiband behavior. A 1 \(\times\) 2 array is implemented with three feeding configurations, where the unidirectional fed model stands out for its omnidirectional radiation pattern and improved gain. This antenna is optimized for integration into IoT systems and RF energy harvesting. [PERSON] et al. [63] presents two antenna designs for RF energy harvesting in an IoT system. The first design is a 2 \(\times\) 2 circularly polarized (CP) antenna array, fabricated \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[66] & 2.45 & — & FR4 & 1.6 \\ \hline
[39] & 2.4 & \(\approx\)200 & RT Duroid 5880 & 3.175 \\ \hline
[60] & 0.8, 2.45 & 1650 & FR4 & 1.5 \\ \hline
[91] & 2.4 & 97 & FR4 & 1.6 \\ \hline
[88] & 1.42, 1.92, 2.42 & 70, 70, 60 & FR4 & 1.6 \\ \hline
[63] & 1.8, 1.9, 2.1, 2.4, 2.6 & — & FR4, Rogers & 1.6, 1.525 \\ \hline
[57] & 2.45, 5.8 & \(\approx\)7500 & Not conventional & 0.8 \\ \hline
[65] & 2.45, 5.8 & \(>\)7600 & Not conventional & 0.8 \\ \hline
[49] & 2.45 & 100 & Rogers RT/duroid 6006 & 2.54 \\ \hline
[84] & 2.479, 2.465 & 44, 61 & RO580, FR4 & 1.57, 1.6 \\ \hline
[53] & 9.9 & 600 & RO4003C & 0.81 \\ \hline
[83] & 2.4 & 24.54, 14.02 & Textil & 0.08 \\ \hline
[80] & 0.95, 1.81, 2.4, 2.90 & 2200 & FR4 & 1.6 \\ \hline
[37] & 2.4 & 320 & FR4 & 1.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Construction characteristics of antennas within the array geometry classification. Resonant frequencies, bandwidth at each frequency, substrate used, and antenna width are included. on an FR4 substrate, operating in the 2.45 GHz band and utilizing a sequential feeding technique to achieve circular polarization. This array offers a multi-directional radiation pattern, ideal for capturing RF energy from sources with unknown orientation. The second design is a dual-linear polarized antenna array (DLPAA), operating in multiple bands with high radiation efficiency (\(\approx\)95%) and dual polarization, maximizing energy capture in environments with polarization diversity. [PERSON] et al. [80] present a quad-band, multiport rectenna. Its primary novelty lies in using multiple frequency-dependent antenna ports. This enables the system to fully exploit available frequency bands (Global System for Mobile Communications (GSM)-900, GSM-1800, third generation (3G), and Wi-Fi), as well as spatial and polarization diversity, maximizing harvested RF energy. The antenna's multiport configuration is crucial for its ability to capture energy from different sources and directions simultaneously, making it a robust, high-performance solution for energy harvesting in urban environments with multiple RF signals present. [PERSON] et al. [83] describe the design, fabrication, and characterization of a \(2\times 2\) textile rectenna array intended for RF energy harvesting in the 2.4 GHz (Wi-Fi) band. The key feature of this work lies in implementing electromagnetically coupled microstrip patch antennas fabricated on a textile substrate, offering significant advantages in terms of flexibility, lightness, and integrability into clothing or wearable devices. As an array, the antenna allows for greater incident power capture and improved overall system efficiency. The electromagnetically coupled configuration also contributes to optimizing the antenna's bandwidth and radiative efficiency, crucial aspects for effective energy harvesting. * Slot  
For applications requiring linear polarization and precise directional control, slot antennas offer a compelling solution.