## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
They are often favored in radar and high-frequency communication systems due to their straightforward design and reliable performance, as summarized in Table 7.  
[PERSON] et al. [30] propose direct integration of an antenna into a passive sensor tag, eliminating the need for external batteries and wires. This E-shaped slotted antenna stands out for its high efficiency in RF energy capture, ensuring reliable operation even in environments with limited power sources. On the other hand, [PERSON] et al. [48] present \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[30] & 0.43 & 60 & FR4 & 1.6 \\ \hline
[70] & 1.81, 2.08, 2.45 & 1050 & FR4 & 1.6 \\ \hline
[43] & 2.45, 5.8 & 1100, 2730 & Textile & 1 \\ \hline
[67] & 2.1 & 150, 500 & FR4 & 1.6 \\ \hline
[108] & 1.71, 1.84, 1.94, 2.14 & — & FR4 & 1.6 \\ \hline
[48] & 1.98, 1.88 & \(\approx\)2250 & FR4 & 1.6 \\ \hline
[61] & 2.45 & 600 & FR4 & 0.8 \\ \hline
[73] & 0.9 & 513.9 & FR4 & 0.508 \\ \hline
[85] & 2.45 & 220 & FR4 & 1.6 \\ \hline
[44] & 2.1, 5.8 & 170, 400 & FR4 & 1.6 \\ \hline
[72] & 1.73, 2.47, 2.53 & 200 & FR4 & 1.6 \\ \hline
[99] & 2.4–2.48 & 680, 400 & RO3010 & 0.635 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Construction characteristics of antennas within the slot geometry classification. Resonant frequencies, bandwidth at each frequency, substrate used, and antenna width are included. an evolved microstrip patch antenna design that incorporates strategically placed circular slots to optimize RF energy harvesting in two frequency bands. These modifications enable superior performance in energy harvesting applications. [PERSON] et al. [61] present a circularly polarized rectenna enhanced through integration of L-shaped slots, optimizing efficiency in wireless energy harvesting by effectively adapting impedance between the antenna and power source. Meanwhile, [PERSON] et al. [73] introduce an antenna with a star-shaped slot, specifically designed for RF energy harvesting in the GSM band. This design, capable of operating across a broad frequency spectrum, proves ideal for capturing GSM signals under diverse conditions. [PERSON] et al. [99] present the design of a circularly polarized antenna intended for microwave wireless power transfer in medical or implantable environments. The antenna incorporates square slots and an additional oval-shaped slot in its final stage, optimizing the capture and conversion of microwave energy into usable power. On the other hand, [PERSON] et al. [108] introduce a circularly polarized antenna with a spiral-shaped slot, designed to operate across multiple frequency bands. This design, optimized for IoT, achieves high efficiency in RF energy harvesting. [PERSON] et al. [43] present a dual-band rectenna design, capable of operating at two distinct frequencies, making it a versatile solution suitable for environments with variable RF signals. Its wearable design allows integration into wearable devices, optimizing efficient energy harvesting. This approach represents a promising solution for powering low-power devices in IoT and wearable applications, offering a sustainable and autonomous energy source. [PERSON] et al. [70] present a study focused on developing an integrated slotted antenna designed for wideband frequency operation. The results demonstrate its effectiveness in RF energy harvesting across a varied spectrum. Additionally, the authors introduce a microstrip antenna design with arrow-shaped slots, equipped with two ports and optimized for wireless power harvesting in the LTE (long-term evolution) band. This dual-port design has shown high efficiency in capturing energy within this band. On the other hand, [PERSON] et al. [44] present an antenna specifically designed to operate in two distinct frequency ranges, achieving effective RF energy harvesting in both bands.