## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
[104] design a multi-channel, implantable cubic rectenna system based on MIMO (multiple-input multiple-output) technology, aimed at enhancing wireless power transfer in biorelemetry applications. The most relevant aspect here is the implementation of antennas with CP diversity distributed in a cubic format. This antenna design enables \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Frequency (GHz)** & **BW (MHz)** & **Substrate** & **Thickness (mm)** \\ \hline
[103] & 2.3–2.5, 3.5–\(>\)9 & 200, \(>\)6000 & FR4 & 0.8 \\ \hline
[104] & 2.45, 5.8 & 1000, 500 & RT Duroid 5880 & 0.508 \\ \hline
[51] & 2.4 & — & Not conventional & 3.18 \\ \hline
[38] & 2.45 & 150 & Textil & 3.94 \\ \hline
[101] & 2.45, 5.8 & 100, 250 & Rogers 5880 & 0.254 \\ \hline
[76] & 1.8, 3.3 & 2200 & Rogers 5880 & 0.787 \\ \hline
[74] & 2.4 & — & Rogers TMM10i & 3.81 \\ \hline
[105] & 2.45 & 46.2 & FR4 & 0.8 \\ \hline
[79] & 0.868, 0.915, 2.4 & 3200 & Textil & 1, 0.4 \\ \hline
[107] & 2.45 & 410 & RO4350B & 0.762 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Construction characteristics of antennas within the circular geometry classification. Resonant frequencies, bandwidth at each frequency, substrate used, and antenna width are included. more robust and efficient RF energy capture by exploiting diversity in orthogonal space, which is crucial for optimizing power transfer within a biological environment. Meanwhile, [PERSON] and [PERSON] [51] introduce a cufflink-shaped rectenna designed to operate within a single frequency range. This study examines the rectenna's effectiveness in capturing and converting RF energy within a specific frequency interval. Results underscore the single-band rectenna's effectiveness in harvesting and converting RF energy into a usable electrical source. In [76], [PERSON] et al. present the design of an ultra-wideband circular rectenna that employs a complementary resonant structure for efficient microwave energy capture. This design is suitable for both transmission and energy harvesting across a broad frequency spectrum, notable for its versatility in microwave applications. On the other hand, [PERSON] et al. [101] introduce research focused on developing and modeling a dual-band circular rectenna, achieving high efficiency in radio frequency energy harvesting. This device utilizes a dielectric resonant antenna array to optimize energy capture across two distinct frequency bands. Finally, [PERSON] et al. [105] present the design of a simple circular rectenna that excels in compactness and efficiency for RF energy harvesting within the ISM spectrum, offering a practical and efficient solution for specific applications. * Spiral  
In particular, spiral antennas offer broadband, frequency-independent performance, making them exceptionally versatile for wideband applications, while also maintaining a compact form, as illustrated in Table 9.  
[PERSON] et al. [35] focus on designing an Archimedean spiral antenna that operates in the UWB and UHF frequency bands. Similarly, research such as that of [PERSON] et al. [55] presents similar designs aimed at localization and energy harvesting applications in the UHF band. Meanwhile, [PERSON] et al. [95] introduce a triple-band spiral antenna optimized for operation in the UHF, GSM-1800, and UMTS-2100 bands. Additionally, [PERSON] et al. [42] evaluate a flexible spiral-shaped antenna, based on Kapton, designed for near-field wireless power transfer. On the other hand, [PERSON] et al. [96] propose a compact and low-cost spiral antenna, specifically designed for implantable medical receivers. * Bow-Tie  
Bow-tie antennas, with their simple yet effective design, provide a wide bandwidth. This makes them particularly useful in applications such as television broadcasting and radar systems. While they do not always achieve the highest efficiency, their simplicity and broad range make them a flexible choice for various systems, as presented in Table 10.