## 4 Results
#### 4.1.1 Antenna Architectures for Energy Harvesting
This approach enables innovative applications like wireless charging of low-power devices, integrating rectennas and booster circuits that maximize energy conversion efficiency. Furthermore, using advanced materials like Kapton substrate and flexible manufacturing techniques enhances these antennas' adaptability and robustness for modern applications. In particular, [PERSON], [PERSON], et al. [71] highlight the design of a compact metallic antenna with an edge structure operating in two frequency bands: 925 MHz and 2450 MHz. This antenna enables efficient ambient RF signal harvesting. It achieves radiation efficiencies of 47% and 89%. Its dual-resonant design enhances RF-to-DC conversion efficiency by optimizing capacitive coupling and impedance matching across both bands. Similarly, [PERSON] et al. [100] present a dual-band implantable antenna (ISM: 900 MHz and 2.4 GHz) designed for medical applications, specifically for RF energy harvesting and data transmission. This antenna utilizes multiple radiating branches and a C-shaped slot to achieve bandwidths of 44.2% and 33.5%. The lower band is used for RF energy harvesting via a rectifier with 52% efficiency at 5 dBm, while the upper band facilitates communication with external medical devices. The system described in [PERSON] et al. [78] operates simultaneously in UHF and frequency modulation (FM) bands, utilizing a rectangular antenna design fabricated on low-cost FR4. This system, which requires no alignment, achieves a radiation efficiency of \(>\)85% and harvests up to 231 \(\upmu\)W and 885 \(\upmu\)W under real-world conditions, with a sensitivity of \(-\)18 dBm. Its innovation lies in its ability to capture energy from all directions and across multiple frequencies, overcoming the limitations of unidirectional designs, making it ideal for autonomous IoT/WSN applications. * Array  
Array antennas, though known for their compact design, tend to lag behind in terms of efficiency. However, they offer a highly directive radiation pattern, which is essential in applications like radar or communication systems where precise beam control is a priority, as shown in Table 6.  
[PERSON] and [PERSON] [37] conducted a study focused on creating a planar dipole antenna array placed over a square reflector. [PERSON] et al. [39], [PERSON] and [PERSON] [53], [PERSON] et al. [84], and [PERSON] et al. [49] concentrate on researching methods and techniques for efficient RF energy harvesting antennas with varied polarizations. Moreover [PERSON] et al. [57] focus on developing a printed antenna that utilizes Hilbert-type metamaterials and is constructed on organic substrates with the objective of improving efficient RF energy harvesting. In another study, [PERSON] [65] employs metamaterials in designing an antenna array capable of operating in ultra-wideband (UWB) frequency ranges, demonstrating the versatility of metamaterials in advanced antenna systems. [PERSON] et al. [66] present a microstrip patch array operating at 2.45 GHz, fabricated on an FR4 substrate, designed to maximize gain and spectral selectivity. This antenna incorporates an electromagnetic bandgap (EBG) structure and an integrated filter, achieving harmonic suppression and high gain. In contrast, [PERSON] et al. [91] utilize a rectangular patch antenna with inset feed in a dual array, also operating at 2.45 GHz. Fabricated on FR4, it achieves a gain of 7.73 dB. Characterized by a reflection coefficient of \(-\)30 dB, its design with a cooperative feeding network enhances efficiency in RF energy capture. [PERSON] et al. [60] employ a fractal array patch antenna designed for operation in the 2.4-2.5 GHz (Wi-Fi) band. Fabricated on an FR4 substrate, this antenna stands out for its miniaturization, achieved through a fractal design, yielding a reflection coefficient of \(-\)32.66 dB at 2.47 GHz. Its fractal structure optimizes RF energy capture in environments with scattered signals, integrated with a Cockcroft-Walton rectifier for hybrid IoT applications. Meanwhile, [PERSON] et al.