## 2 Related Work
The objective was to compile developments of hybrid antennas and devices capable of harvesting energy from various ambient sources simultaneously, a relevant trend for ubiquitous IoT sensors. The authors first presented principles and materials used in each technique, and then discussed projects and prototypes where antenna functionality is synthesized with harvesting devices--for instance, solar rectennas, where a photovoltaic panel simultaneously acts as an antenna's ground plane; or antennas that extract thermal energy from the environment via integrated thermogenerating elements. This study highlights the interdisciplinarity necessary for developing self-powered nodes: it combines radio frequency considerations with materials science/energy. While it provides descriptive tables of different hybrid configurations and points out their achievements, evaluation remains largely qualitative. No consolidated quantitative performance analysis is conducted. The review concludes by emphasizing the need for developing uniform testing methodologies and for further studying antenna-rectifier co-optimization when integrating multiple sources. [PERSON] et al. [26]--this time with an expanded team--published another extensive review specifically focused on RF energy harvesting technologies. This article acts almost as a focused update to their previous work, now concentrating on rectenna design and RF applications, given that RF harvesting has positioned itself as a prominent alternative to other sources in certain scenarios. The authors highlight the abundant ambient RF sources available--from mobile and Wi-Fi signals to radio/TV and local wireless networks--that can be exploited. They systematically cover each component of an RFEH system: receiving antennas, rectifier circuits, impedance-matching networks between antenna and rectifier, and power management modules. For each category, they offer an overview of the most representative recent designs, discussing their pros and cons under various metrics. Likewise, they examine flexible and portable antennas, given the interest in wearables and body-worn IoT devices, commenting on efficiency results under curvature or proximity to body. This review also dedicates sections to recent RFEH applications, from autonomous industrial sensors and active RFID tags to wearable health nodes, describing cases where harvested RF energy complements or replaces batteries. However, similar to previous studies, it does not perform global statistical meta-analyses: performance comparisons are presented on a case-by-case basis but are not normalized under a common framework. The authors emphasize the difficulty of achieving high efficiency with weak ambient signals and the importance of advances in semiconductor technology to improve rectifiers. [PERSON] et al. [27] exclusively focused on the antenna component within RFEH/WPT systems, reflecting increasing specialization in recent reviews. Their article provides a comprehensive overview of advancements in receiving antennas for RF harvesting, organizing antennas into four categories: low-profile antennas, multiband antennas, circularly polarized antennas, and antenna arrays. For each category, they present and compare multiple contemporary designs, analyzing their design principles and reported performance. Regarding low-profile antennas, they discuss the use of high-permittivity dielectric materials or fractal geometries to reduce size; for multiband antennas, they review modified monopole structures and slotted patches with resonant elements, among others, enabling concurrent coverage of multiple frequencies; for circularly polarized antennas, they evaluate slotting or dual-feed techniques to achieve dual polarizations, useful in capturing signals indifferent to orientation; and for arrays, they examine how array configurations increase captured power and directivity. The authors thoroughly discuss the performance of these antennas, consistently drawing upon data from original articles.