## 2 Related Work
Furthermore,they dedicate a section to current design and manufacturing challenges: they point out difficulties such as losses in flexible materials, extreme miniaturization for IoT, and integration of antennas into compact devices. They also identify emerging trends, such as the use of metamaterials and additive manufacturing to create more efficient RFEH antennas. In their final reflections, the authors highlight several open issues: the need to improve the overall conversion efficiency of retennas, the challenge of creating designs that maintain performance in dynamic environments, and the lack of standards for evaluating and comparing RFEH systems. Similar to other reviews, this work does not conduct an aggregated numerical meta-analysis; however, by confining itself to antennas, it was possible to include some direct comparisons. Collectively, these studies (2013-2022) illustrate the evolution of research in RFEH, transitioning from relatively specific analyses to much more comprehensive reviews. Clear trends emerge: earlier reviews prioritized taxonomy and classification (organizing proposals by source type or technique), while more recent ones delve into specific design details and technical challenges (maximizing efficiency at low power, miniaturization, multiband operation). A progressive specialization is also evident. For instance, [PERSON] et al. [5] and [PERSON] et al. [27] focus exclusively on antennas and retennas, reflecting that the subfield of RFEH antennas has matured enough to warrant dedicated reviews. In contrast, works by [PERSON] et al. [25; 26] cover antennas, rectifiers, and power management, attempting to integrate a complete system perspective. Despite the variety in approaches, all studies consistently highlight common challenges: the low power of ambient RF signals, losses in rectification circuits, the necessity of broadband impedance matching, and size limitations when integrating these solutions into real-world IoT devices. Despite these significant contributions, a critical review of the literature reveals common methodological gaps. First, none of the previous studies conducted a statistical meta-analysis of published experimental results. Each review typically summarizes efficiencies or powers obtained in some exemplary works, but there is no consolidated analysis that combines data from multiple studies to draw conclusions with statistical validity. Second, many reviews are limited to qualitative comparison due to heterogeneous conditions: each author evaluates their rectenna under different frequencies and powers, making direct comparison difficult. This lack of normalization is a gap that a new meta-analysis could address by extrapolating efficiencies to common conditions or applying unified models to compare designs. Third, we observe that most reviews segmented problem by discipline, but few fully integrated an interdisciplinary perspective. This is crucial in RFEH: an antenna's performance cannot be divorced from the rectifier it powers. While the analyzed studies have made significant contributions, a need persists for a new systematic review with meta-analysis to address the identified gaps. Recent trends show growing interest and sufficient published data to attempt a rigorous quantitative synthesis.