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51 | These RF material measurements can also be performed on gases and liquids as they flow past a measurement surface or when they are contained in sample containers. Such means can also detect the densities of gases and liquids, as well as other important features of non-solid materials, such as the composition of gases. A common use of RF materials measurements is to test the conformity of materials, such as radomes and aerospace materials. Specifically, RF materials measurements can be used to detect gaps or nonuniformities in a materials matrix, such as imperfections in aerospace composites. Other notable uses of RF materials measurements are detecting soil properties, uses in medical studies, such as that of neurological cell tissue, chemical composition testing, and during manufacturing for quality control of printed circuit board laminates. Wireless Power Transfer and Energy Harvesting Wireless power transfer (WPT) includes techniques such as inductive coupling, capacitive coupling, and RF-based transmission. These techniques enable the transfer of electrical energy without physical connectors. While inductive and capacitive systems are typically used for short-range charging, RF-based energy transmission enables longer-distance power delivery. Although RF-based WPT is less efficient than guided systems, it can be improved through technologies such as resonant antennas, beamforming, and beam steering, which focus and direct energy toward specific receivers. A simple and relatively common WPT method is the use of contactless charging, such as inductive charging. These methods are already used in everyday applications like wireless charging for smartphones and wearables, and are now emerging in electric vehicle charging and industrial automation. NASA is even researching wireless power transfer through its Ultra-Fast Proximity Charging (UFPC) project, which aims to develop contactless charging systems for small devices operating on the moon (Figure 4). With the modern abundance of RF signals used for communication and sensing in most environments, it is possible to harvest energy passively from ambient RF signals. This enables completely wireless and battery-free operation for certain sensors, reducing the need for maintenance or wired infrastructure. High-power RF transmission also has important implications in areas such as system interference testing, RF exposure research, and electromagnetic resilience engineering. Technologies originally developed for long-range RF power transmission, such as phased array antennas, are now being evaluated Figure 4: An extract from a poster on the NASA UFPC project. (Source: NASA) 4