In this paper, we present a novel low-frequency sensing solution based on the manipulation of the near-field distribution by employing a passive holographic magnetic metasurface, excited by an active RF coil placed in its reactive region. In particular, the sensing capability is based on the interaction between the magnetic field distribution produced by the radiating system and the magneto-dielectric inhomogeneities eventually present within the material under test. We first start from conceiving the geometrical set-up of the metasurface and its driving RF coil, adopting a low operative frequency (specifically 3 MHz) to consider a quasi-static regime and able to increase the penetration depth within the sample. Afterwards, since the sensing spatial resolution and performance can be modulated by controlling the metasurface properties, the required holographic magnetic field mask, describing the ideal distribution at a specific plane, is designed. Then, the amplitude and phase of currents, flowing in each metasurface unit-cell and required to synthetize the field mask, are determined through an optimization technique. Next, the capacitive loads necessary to accomplish the planned behavior are retrieved, by exploiting the metasurface impedance matrix. Finally, experimental measurements conducted on fabricated prototypes validated the numerical results, confirming the efficacy of the proposed approach to detect inhomogeneities in a medium with a magnetic inclusion in a non-destructive manner. The findings show that holographic magnetic metasurfaces operating in the quasi-static regime can be successfully employed for non-destructive sensing, both in industrial and biomedical fields, despite the extremely low frequencies.

Resonant inductive Wireless Power Transfer (WPT) offers a practical solution for supplying energy to consumer, industrial and medical devices. However, conventional WPT systems face severe limitations if one is interested to the dynamic and arbitrary control of the magnetic field distribution. Consequently, our paper explores the design and implementation of an electronically reconfigurable 5×5 magnetic metasurface for low-frequency WPT applications, operating at 3 MHz. The reconfigurable array is excited by a resonant transmitting coil operating in its near-field region. Through an analytical approach, the metasurface operation can be arbitrarily driven, obtaining the unit-cells current distribution which optimally reshapes the magnetic field for a desired application. In addition, the method also enables the precise determination of capacitive loads of each unit-cell for effectively synthetizing the metasurface response. Then, the reconfigurability process is accomplished by integrating varactor diodes within each unit-cell, providing real-time control of the currents pattern across the metasurface according to the analytical approach outputs. Finally, numerical simulations and experimental measurements on a fabricated prototype are presented, fully demonstrating the system's capability to efficiently switch between arbitrary configurations, either concentrating the magnetic field in specific areas or creating a uniform distribution. This dynamic adaptability addresses important challenges in WPT, such as reduction in alignment sensitivity and efficiency loss over distance, with enhanced flexibility, reliability, and safety.

Structural composite materials have gained significant appeal because of their ability to be customized for specific mechanical qualities for various applications, including avionics, wind turbines, transportation, and medical equipment. Therefore, there is a growing demand for effective and non-invasive structural health monitoring (SHM) devices to supervise the integrity of materials. This work introduces a novel sensor design, consisting of three spiral resonators optimized to operate at distinct frequencies and excited by a feeding strip line, capable of performing non-destructive structural strain monitoring via frequency coding. The initial discussion focuses on the analytical modeling of the sensor, which is based on a circuital approach. A numerical test case is developed to operate across the frequency range of 100 to 400 MHz, selected to achieve a balance between penetration depth and the sensitivity of the system. The encouraging findings from electromagnetic full-wave simulations have been confirmed by experimental measurements conducted on printed circuit board (PCB) prototypes embedded in a fiberglass-based composite sample. The sensor shows exceptional sensitivity and cost-effectiveness, and may be easily integrated into composite layers due to its minimal cabling requirements and extremely small profile. The particular frequency-coded configuration enables the suggested sensor to accurately detect and distinguish various structural deformations based on their severity and location.

This paper presents a low-frequency metasurface that generates a highly focused magnetic field when excited by an actively fed transmitting coil for long-range resonant inductive wireless power transfer. A theoretical framework based on magnetic dipole theory is derived to calculate the optimal currents required to shape the magnetic field into a confined hotspot at a target working distance. The optimization process maximizes the correlation between the desired field distribution and the combined contributions of the metasurface and transmitter. To verify the design methodology, practical considerations are deployed to effectively design the metasurface unit cell, an asymmetrically twisted double-layer spiral, able to maximize inductance while maintaining the ohmic losses low. Then, a $5\times 5$ metasurface array, covering an overall area of 22 cm $\times $ 22 cm and operating at 4 MHz, undergoes validation through numerical simulations and experimental testing. Simulations revealed a 40 mm diameter hotspot ( $\sim \lambda $ /1800) at a distance of 10 cm from the transmitter, with experimental results from a PCB prototype confirming these findings and estimating a power transfer 18 dB higher in the focusing region with respect to no metasurface configuration. Besides, the system demonstrated robustness by maintaining focus despite variations in the metasurface-to-transmitter distance. This work provides a practical, scalable solution to address distance limitations in magnetic near field focusing, eliminating the need for guiding structures such as ferrites and simplifying design for WPT applications.