arXiv subjects

John S. Ho

Publications and source records attributed to John S. Ho.

5 recordsLinked to original sources

arXiv2025

Conformal Wide-Angle Scanning Leaky-Wave Antenna for V-Band On-Body Applications

Wearable on-body millimeter-wave (mmWave) radars can provide obstacle detection and guidance for visually impaired individuals. The antennas, being a crucial component of these systems, must be lightweight, flexible, low-cost, and compact. However, existing antennas suffer from a rigid form factor and limited reconfigurability. This article presents a low-profile, fast scanning leaky-wave antenna (LWA) operating in the unlicensed V-band (57-64 GHz) for on-body applications such as lightweight portable frequency modulated continuous wave (FMCW) radars. The novel meandering microstrip design allows independent control of gain and scanning rate (rate of change of main beam pointing direction with frequency). Experimental results show that the LWA achieves a realized gain above 10 dB with a fan-beam steering range in the H-plane from -35{deg} to 45{deg} over the operating frequency band, while the half power beamwidth (HPBW) is within 20{deg} in planar condition. To assess the on-body applicability, the antenna's performance is evaluated under bending. When placed on the knee (corresponding to 80 mm radius), the beam steers from -25{deg} to 55{deg} with a maximum realized gain degradation of 1.75 dB, and an increase of HPBW up to 25{deg}. This demonstrates the LWA's robustness in conformal conditions, while maintaining beam-forming and beam-scanning capabilities. Simulations confirm that the LWA's ground plane minimizes user exposure, adhering to international guidelines. Finally, we demonstrate a 2-D spatial scanning by employing an array of twelve LWAs with phased excitation, enabling beam-forming in the E-plane from -50{deg} to 50{deg}, while the HPBW remains below 20{deg}. Mutual coupling analysis reveals that isolation loss and active reflection coefficient remain below 15 dB throughout the operating band.

physics.app-ph↗

arXiv2025

Full-body NFC: body-scale near-field sensor networks with machine-knittable meandered e-textiles

Wireless body networks comprising battery-free on-body sensors and textile-based wireless readers can enable daily health monitoring and activity tracking by continuously monitoring physiological signals across the body. However, previous textile-based wireless networks made of coils or antennas have limited the data and power transmission area because covering the whole body results in undesirable levels of electromagnetic interactions with the body, degrading the scalability, power consumption, and data rate. Here, we report Full-body NFC, digitally-knitted electronic textiles based on a twin meander coil design that enables body-scale near-field communication (NFC) with battery-free sensor tags arbitrarily placed around the body. Full-body NFC features i) a meander coil that enhances the magnetic field intensity on the body's surface while suppressing undesired interactions with deep tissues, in addition to ii) paired identical coil structure that enables highly-sensitive and motion-robust NFC using a differential architecture. Additionally, industrial digital knitting machines loaded with conductive yarn allow the integration of the Full-body NFC system into daily garments supporting approximately $70-80\%$ large-scale NFC-enabled area of the body. We demonstrate Full-body NFC could achieve mW-class energy-efficient near-field sensor networks with hundreds of kbps-class NFC battery-free sensor tags occupying less than $0.3\%$ of the coverage area under severe body movements.

cs.HC↗

arXiv2023

Wireless powering efficiency of deep-body implantable devices

The wireless power transfer efficiency to implanted bioelectronic devices is constrained by several frequency-dependent physical mechanisms. Recent works have developed several mathematical formulations to understand these mechanisms and predict the optimal operating conditions. However, existing approaches rely on simplified body models, which are unable to capture important aspects of wireless power transfer. Therefore, this paper proposes the efficiency analysis approach in anatomical models that can provide insightful information on achieving the optimum operation conditions. First, this approach is validated with a theoretical spherical wave expansion analysis, and the results for a simplified spherical model and a human pectoral model are compared. The results show that although a magnetic receiver outperforms an electric one for near-field operation and both sources could be equally employed in far-field range, it is in mid-field that the maximum efficiency is achieved with an optimum frequency between 1-5 GHz depending on the implantation depth. The receiver orientation is another factor that affects the efficiency, with a maximum difference between the best and worst-case scenarios around five times for the electric source and over 13 times for the magnetic one. This approach is used to analyze the case of a deep-implanted pacemaker wirelessly powered by an on-body transmitter and subjected to stochastic misalignments. We evaluate the efficiency and exposure, and we demonstrate how a buffered transmitter can be tailored to achieve maximum powering efficiency. Finally, design guidelines that lead to optimal implantable wireless power transfer systems are established from the results obtained with the proposed approach.

physics.app-ph↗

arXiv2015

Planar immersion lens with metasurfaces

The solid immersion lens is a powerful optical tool that allows light entering material from air or vacuum to focus to a spot much smaller than the free-space wavelength. Conventionally, however, they rely on semispherical topographies and are non-planar and bulky, which limits their integration in many applications. Recently, there has been considerable interest in using planar structures, referred to as metasurfaces, to construct flat optical components for manipulating light in unusual ways. Here, we propose and demonstrate the concept of a planar immersion lens based on metasurfaces. The resulting planar device, when placed near an interface between air and dielectric material, can focus electromagnetic radiation incident from air to a spot in material smaller than the free-space wavelength. As an experimental demonstration, we fabricate an ultrathin and flexible microwave lens and further show that it achieves wireless energy transfer in material mimicking biological tissue.

physics.optics↗

arXiv2015

Self-tracking Energy Transfer for Neural Stimulation in Untethered Mice

Optical or electrical stimulation of neural circuits in mice during natural behavior is an important paradigm for studying brain function. Conventional systems for optogenetics and electrical microstimulation require tethers or large head-mounted devices that disrupt animal behavior. We report a method for wireless powering of small-scale implanted devices based on the strong localization of energy that occurs during resonant interaction between a radio-frequency cavity and intrinsic modes in mice. The system features self-tracking over a wide (16 cm diameter) operational area, and is used to demonstrate wireless activation of cortical neurons with miniaturized stimulators (10 mm$^{3}$, 20 mg) fully implanted under the skin.

q-bio.NC↗