Alon Ludwig

Convex Optimization of Wireless Power Transfer Systems With Multiple Transmitters

Wireless power transfer systems with multiple transmitters promise advantages of higher transfer efficiencies and focusing effects over single-transmitter systems. From the standard formulation, straightforward maximization of the power transfer efficiency is not trivial. By reformulating the problem, a convex optimization problem emerges, which can be solved efficiently. Further, using Lagrangian duality theory, analytical results are found for the achievable maximum power transfer efficiency and all parameters involved. With these closed-form results, planar and coaxial wireless power transfer setups are investigated.

Impact of surface roughness on the effective dielectric constants and subwavelength image resolution of metal–insulator stack lenses

The effective parallel and perpendicular dielectric constants for a multilayer metal–insulator stack are obtained from numerical simulations and compared with analytical homogenization results as a function of wavelength and number of periods. The influence of inevitable film surface roughness on the homogenized dielectric constants, determined from numerical scattered field calculations, is evaluated as a function of roughness. The impact of this roughness on resolution in a subwavelength imaging application gives smoothness guidelines for material deposition.

Near-Field Antenna Arrays for Steerable Sub-Wavelength Magnetic-Field Beams

A two-dimensional array of resonant capacitively loaded loops for magnetic near-field focusing is presented. The array is designed to produce a narrow magnetic-field beam in the near-field with a reduced residual surrounding electric field. It is shown that the array allows subwavelength beam steering and shaping. The array operation is discussed and explained in detail using a simplified circuit model. The observations made based on the model are then utilized to devise an optimization procedure aimed at fine-tuning the array so as to eliminate the need in multiple excitation sources and maximize the achievable magnetic-field beam intensity. The proposed array may prove useful in a variety of applications including the improvement of wireless power transfer devices performance and the creation of efficient external sources for magnetically mediated therapy.

Optimization and design sensitivity of SISO and MISO wireless power transfer systems

The maximum achievable power transfer efficiency (PTE) of a multi-transmitter wireless power transfer (WPT) system can be obtained directly from its impedance matrix and the load at the receiver through closed-form expressions. This opens the way to both evaluating the performance of a system or an operating mode as well as relating permissible performance degradation to component tolerances. The latter also gives insight into the complexity of realizing and optimally operating a particular system in practice.

FDTD Analysis of Sub-Wavelength Focusing Phenomena in Plasmonic Meta-Screens

The Finite-Difference Time-Domain (FDTD) method is used to study the frequency dependence of the transmission through a plasmonic screen with slots of sizes and spacing that were chosen using the spatially shifted beam approach to achieve subwavelength focusing. It is shown that the electric field beamwidth at the focal plane exhibits a resonance-like behavior in the vicinity of the frequency for which the meta-screen was designed to work. At this frequency range, the power flow path is shown to exhibit circulation around the slotted screen. Results are also compared with those given in [1] for a similar device working at a lower frequency range where the metallic screen behaves as a perfect electric conductor.

Modeling and optimization of coverage in London Underground Subway Tunnels

A semiautomatic model extraction procedure is used together with an image-based ray tracer to study the radio coverage in London Underground Subway Tunnels. The simulated received signal strength is compared with that obtained via measurements. The prospects of optimizing the location of access points to improve the coverage in the tunnel is also investigated.

FDTD analysis of meta-screens for sub-wavelength focusing

The Finite-Difference Time-Domain (FDTD) method is used to simulate the electromagnetic problem of transmission through a metallic screen with slots of sizes and spacing that were chosen using the spatially shifted beam approach to achieve subwavelength focusing. The challenges related with the modeling of such sub-wavelength focusing meta-screens are demonstrated via the study of numerical convergence.

Magnetic near-field focusing and optimal wireless power transfer

The relation between subwavelength focusing near-field arrays and arrays designed to maximize power transfer efficiency is investigated. The intuitive expectation that magnetic field focusing around the receiver may maximize power transfer efficiency is not confirmed, when the fields of capacitively loaded loop arrays, designed for optimal power transfer efficiency, are analyzed. The significant role of loss is outlined and interesting aspects of the underlying physics are illustrated.

Magnetism from carbon nanocoil mixtures

A metamaterial based on carbon nanocoil inclusions is suggested as a means to obtain homogenized magnetism. Demonstrated growth techniques for such nanocoils suggest convenient fabrication is possible. The effective permeability of the medium is studied numerically and shown to exhibit a resonance curve in frequency. By fitting the resonance curve to a Lorentzian-based analytic model and exploring its parameter space, the performance of the nanocoil inclusion configuration is evaluated. These results suggest carbon nanocoils should be effective as magnetic materials in the terahertz frequency regime and possibly beyond.

Closed-form optimization of multi-transmitter wireless power transfer systems

While the physical limitations of single-transmitter wireless power transfer (WPT) systems are well-known, systems with multiple transmitters prove difficult to optimize, in order to find their maximum achievable performance. The optimization process leading to a convex quadratic program is outlined and closed-form expressions solving that problem analytically are reviewed. Comparison of results obtained from these formulations and others aquired from analytical modeling show perfect agreement; thus, it is verified that the formulations are equivalent. Hence, they offer a compact and insightful analytical framework for the design and optimization of emerging multi-transmitter WPT topologies.