Simulation, optimization, and implementation of a capacitor-loaded wireless power transfer structure at 6.78 MHz for a target transfer distance of one meter are presented. First, an investigation into the operating principles behind a capacitor-loaded coupled loop structure is carried out via simulation. By adjusting the structural design parameters, it is found that an optimal configuration for this structure is coplanar. A prototype constructed using thin 18 AWG wire for the loops and a variable capacitor for tuning is used to verify simulation. To reduce losses in the wire, thick 9 AWG wire is implemented and measured. Thick wire is necessary for high efficiency yet undesirable for planarization. Since current flows only on the surface of the wire, ‘unwrapping’ that portion yields copper strips that reduce loss by increasing only the width. Thus, by replacing thick wires with copper strips, a planarized structure can be obtained that can reduce ohmic losses without sacrificing its form factor. Next, additional advantages of a capacitor-loaded system, which include reduced electric near-field and the possibility of resonant frequency tuning, are investigated. It is shown by simulation that the capacitor-loaded structure is not strongly affected by nearby dielectric materials since the stored electric energy is significantly lower than the stored magnetic energy in air at resonance. Finally, further optimizations of the structure are considered along with the analytical expressions for maximum efficiency. / text

Non-radiative wireless power transfer using the coupled mode resonance phenomenon has been widely reported in the literature. However, the distance over which such phenomenon exists is very short when measured in terms of wavelength. In this dissertation, how efficient wireless power transfer can be realized in the radiating near-field region beyond the coupled mode resonance region is investigated. First, electrically small folded cylindrical helix (FCH) dipole antennas are designed to achieve efficient near-field power transfer. Measurements show that a 40% power transfer efficiency (PTE) can be realized at the distance of 0.25λ between two antennas in the co-linear configuration. These values come very close to the theoretical upper bound derived based on the spherical mode theory. The results also highlight the importance of antenna radiation efficiency and impedance matching in achieving efficient wireless power transfer. Second, antenna diversity is explored to further extend the range or efficiency of the power transfer. For transmitter diversity, it is found that a stable PTE region can be created when multiple transmitters are employed at sufficiently close spacing. For receiver diversity, it is found that the overall PTE can be improved as the number of the receivers is increased. Third, small directive antennas are investigated as a means of enhancing near-field wireless power transfer. Small directive antennas based on the FCH design are also implemented to enhance the PTE. It is shown that the far-field realized gain is a good surrogate for designing small directive antennas for near-field power transfer. Fourth, to examine the effects of surrounding environments on near-field coupling, an upper bound for near-field wireless power transfer is derived when a transmitter and a received are separated by a spherical material shell. The derived PTE bounds are verified using full-wave electromagnetic simulation and show good agreement for both TM mode and TE mode radiators. Using the derived theory, lossy dielectric material effects on wireless power transfer are studied. Power transfer measurements through walls are also reported and compared with the theory. Lastly, electrically small circularly polarized antennas are investigated as a means of alleviating orientation dependence in near-field wireless power transfer. An electrically small turnstile dipole antenna is designed by utilizing top loading and multiple folding. The circularly polarization characteristic of the design is first tested in the far field, before the antennas are placed in the radiating near-field region for wireless power transfer. It is shown that such circularly polarized antennas can lessen orientation dependence in near-field coupling. / text

This thesis investigates near-field wireless power transfer using multiple transmitters or multiple receivers. First, transmitter diversity is investigated in terms of the power transfer efficiency (PTE). It is found that an improvement in the PTE can be achieved by increasing the number of transmitters. Furthermore, a region of constant PTE can be created with the proper arrangement of transmitters. Next, receiver diversity is investigated in detail. An improvement in the PTE can be also achieved by increasing the number of receivers. However, it is shown that when two or more receivers are closely located, the PTE is reduced due to mutual coupling between receivers. This is termed a ‘sink’ phenomenon, and it is investigated through measurement and simulation. Finally, to account for more general situations of multiple transmitters and multiple receivers, Monte-Carlo simulation is applied. The cumulative distribution function (CDF) is used to interpret the results of the Monte-Carlo simulation. The transmitter and receiver diversity gain can be found based on the CDF. Moreover, the sink phenomenon can be observed by analyzing the CDF curve. Several strategies for positioning receivers are introduced to reduce the sink phenomenon. The results of the Monte-Carlo simulation also show that a saturation in the transmitter or receiver gain is reached when the number of transmitters or receivers is increased. Therefore, increasing the number of transmitters or receivers beyond a certain number does not help increase the PTE. / text

This dissertation is particularly focused on a novel approach of a wirelessly powered neuromodulation system for chronic patients. The inductively coupled transmitter (TX) and receiver (RX) coils are designed through optimization to achieve maximum efficiency. A power management unit (PMU) consisting of a voltage rectifier, voltage regulator along with a stimulation circuitry is also designed to provide pulse stimulation to genetically modified neurons. For continuous health monitoring purposes, the response from the brain due to stimulation needs to be recorded and transmitted wirelessly outside the brain for analysis. A low-power high-data duty-cycled impulse-radio ultra-wideband (IR-UWB) transmitter is designed and implemented using the standard CMOS process. Another focus of this dissertation is the design of a reverse electrowetting-on-dielectric (REWOD) based energy harvesting circuit for wearable sensor applications which is capable of generating a very low-frequency signal from motion activity such a walking, running, jogging, etc. A commercial off-the-shelf (COTS) based and on-chip based energy harvesting circuit is designed for very low-frequency signals. The experimental results show promising progress towards the advancement in the wirelessly powered neuromodulation system and building the self-powered wearable sensor.