Dissipation Factors of Spherical Current Modes on Multiple Spherical Layers

Radiation efficiencies of modal current densities distributed on a spherical shell are evaluated in terms of dissipation factor. The presented approach is rigorous, yet simple and straightforward, leading to closed-form expressions. The same approach is utilized for a two-layered shell and the results are compared with other models existing in the literature. Discrepancies in this comparison are reported and reasons are analyzed. Finally, it is demonstrated that radiation efficiency potentially benefits from the use of internal volume which contrasts with the case of the radiation Q-factor.Comment: 5 pages, 5 figure

Wireless powering efficiency assessment for deep-body implantable devices

Several frequency-dependent mechanisms restrict the maximum achievable efficiency for wireless powering implantable bioelectric devices. Similarly, many mathematical formulations have been proposed to evaluate the effect of these mechanisms as well as predict this maximum efficiency and the corresponding optimum frequency. However, most of these methods consider a simplified model, and they cannot tackle some realistic aspects of implantable wireless power transfer. Therefore, this paper proposed a novel approach that can analyze the efficiency in anatomical models and provide insightful information on achieving this optimum operation. First, this approach is validated with a theoretical spherical wave expansion analysis, and the results for a simplified spherical model and a bidimensional human pectoral model are compared. Results have shown that even though 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. In addition, 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 an electric source and over 13 times for the magnetic one. Finally, this approach is used to analyze the case of a wirelessly powered deep-implanted pacemaker by an on-body transmitter and to establish the parameters that lead to the maximum achievable efficiency

Small Antenna Options for Ultra-Wideband (UWB) Applications

Ultra-Wideband (UWB) systems provide a means for short range high data rate wireless transmission between electronic devices. Portable devices and in particular, mobile handsets, have the potential to harness the unprecedented connectivity associated with UWB’s high speed, low power data transfer. Over the course of this work, a number of small antenna options for UWB mobile handset applications are presented. Two key subgroups of the 3.1 –10.6GHz UWB band are chosen and suitable antennas designed for both bands. At the upper end of the band, a ceramic planar inverted-F antenna is proposed to cover band groups 3 & 6 (6.3 – 9GHz). At the lower end of the band, a novel Dual-Band PIFA structure is presented and optimised to cover the band group 1 bands (3.1 – 4.8GHz). Design work is carried out using CST Microwave Studio simulation software, and all parameter sweeps of critical dimensions are presented, as well as an in-depth examination of E-fields, Surface Currents and Radiation Patterns for both antennas. Finally measurement prototypes are built up and measured to validate the simulation data. Correlation between measured and simulated results is observed and the performance of the antennas with respect to typical UWB antenna specifications is discussed

Antennas using left handed transmission lines

The research described in this thesis is concerned with the analysis and design of conventional wire antenna types, dipoles and loops, based on the left-handed transmission line approach. The left handed antennas have a unique feature that the wavelength of the induced current becomes shorter with decreasing frequency. The left handed transmission line concept can be extended to construct reduced-size dipole or loop antennas in the VHF frequency band. The use of higher order modes allows orthogonal polarisation to be obtained, which is thought to be a feature unique to these antennas. Efficiency is a key parameter of left handed antennas as the heavy left handed loading increases the resistive loss. A study of the efficiency of small dipole antennas loaded with a left-handed transmission line is specially described, and the comparison with conventional inductive loading dipoles. In a low order mode, the efficiency of L-loading dipole is better with low number of unit cell. If the number of cell increases, CL-loading presents comparable and even better performance. In a high mode the meandered left handed dipole gives the best efficiency due to the phase distribution, presenting orthogonal polarization as well. The optimized dipole loaded with parallel plate capacitors and spiral inductors presents the best performance in impedance and efficiency, even better than the conventional inductive loading. A planar loop antenna using a ladder network of left handed loading is also presented. Various modes can be obtained in the left handed loop antenna. The zero order mode gives rise to omnidirectional patterns in the plane of the loop, with good efficiency. By loading the loop with active components, varactors, a tunable left handed loop antenna with a switchable radiation pattern is implemented. The loop gives an omnidirectional pattern with a null to z axis while working in an n = 0 mode and can switch to a pattern with a null at phi = 45° in the plane of the loop in an n = 2 mode

Miniaturized Antenna Design For Wireless Biomedical Sensors

This thesis is focused on the design and simulation of miniaturized antennas for wireless biomedical sensors. The motivation of the work was to provide a solution for wireless systems that are embedded or placed on the body. Currently, small antennas are on demand to be implanted inside the body or placed closely to the body. The performance of such antennas, gain and efficiency, is affected by the lossy tissues that surround them. The goal of this work was to design antennas that are placed on a living body and integrated with a sensor system implanted in living tissue, to measure the dielectric properties of the tissue. The antenna type that this work was based on is Planar Inverted F Antenna (PIFA). The assumption was that the antenna is placed on skin layer and not embedded inside a tissue layers. A few antennas were designed and simulated. Two major studies were performed. First, an antenna, which was originally proposed in literature for wireless communication systems, was adopted and revised for biomedical applications. The antenna performance while it was on two tissue layers (skin and fat) was studied and optimized. The objective was to understand how miniaturization and the surrounding environment affect the antenna resonance frequency and performance. A second study was performed to design a novel PIFA antenna to improve the performance and reduce the size further

Theory and Simulations of a Conformal Omni-Directional Sub-Wavelength Metamaterial Leaky-Wave Antenna

The detailed theory of a single subwavelength conformal radiator that exploits the resonant properties of thin cylindrical metamaterial shells supporting leaky waves is presented. It is shown and reviewed analytically and numerically how a circularly symmetric resonant leaky mode may be supported by a properly designed subwavelength homogenous cylindrical shell of low negative permittivity. Some physical insights are provided and numerical simulations with a feed point, considering also the finiteness of the antenna in the longitudinal direction, are presented and discussed. Moreover, possibilities and limitations of a practical realization of this setup are mentioned, considering in details the possible anisotropies in the metamaterials

Next Generation of Magneto-Dielectric Antennas and Optimum Flux Channels

abstract: There is an ever-growing need for broadband conformal antennas to not only reduce the number of antennas utilized to cover a broad range of frequencies (VHF-UHF) but also to reduce visual and RF signatures associated with communication systems. In many applications antennas needs to be very close to low-impedance mediums or embedded inside low-impedance mediums. However, for conventional metal and dielectric antennas to operate efficiently in such environments either a very narrow bandwidth must be tolerated, or enough loss added to expand the bandwidth, or they must be placed one quarter of a wavelength above the conducting surface. The latter is not always possible since in the HF through low UHF bands, critical to Military and Security functions, this quarter-wavelength requirement would result in impractically large antennas. Despite an error based on a false assumption in the 1950’s, which had severely underestimated the efficiency of magneto-dielectric antennas, recently demonstrated magnetic-antennas have been shown to exhibit extraordinary efficiency in conformal applications. Whereas conventional metal-and-dielectric antennas carrying radiating electric currents suffer a significant disadvantage when placed conformal to the conducting surface of a platform, because they induce opposing image currents in the surface, magnetic-antennas carrying magnetic radiating currents have no such limitation. Their magnetic currents produce co-linear image currents in electrically conducting surfaces. However, the permeable antennas built to date have not yet attained the wide bandwidth expected because the magnetic-flux-channels carrying the wave have not been designed to guide the wave near the speed of light at all frequencies. Instead, they tend to lose the wave by a leaky fast-wave mechanism at low frequencies or they over-bind a slow-wave at high frequencies. In this dissertation, we have studied magnetic antennas in detail and presented the design approach and apparatus required to implement a flux-channel carrying the magnetic current wave near the speed of light over a very broad frequency range which also makes the design of a frequency independent antenna (spiral) possible. We will learn how to construct extremely thin conformal antennas, frequency-independent permeable antennas, and even micron-sized antennas that can be embedded inside the brain without damaging the tissue.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201