Homogenization of plain weave composites with imperfect microstructure: Part II--Analysis of real-world materials

A two-layer statistically equivalent periodic unit cell is offered to predict a macroscopic response of plain weave multilayer carbon-carbon textile composites. Falling-short in describing the most typical geometrical imperfections of these material systems the original formulation presented in (Zeman and \v{S}ejnoha, International Journal of Solids and Structures, 41 (2004), pp. 6549--6571) is substantially modified, now allowing for nesting and mutual shift of individual layers of textile fabric in all three directions. Yet, the most valuable asset of the present formulation is seen in the possibility of reflecting the influence of negligible meso-scale porosity through a system of oblate spheroidal voids introduced in between the two layers of the unit cell. Numerical predictions of both the effective thermal conductivities and elastic stiffnesses and their comparison with available laboratory data and the results derived using the Mori-Tanaka averaging scheme support credibility of the present approach, about as much as the reliability of local mechanical properties found from nanoindentation tests performed directly on the analyzed composite samples.Comment: 28 pages, 14 figure

Automated design optimisation and simulation of stitched antennas for textile devices

This thesis describes a novel approach for designing 7-segment and 5-angle pocket and collar planar antennas (for operation at 900 MHz). The motivation for this work originates from the problem of security of children in rural Nigeria where there is risk of abduction. There is a strong potential benefit to be gained from hidden wireless tracking devices (and hence antennas) that can protect their security. An evolutionary method based on a genetic algorithm was used in conjunction with electromagnetic simulation. This method determines the segment length and angle between segments through several generations. The simulation of the antenna was implemented using heuristic crossover with non-uniform mutation. Antennas obtained from the algorithm were fabricated and measured to validate the proposed method.This first part of this research has been limited to linear wire antennas because of the wide range and flexibility of this class of antennas. Linear wire antennas are used for the design of high or low gain, broad or narrow band antennas. Wire antennas are easy and inexpensive to build. All the optimised linear wire antenna samples exhibit similar performances, most of the power is radiated within the GSM900 frequency band. The reflection coefficient (S11) is generally better than -10dB. The method of moment (MoM-NEC2) and FIT (CST Studio Suite 2015) solvers were used for this design. MATLAB is used to as an interface to control computational electromagnetic solvers for antenna designs and analysis. The genetic algorithm procedures were written in MATLAB. The second part of the work focuses on meshed ground planes for applications at 900 MHz global system for mobile communications (GSM), 2.45 GHz industrial, scientific, and medical (ISM) band and 5 GHz wearable wireless local area networks (WLAN) frequencies. Square ground planes were developed and designed using linear equations in MATLAB. The ground plane was stitched using embroidery machines. To examine the effect of meshing on the antenna performance and to normalise the meshed antenna to a reference, solid patch antenna was designed, fabricated on an FR4 substrate. A finite grid of resistors was created for numerical simulation in MATLAB. The resistance from the centre to any node of a finite grid of resistors are evaluated using nodal analysis. The probability that a node connects to each node in the grid was computed. The circuit model has been validated against the experimental model by measurement of the meshed ground plane. A set of measurement were collected from a meshed and compared with the numerical values, they show good agreement.</div

Lightweight and Flexible Textile Metasurfaces and Array Antennas via Flat-Knitting

Metasurfaces are a class of electromagnetic devices that shape an outgoing wavefront to realize desired device functionalities. The predominant majority of current generation metasurfaces are compact, planar rigid devices that aim to replace traditional bulk optical components and radio-frequency antennae. The development of lightweight and flexible metasurfaces could enable new classes of conformal metasurfaces that can be used to cover non-planar surfaces in applications such as wearable antennas or carpet cloaks and also help further reduce the weight of existing planar rigid devices and enable easy stowage of very large aperture devices, possibly leading to a new class of lightweight, easily stowable and deployable devices, useful in both terrestrial and space-based communications and sensing applications. Textiles represent an interesting platform for lightweight and flexible metasurfaces. Leveraging highly-established fabric production techniques such as knitting, weaving, and embroidery could enable the production of relatively cheap, flexible devices on an industrial scale. Knitting, in particular, represents a highly-established and highly-flexible fabric production technique capable of engineering the shape and mechanical properties of the fabric alongside the fabric's electromagnetic response via patterning the fabric with metallic antenna archetypes. The works presented in this thesis aim to demonstrate novel designs and means to fabricate flexible, lightweight metasurfaces and array antennas that rely on scalable, established production techniques and commercially available materials. Further aims of this thesis are to provide a detailed account of the fabrication, design, and performance of textile metasurfaces and array antennas and provide limited commentary on the commercial prospects, limitations, and fabrication demands of the works presented herein. The first works detailed in this thesis concern a novel type of lightweight and flexible metasurface produced via flat-knitting using float-jacquard colorwork knitting to pattern the fabric with an array of antenna archetypes. This includes novel demonstrations of a textile metalens and textile metasurface vortex beam generator capable of producing, respectively, focused or collimated Gaussian and vortex beams, albeit with low efficiency. Detailed modeling and analysis of the structure of the float-jacquard knit flexible metasurfaces identify a key cause of the degraded performance of these devices, the irregular float geometry, in particular contact between metallic floats that give rise to a strong specular reflection, offering insight into future avenues for further optimization. The second work detailed in this thesis concerns a flat-knit flexible array antenna consisting of four Yagi-Uda antennas. A different colorwork knitting approach, intarsia knitting, is used to pattern the antenna elements into the fabric. The intarsia knit flexible array antenna performs comparably to other flexible end-fire antennas with a gain of 6.94 dB and forward-to-backward power ratio of 9.25 dB, offering a compelling platform for small arrays of flexible antennas, which can be integrated into wearable garments and textiles. The final work detailed in this thesis concerns another flat-knit flexible metasurface, a metalens, knit using intarsia colorwork. The intarsia knit metasurface has a peak focusing efficiency of 47.46%, and, when oriented as reflectarray, a peak gain of 24.71 dB. The large aperture intarsia knit flexible metasurfaces demonstrate comparable performance to existing flexible metasurfaces, making them far more suitable for commercial applications, albeit with higher fabrication demands than float-jacquard knit devices

Coherent lensless imaging techniques using terahertz radiation

Terahertz (THz) radiation denotes the portion of the electromagnetic spectrum lying between the infrared and microwave bands, corresponding to frequencies in the range 0.1-10 THz. The most intriguing feature of using THz waves is their ability to penetrate several non-conducting and optically opaque materials, such as plastics, textiles, paper, and some building materials as well as intrinsic semiconductors. While this property is also shared by microwaves, THz radiation provides a better spatial resolution thanks to the shorter wavelength, thereby imaging hidden objects with sub-millimeter resolution. The non-ionizing nature of THz radiation when it interacts with living tissues also makes THz imaging techniques promising for biomedical and biological applications. In this thesis, I focus on the development and implementation of THz imaging techniques. All the techniques presented here belong to the realm of coherent lensless imaging, aiming at reconstructing the amplitude and phase of the wavefront diffracted by an unknown object, illuminated with coherent radiation, based on measurements of the intensity of their diffraction pattern recorded with a camera. The fact that the imaging process is carried out fully computationally and without the need of lenses has a crucial impact on the experimental setup, which is therefore compact and can be better tailored to real-life applications. In particular, I am going to discuss both theoretical and experimental aspects of synthetic aperture THz off-axis digital holography, the first experimental demonstration of THz ptychography and how to image objects hidden behind weakly and strongly diffracting barriers. A potential biomedical application for such THz imaging techniques will also be suggested

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium