Tesi sul tema "Microwave holography"

En las pasadas décadas, la investigación en tecnología de terahercios fue únicamente motivada por instrumentación para los campos de astrofísica y ciencias de la tierra. La principal línea de investigación de estos campos comprende la detección, identificación y mapeo mediante espectroscopia molecular de bandas de emisión y absorción de gases a baja presión. Este campo fue el mayor foco de desarrollo que permitió en primer lugar el desarrollo de instrumentación y tecnología a bandas de terahercios. En contraposición con su uso en campos científicos, la radiación de terahercios es una de las bandas de radio-frecuencia menos usadas en el ámbito comercial. La escasez de fuentes, sensores, sub-sistemas e instrumentos ha dificultado en los últimos años la proliferación de aplicaciones para un mayor público de consumo. La combinación de los últimos avances tecnológicos provenientes del campo científico, así como el descubrimiento de nuevas aplicaciones ha despertado de nuevo el interés por este campo, lo que ha supuesto un nuevo impulso económico para el desarrollo a estas frecuencias tanto a nivel público como privado. Además del mencionado interés científico, la radiación de terahercios tiene características muy atractivas como por ejemplo una buena resolución espacial (comparada con menores frecuencias), penetración en materiales, capacidades espectroscópicas, absorción por humedad y niveles bajos de energía. El trabajo desarrollado en esta tesis es parte de un proyecto de investigación a nivel nacional Español denominado Terasense. El interés principal de este proyecto es equipar las instituciones de investigación académicas con un nuevo conjunto de instrumentación y capacidades para poder desarrollar proyectos en el estado del arte en el campo de ondas milimétricas y sub-milimétricas. El objetivo principal de esta tesis es explorar la viabilidad de sistemas de imagen en microondas y ondas milimétricas basados en técnicas holográficas mediante medidas de intensidad. En este documenta se estudia principalmente el uso de la técnica holográfica con referencia desplazada. No solo desde un punto de vista teórico sino especialmente desde un punto de vista experimental y práctico. En la tesis, diferentes experimentos y dispositivos son simulados, diseñados e implementados. La ida y vuelta entre software y hardware ha permitido la creación de un marco de desarrollo para el test de las diferentes técnicas de imagen estudiadas. El rango de frecuencia escogido como meta para este proyecto es la banda de frecuencia W (75-110 GHz). Sin embargo, muchos experimentos han sido desarrollados primero en banda X (8-12 GHz) para desarrollar la experiencia necesaria requerida para trabajar a frecuencias superiores en el rango de las ondas milimétricas.<br>In past decades research in terahertz technology was solely motivated by instruments for topics such as astrophysics, planetary and earth sciences. Molecular line spectroscopy detection, identification and mapping of thermal emission and absorption signatures from low pressure gases comprised the main focus for most scientific requirements and motivated the development of terahertz instrumentation and technology. In spite of the scientific contributions of terahertz radiation, its spectrum is still one of the least used electromagnetics bands in commercial use. The unavailability of sources, sensors, sub-systems and instruments has been a cumbersome issue over the past years for its wide-spread use in commercial instrumentation. The combination of technological advances coming from the space-based community, along with the emergence of new applications, have managed to drive again the interest from both public and private sectors which has renown and skyrocketed the funding and research in terahertz applications. Aside from the aforementioned scientific interest, terahertz radiation has appealing characteristics such as good imaging resolution (as compared to lower frequencies), material penetration, spectroscopic capabilities, water absorption and low energy levels. The work of this thesis is part of a Spanish national research project called Terasense. The main focus of the project is to equip national academic research institutions with a completely new set of instrumentations and capabilities in order to advance towards the current state of the art in millimeter and sub-millimeter wave technologies. The main objective of this thesis is to explore the viability of microwave and millimeter-wave imaging systems based on intensity-only holographic techniques. This dissertation is mostly focused on the Off-Axis Holography technique. Not only from a theoretical perspective but specially from an actual implementation standpoint. In order to do so, different experimental setups and devices have been designed and manufactured. Iteration between hardware and software has created a framework for devising and testing different imaging techniques under consideration. The frequency range W-Band (75-110 GHz) has been chosen as the main goal for all systems under study, however different setups will first be constructed, characterized and tested at X-Band (8-12 GHz) in order to build up the expertise required to work at millimeter-wave frequencies.

Gabor's original holography, which is the basic theory of modern microwave holographic techniques, is introduced. By computer simulations, it is demonstrated that the conventional holographic approach can be used as a tool to reconstruct aperture field distributions of an antenna with some constraints. Computer simulations of the theory and technique of the improved microwave holographic approach originally introduced by Rahmat-Samii et al. are carried out. The results show that it can be used for surface distortion diagnosis of large reflector antennas. The physical optics integral formulation is derived by general solutions of the vector wave equations. The necessary theory , which is needed to reconstruct the aperture field from near-field measurements both in a rectangular coordinate system and in a cylindrical coordinate system is developed. It is based on the plane wave spectrum and the vector wave modal expansion of an electromagnetic field. By using a simple dipole and other well-defined antennas, computer simulations have been performed. The results show that the technique is rigorous and applicable. It is also demonstrated that the sampling intervals and the number of sampling points should be chosen carefully in order to obtain a satisfactory resolution of the reconstructed aperture field. Furthermore, the simulations carried out in this work reveal that the real aperture field distribution of a dipole antenna has a maximum point at each end of the antenna. This characteristic can only be obtained at a very close distance to the antenna. This study also reveals the significant contributions of the evanescent waves to the aperture reconstruction. A simple but effective method for examining the evanescent waves from the measured near-field is also presented. By using dipoles and other well known antennas and antenna arrays, the experiments were carried out. The experimental results provide reasonable good agreements with the simulations. The technique proposed is effective and accurate.<br>Doctor of Philosophy (PhD)

Gabor's original holography, which is the basic theory of modern microwave holographic techniques, is introduced. By computer simulations, it is demonstrated that the conventional holographic approach can be used as a tool to reconstruct aperture field distributions of an antenna with some constraints. Computer simulations of the theory and technique of the improved microwave holographic approach originally introduced by Rahmat-Samii et al. are carried out. The results show that it can be used for surface distortion diagnosis of large reflector antennas. The physical optics integral formulation is derived by general solutions of the vector wave equations. The necessary theory , which is needed to reconstruct the aperture field from near-field measurements both in a rectangular coordinate system and in a cylindrical coordinate system is developed. It is based on the plane wave spectrum and the vector wave modal expansion of an electromagnetic field. By using a simple dipole and other well-defined antennas, computer simulations have been performed. The results show that the technique is rigorous and applicable. It is also demonstrated that the sampling intervals and the number of sampling points should be chosen carefully in order to obtain a satisfactory resolution of the reconstructed aperture field. Furthermore, the simulations carried out in this work reveal that the real aperture field distribution of a dipole antenna has a maximum point at each end of the antenna. This characteristic can only be obtained at a very close distance to the antenna. This study also reveals the significant contributions of the evanescent waves to the aperture reconstruction. A simple but effective method for examining the evanescent waves from the measured near-field is also presented. By using dipoles and other well known antennas and antenna arrays, the experiments were carried out. The experimental results provide reasonable good agreements with the simulations. The technique proposed is effective and accurate.

Thesis (Ph.D.) -- University of Western Sydney, 2001.<br>"Submitted in fulfilment of requirements for the degree of Doctor of Philosophy, School of Engineering and Industrial Design, University of Western Sydney" Includes bibliography.

"Near-field diagnosis of antenna arrays is often done using microwave holography; however, the technique of near-field to near-field back-propagation quickly loses its accuracy with measurements taken farther than one wavelength from the aperture. The loss of accuracy is partially due to windowing, but may also be attributed to the decay of evanescent modes responsible for the fine distribution of the fields close to the array. In an effort to achieve better resolution, the difference between these two phase-synchronized near-field measurements is used and propagated back. The performance of such a method is established for different conditions; the extension of this technique to the calibration of small antenna arrays is also discussed. The method is based on the idea of differential backpropagation using the measured/simulated/analytical data in the near field. After completing the corresponding literature search authors have found that the same idea was first proposed by P. L. Ransom and R. Mittra in 1971, at that point with the Univ. of Illinois. This method is basically the same, but it includes a few distinct features: 1. The near field of a (faulty) array under test is measured at via a near field antenna range. 2. The template (non-faulty) near field of an array is simulated numerically (full-wave FDTD solver or FEM Ansoft/ANSYS HFSS solver) at the same distance - an alternative is to use measurements for a non-faulty array. 3. Both fields are assumed (or made) to be coherent (synchronized in phase). 4. A difference between two fields is formed and is then propagated back to array surface using the angular spectrum method (inverse Fourier propagator). The corresponding result is the surface (aperture) error field. This approach is more precise than the inverse Rayleigh formula used in Ransom and Mittra's paper since the evanescent spectrum may be included into consideration. 5. The error field magnitude peaks at faulty elements (both amplitude and phase excitation fault). 6. The method inherently includes all mutual coupling effects since both the template field and the measured field are full-wave results."

Breast microwave imaging (BMI) is a novel modality that complements current breast screening tools. Microwave radar imaging creates a radar cross-section (reflection) map of the breast. The difference in permittivity between healthy and malignant tissue is between 10-50%. This contrast is significantly higher than that obtained with x-rays and supports the use of microwave imaging for breast cancer diagnosis. Prior to widespread clinical use, some areas require further study. Firstly, the performance of three different antennas was carried out, to assess their suitability for a BMI system. Secondly, the sampling constraint of a circular scan geometry was studied and tested using experimental phantoms and these antennas. For accurate breast BMI reconstruction, the transmission speed of the radio waves inside the breast must be determined. The tissue composition of each patient is different, making this task challenging. This work presents an algorithm for wave speed estimation in different mediums.<br>February 2017

Free-space optics (FSO), or wireless optical communications, has received extensive research due to its promise of practically limitless bandwidths. However, FSO has challenges yet to be met for a cost effective realisation. This D.Phil thesis explores a solution using a ferro-electric liquid crystal spatial light modulator (FLC SLM) and binary phase holograms to significantly reduce the hardware complexity of an FSO system with auto-alignment and turbulence compensation. The theory of binary phase hologram is presented and extended to obtain a new algorithm that is suitable for a FLC SLM. The algorithm is able to be used in a demonstration system to broadcast data streams to multiple receivers, showing the capability of using FLC SLM to form any beam configuration. An FSO transmitter is then developed that uses retro-reflectors as markers for the receivers. The transmitter combines an imaging system with the FLC SLM as a reconfigurable beam steering system for acquiring the retro-reflector location. The FLC SLM is also used to reduce aberrations in the optics, resulting in a significant increase in the transmitted beam power density. The accuracy of the acquisition is measured to give a small steering error without the use of a closed loop controller. An optical turbulence simulator, using the principals of binary phase hologram, is constructed to simulate optical beam propagation in turbulent conditions. The simulator accurately produces aberrations that have the same statistics with the theoretical prediction. Analysis of the phase distortion due to turbulence is performed and a wavefront sensorless turbulence compensation method based on the FLC SLM gives significant reduction in calculated bit error rates. New scintillation index derivation for multiple optical beams is described and then used to demonstrate further decrease in bit error rates.

This master’s thesis deals with principle of microwave holography, methods of microwave hologram recording and analyzes some types of holographic antennas from structural and functional point of view. It also includes simulations of selected holographic antennas and their modi?cations, applies optimization on three types of antennas. Finally it reviews measured results of three costructed antennas.

The retrieval of surface profile of a reflector antenna is an important task, mainly for radio astronomical application. Up to now the microwave holographic reconstruction technique has been applied to far field data either directly measured or extrapolated from near field measurements by using the Fourier transform inversion. The main step of this technique is the computation of the induced currents on the reflector surface. In this thesis the relation between the far field (or near field) and the current on the parabolic reflector surface is formulated as a discrete data inverse problem. Since this problem is strongly ill conditioned, an alternative technique is proposed to find a solutionin the least squares sense and to regularize it by using a Singular Value Decomposition. This technique supplies a flexible regularization scheme, able to take into account also the noise level of the data. Moreover it allows to reconstruct a surface distortions map using either far field data or the direct use of Fresnel zone field data, i.e. these data are not used to extrapolate the far field pattern, but directly used by the procedure. A comparison with the standard technique based on the FFT inversion algorithm has been made on a number of different cases, with noise free and noisy far field data generated by a commercial software. The results show that this alternative procedure is able to delect, with a very good accuracy, both the position and the amount of the distortions of the reflector surface starting from both far field and near field data.

The rate at which new communications technologies are being developed has been immense and will continue to intensify for the foreseeable future. This evolution is fuelled by the desire to meet the wants and needs of the global community, by developing devices able to offer ever-increasing functionality, with greater complexity. To achieve this designers are forced to move to higher and higher frequencies. The antenna, as one of the fundamental building blocks of any radiated wave system, and as such, must develop along with the evolution of the communication system be it for, mobile, satellite or point to point systems. Antenna designers need to be able to test antenna, to ensure they exhibit the characteristics to which they were designed. Antenna test becomes progressively more difficult and costly as both, the operating frequency and the size of the antenna increase, especially for the measurement of the antenna Far-Field radiation pattern. Either the distance over which the measurement must be made becomes unfeasibly large or expensive measurement equipment is required to attain the phase component of the antenna field, where traditional methods for measuring close to the antenna are applied. Techniques have been developed to eliminate the need for the expensive phase measurement at reduced distances. Specifically of interest in this thesis, is the optical process of Off-Axis holography. The process allows phase information to be retained in a scalar measurement by use of a phase coherent known reference source. The reference desired reference source is a plane wave, which although possible at optical frequencies with the use of lasers is problematic at microwave frequencies. To date the plane wave characteristic required has been approximated using conventional radiating elements, which degrades the quality of the recorded holographic image. This thesis proposes a novel implementation of the Off-Axis hologram system, for application in the microwave frequency region. The novel system developed here addresses the problem of generating the desired plane wave characteristic. The conventional radiating element used to provide the near plane wave reference is replaced by a synthetic equivalent, which allows the magnitude and phase of the reference beam to be directly controlled at every measurement location required. Practical verification of the novel system proposed has been performed, with comparisons made between the results obtained from the novel technique and standard techniques used in industry. The comparisons show that the novel implementation is valid and able to provide good repeatable results.

Ce travail est concentré sur l'étude de la cosmologie primordiale et en particulier sur l'étude de l'inflation. Après une introduction sur la théorie standard du Big Bang, nous discutons de la physique du CMB et nous expliquons comment ses observations peuvent être utilisées pour définir des contraintes sur les modèles cosmologiques. Nous introduisons l'inflation et nous expliquons sa réalisation la plus simple. Nous présentons les observables et les contraintes expérimentales qui peuvent être utilisées pour mettre des contraintes sur les modèles d'inflation. La possibilité d'observer des ondes gravitationnelles primordiales (GW) produites au cours de l'inflation est examinée. Nous présentons les raisons pour définir une classification des modèles d'inflation et pour introduire le formalisme de la fonction 13 pour décrire l'inflation. En particulier nous expliquons pourquoi, dans ce cadre, nous pouvons naturellement définir un ensemble de classes d'universalité pour les modèles d'inflation. Les motivations théoriques pour soutenir la formulation de l'inflation en termes de ce formalisme sont présentées. Certains modèles généralisés d'inflation sont introduits et l'extension du formalisme de la fonction (3-formalisme à ces modèles est discutée. Enfin, nous nous concentrons sur l'étude des modèles où l’inflation (qui es assumé être pseudo-scalaire) est couplé non-minimalement à des champs de jauge abéliens qui peuvent être présents lors de l'inflation. L'analyse du problème est effectuée en utilisant une caractérisation de modèles d'inflation sur la base de leur comportement asymptotique. Un large éventail d'aspects théoriques et des conséquences d'observation est discuté<br>This work is focused on the study of early time cosmology and in particular on the study of inflation. After an introduction on the standard Big Bang theory, we discuss the physics of CMB and we explain how its observations can be used to set constraints on cosmological models. We introduce inflation and we carry out its simplest realization by presenting the observables and the experimental constraints that can be set on inflationary models. The possibility of observing primordial gravitational wave (GW) produced during inflation is discussed. We present the reasons to deftne a classification of inflationary models and introduce the [3-function formalism for inflation by explaining why in this framework we can naturally define a set of universality classes for inflationary models. Theoretical motivations to support the formulation of inflation in terms of this formalism are presented. Some generalized models of inflation are introduced and the extension of the (3-function formalism for inflation to these models is discussed. Finally we focus on the study of models where the (pseudo-scalar) inflaton is non-minimally coupled to some Abelian gauge fields that can be present during inflation. The analysis of the problem is carried out by using a characterization of inflationary models in terms of their asymptotic behavior. A wide set of theoretical aspects and of observational consequences is discussed

This thesis describes experiments in Microwave Holography using the 26 m radio telescope at the Mount Pleasant Observatory, and the development of data processing methods to overcome a major limitation of traditional holographic measurements. Microwave Holography is a fast and effective technique for measuring the surface profile of reflector antennas. It requires measurement of the amplitude and phase of the antenna's far field radiation pattern. The Fourier transform relationship of the far field pattern to the aperture current function is used to estimate the aperture phase profile, which can be related directly to the surface profile. Using an unmodulated 12.7 GHz carrier signal transmitted by the AUSSAT-1 geostationary satellite, the 26 m diameter surface was measured to an accuracy of ±53 μm with a resolution of 0.6 m. Using a beacon signal, weaker by 20 dB, gave a comparable accuracy of ±64 μm. Each map took about 4 hours to record. In practice only a small part of the complete radiation pattern can be measured, so information about high spatial frequencies in the aperture is lost. This causes detailed structure in the surface error map to be smoothed out. Most large reflectors are constructed with panels, and misaligned panels give discontinuities in surface error which cannot be resolved. This can lead to incorrect assessment of panel positions. Significant improvement can be obtained by recognizing at the outset that the reflector surface is not smooth and continuous, but a collection of individual rigid panels. The Method of Successive Projections is an algorithm, with a simple geometrical interpretation, which allows information about the panel boundaries to be readily incorporated into the data reduction process. The algorithm is straight-forward to apply and is very flexible: aperture blockage effects do not disturb its operation, and panel distortions can be included in the analysis with minimal extra effort. Good results may be achieved while minimizing measurement time. This is an important consideration for high-performance antennas with a busy operating programme.

Microwave holography is a well established method of using the Fourier relationship between an antenna’s current distribution and its complex beam-pattern to produce surface maps of large parabolic antennas. As the final part of a surface upgrade, a holographic map of the HartRAO 26 m telescope was produced. This showed that the surface has an RMS error of 0.45 mm. The measurement used a small reference dish to correlate against and retrieve amplitude and phase values. Due to system phase instabilities, this dish had to be attached to the measured antenna in order to enable sharing a high frequency local oscillator (LO). The movement was modelled and corrected for. However, a slight distortion remained. It is recommended that, either the LO distribution system is stabilised by using multiple PLLs or amplifiers and low loss cables are used to enable moving the reference antenna to a stationary position.

Per mezzo del finanziamento ottenuto con il progetto North Atlantic Treaty Organization, Science for Peace and Security (NATO SPS) G-5014 è stata sviluppata una piattaforma robotica multi-sensore in grado di individuare oggetti sepolti plastici e metallici e generare dati per la successiva classificazione degli ordigni attraverso l’analisi di operatori specializzati. Utilizzare una piattaforma robotica permette di aumentare la sicurezza per gli operatori, perché completamente controllabile in remoto tramite un’interfaccia software web e permette di utilizzare diversi sensori per massimizzare la probabilità di rivelazione delle mine, mantenendo minima la probabilità di ricevere falsi allarmi. I sensori principali installati sono due RADAR operanti nello spettro delle microonde ( ≃2 GHz): un UWB Ground Penetrating RADAR (GPR), sviluppato appositamente per rilevare la posizione dell’oggetto sepolto all’interno dell’area illuminata, capace di rilevare oggetti sepolti durante il moto della piattaforma e un Holographic Subsurface RADAR (HSR), operante ad onda continua e singola frequenza, in grado di generare immagini olografiche che permettono di osservare la forma e le dimensioni degli oggetti sepolti nei primi 15 - 20 cm del sottosuolo e, tramite l’elaborazione con algoritmi di inversione del campo elettromegnetico, permette di ricostruire la scena tridimensionale che si trova di fronte all’apertura sintetica del RADAR. Le immagini che genera questo dispositivo consentono di discriminare gli ordigni da altri oggetti riflettenti le microonde ma del tutto inoffensivi (clutter). Il HSR progettato nel corso del progetto NATO SPS G-5014 costituisce un primo prototipo che soddisfava i requisiti richiesti dal progetto. Il frutto di questo lavoro ha riscosso interesse nella comunità scentifica e presso NATO SPS, generando un seguito: il progetto NATO SPS G-5731, tutt’ora in corso. Nell’ambito di quest’ultimo progetto si colloca il mio lavoro: ho contribuito allo sviluppo di un sistema RADAR per immagini a microonde in grado di migliorare, in termini di qualità di immagini prodotte (incrementando il rapporto segnale-rumore e la risoluzione) e di profondità di penetrazione (studiando le caratteristiche elettromagnetiche del suolo di interesse), le prestazioni del HSR. Mi sono occupato di individuare i parametri su cui poter intervenire: la risoluzione ottenibile applicando la matematica dell’olografia, le tecniche e gli algoritmi di inversione del campo elettromagnetico, lo studio dell’ambiente elettromagnetico irradiato e i requisiti dell’elemento radiante (tipo di antenna, forma, dimensioni, potenza irradiata) reailzzandone uno con la tecnologia della stampa tridimensionale. Ho valutato e studiato una soluzione per migliorare la compatibilità elettromagnetica con il sistema robotico su cui dovrà operare il RADAR. Per realizzare un prototipo funzionante mi sono occupato di definire i requisiti dell’elettronica di pilotaggio e della programmazione dei dispositivi implementati. Questo testo si conclude con la dimostrazione, mediante l’esposizione di prove sperimentali in ambiente controllato, delle prestazioni del nuovo RADAR, evidenziandone le differenze rispetto al HSR originale. ------------- Thanks to the funding obtained with the North Atlantic Treaty Organization, Science for Peace and Security (NATO SPS) G-5014 project, a multi-sensor robotic platform was developed capable of identifying buried plastic and metal objects and generating data for subsequent classification. of ordnance through the analysis of specialized operators. Using a robotic platform allows you to increase safety for operators, because it can be completely remotely controlled via a web software interface and allows you to use different sensors to maximize the probability of mine detection, while keeping the probability of receiving false alarms to a minimum. The main sensors installed are two RADARs operating in the microwave spectrum (≃2 GHz): a UWB Ground Penetrating RADAR (GPR), specially developed to detect the position of the buried object within the illuminated area, capable of detecting buried objects during the motion of the platform and a Holographic Subsurface RADAR (HSR), operating at continuous wave and single frequency, capable of generating holographic images that allow to observe the shape and dimensions of the objects buried in the first 15 - 20 cm of the subsoil and, through the processing with electromagnetic field inversion algorithms, it allows to reconstruct the three-dimensional scene that is in front of the synthetic opening of the RADAR. The images that this device generates allow to discriminate the bombs from other objects reflecting the microwaves but completely harmless (clutter). The HSR designed during the NATO SPS G-5014 project constitutes a first prototype that met the requirements of the project. The fruit of this work has attracted interest in the scientific community and at NATO SPS, generating a sequel: the NATO SPS G-5731 project, which is still underway. My work is part of this last project: I contributed to the development of a RADAR system for microwave images capable of improving, in terms of the quality of images produced (by increasing the signal-to-noise ratio and resolution) and depth of penetration (studying the electromagnetic characteristics of the soil of interest), the performance of the HSR. I worked on identifying the parameters on which to intervene: the resolution obtainable by applying the mathematics of holography, the techniques and algorithms of electromagnetic field inversion, the study of the radiated electromagnetic environment and the requirements of the radiant element (type of antenna , shape, size, radiated power) by realizing one with the technology of three-dimensional printing. I have evaluated and studied a solution to improve the electromagnetic compatibility with the robotic system on which the RADAR will have to operate. To create a working prototype, I worked on defining the requirements of the driving electronics and programming of the implemented devices. This text ends with the demonstration, through the display of experimental tests in a controlled environment, of the performance of the new RADAR, highlighting the differences compared to the original HSR.

abstract: To detect and resolve sub-wavelength features at optical frequencies, beyond the diffraction limit, requires sensors that interact with the electromagnetic near-field of those features. Most instruments operating in this modality scan a single detector element across the surface under inspection because the scattered signals from a multiplicity of such elements would end up interfering with each other. However, an alternative massively parallelized configuration, consisting of a remotely interrogating array of dipoles, capable of interrogating multiple adjacent areas of the surface at the same time, was proposed in 2002. In the present work a remotely interrogating slot antenna inside a 60nm silver slab is designed which increases the signal to noise ratio of the original system. The antenna is tuned to resonance at 600nm range by taking advantage of the plasmon resonance properties of the metal’s negative permittivity and judicious shaping of the slot element. Full-physics simulations show the capability of detecting an 8nm particle using red light illumination. The sensitivity to the λ/78 particle is attained by detecting the change induced on the antenna’s far field signature by the proximate particle, a change that is 15dB greater than the scattering signature of the particle by itself. To verify the capabilities of this technology in a readily accessible experimental environment, a radiofrequency scale model is designed using a meta-material to mimic the optical properties of silver in the 2GHz to 5GHz range. Various approaches to the replication of the metal’s behavior are explored in a trade-off between fidelity to the metal’s natural plasmon response, desired bandwidth of the demonstration, and ii manufacturability of the meta-material. The simulation and experimental results successfully verify the capability of the proposed near-field sensor in sub-wavelength detection and imaging not only as a proof of concept for optical frequencies but also as a potential imaging device for radio frequencies.<br>Dissertation/Thesis<br>Doctoral Dissertation Electrical Engineering 2016

碩士<br>國立臺灣大學<br>電信工程學研究所<br>102<br>A novel probe-compensated microwave holographic imaging method is proposed. Planar scanning by a directive probe antenna is adopted to reconstruct the images of unknown metallic or dielectric targets. Unknown targets are first illuminated by the probe antenna, and then the fields scattered from the targets are received by the probe. Directive or moderate-gain probe antennas are adopted since they are more sensitive to the scattered fields. To enhance the image quality, the proposed approach necessitates the detailed radiation information of the probe antenna and its environment. Theoretical formulation for image reconstruction is derived and verified by both simulations and experiments. A series of imaging results based on FEKO simulations is presented to verify the efficacy of the proposed imaging approach. The effects of sampling rate on the image reconstruction are also investigated. Moreover, the experimental imaging system is also built up. A microstrip patch antenna that operates at X-band is chosen as the moderate-gain probe antenna. In addition to the monostatic imaging system, a quasi-monostatic system is also demonstrated. The quasi-monostatic system is more feasible and is capable of measuring the relatively weak fields scattered from dielectric targets.

博士<br>國立臺灣大學<br>電信工程學研究所<br>106<br>A Novel 3-D microwave holographic imaging algorithm compatible with both single-probe and dual-probe scanning setups is proposed for imaging dielectric targets. In some cases, even with only one probe antenna, the quality of the images reconstructed using the proposed algorithm is comparable or even superior to those using dual-probe-based methods. The proposed algorithms can be divided into four parts. First, starting from the open-circuit voltage of an antenna, the proposed algorithm compensates for both transmitting and receiving properties of the probe antenna, leading to least squares problems to be determined. However, the problem tends to be ill-conditioned in the single-probe setup (irrelevant to the dual-probe one). Second, an auxiliary equation based on lossless condition of an unknown target is thus derived and exploited to effectively improve the numerical stability. Third, to accurately locate the unknown target in range direction, a phase compensation method that requires only phase correction to the associated entries of the kernel matrices is proposed based on a simple ray model. Last but not least, considering both a finite size of a scanning aperture and beamwidth of the probe antenna, a numerical low-pass filter in the spatial-frequency domain is devised to effectively locate the worth-solving area and thus reduce computational time substantially. For verification, a series of images reconstructed from the simulated and measured scattering data using the proposed algorithm are presented. Thanks to the enhanced image quality, the geometry, location, and dielectric constant of the target can readily be retrieved. The range and cross-range resolutions achieved are about /10 and /4,