The desire to incorporate multiple frequency bands of operation into personal communication devices has led to much research on reducing the size of antennas while maintaining adequate performance. GPS is one such application, where dual frequency operation, bandwidth and circular polarization pose major challenges when using traditional miniaturization techniques. Various loading methods have been studied to reduce the resonant frequency of the antenna – high permittivity dielectric loading, slot loading and cavity loading – while examining their effects on bandwidth and gain. The objective of this thesis is to provide guidelines on what is achievable using these miniaturization methods and insight into how to implement them effectively.

The desire to counter multipath effects and improve communication links between mobile wireless systems in dense environment has led to much research in implementing antenna diversity. Space diversity, utilizing two or more antennas separated several wavelengths from one another, is one of the most popular method to achieve this operation. Meanwhile, polarization diversity, utilizing two orthogonal polarizations, has become more attractive in reducing cost and size of antenna systems. Polarization diversity is achieved using two orthogonal feeds to excite the two orthogonal polarization planes of the antenna. The challenge associated with designing dual polarized antennas is the need to reduce isolation between the feed and cross polarization level while maintaining a high efficiency. While a number of studies are successful in realizing polarization diversity, their antenna structure typically present more complex structures involving multiple layers. This thesis presents a novel method to implement polarization diversity on a miniature antenna using a simple planar structure. The antenna structure uses two crossed slots further miniaturized using a method derived from a recent study on miniaturized spiral slot antenna. At an operating frequency of ~ 1 GHz, the antenna is capable of achieving efficiency greater than 90% with a size as small as 0.08 .? x 0.08? The dual polarization operation is achieved by exciting the magnetic currents of the crossed slots with two orthogonal coplanar waveguide feeds. Simulation results of the proposed antenna yield an isolation > 15 dB with cross polarization levels > 10 dB. Theantenna structure was designed using CST Microwave Studio and the simulations were performed using IE3D simulation software.

Pour faire cohabiter les nombreuses technologies radios, les terminaux mobiles nécessitent une miniaturisation de plus en plus poussée des antennes. Toutefois, les performances d'antennes ont des limites fondamentales liées à leurs dimensions physiques. La littérature met en évidence que les réseaux superdirectifs permettent de dépasser la limite de Harrington sur la directivité et que des antennes adaptées par des circuits non-Foster peuvent dépasser la limite de Bode-Fano sur la bande passante. Les contributions essentielles de ce travail de thèse consistent en la conception deréseaux d'antennes superdirectifs et d'antennes adaptées par des circuits non-Foster comme solutions possibles pour l'amélioration des performances des Antennes Electriquement Petites (AEP). Dans une première partie, un convertisseur d'impédance négative est réalisé pour obtenir des condensateurs de valeurs négatives de façon à adapter des antennes miniatures sur une large bande de fréquence. Dans la deuxième partie de ces travaux, les limites théoriques des réseaux d'antennes superdirectifs sont évaluées et une approche simple et pratique permettant la conception de ces réseaux à partir d'éléments parasites est proposée. L'intégration des AEP superdirectives sur des cartes de circuit imprimé est étudiée et les difficultés de mesure de ce type d'antenne sont évaluées. A partir de ces résultats, une nouvelle stratégie pour réaliser des réseaux compactes 3D ou planaires à polarisation linéaire ou circulaire en utilisant des éléments superdirectifs est présentée. / For supporting different wireless technologies, mobile terminals require significant miniaturization of antennas. However, antennas performance has some fundamental limits related to their physical dimensions. The available theory shows that superdirective arrays can exceed Harrington’s limit on antenna directivity and non-Foter matched antennas can surpass Bode-Fano limit on antenna bandwidth. Therefore, this work focuses on the design of superdirective antenna arrays and non-Foster matched antennas as possible solutions for improving the performance of Electrically Small Antennas (ESAs). In the first part: a Negative Impedance Converter (NIC) is designed to have a very small negative capacitor. The circuit is evaluated in terms of gain, stability and linearity. Then, the circuit is used to match several small antennas in the UHF band. In the second part: the theoretical limits of superdirective antenna arrays are studied. A simple and practical approach to design parasitic antenna arrays is proposed. The integration of superdirective ESAs in Printed Circuit Boards (PCBs) is studied and the difficulties of measuring this type of antennasare evaluated. A new strategy for the design of 3D or planar compact arrays, with linear or circular-polarization, using superdirective elements is presented.

This project is focused on computer modeling of so-called meta-materials, and on the exploitation of metamaterials in the design of electrically small antennas. For modeling, COMSOL Multiphysics 3.3 was used. Simulations were focused on impedance matching of antennas. Antennas with metamaterials were compared with corresponding conventional antennas without metamaterial layers. The project does not investigate the creation of metamaterials; the project concentrates on their influence on crucial parameters of antennas. Next step this project is focused on optimalization this structure in program Matlab version R2009b. We used optimalization method PSO (swarms of particles) and results are comparing whit results calculating in COMSOL program.

This work describes an electrically small antennas used for mobile devices in the ISM band. The aim of this work is to select an antenna for a universal transmitter/receiver working in the 868 MHz band. The work contains description of the universal transmitter/receiver, and a list of suitable antennas. Tolerance analysis is performed by numerical model. The proposed antenna is manufactured and measured.

No / A miniaturized conical helix antenna is presented, which displays vertical polarization with electrically small dimensions of 10mm×10mm×45mm. The resonance of the antenna is made tunable by adding a variable digital MEMS capacitor load at the bottom of the helix, giving a tuning range of 316 MHz to 400 MHz. The antenna demonstrates considerable impedance matching bandwidth and gain over the entire tuning frequency band. Most importantly, the antenna is capable of compact, flexible and easy integration into a wireless device package or for platform installation. / Datong of Seven Technology Group, for their support under the KTP project grant No. 008734.

Les antennes présentes dans la plupart des systèmes communicants comme les véhicules automobiles, les avions et les trains se multiplient et sont soumises à une contrainte d’intégration de plus en plus sévère. De nombreuses techniques de miniaturisation d’antennes existent et passent toutes par un compromis entre la taille et les performances (bande passante et/ou rendement de rayonnement). Pour les systèmes cités ci-dessus, les antennes sont souvent placées devant ou à proximité d’un réflecteur métallique (toit de véhicule, carlingue d’aéronef). Dans ce cas, l’épaisseur de système antennaire est une contrainte majeure et les métamatériaux de type Conducteur Magnétique Artificiel (CMA) ouvrent des perspectives intéressantes grâce à leurs propriétés électromagnétiques non conventionnelles. Cependant, pour les applications sub-GHz (RFID, LTE, PMR…), les CMA sont limités par les dimensions des cellules unitaires nécessaires à leur mise en œuvre (λg/4) ainsi que leur bande réduite de fonctionnement. Réduire leurs dimensions permet de rendre leur utilisation compatible avec le contexte des antennes miniatures intégrées. Ajouter l’agilité fréquentielle permet de palier le problème de la bande passante réduite dans le cas des antennes et des CMA miniaturisés en ajustant le fonctionnement du système antennaire sur une large bande passante. Cette thèse de doctorat propose d’étudier et de développer un nouveau système antennaire à faible profil composé d’une antenne miniature associée à une métasurface compacte reconfigurable en fréquence et compatible avec le standard NB-IoT dans la bande basse LTE (700 MHz – 960 MHz). / Antennas are now very integrated in several connected systems like cars, airplanes and trains. Many antenna miniaturization techniques exist and all go through a compromise between size and performance (bandwidth and/or radiation efficiency). For the systems mentioned above, the antennas are often placed near a metallic reflector (vehicle roof, aircraft cabin). Within this context, Artificial Magnetic Conductors (AMC) present an attractive reflector for low profile antennas which can take advantage of intrinsic zero reflection phase response to boost antenna performance without the need for thick quarter wave backplane. However, for sub-GHz applications (RFID, LTE, PMR ...), AMC are limited by the size of the unit cells necessary for their implementation (λg/4) as well as their reduced operating bandwidth. AMC miniaturization makes their use compatible with small antennas. Adding tunability restores the possibility of adjusting the operating frequency over a large bandwidth. This PhD thesis proposes to study and develop a new electrically small, low-profile antenna based on miniaturized and tunable AMC for the NB-IoT standard in low LTE band (700 MHz – 960 MHz).

The concept for a novel new antenna design is presented and investigated for application to ground penetrating radar (GPR). The proposed new antenna design is called the shielded magnetic bowtie antenna (MBA). As the name suggests, it is predominately constructed from a bowtie-shaped volume of magnetic material that is fed from the centre of the structure by a small magnetic loop antenna. This thesis develops the magnetic antenna concept and investigates its potential for GPR predominately through numerical modelling. However, a significant part of the investigation concentrates on validating the numerical modelling technique developed to study the shielded MBA by comparing the results with measurements obtained from a scale model constructed to operate in the watertank antenna test facility, a controlled environment for GPR antenna research. The broadband properties required for GPR antennas are achieved uniquely with the shielded MBA design by a combination of the antenna shape being defined in terms of angles and an inherent magnetic loss mechanism within the antenna material structure. The design also affords an intrinsically placed antenna shield that has the potential for mitigating problems typically experienced with shielding electric dipole antennas. Antenna shielding is an important consideration for GPR antenna designers, especially given the recent US government (FCC) changes that restrict radiated energy emissions within the regulated spectrum used by GPR systems. In addition to providing the intended directional radiation properties, the magnetic antenna shield also provides an elegant solution for a low-loss wideband balun, allowing the antenna to be effectively fed from an unbalanced coaxial transmission line. Other important aspects of the proposed design are discussed in relation to the requirements for GPR antennas. Numerical models of the magnetic antenna concept show encouraging bandwidth results. For example, from a simple comparison with an equivalent sized electric bowtie antenna model, the effective gain bandwidth of the magnetic antenna is found to be at least 3-octaves compared to approximately 2-octaves for the electric bowtie. The shielded magnetic antenna achieves a gain of approximately 2 dB, compared to 5 dB for the unshielded electric bowtie antenna. However, it is noted that the magnetic antenna models contain significantly more loss compared to the electric bowtie model. The shielded MBA design emerged from a theoretical investigation of electrically small GPR antennas, given that the initial thesis objective was to investigate ways of improving low frequency GPR antennas. In general, GPR systems are operated with electric dipole antennas, such as the electric bowtie. Interestingly, the electrically small antenna investigation revealed that only the small magnetic loop (i.e., magnetic dipole) antenna can be constructed to approach, arbitrarily closely, the fundamental bandwidth limit for small antennas. This surprising and counter intuitive result is shown to be theoretically achievable with the use of magnetic materials. For the small loop antenna, energy stored within the antenna structure can be avoided by filling the antenna sphere with a perfect magnetic material. This theoretical argument is discussed and supported by numerically modelled results. The electrically small antenna investigation presented in this thesis extends to include the influence that proximity to a lossy dielectric half-space has, on improving the antenna impedance bandwidth. This investigation is of general interest for GPR; it is performed numerically and supported by measurements conducted on an experimental loop antenna situated at various heights above the ground. These results provide support for the hypothesis that a magnetic antenna may experience less influence from near-field changes in the dielectric properties of the ground compared to the equivalent sized electric field antenna.

В работе представлены результатыисследования метода, который позволяет увеличить полосу частот электрически малой антенны с помощью “нефостеровской цепи”на основе операционного усилителя. Были получены графики, которые позволяют оценить входное реактивное сопротивление и полосу частот электрически малой антенны с представленным методом расширения полосы частот. / The paper presents the results of a study of the method, which makes it possible to increase the frequency band of an electrically small antenna by means of a "Non-foster circuit"with operational amplifier. The graphs were obtain, which allow estimating the input reactance and the bandwidth of an electrically small antenna with the method of bandwidth extension represented.

В работе представлены результатыисследования метода, который позволяет увеличить полосу частот электрически малой антенны с помощью “нефостеровской цепи”.Также были представлены классические методы расширения полосы частот антенны с помощью индуктивности и колебательного контура, чтобы сравнить их эффективность с исследуемым методом. Были получены графики, которые позволяют оценить входное реактивное сопротивление и полосу частот электрически малой антенны со всеми представленными методами расширения полосы частот. / The paper presents the results of a study of the method, which makes it possible to increase the frequency band of an electrically small antenna by means of a "Non-foster circuit". Also, classical methods for extending the frequency band of the antenna with the help of inductance and a vibrational circuit were presented to compare their effectiveness with the method being studied.