Small antennas have attracted significant attention due to their prolific use in consumer electronics. Such antennas are highly desirable in the healthcare industry for imaging and implants. However, most small antennas are not highly directive and are detuned when in the presence of a dielectric. The human body can be compared to a series of lossy dielectric media. A novel antenna design, the orthogonal coil, is proposed to counter both of these shortcomings. As loop antennas radiate primarily in the magnetic field, their far field pattern is less influenced by nearby lossy dielectrics. By exciting two orthogonal coil antennas in quadrature, their beams in the H-plane constructively add in one direction and cancel in the other. The result is a small, yet directive antenna, when placed near a dielectric interface. In addition to present a review of the current literature relating to small antennas and dipoles near lossy interfaces, the far field of the orthogonal coil antenna is derived. The directivity is then plotted for various conditions to observe the effect of changing dielectric constants, separation from the interface, etc. Numeric simulations were performed using both Finite Difference Time Domain (FDTD) in MATLAB and Finite Element Method (FEM) in Ansys HFSS using a anatomically accurate high-fidelity head mesh that was generated from the Visible Human Project® data. The following problem has been addressed: find the best radio-frequency path through the brain for a given receiver position - on the top of the sinus cavity. Two parameters: transmitter position and radiating frequency should be optimized simultaneously such that (i) the propagation path through the brain is the longest; and (ii) the received power is maximized. To solve this problem, we have performed a systematic and comprehensive study of the electromagnetic fields excited in the head by the aforementioned orthogonal dipoles. Similar analyses were performed using pulses to detect Alzheimer’s disease, and on the femur to detect osteoporosis.

"In the design of any antenna radiator, single or multi-element, a significant amount of time and resources is spent on impedance matching. There are broadly two approaches to impedance matching; the first is the distributed impedance matching approach which leads to modifying the antenna geometry itself by identifying appropriate degrees of freedom within the structure. The second option is the lumped element approach to impedance matching. In this approach instead of modifying the antenna geometry a passive network attempts to equalize the impedance mismatch between the source and the antenna load. This thesis introduces a new technique of impedance matching using lumped circuits (passive, lossless) for electrically small (short) non-resonant dipole/monopole antennas. A closed form upper-bound on the achievable transducer gain (and therefore the reflection coefficient) is derived starting with the Bode-Fano criterion. A 5 element equalizer is proposed which can equalize all dipole/monopole like antennas. Simulation and experimental results confirm our hypothesis. The second contribution of this thesis is in the design of broadband, small size, modular arrays (2, 4, 8 or 16 elements) using the distributed approach to impedance matching. The design of arrays comprising a small number of elements cannot follow the infinite array design paradigm. Instead, the central idea is to find a single optimized radiator (unit cell) which if used to build the 2x1, 4x1, 2x2 arrays, etc. (up to a 4x4 array) will provide at least the 2:1 bandwidth with a VSWR of 2:1 and stable directive gain (not greater than 3 dB variation) in each configuration. Simulation and experimental results for a solution to the 2x1, 4x1 and 2x2 array configurations is presented. "

En focalisant le rayonnement dans les directions utiles, les antennes directives ouvrent de nouvelles perspectives pour les applications sans-fil en termes de sélectivité spatiale, d'impact environnemental électromagnétique et de modes d'utilisation. Cependant les techniques classiques pour augmenter la directivité aboutissent souvent à une augmentation de la taille de l'antenne rendant difficile l'intégration dans les petits objets communicants. Cette difficulté est particulièrement critique pour les gammes de fréquences inférieures au gigahertz, lorsque l'on vise une intégration dans des objets dont les dimensions sont limitées à quelques centimètres. Le contrôle du rayonnement reste un enjeu important pour les radiocommunications futures afin de réduire les pollutions électromagnétiques qui limitent l'acceptabilité des communications sans-fil et la cohabitation des systèmes. L'état de l'art récent dans le domaine des antennes miniatures a montré de nouvelles perspectives pour l'établissement de super-directivité ; c'est-à-dire pour dépasser la directivité naturelle faible des systèmes antennaires miniatures. Ces perspectives reposent sur l'utilisation de réseaux d'antennes parasites miniatures pour construire un rayonnement directif. De plus, les activités de recherche dans le domaine des antennes actives ces dernières années permettent d'envisager une approche moderne aux problématiques liées à la directivité des antennes miniatures. Ces travaux de thèse ont pour objectif d'évaluer les perspectives d'améliorations qu'apporte l'électronique active aux problématiques des antennes miniatures directives. Des circuits au comportement particulier sont notamment mis en œuvre et évalués expérimentalement pour concrétiser ces nouvelles perspectives. / Directive antennas, used to focus the radiation in useful directions, offer new perspectives for wireless applications in terms of spatial selectivity, electromagnetic environmental impact and possible uses. Nevertheless, usual techniques to enhance antennas’ directivity lead to larger antennas and their integration into small objects would be difficult. That becomes critical when antennas working at less than 1 GHz frequencies have to be integrated in small objects (around few centimeters). Radiation control through directive antennas is being an important issue for the future communications. This kind of antennas allows reducing electromagnetic pollutions which limit wireless systems and communicants objects cohabitation. Recent state of the art shows new perspectives to establish small antennas with super directive radiation using parasitic antenna arrays. A super directive antenna is a small antenna that exceeds its natural and low directivity. Moreover, these last few years, researches on active antennas and associated results could be considered for modern approach to deal with small and directive antennas’ issues. In this work, we propose to evaluate the enhancement perspectives brought by the active electronic circuits to solve the small and directive antennas’ issues. Typically, special active circuits are designed and experimentally evaluated to materialize those antennas perspectives.

Radiation Efficiency improvement of miniaturized microstrip antenna is studied in this thesis. It is shown that, the loss reduction in miniaturized Microstrip Antenna can be achieved through two possible ways. The first is by modifying the materials used for building the antenna, and the second method is by increasing the radiation conductance of the antenna. Material modification at nano/micro scale by replacing conductors with Metallo-Dielectric one dimensional medium for applications in loss reduction is investigated first. It is shown by the Transfer Matrix Method and using simulations that, for a one dimensional medium replacing very thin conductors (less than skin depth) by laminated multilayered conductors reduce losses. However, the improvement does not exceed the case of single conductor which is a few times thicker than skin depth. Secondly, the efficiency improvement of a small H-Shaped patch antenna by using closely coupled stacked parasitic resonators is studied. It is shown that significant improvement in efficiency can be achieved with minimal changes in the foot print, radiation pattern and cross polarization levels of the antenna. The effect of the overall thickness and superstrate dielectric constant on the efficiency improvement is studied parametrically. It is shown that by using 5 radiating resonators and appropriate choice of inter-conductor dielectric constant, for a small increase in thickness of 0.127mm (5mil), the radiation efficiency can be increased from 2.34% to 6.3%. This efficiency improvement can be made very significant from 2.4% to 33%, by increasing the height to 1.27mm (50mil). These translate to a gain improvement of 4dB and 13dB, respectively. This technique is also demonstrated experimentally in H-Shaped antennas with two different levels of miniaturizations. / October 2015

Antennas in hand-held radios must be compact and unobtrusive. Electrically small and low-profile antennas experience high input reactance, low input resistance, and low radiation efficiency.Further degradation of radiation efficiency occurs in hand-held radios due to size-reduced ground planes, losses within the plastic device casing, and losses due to coupling with the tissue of the user. These factors may also affect the radiation pattern of the antenna. This discussion reports on antenna designs that are well suited for hand-held radios. The design issues are covered for electrically small antennas and the hand-held environment. A review of Microstrip Antenna (MSA) theory, and the theory of the Inverted-L Antenna (ILA), and variations on the ILA including the Inverted-F Antenna (IFA), Planar Inverted-F Antenna (PIFA), and Dual Inverted-F Antenna (DIFA) is included. Two specific antenna designs are presented: the DIFA and the Proximity-Coupled Rectangular Patch MSA. The radiation patterns and input impedance of the DIFA are calculated numerically and measured empirically. The Proximity-Coupled Rectangular Patch Microstrip Antenna is treated numerically. / Master of Science

An indirect impedance measurement approach that does not require direct cable attachment or large space using a two-port network is presented. Using a straight wire monopole as an interrogating antenna and measured impedances of three calibration standards, the input impedance of a small spherical helix dipole over a ground plane is retrieved. It is found that accurate result is obtained around the dipole resonance frequency. The accuracy and sources of error are discussed.

Without noise, a wireless system would be able to transmit and receive signals over an arbitrary long-distance. However, practical wireless systems are not noise-free, leading to a limited communication range. Thus, the design of low-noise devices (such as antennas, amplifiers, and filters) is essential to increase the communication range. Also, it is well known that the noise performance of a receiving radio is primarily determined by the frontend including the antenna, filter, and a low-noise amplifier. In our first design, we intend to reduce the noise level of the receiving system by integrating a parametric amplifier into the slot antenna. The parametric amplifier utilizes nonlinear and/or time-variant properties of reactive elements (capacitors and/or inductors) to amplify radio frequency signals. Also, the parametric amplifier offers superior noise performance due to its reactive nature. We utilize the parametric amplifier to design a low-noise active matching circuit for electrically small antennas in our second design. Using Chu's limit and the Bode-Fano bound, we show a trade-off between the noise and bandwidth of the electrically small antennas. In particular, to make the small antenna wideband, one needs to introduce a mismatch between the antenna and the amplifier. Due to the mismatch, the effect of the low-noise amplifier becomes even more critical and that is why we choose the parametric amplifier as a natural candidate. As a realized design, a loop antenna is configured as a receiver, and the up-converter parametric amplifier is connected to it leading to a low-noise and wideband active matching circuit. The structure is simulated using a hybrid simulation technique and its noise performance is compared to the transistor counterpart. Our simulation and measurement results show more than 20 times bandwidth improvement at the expense of a 2 dB increase in the noise figure compared to the passive antenna counterpart. / Doctor of Philosophy / Nowadays, there is a high demand for compact and high-speed electronic devices such as cellphones, tablets, laptops, etc. It is therefore essential to design a miniaturized wideband antenna. Unfortunately, a trade-off exists between the bandwidth and gain of small antennas. The trade-off is based on some fundamental limits and extends to all small and passive antennas, regardless of their shape or structure. By using an active component such as an amplifier, the gain-bandwidth trade-off can be improved. However, we show that the active component adds noise to the receiving system leading to a new trade-off between noise and bandwidth in the receiving structures. In other words, utilizing the active component does not solve the problem and just replaces the gain-bandwidth trade-off with the noise-bandwidth trade-off. To improve the noise-bandwidth trade-off, we propose a new receiving structure in which we use the parametric amplifier instead of a commercially available transistor amplifier. The noise performance of the parametric amplifier is extremely better than the transistor amplifier leading to lower noise for the specified bandwidth. In particular, we improved the noise performance of the receiving system by 3 dB leading to doubling the communication distance.

With the current scenario of development of antennas in the wireless communication field, the need of compact multiband, multifunctional and cost effective antenna is on the rise. The objective of this thesis is to present fixed and reconfigurable techniques and methods for small and slim multiband antennas, which are applicable to serve modern small and slime wireless, mobile and cognitive radio applications. In the fixed designs, independent control of the operating frequencies is investigated to enhance the antennas capabilities and to give the designer an additional level of freedom to design the antenna for other bands easily without altering the shape or the size of the antenna. In addition, for mobile phone antenna, the effect of user’s hand and mobile phone housing are studied to be with minimum effect. Although fixed multiband antennas can widely be used in many different systems or devices, they lack flexibility to accommodate new services compared with reconfigurable antennas. A reconfigurable antenna can be considered as one of the key advances for future wireless communication transceivers. The advantage of using a reconfigurable antenna is to operate in multiband where the total antenna volume can be reused and therefore the overall size can be reduced. Moreover, the future of cell phones and other personal mobile devices require compact multiband antennas and smart antennas with reconfigurable features. Two different types of frequency reconfigurability are investigated in this thesis: switchable and tunable. In the switchable reconfigurability, PIN diodes have been used so the antenna’s operating frequencies can hop between different services whereas varactor diode with variable capacitance allow the antenna’s operating frequencies to be fine-tuned over the operating bands. With this in mind, firstly, a switchable compact and slim antenna with two patch elements is presented for cognitive radio applications where the antenna is capable of operating in wideband and narrow bands depending on the states of the switches. In addition to this, a switchable design is proposed to switch between single, dual and tri bands applications (using a single varactor diode to act as a switch at lower capacitance values) with some fine tuning capabilities for the first and third bands when the capacitance of the diode is further increased. Secondly, the earlier designed fixed antennas are modified to be reconfigurable with fine-tuning so that they can be used for more applications in both wireless and mobile applications with the ability to control the bands simultaneously or independently over a wide range. Both analytical and numerical methods are used to implement a realistic and functional design. Parametric analyses using simulation tools are performed to study critical parameters that may affect the designs. Finally, the simulated designs are fabricated, and measured results are presented that validate the design approaches.

L’essor de l’internet des objets engendre le développement et l’introduction de nouveaux capteurs dans l’industrie mais aussi dans les objets du quotidien. Bâtiments et usines mais aussi réfrigérateurs et lunettes regorgeront bientôt de capteurs sans fils connectés. L’utilisation de capteurs sans fils implique le développement d’antennes compactes à la conception complexe. Diverses technologies de capteurs sans fils et passifs (sans batterie) ont été étudiées ou sont d’ores et déjà disponibles dans le commerce. L’utilisation d’antennes miniatures dans ces technologies de capteur a été traitée dans la thèse. Plus intégrables, les antennes miniatures imposent des contraintes en termes d’efficacité et de facteur de qualité qui peuvent détériorer les performances du capteur. La conception, la modélisation ainsi que l’optimisation d’une antenne miniature particulièrement intéressante pour les capteurs sans fils et passifs sont présentées. Les résultats obtenus montrent l’intérêt de la méthode décrite pour améliorer les performances de telscapteurs / The Internet of Things is a promising research area for the next years. 50 billions of connected objects are expected in 2020. Among them, wireless sensors are required for appli- cations like body and environmental monitoring. This implies a lot of research works such as size reduction, efficiency im- provement and energy harvesting. The variety of applications and environments leads to a huge panel of constraints. In this context, chipless and passive sensors present numerous advantages such as long lifetime, low cost and robustness. This thesis has two aims : show the interest of using electrically small antenna in wireless sensors and developpe a method to improve sensors performance (range and sensibilitty)

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