This dissertation details the development of a novel reduced size axial mode helical antenna called the Stub Loaded Helix (SLH). The SLH achieves a significant reduction in helix size, both diameter and length, compared to the conventional axial mode helix antenna with only small compromises in performance. The SLH achieves this entirely through the use of a unique geometry. The performance characteristics of the SLH are explored through the use of computer simulations using NEC (Numerical Electromagnetics Code) to study the effects of design parameter variations. Based on those simulations, design guides are developed. The numerical simulations are verified though measurements of experimental prototypes. The program of experimental prototypes included the development of an appropriate impedance matching network for the SLH, which is also detailed. / Ph. D.

The genetic algorithm (GA) is used to design helical antennas that provide a significantly larger bandwidth than conventional helices with the same size. Over the bandwidth of operation, the GA-optimized helix offers considerably smaller axial-ratio and slightly higher gain than the conventional helix. Also, the input resistance remains relatively constant over the bandwidth. On the other hand, for nearly the same bandwidth and gain, the GA-optimized helix offers a size reduction of 2:1 relative to the conventional helix. The optimization is achieved by allowing the genetic algorithm to control a polynomial that defines the envelope around which the helix is wrapped. The fitness level is defined as a combination of gain, bandwidth and axial ratio as determined by an analysis of the helix using NEC2. To experimentally verify the optimization results, a prototype 12-turn, two-wavelength high, GA-helix is built and tested on the Virginia Tech outdoor antenna range. Far-field radiation patterns are measured over a wide frequency range. The axial-ratio information is extracted from the measured pattern data. Comparison of measured and NEC-2 computed radiation patterns shows excellent agreement. The agreement between the measured and calculated axial-ratios is reasonable. The prototype GA-helix provides a peak gain of more than 13 dB and an upper-to-lower frequency ratio of 1.89. The 3-dB bandwidth of the antenna is 1.27 GHz (1.435 GHz - 2.705 GHz). Over this bandwidth the computed gain varies less than 3 dB and the axial-ratio remains below 3 dB. / Master of Science

A new variation of the spherical helical antenna made of a wire wound over a hemispherical surface and backed by a conducting ground plane is introduced. A constant spacing is maintained between the turns of winding. The geometry of this antenna is fully described by the number of turns and the radius of hemispherical surface. In addition to the hemispherical geometry, truncated double spherical helices are also examined. Radiation properties of the proposed antennas are studied both theoretically and experimentally. The wire antenna code ESP (electromagnetic surface patch), which is based on the method of moments, is used to obtain simulation results. The results for far-field patterns, gain, axial ratio, bandwidth, and input impedance are presented. Several prototypes of this antenna were constructed and tested using an outdoor antenna range. Far-field patterns were measured over a wide range of frequencies. The measured and calculated radiation patterns are in good agreement. A unique property of the hemispherical helix is its broad half-power beamwidth. Furthermore, this antenna provides circular polarization and relatively high gain over a narrow frequency range. The results of this research show that, for example, a 4.5-turn hemispherical helix with a radius of 0.02 meter designed for operation around 2.8 GHz provides a half-power beamwidth of about 90 degrees, more than 9 dB gain, and less than 3 dB axial ratio over a 300 MHz bandwidth. The input impedance of the antenna is largely resistive and is about 150 ohms in the above frequency range. Compared with a full spherical helix, the hemispherical helix provides comparable radiation characteristics, but occupies only half the volume. The compact size of this antenna makes it attractive to mobile communication applications / Master of Science

A unique property of spherical and hemispherical helical antennas is that they provide very broad half-power beamwdiths and circular polarization over a narrow bandwidth. In this thesis, new designs for hemispherical helical antennas are introduced that provide significant improvement in bandwidth, while maintaining the directivity and half-power beamwidth of the basic design. In the basic design, a simple wire of circular cross section is wound on the surface of a hemisphere, whereas in the proposed new designs a metallic strip forms the radiating element. Furthermore, the metallic strip may be tapered and tilted relative to the hemispherical surface, allowing wider bandwidth to be achieved. The antenna is fed by a coaxial cable with the inner conductor connected, through a matching section, to the radiating strip and its outer conductor connected to a ground plane. Radiation properties of the proposed hemispherical helical antennas are studied both theoretically and experimentally. A commercial software, based on the method of moments, is used to perform the numerical analysis of these helices. Three-dimensional far-field patterns, axial ratio, directivity, and voltage standing-wave ratio (VSWR) are calculated for several designs. The impacts of tapering as well as tilting of the metallic strip on radiation characteristics are examined. Also, matching of the proposed hemispherical antennas to 50â ¦ transmission lines is addressed. A 4.5-turn hemispherical helix with tapered radiating element and zero degree tilt angle, (metallic strip is perpendicular to the hemisphere axis of symmetry) provides the largest overall bandwidth. A nonlinearly tapered matching section is incorporated into the design in order to reduce the VSWR. For this design, an overall measured bandwidth of about 24% at a center frequency of 3.35 GHz is achieved. Over this bandwidth, the axial ratio remains below 3 dB, the VSWR is less than 2, and the directivity is about 9 ±1 dB. A half- power beamwidth of 70° is also obtained. A prototype of the best design was fabricated and tested using the VT indoor antenna range. Radiation patterns, the scattering parameter S₁₁, and the axial ratio were measured. The measured and simulated results agree reasonably well. In particular, agreements between measured and calculated far-field patterns and VSWR are quite remarkable. This compact, low profile antenna might find useful applications in avionics, global positioning systems (GPS), and high data rate wireless communication systems. / Master of Science

No description available.

Novel antennas attractive to fit in the internal space of a PDA (Personal Digital Assistant) for WLAN (Wireless Local Area Network) operations are presented in this dissertation. The proposed antennas have in common good impedance bandwidth (defined by 10 dB return loss), covering the dual-band WLAN operation in the 2.4/5.2 GHz bands. Two novel designs of foam-base surface-mount antennas are proposed in Chapters 2 and 3. Surface-mountable antennas, compared with ceramic antennas, are generally low cost in fabrication and rigid in nature. Low-profile and good dual-band operation of the proposed surface-mountable antennas can be observed in Chapters 2 and 3, and in addition, a few present-day WLAN bands at 5 GHz are covered in the operating bandwidths of the proposed foam-base surface-mountable shorted monopole antenna, shown in Chapter 3. Finally, in Chapter 4, a novel planar helical antenna printed on both surfaces of a dielectric substrate is demonstrated. This patent-pending helical antenna is very suitable to print and integrate on a circuit board of a PDA device for 2.4/5.2 GHz WLAN operation.

There are a number of global navigation satellite systems (GNSS), in use or planed, which are used for navigation on earth but also for autonomous navigation of satellites in low earth orbit (LEO). It would be desirable to also have autonomous navigation in geosynchronous earth orbit (GEO) to reduce costs and make it possible to get higher accuracy on the position of the satellite. One part of the navigation system is the GNSS antenna which is examined in this master thesis. The specifications of the antenna were first decided and then three antenna alternatives were investigated in greater detail: a monofilar helix antenna, a three element circular array antenna and a twelve element circular array antenna. The result was that they would all work as a GNSS antenna in GEO but none could be judged to be the best under all circumstances. The size requirement for the mission and the used GNSS receiver would primarily decide which fits the mission best. / Det finns ett antal världstäckande navigeringssystem (GNSS), i användning och planerade, som används för navigation på jorden fast också för autonom navigation för satelliter i låg bana runt jorden. Det skulle också vara önskvärt att använda autonom navigation för satelliter i geostationär omloppsbana (GEO) för att reducera kostnaden och få högre positions noggrannhet. En del av navigationssystemet är GNSS antennen vilken är undersökt i detta examensarbete. Specifikationerna för antennen bestämdes först och sedan undersöktes tre olika antennalternativ i detalj: en monofilär helixantenn, en tre elements cirkulär gruppantenn och en tolv elements cirkulär gruppantenn. Resultatet var att alla alternativen skulle fungera som en GNSS antenn i GEO-bana fast inget av alternativen är bäst i alla förhållanden. Storlekskraven för uppdraget och vilken GNSS mottagare som skall användas påverkar vilket av alternativen som passar uppdraget bäst.

No / An investigation of the surface current distribution in a normal-mode helical antenna (NMHA) is reported. This enables precise prediction of the performance of NMHAs, since traditional wire-antenna simulations ignore important details, such as non-uniform and transverse current distributions. A moment-method formulation is developed, using two-dimensional basis functions to represent the total non-uniform surface current distribution over the surface of the wire of the helix. Piecewise-sinusoidal basis functions are employed in two normal directions, with an exact kernel formulation and application of Galerkin's solution method. The numerical solution of the singular integrals associated with self-impedance terms was computed with a very low relative error. The surface current distribution was computed for different helix geometries. It was found that the axially-directed component of the current distribution around the surface of the wire was highly non-uniform and that there was also a significant circumferential current flow due to inter-turn capacitance, both effects that are overlooked by standard filamentary current representations.

Au cours de ces dernières années, les communications sans fil connaissent une croissance vertigineuse, avec le développement de standards de communication de plus en plus nombreux, qui ouvrent la voie à de multiples applications telles que : la téléphonie mobile, le biomédical, le maritime, le civil et le militaire. De nos jours, les communications sans fil se sont diversifiées et multipliées. Cela entraîne la conception d’antennes toujours plus innovantes, performantes et de taille de plus en plus réduite (miniaturisation). Le projet FLEXBEA (FLEXible BEAcon) a pour but le développement d’un nouveau concept de balises de détresse miniatures (AIS et COSPAS-SARSAT), faible coût, intégrées dans des équipements de sécurité maritime tels qu’un radeau de survie et un gilet de sauvetage. Ces équipements sont destinés aux professionnels de la mer et aux plaisanciers. L’atout majeur de ce nouveau concept est l’intégration dans des équipements de sécurité maritime d’une fonction de détresse en cas de problème majeur : homme à la mer (MOB, Man OverBoard) par exemple lors d’un naufrage. Différentes antennes ont été étudiées. Nous présentons des antennes planaires (de type dipôle ou monopôle imprimé) développées dans la bande UHF : une solution de dipôle avec brins repliés est proposée afin de réduire l'encombrement, et deux modes d'alimentation (symétrique / dissymétrique) sont comparés. Des exemples d'antenne monopôle sont ensuite présentés avec une modification de leur géométrie (structures de type Bow-tie ou méandre) pour assurer une miniaturisation optimale. Puis les antennes filaires retenues pour le projet, avec une modélisation de ces antennes par un circuit équivalent (RLC). Des formules analytiques sont proposées afin de déterminer les valeurs de composants RLC qui interviennent dans le modèle circuit. Ensuite, nous sommes passés à la conception de l’antenne de la balise. Deux antennes ont été conçues et mesurées. Un monopôle ruban avec introduction de composants localisés pour la balise AIS et COSPAS-SARSAT, et une antenne hélice fonctionnant dans la bande AIS, intégrée dans la balise "SIMY". De nombreuses réalisations et mesures ont été effectuées pour caractériser ses antennes. / Over the last few years, wireless communications have grown dramatically, with the development of more and more communication standards, which open the way to multiple applications such as: mobile telephony, biomedical, maritime, the civilian and the military. Today, wireless communications have diversified and multiplied. This leads to the design of antennas that are always more innovative, more efficient and smaller in size (miniaturization). The FLEXBEA project (FLEXible BEAcon) aims to develop a new concept of low cost miniature distress beacons (AIS and COSPAS-SARSAT) integrated into marine safety equipment such as a life raft and a lifejacket safety. This equipment is intended for professionals of the sea and boaters. The main advantage of this new concept is the integration in maritime safety equipment of a distress function in case of major problem: man overboard (MOB, Man OverBoard) for example during a shipwreck. Different antennas have been studied. We present planar antennas (dipole type or printed monopoly) developed in the UHF band: a dipole solution with folded strands is proposed to reduce the bulk, and two modes of supply (symmetrical / asymmetrical) are compared. Examples of monopole antennas are then presented with a modification of their geometry (Bow-tie or meander type structures) to ensure optimal miniaturization. Then the wired antennas selected for the project, with a modeling of these antennas by an equivalent circuit (RLC). Analytical formulas are proposed to determine the RLC component values ​​involved in the circuit model. Then we went to the design of the beacon antenna. Two antennas were designed and measured. A ribbon monopoly with introduction of localized components for the AIS and COSPAS-SARSAT beacon, and a helix antenna operating in the AIS band, integrated into the "SIMY" beacon. Many achievements and measurements have been made to characterize its antennas.

Sewer and oil pipeline spillage issues have become major causes of pollution in urban and rural areas usually caused by blockages in the water storage and drainage system, and oil spillage of underground oil pipelines. An effective way of avoiding this problem will be by deploying some mechanism to monitor these installations at each point in time and reporting unusual liquid activity to the relevant authorities for prompt action to avoid a flooding or spillage occurrence. This research work presents a low cost energy efficient liquid level monitoring technique using Radio Frequency Identification Technology. Passive UHF RFID tags have been designed, modelled and optimized. A simple rectangular tag, the P-shaped tag and S-shaped tag with UHF band frequency of operation (850-950 MHz) has been designed and modelled. Detailed parametric analysis of the rectangular tag is made and the optimised design results analysed and presented in HFSS and Matlab. The optimised rectangular tag designs are then deployed as level sensors in a gully pot. Identical tags were deployed to detect 4 distinct levels in alternate positions and a few inches in seperation distance within the gully pot height (Low, Mid, High and Ultra high). The radiation characteristic of tag sensors in deployment as modelled on HFSS is observed to show consistent performance with application requirements. An in-manhole chamber antenna for an underground communication system is analysed, designed, deployed and measured. The antenna covers dual-band impedance bandwidths (i.e. 824 to 960 MHz, and 1710 to 2170 MHz). The results show that the antenna prototype exhibits sufficient impedance bandwidth, suitable radiation characteristics, and adequate gains for the required underground wireless sensor applications. Finally, a Linearly Shifted Quadrifilar Helical Antenna (LSQHA) designed using Genetic Algorithm optimisation technique for adoption as an RFID reader antenna is proposed and investigated. The new antenna confirms coverage of the RFID bandwidth 860-960 MHz with acceptable power gain of 13.1 dBi.