In this thesis the focus was on the design, fabrication, and tests of the feeding networks individually and within an array system. The array feeding network is a corporate-fed type utilizing equal-split, stepped-multiple sections of the conventional Wilkinson power divider in microstrip form with a unique topology. The feeding network was specifically designed for a broadside relatively small linearly-polarized wideband UHF non-scanning array for directed power applications that uses an array radiator with a new volumetric ribcage dipole configuration. The array has a large impedance bandwidth and consistent front lobe gain over the wide frequency band. Theoretical and experimental results describing the performance of the array feeding network and the array are presented and discussed.

Abstract. The problem of Electromagnetic compatibility is a popular topic. It is divided into two categories, one is conducted electromagnetic interference, another is radiated interference. We can use LISN to measure the signal of conducted electromagnetic interference , and use antenna to measure the signal of radiated interference. This paper will focus on the antenna. The frequency range of radiated Electromagnetic Interference measurement is so wide that using the halfwave dipole antenna will be very time-consuming, Therefore broadband antennas are often used in lieu of the halfwave dipole antennas. The design procedure of the log-periodic dipole array antenna is introduced in this article. Simulation data of such antenna using the NEC2 software, including the input impedance and the antenna pattern are also provided. Furthermore, based on the parameters obtained from the simulation, a log-periodic dipole array antenna can be fabricated. In doing so, the simulation results should be modified to take into considerations of the specification of the material used, such as diameters of the materials, and lengths of the transmission lines. Once the construction of the log-periodic dipole array antenna is completes, it can be compared with the simulation results, and the difference between them should be investigated to find out the optimal design parameters. Finally, the antenna factor can be calculated to compare it with the measurement data.

This doctoral thesis deals with designs and developments of multi(wide)-band multifunctional antennas based on folded dipoles. In the beginning, the concept of Orthogonal Folded Dipoles (OFD) are put forward. Orthogonal folded dipoles are formed by two identical two-port folded dipoles orthogonally joining with each other at the center, and can be fed through different combinations of feed nodes to offer dual- and wide-band modes, respectively. The impedances of the 2 modes are studied both by analytical models and by commercial electromagnetic simulation tools. The properties of the linearly polarized radiation patterns in the two modes vary quite little, except for ±45◦  rotation of two principal planes. In this way, orthogonal folded dipoles can possibly provide pattern reconfigurability in a context of switchable types and orientations of polarizations. Next, the concept of Log-Periodic Folded Dipole Array (LPFDA) is proposed. It stems from the traditional log-periodic dipole array, whereas folded dipoles instead of straight dipoles are applied as the elements of the array. Two configurations, i.e. partly-scaled LPFDA and fully-scaled LPFDA, are studied through simulations and optimizations. The comparison shows that the latter outperforms the former in terms of higher directivity, reduced front-to-back ratio and lower crosspolar level. The key parts of this thesis focus on exploiting Eleven antennas, based on the LPFDAs, in a variety of applications. First, the 4-port L-band lab model for use in satellite terminals demonstrates that the radiation patterns for monopulse tracking can be achieved through different excitation combinations of the multiport Eleven antenna. Second, a decade bandwidth, an unchanged phase center and nearly constant directivities over the whole band can be regarded as the major figure-of-merit of the Eleven antenna, which makes it suitable as feed for prime-fed reflector antennas. Through endeavors of using Genetic Algorithms, the wideband Eleven antennas have been gradually optimized in term of matching as well as efficiencies. Besides, the rotationally symmetric circular Eleven feed is a very promising solution for improving the BOR1 efficiency and therefore the aperture efficiency by a fraction of dB. Last but not least, the two multiport L-band lab models measured in a reverberation chamber demonstrate that the Eleven antenna with MIMO diversity ports can possibly overcome narrowband multipath fading in a real radio link and improve the link quality in terms of a significant diversity gain and high maximum available capacity.

The purpose of this research is to find ways to create an omnidirectional antenna with high directivity in the vertical plane. The investigation is based on computer simulation using the program CST 2011. The objective is a narrow-band antenna that is omnidirectional in the horizontal plane and has maximum achievable directivity for a fixed size. Three of the most promising antenna designs are selected based on the current state of antenna technology. Their maximum directivities are estimated by means of well known relations in antenna theory. It is shown that the most suitable design is an omnidirectional antenna in the form of a cylindrical dipole antenna array with an active central dipole. For this antenna, excitation by means of a radial traveling wave, with a phase velocity smaller than speed of the light, is possible. It is found that for a certain value of a moderating factor it is possible to obtain a directivity that is 2.5 – 3 dB larger than that of a dipole or a linear antenna with uniform excitation. The antenna structures are modeled to determine the number of dipoles, their dimensions and the spacing between them that maximizes the directivity.

Novel and low-cost antennas suitable for WLAN access points are presented. The operating bandwidths of the proposed antennas can cover the 2.4/5.2/5.8 GHz WLAN bands, and the antenna gain is larger than 2 and 4 dBi in the 2.4 and 5 GHz bands; respectively. Besides, they can also generate the good omnidirectional radiation patterns in the azimuthal plane. Dipole antenna, folded dipole antenna, monopole antenna and circularly polarized antenna has been applied in the proposed designs, and good antenna performance has been obtained.

Abstract Whether an Open Area Test Site (OATS) is qualified is based on the Normalized Site Attenuation (NSA). The purpose is to eliminate the influence of Antenna Factor (AF). Usually the AF provided by the manufacturer adopts the Standard Site Method (SSM) and is quoted from measurements at a 10-m range. In practice, the AF varies with the measurement conditions. This uncertainly will translate into error in NSA measurements. Currently radiated electromagnetic interference measurement mostly adopts the broadband antennas, and LPDA is one of the antennas used extensively. However, the AF provided by the manufacturer does not consider shifts in the phase center of LPDA with frequency. In the meantime the radiation pattern of LPDA is different from that of a short dipole. The investigation of this thesis will focus on these two parts. In determining the phase center of LPDA antenna we adopt the average shift of phase center to improve the AF. The numerical simulation results show that such an arrangement can result in improvement. We also use the method of PCPM (Phase Center and Pattern Matching) to modify the AF under different conditions of measurement when a ground plane is present. Our study, by using the numerical simulation and measurement, shows that this indeed improves the variation of AF over that obtained by SSM. In addition, efforts are also made to have a detailed discussion in mutually coupling effects between the LPDA antenna and the ground plane, in order to distinguish the impact on AF due to the coupling of the LPDA antenna and its image. Our numerical simulation results indicate that it does not have a significant effect.

No description available.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This paper presents the implementation of an artificial magnetic ground-plane with a low profile Log-periodic Dipole Array (LPDA) antennas. After the properties of three typical Electromagnetic Bandgap (EBG) structures are investigated and their bandwidth properties are studied, a mechanism is presented to improve the band-width over which the EBG surface acts as a perfect magnetic conductor (PMC). A low profile LPDA is modeled above this surface and the results indicate an improved band-width region. Compared with a LPDA in free space the frequency band is shifted higher by the EBG surface and the gain pattern is shifted from a horizontal orientation to a vertical orientation. / AFRIKAANSE OPSOMMING: Hierdie dokument stel voor die implementering van kunsmatige magnetiese grondvlakke met Logaritmiese Periodiese Dipool Samestelling (LPDS) antennas. Die eienskappe van drie tipiese Elektromagnetiese Bandgaping (EBG) strukture word ondersoek en hul bandwydte eienskappe word bestudeer. ’n Meganisme word voorgestel om die bandwydte te verbeter waar die EBG oppervlakte soos n perfekte magnetiese geleier optree. ’n Lae profiel LPDS word bo hierdie oppervlakte geplaas. Die resultate dui aan ’n verbetering in the bandwydte. In vergelyking met ’n LPDS in vrye ruimte skuif die frekwensie band ho¨er as gevolg van die EBG oppervlakte en die aanwins patroon skuif van ’n horisontale orientasie na ’n vertikale orientasie.

Student Number : 9713144D - MSc Dissertation - School of Electrical Engineering - Faculty of Engineering and the Built Environment / The performance of the Log Periodic dipole array antenna has been characterized, in the form of parametric curves available in most antenna design handbooks and other relevant literature. These characteristic curves are often limiting in scope, as for instance they do not contain parametric curves giving the relationship between the boom-length 'L' and the number of dipole element 'N' for any given bandwidth, even when it is known that these two parameters are the main cost determinants of a LPDA Antenna. The concept of convergence is introduced to aid cost optimization of the LPDA Antenna in terms of number of dipole element 'N'. Although 'N' is used as the minimization criterion, the criteria for establishing convergence encompass all the main electrical characteristics of the LPDA Antenna, such as VSWR, gain and radiation patterns. Lastly, the effects of boomimpedance 'Zo' and length to diameter ration 'Ln/Dn', on the performance characteristics of the LPDA Antenna was investigated with the view to determining if neglecting the effects of these two parameters were responsible for the disparity in the directive gain values obtained by R. L Carrel compared to those obtained by later researchers. The investigation indicates that if an LPDA Antenna is converged, then the effects of Zo and Ln/Dn ratio though significant can not alone account for the fairly large disparity in the gain values. In other to perform these investigations, a modern scientific tool in the form of numerical modeling by method of moments based, Super Numerical electromagnetic code version2 was utilized. The numerical modeling tool was first validated by agreement between measured values and the values as predicted by the modeling tool. Next, simulation of the performance of LPDA antennas under variations of their number of elements was done. Thereafter, the means and standard deviations of the gain were extracted from the simulated numerical models. Trends in the pattern of variation of the means and standard deviations of the gain are used as the basis for deciding the value of number of element at which the antenna can yield acceptable performance (convergence criteria). These are presented as convergence curves, which gives for any given boom-length and operating bandwidth, the minimum number of elements required for the antenna to yield acceptable performance. Finally, the effect of length to diameter ratio and boom-impedance on the gain of optimized LPDA antennas are presented as parametric curves.

Several microwave devices and antennas that are based on negative-refractive-index transmission-line (NRI-TL) metamaterials are presented in this thesis, which exhibit superior performance features compared to their conventional counterparts. These are a Wilkinson balun, a 1:4 series power divider, a four-element printed dipole array, a leaky-wave antenna, and an electrically small folded-monopole antenna. The Wilkinson balun employs +90° and −90° NRI-TL metamaterial lines at the output branches of a Wilkinson divider, to achieve a six-fold increase in the measured differential output phase bandwidth compared to that of an analogous balun employing transmission lines, while occupying only 55% of the area. The 1:4 series power divider comprises four non-radiating 0° NRI-TL metamaterial lines, each with a compact length of λ0/8, to provide equal power split to all four output ports. Compared to a conventional series power divider employing one-wavelength long transmission lines, the metamaterial divider provides a 154% increase in the measured through-power bandwidth, while occupying only 54% of the area. The metamaterial series power dividing concept is also applied to a four-element fully-printed dipole array that is designed to radiate at broadside, in order to demonstrate that the array exhibits reduced beam squinting characteristics. It is shown that the metamaterial-fed array has a measured scan-angle bandwidth that is 173% greater than an array that is fed using a conventional low-pass loaded line. The reduced-beam squinting property that NRI-TL metamaterial lines offer is subsequently exploited to create a leaky-wave antenna that radiates a near-fixed beam in the forward +45° direction, with an average measured beam squint of only 0.031°/MHz. This is achieved by operating the antenna in the upper right-handed band where the phase and group velocities are the closest to the speed of light. Finally, an electrically small antenna comprising four 0° NRI-TL metamaterial unit cells is presented which supports a predominantly even-mode current, thus enabling it to be modeled as a multi-arm folded monopole. This significantly increases its radiation resistance, which allows it to be matched to 50 Ω, while maintaining a high measured efficiency of 70%.