Investigation On Technology And Design Of Fractal Antennas

Antenna is a vital component of radio communications and radar systems, of whichcommunication quality and detection ability greatly depend upon the antennas’performances. With incessant growth of modern wireless services, radio systems are moreand more compact with multiband, broadband, lightweight, and low cost. Consequently,antenna technology becomes one of the biggest challenges to performance enhancement ofthe systems day by day. Miniaturization, multiband, wideband, high efficiency, high gain,reconfigurability, and self-adaptability have already been the major trends of developmentof antenna technology at present and in the future. Fractal antenna is a significantbreakthrough of modern antenna technologies. It has presented many unique traits, such asminiaturization, multi-resonances, multi-modes, self-matching, array effect of elementantenna, aesthetics, and broad suitability for various antennas, etc. It is a mixtured productof fractal geometry and antenna technique. Fractal antennas are a kind of antenna, whoseradiator, ground or reflector is fractal-shaped or the fabrication materials have some fractalcharacteristics. It is a unique antenna technology rather than an antenna type withspecified geometrical configuration, such as microstrip antennas, helix or spiral antennas,and parabolic reflector antennas etc. It can be used for design of various types of antennas,such as wire antennas and planar antennas, etc. So, it will have broad applications and abright future. Usually, a fractal antenna has some odd radiation properties in its high orderresonances, which can be utilized for design of some novel antennas, such as the1.5·λaxial-mode and coplanar omni-directional mode fractal dipoles, and the down-sizedmultiband multimode fractal helix antenna. It can also be used for performanceenhancement of variety of conventional antennas. For instance, fractal microstrip antennascan operate in multiband with shrunk dimension; fractal loop antennas are multiband,multimode and high gain; the0.5·λ fractal dipole can operate in normal-mode but withsmaller size; fractal dielectric resonant antennas (DRAs) are high gain, multiband or ultra-wideband; the fractal ultra-broadband monopoles have high gain, high efficiency,and consistent directivity within the impedance bandwidth; fractal conformal antennashave size shringkage, novel radiation mode and insensitivity to the surface curvature;fractral array antennas have compactess, high gain, and low sidelobe level etc.Just since the concept “fractal antennas” was put forth by Dr. Cohen from CornellUniversity in1991, fractal antenna has drawn vast attentions from researchers around theworld and has been commercialized in small scale. In the past two decades, fractal antennahad gained abundance in types, advances in fabrication technics, and extension inapplications. However, for fractal antennas, more novel properties haven’t beendiscovered, strict theoretical basis has not been established, design methodology stillneeds to be improved, and the challenge of ultra-wideband remains existent. Many fancypeculiarities, such as multiple radiation modes, high gain, good efficiency, array effect ofelement antenna, and mutilation resistivity have never arrested researchers’ attentions.Relationships between resonances, miniaturization, adjacent frequency ratio, radiationproperties, and the fractal geometry have not been formulated mathematically. Thefabrication methods of fractal geometry, optimal fractal geometry for antenna design, andthe best feeding way have not been extensively studied. Efforts for ultra-wideband byoverlapping of multiple bands remain demanded. The subject of designing conformalantennas utilizing fractal geometries has not been initiated. Furthermore, investigation onfractal antennas has only focused on monofractal up to now. As regards multifractalantennas, which are combined with two or more monofractals, with some uniqueproperties never possessed by monofractal antennas need to be deeply explored.Non-rectilinearly bent fractals, such as circularly arced fractals have never been proposed.Thus far, the available types of fractals are still insufficient, outstanding performances arestill scarce, and the resonsnt banwidth is inherently narrow. These may explain that eventhough fractal antenna has been studied for many years, it hasn’t been applied vastly inengineering. Therefore, some researchers said “For a fractal antenna, its beautiful shape is always more attractive than the performances”. In some sense, this word reflects theinsatisfied development status of fractal antennas really. In this thesis, the author tries hisbest to just change the circumstance, overcome the innate shortcomings, devise originalfractal geometries, and explore peculiar traits of fractal antennas. Many novel results havebeen acquired from the extensive investigations and explorations. And application valuesof fractal antennas will be upgraded and aggrandized substantially with thesearcheivements.The main contribution of this dissertation to research of fractal antennas includesbreakthrough in theory and expansion in applications. In theory, the relationships betweenfractal antenna’s electrical properties and its geometrical characteristics have beenrevealed exhaustively and scientifically for the first time, namely the relationshipsbetween resonant properties such as miniaturization, adjacent resonance ratio, andresonant impedances etc. and geometrical parameters like iteration number, physical scaleratio, fractal scale ratio, and fractal dimension have been precisely formulated. Inapplications, Guiseppe Peano miniaturized axial-mode helix antenna, Koch-like sidedbow-tie multiband/ultra-wideband dipole antenna, KSSG/SKLB/KLSHC multifractalmonopole/dipole antennas, CAKC/Guiseppe Peano fractal multiband multimodemonopole/dipole antenna, CABFL fractal multiband multimode loop antenna, CABFHfractal multiband multimode helix antenna, Pyriform shaped ultra-wideband dipoleantenna, and Sierpinski curve coplanar omni-directional mode/axial-mode dipole antennahave been originally invented. For the aforementioned fractal antennas, size shrinkage isgenerally realized in fundamental resonance and fundamental-like mode even novelradiation modes usually emerge in high resonances. For example, the CAKC penta-band,tri-mode monopole antenna, its first and second resonance are omni-directional modes, thethird and fourth resonances are axial-mode with high gain, and the fifth mode isoffset-axial omni-directional mode; the K2-iterated CABFH penta-band, tri-mode,dual-sensed circular-polarizations fractal helix antenna, its first, second, and fifth resonance are RHCP axial-mode, the third resonance is LHCP axial-mode, and the fourthresonance is RHCP offset-axial mode. Especially, the fifth resonance presents remarkable“array effect of element antenna” with axial gain as high as a4-element cylindrical helixarray antenna. What difference from other literatures on fractal antennas which only focusupon theory studies, is that the design instances in this dissertation not only well elucidatetheory of fractal antennas but also have good practicability. For instance, Sierpinski curveco-axial dipole array antenna with horizontal omni-directional high gain has very lowaxial height; CAKC monopole can be used for omni-directional radio broadcasting andhigh gain directive telecommunications in its normal and axial mode, respectively;K2-iterated CABFH fractal helix antenna can supersede multiple cylindrical helix antennas.With these miniaturized, multiband, multimode, and high performance fractal antennas,numbers of antennas, cost of the system, burdened weight of the platform, andelectromagnetic coupling among multiple antennas in wireless communication systemscan be significantly reduced. They prove themselves possessing of good practical valueand promote applications of fractal antennas in engineering and commerce.