Wideband Unidirectional Antennas And Tightly Coupled Antenna Arrays

With the continuous advancement of modern wireless communications, radar and electronic warfare systems, the electrommunication devices are promoted to be more and more integrated, miniatured and multi-functional. Thus, it exists higher requirements on the characteristics of impedance matching bandwidth, radiation and size of antenna and antenna array. Combining the scientific project with hotspot of antenna technology, this dissertation is engaged in the research on the wideband unidirectional antenna with improved dipole, wideband high-gain antenna and tightly coupled dipole antenna array. The major contributions are listed as follows:1. Wideband unidirectional antennas with improved dipoles are investigated for the wideband wireless communication system. a) Wideband foursquare dipole antenna:through properly designing the feeding balun and gap between parasitic patches and radiating loops, a wideband foursquare dipole antenna can be achieved. By adding a square metal cavity, a small-size wideband foursquare dipole antenna is obtained with good radiation characteristic. b) Wideband unidirectional folded dipole antenna:based on the folded dipole with integrated balun, wideband dual-element folded dipole can be obtained by multi-section microstrip feedline, the optimized width of two dipoles in folded dipole and distance between the two dipoles. Reflected by a metal plate with bending edge, the dual-element folded dipole can be evolved to wideband unidirectional folded dipole antenna with good radiations. By utilizing coaxial line to improve the feeding structure of wideband unidirectional folded dipole, a dual-band low-profile antenna array with high gain can be designed for base-station applications.2. Instead of developing antenna array for high gain, we adopt three different ways to improve the directivity of a single antenna.a) Wideband high-gain Yagi-Uda antennas: through the study of printed Yagi-Uda antenna, the one-dimensional director array is extended to two and three-dimensional director array for greatly improving the directivity and stability of directivity. Then, wideband dual-element folded dipole and metal plate are utilized as driven element and reflector, respectively, for constructing two types of wideband high-gain Yagi-Uda antennas.b) Artificial electromagnetic material-loaded Vivaldi antenna: Through the EM simulation of E-shape structure, it can be known that the arrangement of E-shape element can be treated as the material with index of refraction being larger than 1 in wide band, which makes the Vivaldi antenna have high gains in wide band. c) Fabry-perot resonator antenna:By utilizing the partially reflective surface with positive reflection phase gradient, the resonator condition of Fabry-perot cavity can achieve in wide band. With the help of rectangular loop, the wideband Fabry-perot resonator antenna with a good 3-dB gain bandwidth can be obtained.3. The research on ultra-wideband tightly coupled dipole array is proposed. The bowtie dipole is chosen as an array element with the metal patch layer being impedance matching layer for low cost and easy fabrication. The element is fed by the balun from microstrip feedline to BPS, while the metal shorting cylinder is used to improve the H-plane radiation of 1-D infinite model of element. The elements are arranged for constructing 2×8 tightly coupled dipole antenna array. The mutual coupling coefficient of its elements is also analyzed. Then, perpendicular shorting metal plate and extended radiating arm are used to solve the edge effect of the antenna array, respectively. The array is fed by a properly designed 1 to 16 feeding network. Lastly, the two final array models are fabricated for verifying simulations.4. Wideband horizontally polarized omnidirectional (HPO) antennas with mutual coupling method:In order to solve the edge effect of tightly coupled array, some arc dipoles are formed as a ring to simulate the tightly coupled array, which makes arc dipoles being a circular loop antenna with good impedance matching and radiation omnidirectionality performance. Based on this designing method, two wideband HPO antennas are designed, a) 12-element HPO antenna design:The dipole is chosen as array element with its edge overlap for tightly mutual coupling. The effect of element numbers on impedance matching performance and the horizontal-plane radiation is analyzed. Lastly, the 12 arc dipoles are utilized to construct a circular loop antenna, being fed by a 1 to 12 feeding network, while 12 arc director elements are added to enhance elevation radiation, b) 6-element HPO antenna design:The dipole is chosen as array element with its interdigital edge for tightly mutual coupling. The 6 arc dipoles form a circular loop antenna which is fed by a novel 1 to 6 feed network. Compared with 12-element HPO antenna, this 6-element HPO antenna can simultaneously cover 2G/3G/4G bands with a 36% reduction of size.