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Title: Design of magneto-electric dipole antennas
Other Titles: Ci dian ou ji zi tian xian de she ji
Authors: Wu, Biqun ( 吳壁群)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2010
Publisher: City University of Hong Kong
Subjects: Antennas, Dipole -- Design and construction.
Electromagnetic theory.
Notes: CityU Call Number: TK7871.67.D56 W8 2010
vii, 150 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves 145-147)
Type: thesis
Abstract: This thesis presents proposals for the design of new wideband magneto-electric dipole antennas for various wireless communications. The standard magneto-electric dipole consists of a vertically-oriented quarter-wave shorted patch and a planar dipole, which are equivalent to a combination of a magnetic dipole and an electric dipole, respectively. Our findings demonstrate good electrical characteristics for the proposed designs, including low back-lobe radiation, stable antenna gain across the operating band and symmetrical radiation patterns in the E- and H-planes. In addition to single linear polarization designs, we also reviewed dual polarization, quad polarization and diversity designs in developing the new proposed designs. Our study of prior designs leads us to propose two novel structures of magneto-electric dipole elements, each composed of a top-driven loop magnetic element and a planar dipole, or center-feed mono-loop and a planar dipole, for different applications. We examine a number of feeding techniques designed to enhance the impedance bandwidth of magneto-electric dipoles, including coaxial-to-patch probe feeding, and transition of microstrip-to-parallel strip line feeding. Based on the original magneto-electric dipole design with an L probe-feed, an improved ultra-wideband unidirectional antenna element composed of a magnetic dipole and an electric dipole excited by a Γ-shaped strip was designed and studied, with a pair of upper trapezoidal-shaped patches acting as an electric dipole, and two pairs of right-angle corner vertical walls acting as a magnetic dipole. The proposed antenna element achieves an impedance bandwidth of 91% (SWR ˂ 2). Due to the complementary nature of the antenna and the incorporation of a modified grounded structure, the proposed antenna has a relatively stable broadside radiation pattern with low cross-polarization and low back-lobe radiation over the operating band. The gain of the antenna is about 9.3 dBi. In addition to this first proposed design, a second dual-polarized magneto-electric dipole antenna excited by two Γ-shaped strips is presented. This second antenna achieves an overlapped impedance bandwidth of 65.9% (SWR ˂ 2) at both input ports. The antenna has excellent performance in isolation (more than 36 dB between the two input ports) and the gain of the antenna is about 9.5 dBi. The radiation pattern and bandwidth over the operating frequency band for this second proposed antenna design also are very stable. After a review of these models, we propose a novel horizontally polarized conical beam antenna consisting of four magneto-electric dipoles in a ring configuration above a ground plane. The antenna is low in profile (H=0.1867 λₒ) and has about 38% impedance bandwidth (SWR ˂ 2) and average antenna gain of 5 dBi. The radiation patterns are stable across the operating band. Good agreement between simulated and measured results is observed. Based on the proposed magnetic monopole structure, a wideband four-port diversity antenna that is capable of exciting four different radiation patterns is presented by simply adding a vertical copper wire on the center of the conducting plane. A matching network is used to combine the signals from the four magneto-electric dipoles to produce two orthogonal broadside modes and one conical mode with horizontal polarization. The electric monopole is used to generate a conical beam with vertical polarization in electric feed. The performance of the antenna is studied both computationally and experimentally. A prototype operated at around 2.4 GHz was constructed and tested. Experimentally, the antenna exhibits about 22.2% overlapped impedance bandwidth of the four ports. The two orthogonal broadside modes saw about a 10 dBi average gain, and the two orthogonal conical modes achieved an average gain of 6 dBi. The measured radiation patterns of the four degenerate modes each are stable within the operating band. The measured isolations between different ports are generally less than -26 dB within the overlapped bandwidth. Finally, several broadband techniques are employed to develop a novel profile magneto-electric dipole element (TYPE I) composed of an electric dipole and a top-driven triangular-shaped patch loop with coaxial-to-patch probe feeding. The height of this proposed antenna is only 21 mm (0.16 λₒ, λₒ refers to the center frequency of operation). This represents a reduction of about 0.09 λₒ from the height of the original magneto-electric dipole. The proposed antenna also is low in profile (H = 0.161 λₒ) and has about 40% impedance bandwidth (SWR ˂ 1.5) and antenna gain of 8.4dBi with 0.2 dBi variation. The radiation patterns are stable across the operating band. Good agreement has been observed between simulated and measured results. For ultrawideband (UWB) operations, a second type of magneto-electric dipole (TYPE II) is proposed, consisting of a bowtie-shaped electric dipole and center-feed tapered mono- loop excited by an elliptically tapered microstrip line-to-balanced strip line. This TYPE II antenna achieves a simulated impedance bandwidth of 113% (SWR ˂ 2). Due to the complementary nature of the antenna structure, the antenna has a relatively stable broadside radiation pattern with low cross-polarization and low back-lobe radiation over the operating band. The antenna gain is 5 - 10.8 dBi. Additionally, a novel ±45° dual-polarized unidirectional antenna element composed of two cross center-fed tapered mono-loops and two cross electric dipoles located against a reflector for ultra-wideband applications is presented, based on an operating principle that includes the use of an elliptically tapered microstrip line feed for dual-polarization. Prototypes with different reflectors, planar or conical, were investigated experimentally and numerically, and demonstrated a measured overlapped impedance bandwidth of 126.2% (SWR˂2). Due to the complementary nature of the antenna structure, the antenna has a relatively stable broadside radiation pattern with low cross-polarization and low back-lobe radiation over the operating band. The measured gain of the proposed antenna varies from 4 to 13 dBi, and 7 to 14.5 dBi from port 1 and port 2, respectively, over the operating band, when mounted on a conical backed reflector. The measured isolation between the two ports is below -25dB over the operating band.
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