안테나 소형화와 배열급전구조 연구

Abstract

This dissertation describes a theoretical investigation of material effects on the bandwidth of a small antenna, demonstrates miniaturized antenna designs for dual-band mobile phones, and proposes switched beam forming networks for array antennas. Various materials are attached to mobile antennas to support the mechanical strength and to reduce the antenna size, but the presence of a material affects the antenna performances. The radiation quality factor of an infinitesimal electric dipole antenna surrounded by a small sphere filled with a homogeneous material is derived and analyzed to illustrate how the bandwidth of the dipole is affected by the permittivity and permeability of the material. The stored energy inside the sphere is augmented by the inclusion of the reflected wave from the material and vacuum boundary. The analysis reveals that among the four different materials, namely, vacuum, dielectric, magnetic, and magneto-dielectric materials, vacuum always yields the greatest bandwidth, and dielectric materials come next for small dipoles. When the dipole size becomes larger, magneto-dielectric materials come next to vacuum. Magnetic materials always produce the narrowest bandwidth. These orders are reversed for small loop antennas.A twisted line inverted-F antenna is designed on a thin printed circuit board for dual-band mobile phones. The antenna features a twisted line for dual bands and a parasitic line for an additional resonance. This antenna has a wideband characteristic in the high frequency band due to the mergence of resonances by the twisted line and the parasitic line. The electric size of the antenna is 0.109 λ × 0.025 λ × 0.0025 λ at 0.9 GHz, and its length is about 44 % that of a conventional inverted-F antenna. The genetic algorithm is utilized to optimize two types of antenna for dual-band mobile phones. One is to optimize the antenna topology, and the other is to optimize the locations and values of two inductors to be mounted on a given loop antenna. The performances of candidate designs are evaluated using an objective function that gives a larger weighting value as the reflection coefficient decreases in the target band. These antennas are fabricated to demonstrate their dual-band operations, and the measured results agree well with the simulation.Three different types of beam forming feed networks and a microstrip phase inverter are proposed. The first feed network is a simplified four-beam Butler matrix feeding a 3×1 array antenna, and it consists of one 90° hybrid, two switches, and delay lines. By controlling the switches, the three elements can produce four different beam patterns with the phase differences between adjacent radiating elements of ±135° and ±45° at 11.2 GHz. The second one is a 4×4 Hadamard matrix network feeding a 4×1 array antenna. The excitation of the array follows the elements in a row of the matrix. The array produces four different two-lobed beam patterns, whose selection depends on the choice of the input port of the feed network. The third feed network forms a sectoral conical beam at 2.57 GHz when the feed network is used for a 2×2 array antenna. The maximum radiation occurs at the 45° elevation angle and at the 45°, 135°, 225°, and 315° azimuth angles, depending on the choice of the input port of the feed network. A microstrip phase inverter, which consists of two microstrip lines, two via holes, and a meander line slot on the ground, is proposed. The pass band of the phase inverter corresponds to the resonant frequency of the meander line slot, which is determined by the length of the meander line. One fabricated phase inverter has a bandwidth of 11 %, an insertion loss of 0.39 dB, and a phase difference of 193 degrees at the resonant frequency of 2.34 GHz.