| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 101 === The microwave dielectric resonators (DR) having a high dielectric constant, high quality factor and near-zero temperature coefficient of resonant frequency and other features, suitable for a dielectric resonator antennas, filters, oscillators, and duplexers. In recent years, due to the evolution and development of microwave communication system, miniaturized, high performance and low cost has become the main demand of the microwave components. Therefore, the high quality factor dielectric material often used in communications systems. With low loss characteristics performance, often designed for high-performance microwave components. However, the high dielectric constant ceramic material can also achieve the purpose of miniaturization, so the choice of material is also a very important issue. As mentioned above, the main study of this dissertation is divided three parts which preparation of high dielectric constant, high quality factor, design and fabrication of the microwave dielectric resonator antennas.
1.Development of High Q Microwave Dielectric Materials
[a]Study of Mg1.8Ti1.1O4 Ceramics:
(1)Binary titanate microwave dielectric ceramic Mg1.8Ti1.1O4 (er~ 15.7, Q×f~141,000 GHz at 10.57 GHz, and τf~–52.4 ppm/°C), having an extremely low dielectric loss and a low cost, were reported as suitable materials for global positioning system (GPS) and wireless local area network (WLAN). Therefore, the spinel-structured Mg1.8Ti1.1O4 is worthy to investigate its microwave properties.
(2)In this dissertation, with partial replacement of Mg by Co or Zn, the Q×f of the dielectrics (Mg0.95Co0.05)1.8Ti1.1O4 (er~ 16.1, Q×f~207,500 GHz at 10.72 GHz, and τf~–52.6 ppm/°C) and (Mg0.94Zn0.06)1.8Ti1.1O4 (er~16.5, Q×f~210,700 GHz at 10.52 GHz, and τf~–62.3 ppm/°C) can be easily boosted to a value higher than 200,000 GHz and retain compatible and τf.
(3)In order to achieve temperature-stable materials, CaTiO3 was added to form the (1–x) Mg1.8Ti1.1O4–xCaTiO3 ceramic system. A three-phase system was confined by X-ray diffraction patterns and EDX analysis. The microstructures of the ceramics were characterized by SEM. The microwave dielectric properties of the ceramics can be effectively controlled by varying the x value. For practical applications, a fine combination of microwave dielectric properties (er~19.6, Q×f~72,700 GHz at 9.03 GHz, τf~–3.7 ppm/°C) was achieved for 0.91Mg1.8Ti1.1O4–0.09CaTiO3 ceramics sintered at 1330°C for 4 h, which makes it is a very promising candidate material for applications in dielectric resonator antenna.
[b]Study of Li2TiO3 Ceramics:
(1)Development of Li2MgTiO4 Ceramics
Rock-salt-structured Li2MgTiO4 ceramic was prepared by the conventional mixed oxide route and its microwave dielectric properties were investigated. The microstructures of the ceramics were characterized by SEM. The dielectric properties of the ceramics exhibited a significant dependence on the sintering condition and crystal structure. A new microwave dielectric material, Li2MgTiO4 sintered at 1360°C has a dielectric constant (er) of ~17.3, a Q×f of ~97,300 GHz (where f = 9.86 GHz, is the resonant frequency) and a τf of ~–27.2 ppm/°C. The microwave dielectric properties of the ceramic are reported for the first time.
(2)Partial replacement of Li2TiO3 by ZnO
The microwave dielectric properties of the (1–x)Li2TiO3–xZnO (x = 0.1–0.5) ceramic system prepared by mixed oxide route have been investigated. The rock-salt structured (1–x)Li2TiO3–xZnO were confirmed by using X-ray diffraction spectra, scanning electron microcopy (SEM). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the specimens. The decrease of Q×f value at high-level ZnO addition (x 〉 0.3) was owing to the intensity of the (002) superstructure reflection decreased and became disordered rock-salt structure. For practical applications, a new microwave dielectric material 0.7Li2TiO3–0.3ZnO is suggested and it possesses a good combination of dielectric properties with an er of ~ 23, a Q×f of ~99,800 GHz (measured at 8.91 GHz), and a τf of ~0 ppm/°C. A low-loss dielectric resonant antenna using aperture-coupled cylindrical dielectric resonant was designed and fabricated using the proposed dielectric to study its performance.
2.Research of High K Microwave Dielectric Materials
High-dielectric-constant and low-loss ceramics in (1–x)Ca4MgNb2TiO12–xCaTiO3 system have been prepared by the conventional solid-state route. The forming of complete (1–x)Ca4MgNb2TiO12–xCaTiO3 solid solutions were confirmed by the X-ray diffraction pattern analysis and the measured lattice parameters,which linearly varied from a = 5.5478 Å, b = 7.7710 Å, and c = 5.4543 Å for x = 0.1 to a = 5.4718 Å, b = 7.6799 Å, and c = 5.4262 Å for x = 0.9. By increasing x, not only could the τf of the ceramics be turned to a near-zero value (~ –6.9 ppm/°C) at x = 0.3, a substantial Q×f (~ 20,200 GHz) and er (~ 43.9) could aslo be achieved simultaneously.
3. Design and Fabrication of Microwave Dielectric Resonator Antennas
A new triple-band dielectric resonator antenna (DRA) fed by a coplanar waveguide (CPW) is presented. The proposed antenna, composed of a high permittivity dielectric resonator and printed on FR4 substrate, is fed by a 50 Ω coplanar waveguide transmission line. In order to achieve wideband applications, the antenna with two parasitic inverted-L strips is demonstrated to generate two resonant frequencies covering 3.5- and 5.2-GHz. The measured results show that the antenna covers the frequency bands 2.30–2.72, 3.45–3.61, and 5.05–6.23 GHz with less than –10 dB of S11. The frequency response of the simulation results shows good agreement with the measured data. Good antenna gain, radiation efficiency and radiation patterns of the proposed antenna have also been observed across the operation band. Details of the proposed antenna design and experimental results are presented and discussed.
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