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In this dissertation, several multi-band antenna with reconfigurable techniques are presented. The first part of the dissertation focuses on using varactors in the antenna to achieve active reconfigurable operation. In Chapter II, a novel design of dual-band reconfigurable slot antenna is proposed. The configuration of the proposed antenna is based on a folded slot with a branch edge formed by multiple strips loaded with circuit components. In the design, the antenna is decomposed into networks, and the corresponding equivalent circuits are deduced. By applying resonant condition on the equivalent circuit at the desired center frequencies, the circuit components can be determined. The proposed design is later transformed into a reconfigurable antenna by using varactors. The antenna configuration is also modified for applying biasing voltages to varactors. Dual-band reconfigurable operation of antenna is studied and demonstrated for S-band applications at 2.45 GHz and GPS at 1.227/1.381/1.575 GHz. The design approach is described in details. Also, antenna measurements are conducted for design validation. In Chapter III, novel resonant modules for constructing multi-band mobile antennas are proposed. The basic configuration of the module, named as the prototype module, is designed based on a series LC resonant circuit. By adding a varactor to the module, it can be transformed into a reconfigurable module. By introducing an extra resonant mode with capacitive coupling to enhance the bandwidth, a wide-band module can be obtained. These modules have different frequency ranges, and can be combined into multi-band antennas for various applications. The proposed modules are demonstrated by constructing a 4G-LTE antenna. The reconfigurable module, which is of 20×20 mm2, is designed to switch between LTE700 (698-787 MHz) and GSM850 (824-960 MHz) bands. The wide-band module, which is of 10×10 mm2 and 49.3% bandwidth, is designed for GSM1800/ GSM1900/ UMTS/ LTE2300/ LTE2500 (1.71-2.59 GHz) applications. All designs have been realized and measured for validation. The proposed antenna modules, which are simple and convenient to use, can provide flexibility, especially in antenna deployment, in multi-band mobile antenna designs.
The second part of the dissertation focuses on switchable polarization. In Chapter IV, a design of dual-band cavity-backed slot antenna loaded with a spurline is presented for GPS and S-band radar applications. In the design, a very thin cavity (0.017λ0 in thickness at 1.575 GHz) is used to achieve unidirectional radiation. The dual-band responses of the antenna are excited by the slot resonance at 1.575 GHz and waveguide transition effect at 2.4 GHz. Even with very small cavity thickness, reasonable impedance bandwidths are obtained as 1.6% (26 MHz) at 1.575 GHz and 8.4% (203 MHz) at 2.4 GHz. The proposed slot can also be loaded with a spurline to adjust the antenna center frequency and the phase of the radiated field. A switch using a PIN diode is deployed with the spurline. Since the spurline is in serial connection with the slot, the spurline and the corresponding bias circuit for the PIN diode have little effect on the antenna radiation. Two proposed slots are combined to form an antenna which is right-hand circularly-polarized at both 1.575 and 2.4 GHz. A phase delaying system consisting of delay line and the spurline is designed to achieve 90° phase difference at both frequencies. The axial-ratio bandwidths are obtained as 1.5% (24 MHz) at 1.575 GHz and 4.5% (110 MHz) at 2.4 GHz. Compared to the traditional designs in which the cavity thickness is often more then 0.25 , the proposed design with thin cavity is more suitable to be deployed on the vehicle surface.
The last part of the dissertation focuses on the reconfigurable antenna with passive components. In Chapter V, a frequency reconfigurable tri-band antenna using resonant circuit is proposed. In the design, the resonant frequency of the LC parallel resonator is designed at 2.4 GHz, which formed a band-stop filter. With the frequency selective property of the band-stop filter, the configuration of the proposed antenna is transformed into a monopole at 2.4 GHz, and quarter-wave slots at 3.3-3.7, and 5.15-5.85 GHz, respectively. At 2.4 GHz, the monopole is the main radiator, the quarter-wave slots are performed as ground plane. Besides, at 3.3 and 5.5 GHz, the monopole is the ground plane of the quarter-wave slots. Thus, the mutual coupling effect of the antennas are not of concern. The fabricated dual-band and tri-band antenna have compact dimensions of 8.5×10 mm2 and 14.5×10 mm2, respectively. Both the dual-band and tri-band antennas are tested experimentally. Measured and simulated return loss and radiation patterns are in good agreement. | en_US |