Research On The Miniaturization And Reconfigurability Of Compact Antennas

All the works in the dissertation are based on the concept of miniaturization. These works focus on several hot spots, such as multiband antennas, reconfigurable antennas and MIMO (Multi-input multi-output) systems. The design rules and processes are discussed theoretically, thereafter, some antennas are designed after simulation and optimization. The organization of this dissertation is as follows. Firstly, Microstrip and printed antennas are used to design multiband antennas with wide bandwidths. Secondly, through the analysis of resonant mechanisms, reconfigurable antennas are studied with PIN diode in the proper position. The related parameters of the antennas are analyzed as well. Several antennas with switchable frequency or polarization are designed and fabricated. Lastly, spatial diversity is considered. Some structures are discussed to reduce the mutual coupling between two antennas in the MIMO system. The main academic contributions of this dissertation are as follows:1. An axe-shaped antenna is designed to realize dual-frequency radiation. The antenna works at the frequencies of 1.77GHz and 3.7GHz. The frequency ratio is between 1.639 and 2.616. The radius of the patch is only 8.42mm, which is reduced by 85% compared to conventional circular microstrip antennas. To solve its problem of narrow bandwidth, a printed folded antenna is designed. By making use of some special techniques, the X/4 resonance is excited. This antenna can operate at frequency bands of 1.26-1.3GHz(Galileo E6), 2.49-2.92GHz(Wimax)and 5.04-6GHz(WLAN). Because several resonances are merged into one band, the antenna has wide bandwidths in the middle and high frequency bands. Meanwhile, the antenna has a very compact size of 0.1λ0×0.0036λ0×0.0034λ0 at 1.28GHz.2. A frequency reconfigurable antenna is designed using the combination of a ring antenna and an inverted-L antenna. The shorted side of ring antenna is switched for different frequencies. The antenna resonates at 1.49-2.58GHz in switching-off state and 3.78-9.28GHz when switching on the diode. The antenna is only 0.15λ0×0.066λ0×0.005λ0 at 1.49GHz.3. Defected ground structure (DGS) and artificial transmission line cells (ATLC) are used to miniaturize the proximity coupled antenna. The antenna size is reduced by 68% as compared to the conventional microstrip antenna. By controlling the terminal of the feed line using one PIN diode, the coupling mechanism is changed, so that two orthogonal linear polarizations are realized.4. Conventional aperture coupled antennas are linearly polarized. By adding two open stubs on the feed line, the antenna can radiate one linearly or two circularly polarized waves using two PIN switches. All the bias circuits are on the feed line and shielded by the ground plane. The antenna resonates at 2.5GHz. The impedance bandwidth is 10.5%. For circularly polarization, the bandwidth of 3dB axial ratio is 3.6%.5. The mutual coupling of two T-shape coupled-fed antennas is researched. By inducing the method of desiging bandstop filters, a multiband isolation structure is analyzed and compactly inserted in the antenna system. The system operates at 2.42,3.43 and 5GHz bands. The S21 of each band is below-10dB.6. Center-shorted neutralization line (CSNL) is used to design reconfigurable MIMO antennas. The CSNL can induce a notch on the S21 curve, so that the mutual coupling is largely reduced. The size of the antenna system is 0.195λ0×0.208λ0×0.03λ0 at 1.6GHz. The PIN diode is switched on or off for 1.616GHz or 2.51 GHz radiation, with a maximal isolation which is better than 24dB in each band. Lastly, CSNL method is compared with the NL method in published papers. It is proved that the-10dB isolation can be achieved in a more compact size. Also, the isolation method will not induce new interferential frequencies.