Miniaturized Antennas For Wireless Communications

Antenan miniaturization has benn a hot topic throughout the years. In the past ten or twenty years, with the the rapid development of wireless communication, increasing requirement for miniaturized antennas has been proposed. Based on antenna and microwave circuit theories, miniaturization of loop and dipole antennas, reconfigurable antenans as well as omnidirectional antennas of circular polarizations are studied in this dissertation. The main academic contributions are summarized as follows.1) The design method of miniaturization of loop antennas through capacitive load and coupled feed has been summarized. The equivlent circuits of capacitively loaded, inductively and capacitively coupled fed miniaturized loop antennas are proposed. Based on the equivlent circuits, the operating frequency, impedance matching as well as radiation efficience have been well explained and the design procedure of the miniaturized loop antennas has been summarized.2) For the requirements of anti-interference and secure communications, a frequency reconfigurable antenna with a very wide tuning band is proposed. The tuning band of the antenna is significantly enhanced with a coupled loop in the feeding structure, and this has been explained through the equivalent circuit analysis.3) Three miniaturized antennas for mobile terminals has been proposed. The first and second capacitively loaded loop antennas are coplanar with the system ground and is inductively and capacitively coupled fed, respectively. Both of them have high radiation efficiency and the potential of frequency reconfigurability. The excited currents on the system ground are concentrated on the radiating loop and barely flow toward the ground, which indicats potential use of the two antennas in MIMO antenna systems. The third antenna are composed of two inductor loaded monopoles. The broad bandwidth which covers LTE band14, GSM850and GSM900is realized through two resonance of the monopoles.4) For2.4-GHz WLAN access-point or application, a compact omnidirectional antenna of circular polarization (CP) is presented. The antenna consists of four bended monopoles, which are simultaneously excited through an associated feeding network. The radiated horizontal and vertical field components are derived based on an analytical model and the90°phase difference between them is shown to be naturally guaranteed. The design procedure is also given.