The tremendous advancement in general communication technologies together with the growing on consumer have driven strong demands for small handsets but applicable for multiband on a single device. A recent trend shows that modern mobile handsets are required to operate at multiple frequency bands and to be able to cover various commercial communication services. The requirement of compact size, light weight, reduced height, and applicable of multifunctional handsets puts more stringent requirements on the antenna design and more challenging for antenna designers. Antenna is one of important elements and plays a significant role in wireless communication systems. Thanks to the development of modern integrated circuit technology, the size and weight of mobile handsets have been rapidly reduced. Generally, though broadband and multiband antenna designers have achieved advantages to reach the above target, the number of wireless communication systems is increasing dramatically, meaning that adjacent interference signals between those systems for the broadband and/or multiband antennas are also increasing. It is desired that the antenna can be tunable in active ways to meet the requirements of covering multiband individually or simultaneously while being able to minimize or eliminate unwanted radio frequency interferences. With advances in high density radio frequency and microwave circuit packaging, integrated components for antennas have been studied widely for tuning or reconfiguring purposes. It is shown that a single well-designed antenna having efficient bandwidth and enough tuning range can be used for different operating frequency bands.
This thesis contributes another effort for tunable/reconfigurable antennas. In scientific literatures, several techniques are proposed for tunable and reconfigurable antennas such as RF switches, MEMS switches, PIN diodes, varactor diodes, and screw. We review the advantages, disadvantages and tuning/reconfiguring mechanisms of all proposed designs. Among them, we realize that varactor diode technique is very promising for designing antennas tunable over wide frequency ranges thanks to its excellent DC voltage controlled reactants and continuity properties. By using a single typical varactor diode, two compact tunable internal PIFA-type antennas are proposed for Personal Communication Handsets. Both antennas have a significant small dimension, and conformal structure. They can be easily integrated inside modern slim mobile handsets. The electrical characteristic of these two proposed antennas make it attractive for use in 4G handset applications.
Firstly, a simple PIFA slot type tunable for Personal Communication Handsets is presented. With this design, the antenna can operate at the following frequency bands DCS (1710-1880MHz), PCS (1750-1870MHz, 1850-1990MHz), UMTS (1920- 2170MHz), Bluetooth (2400-2500MHz), WiBro (2300-2400MHz), WLAN (2400, 5200, and 5800MHz), and ISM band (2500-2700MHz). The proposed antenna consists of three main slots to generate multi-resonance frequency. The varactor diode is located in a proper location between one of these slots to get the tuning at the target frequency. The antenna size is about 0.741 $cm^3$ and mounted on a FR-4 composite ($\epsilon_r$=4.6) PCB board with size of 80 $\times$ 45 $mm^2$. The return losses, the surface current distributions, and the radiation patterns are simulated and measured to illustrate the antenna performance.
Secondly, to overcome a limitation of the first design in terms of the efficient bandwidth of each individual tuning band around 2GHz, the second compact tunable internal antenna for Personal Communication Handsets is proposed. In the second design, the antenna size is about 1.04 $cm^3$, and mounted on the same FR-4 substrate PCB board size. The proposed antenna and ground are located in one side of PCB board while DC bias circuit is put in the other side. The antenna consists of a main radiating patch and a parasitic patch. The parasitic patch is put closed to the main patch to excite the electromagnetic field and enhance the efficient bandwidth. Slots are created in the main patch to generate multi-resonance frequency. Varactor diode is used to connect parasitic patch and the main radiating patch at proper location in order to achieve tuning property at target frequency. The antenna cover these following band frequencies DCS (1710-1880MHz), PCS (1750-1870MHz, 1850-1990MHz), UMTS (1920-2170MHz), WiBro (2300-2400MHz), WiMax, WiBro phase III, and other MIMO applications at 3200-4500MHz, and WLAN (5200MHz). The antenna's geometry, the return losses, the surface current distributions, and the radiation patterns are simulated and measured to illustrate the antenna performance.
Finally, a conclusion of thesis and future works, references and acknowledgments are presented.
