Fractal Antennas And The Research Of Their Multi-band Characteristics

The rapid expansion of wireless technology has set new demands on antennas, such as miniaturization and multiband. The solution to this need can be fulfilled by applying fractal concepts on antenna designing. Fractal has space self-filling ability and self-similarity ability.When applying fractal to the design of antenna fractal antenna will be gained. Fractal antennas have been proven to possess the characteristics of miniaturization and multi-frequency, so they are well suited to the development of broad band and super-broad band wireless communication.Current research of fractal antenna is mainly focused on basic fractal geometry.Practical fractal antennas are pre-fractal antennas with deterministic iterative algorithm and are obtained through a finite number of iteration.It would lead to truncation effect which has a great influence on the performance of fractal antennas, however. So, in order to exploit the merits of fractal antenna and obtain satisfactory performance, some corresponding modifications on the basic fractal structures should be done.The traditional Sierpinski gasket monopole antenna is well known for its multiband behavior, but the ratio between its first two resonant frequencies is in the order of 3.5, not equal to its scale factor. In this thesis, a new loaded Sierpinski gasket monopole is first investigated and it is found that the ratio has been greatly lowered to be close to its scale factor 2.Meanwhile, all of the upper resonant frequencies of the loaded monopole have been significantly shifted toward lower frequencies which is favorable when the second band and above are to be employed.The effect of some important parameters on the performances of the antenna is also studied and a conclusion is drawn that increasing the flare angle can help lower its resonant frequencies. Hybrid fractal antennas constructed by loading Koch element on Sierpinski gasket are then proposed and analyzed, they can lower the resonant frequencies ulteriorly without affecting its multiband behavior as they have taken advantage of the merits of the two fractals.At last, some MEMS reconfigurable antennas based on Koch loaded Sierpinski gasket are investigated, the results indicate that they can increase operational bands while maintaining the radiation pattern.Traditional Sierpinski gasket monopole has only one scale fator which can not be altered, it would confine the ratio between its adjacent resonant frequencies and thus its practical use, however.In this thesis multi-scale factor Parany gasket antennas are proposed, they are all obtained by modifying the Sierpinski gasket in a simple way and fed through a microstrip line attached at the base without any matching network while with a similar multiband behavior. It is very convenient to allocate the scale factors flexibly to obtain the bands of interest with different ratios, so a Parany gasket antenna and a 2-order Koch loaded Parany gasket antenna are designed separately to accomplish the communication demands of GSM, 3G and WiMAX.