Research And Design Of Microwave Fractal Antennas

'Fractal' was first presented by Mandelbrot in 1975. 'Fractal' can be used to characterize unique occurrence in nature which were difficult to define with Euclidean geometries. 'Fractal' has space filling ability and self-similarity ability. When applying the 'Fractal' to the design of antennas, we get the fractal antennas. Fractal antennas have shown the possibility to miniaturize antennas and improve input matching. They can also be configured to operate effectively at various frequency. So, they are well suited to the development of broad band and super-broad band wireless communications.Current research of fractal cell antennas mainly focused on the basic fractal geometry. But in practice we must consider the 'truncation' effect. We can only get the fractal which is defined by several iterations, not infinite iterations. This type of fractal is named prefractal and present 'truncation' effect. 'Truncation' has a great influence on the performance of fractal antennas, if we don't do some modifications on the basic fractal geometry, usually we can't expected good performance. So, in order to materialize the merits of fractal, it is necessary to do some modifications on the basic structures, meanwhile, we must understand the effect of some important parameters on the performances of antennas. When it comes fractal array, current research mostly take the fractal cells for ideal current sources and don't consider the effect of coupling, which also have great differences with practical cases.In this thesis, we first investigated the Sierpinski monopole, because of 'Truncation' effect, the ratio between the first resonant frequency and the second resonant frequency is about 3.5, this thesis presented some modifications and lowered the ratio to 2.2. The modified Sierpinski monopole can be used in the design of GSM850 and PCS dual frequency antenna.When introduced the fractal to the design of Sierpinski microstrip antenna, we found out that the first resonant frequency is higher than the conventional triangle microstrip antenna. This thesis analyzed the reasons and presented some modifications. The modified Sierpinski microstrip antenna successfully lowered the first resonant frequency but still possess multiband characteristics; at the same time, the size is comparable with conventional triangle antenna.The Koch monopole has good characteristics, this thesis presented a novel curved Kochmonopole which are suitable to DCS+PCS band. After introduced the 90° right angle, the heightof the antenna can be lowered to 40% of the conventional — monopole, but the impedancebandwidth is increased to 17%.In the last of this thesis, we investigated the characteristics of the Koch Yagi antenna, the emphasis is on the effect of coupling. The result shows Koch Yagi antenna can lower the resonant and at the same time maintain the good radiation pattern.