【作者】 薛子光；

【导师】 王德苗；

【作者基本信息】 浙江大学 ， 电子科学与技术， 2005， 硕士

【摘要】 “分形”是法国数学家Mandelbrot于1975年提出的全新几何分支，它可有效地用于描述自然界中广泛存在的一大类Euclid几何所难以定义的复杂对象。分形具有空间填充能力和自相似属性。当把分形的概念用于天线的设计时，就得到了分形天线。分形天线具有小巧、轻便、易于匹配的优点，同时有的还具有多频带、宽频带特性，因此，它特别适合于当前宽带、超宽带通信领域。 当前，对分形单元天线的研究主要集中在对基本分形性质的了解。但在实际应用中，必须考虑到有限截取效应，即实际制作的天线是有限次迭代的结果，而不是数学上无穷迭代的概念；有限截取效应对分形天线的性能有着重大的影响，如果对基本分形不做相应的改变，往往得不到预期的性能。因此，要充分体现出分形的一系列优点，就必须做一些结构上的改变，同时考虑到实际需要，还需详细了解一些重要参数对天线性能的影响。至于分形阵列，当前的研究基本上把天线单元处理成理想的电流源，而没有考虑各单元之间的互耦，这与实际情况也有较大的差异。 本文首先研究了传统的Sierpinski单极子天线，由于它存在有限截取效应，其第二个谐振频率与第一个谐振频率之比约为3.5。文章通过将Sierpinski单极子的上半部分改成三角环，使两频率的比值下降到2.2，改进后的天线可用于GSM850和PCS双频天线的设计。 当把分形的概念直接运用到Sierpinski微带天线的时候，发现其第一个谐振频率点相对普通的三角形贴片天线有显著的提高。文章分析了其中的原因，并通过加顶层网格并截去其边角，使改进后天线的第一谐振频率显著降低，但仍有多个谐振频带，同时尺寸没有增加。 Koch单极子具有良好的性质，本文设计了一种应用于DCS+PCS频段的新型折叠型Koch单极子，通过在恰当的地方引入90°弯角，天线的高度可减小到普通λ/4单极子天线的40％，而带宽却增加到17％。 文章最后还研究了分形天线在最简单的阵列天线——八木天线中的应用，并对其进行了全阵列电磁仿真，重点是考虑各单元之间的互耦对天线性能的影响。研究结果表明，相对一般的八木天线，Koch八木天线的谐振频率显著降低，但辐射方向图仍保持类似。文章还详细给出了各主要参数对Koch八木天线性能的影响。更多还原

【Abstract】 ’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.更多还原