| Fractal microstrip antennas have the feature of multiband frequency operation, which is favorable in the development of antenna integration. Fundamental theories and engineering principles of designing microstrip antennas, including classic theoretical models,numerical methods for electromagnetic fields as well as some empirical formula about antenna design, are briefly introduced. Several popular commercial software packages for electromagnetic structure analyses are also reviewed. Based on these advanced software packages, such as HFSS and Ensemble, the following research work about fractal microstrip antennas are fulfilled: (1) Mechanism of multiband frequency behavior of typical fractal microstrip antennas is explored by investigating the behavior of 1, 2 and 3-order Sierpinski carpet fractal microstrip antennas. The influence of different feeding positions on the performance of the fractal microstrip antennas are analyzed by simulation. Frequency response of S11 at the input port, impedance bandwidth and directivity are calculated and analyzed in detail. Some Sierpinski carpet fractal antennas with different orders are manufactured and measured to verify the simulation results. Good agreement is achieved. (2) In order to eliminate the drawback of narrow band of single layer fractal microstrip antenna, a novel quasi-fractal multilayer microstrip antenna structure is developed based on multiband property of fractal antennas. The impedance bandwidth is effectively extended. Sierpinski carpet patches are applied to this new structure and modified to tune resonant frequencies of patches lied in different layers. After mass simulation and calculation, an optimization structure is selected. A two-layer and a three-layer fractal microstrip antenna evolved from this structure are both fabricated and measured. Good agreement between measurement data and simulations is obtained, with a relative bandwidth of 28% and 24% respectively. Compared to the single layer fractal microstrip antenna, the impedance bandwidth of the new multilayer fractal antenna is significantly extended. It is concluded that advantages of fractal microstrip antenna integrated with traditional bandwidth-extending techniques are useful to the design of broadband fractal microstrip antenna, which expands the application of microstrip antenna, and are highly valuable for antenna integration. As a result of adopting commercial software for electromagnetic structure calculation, dependability, effectivity of designing microstrip antennas are enhanced. |
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