| In recent years, mobile communication systems make a rapid development towards miniaturization, multi-function and intelligence. Antenna as a key device is faced with many new demands for miniaturization, multi-frequency and broad-band. Planar monopole antenna processes many advantages of microstrip antenna, such as small volume, low profile, light weight and easy to be integrated with other microwave circuits. Meanwhile, planar monopole antenna avoids some of the shortcomings of the microstrip antenna, for example, the bandwidth is narrow and the performance of antenna is vulnerable to the substrate.This thesis introduces the basic theories and analytical methods of the planar antennas, and summarizes the current techniques to achieve antenna miniaturization and multi-band. Then, in view of the current antenna design trend and hot topics, the design ideas are studied and three novel miniature multi-frequency planar monopole antennas are proposed.Among the three schemes of miniature multi-frequency antenna, one design is a dual-band planar quasi-fractal antenna, another design is an arc-shaped dual-band printed monopole antenna and the last one is an arc-shaped triple-band planar monopole antenna. The dual-band planar quasi-fractal antenna has advantages of compact structure, simple feed and easy fabrication. It can work at 2.4GHz/5.2GHz/5.8GHz, which is suitable for the WLAN (Wireless Local Area Network) applications. For expanding the bandwidths, a cambered defected ground is adopted by the arc-shaped dual-band printed monopole antenna, so the antenna can completely cover the three bands of WLAN. The triple-band antenna excites the third resonant mode based on the arc-shaped dual-band monopole antenna, so that it meets both 2.4GHz/5.2GHz/5.8GHz WLAN and 2.5GHz/3.5GHz/5.5GHz WiMAX (Worldwide interoperability for Microwave Access) demands. Compared to the dual-band arc-shaped antenna, the triple-band arc-shaped antenna has smaller volume. The three presented antennas all employ defected ground planes to obtain good impedance matching (especially for the high frequency band). In addition, three antennas have wide bandwidths, high gains and good omni-directional radiation patterns. All the antennas are fabricated and tested. The simulated and measured results agree well, indicating that these antennas are suitable for current wireless communication systems. |
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