Broadband Planar Antennas: Design and Applications

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Planar Monopole Antennas and Ultra-wideband Applications

5.1 INTRODUCTION

Besides planar transmission-line antennas such as microstrip patch antennas, SPAs, and PIFAs or PILAs, dipoles and monopoles are the most basic types of antenna and have been used widely since Guglielmo Marconi, an Italian inventor, sent and received his first radio signal in Italy in 1895. A straight wire monopole vertically installed above a ground plane features simple structures but pure vertical polarization and horizontal omnidirectional radiation. The impedance bandwidth of simple thin-wire monopoles can be increased by modifying their geometry, such as thickening or loading or folding their wire elements. Typical designs include conical or skeletal conical, cage, and various loading monopoles.^1–4 However, as compared with the thin-wire monopoles, the conical or rotationally symmetric monopoles are much more bulky. Alternatively, planar elements have been used to replace the wire elements of the monopoles to broaden the impedance bandwidth.^5–11

With planar radiators, the impedance bandwidth of monopole antennas can be broadened, typically up to more than 70%, or even high-pass. Therefore, broadband planar antennas are becoming very attractive to promising developments such as software-defined radio systems, reconfigurable wireless communication systems and ultra-wideband (UWB) systems. Usually, the broadband design of antennas is much simpler than multiband design of narrowband antennas, such as ...