Broadband Planar Antennas: Design and Applications

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Planar Inverted-L/F Antennas

4.1 INTRODUCTION

Mobile and wireless communication systems are extremely popular, some device examples being cellular phones, laptop computers, PDAs, in-building access points of WLANs and GPS terminals. Wireless devices for personal or vehicular applications need to be portable, so they must be small in size and light in weight. The antenna for these devices has been the largest component except for the battery and display. As a result, there is a strong demand for reducing the size of antennas.

An antenna with a low profile is one type of miniaturized design that is widely applied in vehicle-mounted devices. To reduce the height of a conventional wire monopole, the top end of the wire monopole is folded such that the monopole consists of two radiating segments, one vertical and the other horizontal. However, with the reduction in the height of the monopole, its operating mode changes from series resonance, whereby the monopole length is about a quarter of the operating wavelength, to parallel resonance, whereby the overall monopole length is about half the operating wavelength. A quarter-wavelength thin-wire monopole usually has half the impedance of a half-wavelength dipole, namely (73 + j42.5)/2 Ω, and operates around its first resonance. To change the length and/or thickness of the monopole, the imaginary part will be cancelled out and the pure resistive impedance will be around 50Ω. It is easy to match the monopole to transmission systems such ...