The rapid development of wireless communication systems is bringing about a wave of new wireless devices and systems to meet the demands of multimedia applications. Multi-frequency and multi-mode devices such as cellular phones, wireless local area networks (WLANs) and wireless personal area networks (WPANs) place several demands on the antennas. Primarily, the antennas need to have high gain, small physical size, broad bandwidth, versatility, embedded installation, etc. In particular, as we shall see, the bandwidths for impedance, polarization or axial ratio, radiation patterns and gain are becoming the most important factors that affect the application of antennas in contemporary and future wireless communication systems.
Table 1.1 shows the operating frequencies of some of the most commonly used wireless communication systems. The bandwidths vary from 7% to 13% for commercial mobile communication systems, and reach up to 109% for ultra-wideband communications. The antennas used must have the required performance over the relevant operating frequency range. Antennas for fixed applications such as cellular base-stations and wireless access points should have high gain and stable radiation coverage over the operating range. Antennas for portable devices such as handphones, personal digital assistants (PDAs) and laptop computers should be embedded, efficient in radiation and omnidirectional in coverage. Most importantly, the antennas should be ...