Introduction and Overview 1.1 Introduction

Conventional microstrip antennas in general have a conducting patch printed on a grounded microwave substrate, and have the attractive features of low profile, light weight, easy fabrication, and conformability to mounting hosts [1]. However, mi-crostrip antennas inherently have a narrow bandwidth, and bandwidth enhancement is usually demanded for practical applications. In addition, applications in present-day mobile communication systems usually require smaller antenna size in order to meet the miniaturization requirements of mobile units. Thus, size reduction and bandwidth enhancement are becoming major design considerations for practical applications of microstrip antennas. For this reason, studies to achieve compact and broadband operations of microstrip antennas have greatly increased. Much significant progress in the design of compact microstrip antennas with broadband, dual-frequency, dual-polarized, circularly polarized, and gain-enhanced operations have been reported over the past several years. In addition, various novel broadband microstrip antenna designs with dual-frequency, dual-polarized, and circularly polarized operations have been published in the open literature. This book organizes and presents these recently reported novel designs for compact and broadband microstrip antennas. 1.2 COMPACT MICROSTRIP ANTENNAS Many techniques have been reported to reduce the size of microstrip antennas at a fixed operating frequency. In general, microstrip antennas are half-wavelength structures and are operated at the fundamental resonant mode TM 01 or TM 10 , with a resonant frequency given by (valid for a rectangular microstrip antenna with a thin microwave substrate) f ∼ = c 2L √ ε r , (1.1) 2 INTRODUCTION AND OVERVIEW FIGURE 1.1 Circularly polarized corner-truncated square microstrip antennas for GPS application at 1575 MHz. (a) Design with a microwave substrate (ε r = 3.0, h = 1.524 mm); (b) design with a ceramic substrate (ε r = 28.2, h = 4.75 mm). Dimensions are in millimeters and not to scale. where c is the speed of light, L is the patch length of the rectangular microstrip antenna, and ε r is the relative permittivity of the grounded microwave substrate. From (1.1), it is found that the radiating patch of the microstrip antenna has a resonant length approximately proportional to 1/ √ ε r , and the use of a microwave substrate with a larger permittivity thus can result in a smaller physical antenna length at a fixed operating frequency. Figure 1.1 shows a comparison of the required dimensions for two circularly polarized corner-truncated square microstrip antennas with different substrates for global positioning system (GPS) application. The …