Antenas microstrip

CHAPTER

14

Microstrip Antennas

14.1

INTRODUCTION

In high-performance aircraft, spacecraft, satellite, and missile applications, where size, weight, cost, performance, ease of installation, and aerodynamic profile are constraints, low-profile antennas may be required. Presently there are many other government and commercial applications, such as mobile radio and wirelesscommunications, that have similar specifications. To meet these requirements, microstrip antennas [1]–[38] can be used. These antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to manufacture using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs, and when the particular patch shape and mode areselected, they are very versatile in terms of resonant frequency, polarization, pattern, and impedance. In addition, by adding loads between the patch and the ground plane, such as pins and varactor diodes, adaptive elements with variable resonant frequency, impedance, polarization, and pattern can be designed [18], [39]–[44]. Major operational disadvantages of microstrip antennas are their lowefficiency, low power, high Q (sometimes in excess of 100), poor polarization purity, poor scan performance, spurious feed radiation and very narrow frequency bandwidth, which is typically only a fraction of a percent or at most a few percent. In some applications, such as in government security systems, narrow bandwidths are desirable. However, there are methods, such as increasing the height of thesubstrate, that can be used to extend the efficiency (to as large as 90 percent if surface waves are not included) and bandwidth (up to about 35 percent) [38]. However, as the height increases, surface waves are introduced which usually are not desirable because they extract power from the total available for direct radiation (space waves). The surface waves travel within the substrate and they arescattered at bends and surface discontinuities, such as the truncation of the dielectric and ground plane [45]–[49], and degrade the antenna pattern and polarization characteristics. Surface waves can be eliminated, while maintaining large bandwidths, by using cavities [50], [51]. Stacking, as well as other methods, of microstrip elements can also be used to increase the bandwidth [13], [52]–[62]. Inaddition, microstrip antennas also exhibit large electromagnetic signatures at certain
Antenna Theory: Analysis Design, Third Edition, by Constantine A. Balanis ISBN 0-471-66782-X Copyright  2005 John Wiley & Sons, Inc.

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MICROSTRIP ANTENNAS

frequencies outside the operating band, are rather large physically at VHF and possibly UHF frequencies, and in large arrays there is atrade-off between bandwidth and scan volume [63]–[65].
14.1.1 Basic Characteristics

Microstrip antennas received considerable attention starting in the 1970s, although the idea of a microstrip antenna can be traced to 1953 [1] and a patent in 1955 [2]. Microstrip antennas, as shown in Figure 14.1(a), consist of a very thin (t λ0 , where λ0 is the free-space wavelength) metallic strip (patch)placed a small fraction of a wavelength (h λ0 , usually 0.003λ0 ≤ h ≤ 0.05λ0 ) above a ground plane. The microstrip patch is designed so its pattern maximum is normal to the patch (broadside radiator). This is accomplished by properly choosing the mode (field configuration) of excitation beneath the patch. End-fire radiation can also be accomplished by judicious mode selection. For a rectangular patch,the length L of the element is usually λ0 /3 < L < λ0 /2. The strip (patch) and the ground plane are separated by a dielectric sheet (referred to as the substrate), as shown in Figure 14.1(a). There are numerous substrates that can be used for the design of microstrip antennas, and their dielectric constants are usually in the range of 2.2 ≤ r ≤ 12. The ones that are most desirable for good...