The small bandwidth, however, is adequate in a large class of communication applications. Therefore, the bandwidth of the radiation is poor compared to the bandwidth of antennas discussed earlier. However, the structure is principally a resonant cavity, with only limited fringing radiation. The fringing fields through the open sidewalls are responsible for the radiation. The microstrip antenna can be thought of as a rectangular cavity with open sidewalls. The width of the antenna, W, on the other hand, determines the level of the input impedance. For a rectangular microstrip antenna, this requires the length of the antenna, L, to be about half a wavelength in the dielectric medium. The size of the antenna is usually designed such that the antenna resonates at the operating frequency, producing a real input impedance. The microstrip antenna produces maximum radiation in the broadside (perpendicular to the substrate) direction and ideally no radiation in the end-fire (along the surface of the substrate) direction. The microstrip line may be connected to a feeding circuitry or directly fed by connecting a signal source across the microstrip line and the ground plane. One common approach is to feed from a microstrip line, connecting the microstrip antenna at the center of one of its edges. The antenna may be excited using various methods (Pozar, 1992 Pozar and Schawbert, 1995). The metal patch on the front surface can have various shapes, although a rectangular shape, as shown in Figure 6.17, is commonly used. The geometry of a microstrip antenna consists of a dielectric substrate of certain thickness d, having a complete metalization on one of its surfaces and of a metal “patch” on the other side. Besides other resulting advantages, the integrated-circuit technology for the antenna fabrication allowed high dimensional accuracy, which was otherwise difficult to achieve in traditional fabrication methods. It was invented to allow convenient integration of an antenna and other driving circuitry of a communication system on a common printed-circuit board or a semiconductor chip (Carver and Mink, 1981 Pozar, 1992). The microstrip antenna is a relatively modern invention.
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