This invention relates to the use of electromagnetic waves and, more particularly, relates to devices for changing the propagation direction of an electromagnetic wave.
In the years since the development of radar systems for military applications during World War II, radio ranging systems have undergone extensive refinement and have been employed in many different environments, both military and civilian. One component required by such systems, and more broadly by most electronics systems employing the use of electromagnetic waves in general, is a device capable of directing the wave energy in a particular direction. In a radar system, for example, the beam frequently must be swept in azimuth and elevation, while the antenna in a communications system often must be directed toward a receiving unit.
Although steering devices adequate to handle the needs of conventional radars are known in the art, the recent trend toward the development of radar systems utilizing radio waves in the millimeter wavelength region has complicated the design of the beam steering function.
Millimeter wave active and passive radars possess an inherent allweather capability. Because of this feature, defense organizations have emphasized the development of millimeter wave systems for a number of major weapons systems. Millimeter wave radars have been contemplated, for example, for use in terminal guidance for mobile anti-tank, anti-aircraft, and air-to-surface projectile and missile systems, surface-to-air guidance and tracking, "smart" disposable warheads, and moving target identification and tracking. Particularly where such radars are designed for uses requiring a compact radar unit, such as on a ballistic missile, the small size of a millimeter wave antenna (due to the reduced wavelength as compared to conventional radars) is an additional attractive feature of the millimeter wave radar.
Conventional mechanical radar antenna sweeping systems, however, are inherently slow in response, relatively large in size, excessively heavy, and tend to be mechanically unstable, each of these attributes making these types of antennas unsuitable for use in the millimeter wave region missile radars. Furthermore, the discrete element phased array antennas used at longer (centimeter) wavelengths are not practical in the millimeter wavelength region because of the small physical dimensions, the large number of individual elements, and the exact tolerances which are there required. A ferromagnetic phased array, whether of the discrete element or space fed mosaic design, includes costly magnetic control coils in the antenna structure, and in addition is difficult to fabricate and tends to be less compact than is desired. Semiconductor diode phase shifters are limited in speed by carrier recombination time and require complicated structures for their implementation.
Consequently, a need has developed in the art for a beam steering device which is fast, reliable, relatively low in cost, compact, and light-weight in millimeter wavelength embodiments.