Phase-scanning experiments with two-reflector antenna systems

The two-reflector antenna system is widely used in its Cassegrain form for many applications because it has considerable versatility in the optimization of its design parameters. By replacing either one of its continuous reflecting surfaces with a discrete-element phased array, additional versatility is created. Both cases are discussed herein and key experiments are reported which verify that such modifications are practical. When the main reflector becomes a phased array, a fairly large region of space can be scanned rapidly. The configuration investigated indicates that a modest reduction in the number of array elements, as compared to conventional arrays, can be obtained by absorption of the array factor grating lobe structure. Excellent monopulse pattern characteristics are also shown throughout the scan region. When the subreflector becomes a phased array, a small region of space can be scanned rapidly with very few array elements. The configuration investigated indicates that phase control based on geometrical optics analyses is sufficient to overcome the usual aperture phase aberrations which occur with displaced and/or multiple feed antenna geometries. Conventional array beam-steering expressions become somewhat complex when exact results are to be implemented. A major advantage of the two-reflector antenna wherein one surface is a phased array is the possibility of making modifications to existing systems at reasonable costs through a redesign of only one subunit in the configuration.