The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to antenna systems and more particularly to limited scan phased array antenna systems.
2. Description of the Prior Art
Phased array antenna systems are well known in the prior art. The usual phased array system scans a narrow beam many beam widths within a sector of perhaps .+-.60.degree. from broadside. A limited scan antenna system which is the subject of the present invention scans a narrow beam only a few beam widths. Limited scan antennas have found application in radars for locating projectiles such as mortar and artillery fire. The object of a projectile locator is to detect and ascertain the location of the source by accurate trajectory measurements early in the flight of the projectile. Thus, this type of radar need only scan a few beam widths from the horizon. High gain beams are required in order to combat noise and minimize multipath effects.
Another application of limited scan antenna systems is in the aircraft approach and landing system, such as a Category III Instrument Landing System (ILS), which allows an aircraft to be flown onto the ground without visual ground reference. Generally an aircraft on ILS approach to landing is flown to within a predetermined distance of landing and to a preselected altitude above the landing spot by reference only to instruments. Upon obtaining visual reference of the runway, the pilot in command lands by reference to the ground. In the advanced ILS, an aircraft may be flown to touchdown without any visual ground reference.
A third application is in the field of satellite communication systems which utilize a high gain antenna having a narrow beam width emanating from the satellite and covering only a portion of the earth. Such coverage may be limited to half a continent. Satellite communications systems with viewing angles of approximately 18.degree. require a small number of beams to cover the earth.
Limited scan antenna systems are generally known in the prior art. An optical antenna which provides limited scan with a minimum number of active elements is the Luneberg lens. The Luneberg lens is spherically symmetric and has the property that a plane wave incident on the sphere is focused to a point on the surface at the diammetrically opposite side. Likewise, a transmitting point source on the surface of the sphere is converted to a plane wave on passing through the lens. Due to the spherical symmetry of the lens, the focusing property does not depend upon the direction of the incident wave. A Luneberg lens may provide a limited number of scan beams by utilizing an equal number of feed horns. Also, this lens may be used in conjunction with an intermediate lens and confocal with an aperture lens. For a more detailed explanation of a Luneberg lens, refer to R. C. Hansen "Microwave Scanning Antennas," Vol. 1, pages 214-218 and 224, Academic Press, New York. U.S. Pat. No. 3,835,469 issued to the assignee herein, describes the utilization of a Luneberg lens with confocal lenses.
One of the drawbacks of optical devices is that they occupy a relatively large volume. Also, this type of optical lens presents deployment and alignment problems such as moving a large Luneberg lens to an operational position while maintaining the proper alignment. Consequently, optical lenses may not be suitable for transportable equipments or systems.
Another antenna network which is well known in the prior art is the Butler matrix, which has the number of active inputs (phase shifters) equal to the number of beams. The Butler system provides ideal performance; i.e., maximum realizable gain consistent with the aperture size and no grating or other spurious lobes. The limitation of the Butler system is that it is very complicated and expensive to build due to the large number of hybrids and transmission line crossovers. For a more detailed explanation of the Butler antenna, refer to "Microwave Scanning Antenna," supra, page 262.
Still another antenna array utilizes a "thinned" array of phase shifters coupling an input corporate feed and an array of sub-array corporate feeds which are in turn coupled to periodic arrays of radiating elements. A "thinned" array refers to an antenna feed system having fewer phase shifters than radiating elements. For example, a prior art thinned array antenna may have a corporate feed with four output elements coupled to four phase shifters. The phase shifter output terminals are in turn coupled to the input terminals of sub-array corporate feeds which are each connected to three radiating elements. The sub-array corporate feeds are coupled only to their respective radiating elements and not to the elements of other sub-arrays. Since the sub-arrays do not overlap, there is no combining loss and all the energy is radiated. Gain degradation occurs due to grating lobes rising as the beam is scanned off the broad side direction. Grating lobes, as is well known, are beams or secondary principle maxima which have an amplitude equal to that of the main beam unless the sub-arrays are properly configured. Grating lobes are caused when the radiation from the elements add in phase in those directions from which the relative path lengths are integral multiples of a wavelength. For six radiating elements per conventional sub-array, there are no grating lobes when the beam is perpendicular to the plane of the radiating elements. As the beam is steered from the perpendicular position, grating lobes begin to appear and their level rises rapidly to -12 dB for an intersub-array phase of 72.degree..