Conformal radar antenna

#The present invention pertains generally to electronically scanned antennas and more particularly to scanned directive antenna arrays. An outstanding problem in naval fire control is the simultaneous tracking of multiple targets. This problem is partially solved by the use of electronically scanned antenna arrays. However, due to the complexity (mutual impedance and complex computerized steering commands) and resultant cost of these systems, their use has been fairly limited in the Naval Fleet.

#Another configuration which has been considered is a series of single radar beams produced by directive antenna elements which are selectively addressed in accordance with their placement in space to effectuate a steered output beam. End fire antenna elements, e.g., dielectric rods, have advantages over alternative beam directors such as parabolic reflectors, lenses, and antenna subarrays since they occupy considerably less cross-sectional surface area than the others. The physical length of the end fire antenna elements however have virtually eliminated their use.

#In studying the physical characteristics of end fire rod antennas, diffraction theory indicates that if D represents the maximum antenna diameter and .lambda. the responsive antenna wavelength, then the minimum angle .theta. of the antenna beam within which radiation can be concentrated is proportional to .lambda./D. Therefore, to achieve small angles, .lambda. must be small and D large. However, both .lambda. and D are constrained by other system characteristics.

#FIG. 1 is an illustration of a conventional Butler matrix.

#FIG. 2 is an illustration of one embodiment of the present invention.

#FIG. 3 is an illustration of the application of an embodiment of the present invention with an array of a plurality of directive antenna elements, each with an unique orientation.

#FIG. 1 discloses a typical Butler matrix scanning configuration as disclosed on "Beam Forming Matrix Simplifies Design Of Electronically Scanned Antennas" by J. Butler and R. Lowe, Electronic Design, Vol. 9, pp. 170-173, Apr. 12, 1961. The Butler matrix 10 shown in FIG. 1, is an array of hybrid couplers and fixed phase shifters that have an equal binary number of inputs 16 to 22 and outputs 24 to 30. In its usual mode, a linear array of antenna elements 32 to 38 are connected to output terminals 24 to 30. When a coherent power source 12 is connected to one of the input terminals 16, the matrix distributes the input power signal to all the outputs 24 to 30 and from there, to the antenna elements 32 to 38, with a linear phase shift between each adjacent terminal. For example, when source 12 is connected to input 16, the signal appears on each of the output terminals 24 to 30 with a progressive linear phase shift between adjacent terminals. Therefore, if output 24 has a zero phase shift when compared to the input signal, then output 26 might have a 10.degree. phase shift and output 28 might have a 20.degree. phase shift and output 30 a 10 .times. 2.sup.n degree phase shift (where n is the number of terminals) when compared to the input signal thereby rendering a progressive linear phase shift of 10.degree. between each adjacent output terminal. As the source 12 is applied to different input terminals 18 to 22, the only change at the output terminals is the amount of phase shift that exists between adjacent terminals, i.e., input 18 may produce 20.degree. phase shifts between adjacent output terminals, input 20, 30.degree. phase shifts, etc., or any suitable progression of phase shifts desired. Since the array of antenna elements 32-38 interact mutually, an output beam from the array can be steered across space. The matrix is therefore capable of producing 2.sup.n distinct output beam positions in space from the antenna array with each position uniquely defined by a predetermined input position.

#FIG. 2 discloses the preferred system of the present invention. The system comprises a Butler matrix 12 used in a reverse manner, a series of directive antenna elements 40, a phase shifter 42 and a coherent power source 12. Suitable directive antenna elements for use with the preferred embodiment are disclosed in U.S. application Ser. No. 565,292, entilted "Conformal Radar Antenna Utilizing Directive Antenna Elements" filed on Mar. 25, 1975, by W. E. Anderson, A. D. Krall, A. M. Syeles, and O. J. VanSant. The directive antenna elements disclosed therein have a sufficiently small propagation angle and are physically small enough to make the present system feasible.

#Since the Butler matrix 10 is passive and linear it may be used in a reciprocal manner as shown in FIG. 2. By applying an input signal to what would normally be the outputs 24 to 30, the input can be summed to appear on one of the antenna ports 16 to 22 by adjusting the phase of the input signals. Using a prior example, if a linearly progressive phase change of 10.degree. were introduced between 24, 26, 28, . . . 30, the entire signal would be summed to appear at antenna port 16 of FIG. 2. By changing the amount of phase shift, alternate or multiple ports can be selected. Since each antenna port is terminated by a narrow beam antenna, the propagated beam can be positioned to point anywhere in space. The antennae have no mutual dependance or coupling between each other and can therefore be mounted in any desired manner rendering the system fully conformal. The physical destruction of any group of antennas results in a loss of communication only from the assigned space coverage of that group.