Multiple Beam Retrodirective Array With Circular Symmetry

Abstract

A method and apparatus for obtaining automatic, selective retrodirective formance from a circularly symmetric antenna array. This system may be employed in an active or passive manner and accomplishes selective retrodirectivity by manipulation of beam terminals of a multiple beam matrix which in turn controls a multimodal network. The combination of the two matrix networks provide N separate beams from the circular antenna array. Also, by providing gain networks, control of the reradiated beam pattern is possible. This system has the ability to identify the angle of incidence of any particular transmission, and is particularly suited for navigational beacon systems since the reradiated signal can provide bearing information in response to interrogation.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The most well known method of obtaining retrodirective beam radiation capability is disclosed by Van Atta in U.S. Pat. No. 2,908,002. Van Atta shows a passive linear array of elements, interconnected in a manner such that an electromagnetic beam is radiated at substantially the same angle from which it came. It has become possible to construct an active Van Atta array, and thus, the use of such an array has been found to be an effective and practical way to obtain retrodirectivity. However, the Van Atta array is basically limited to the linear or planar discrete arrays. Therefore, it effectively lacks the capability of operating with a circularly symmetric array, and is unable to provide 360.degree. coverage.

Another method of obtaining retrodirective performance is explained in IEEE Transaction on Antennas and Propagation, March 1964, entitled "Self-Phasing Array Antennas" by M. Skolnik and D. King. This system operates on the incident wavefront in such a manner that when it is retransmitted it returns whence it came as a coherent wavefront, irrespective of the original phase distribution incident on the array. Although this method is applicable to very general arrays, information regarding the angle of arrival cannot be made available as a result of an interrogation.

Finally, a retrodirective circularly symmetric antenna system has been described in my copending application ser. No. 220,663, filed Jan. 25, 1972. Although 360.degree. retrodirectivity is possible, as is control of the beam pattern, that device lacks flexibility when compared to this system. For example, particular sectors of this system may be inhibited from response, and additionally, a ready means of identifying the original angle or incidence upon retransmission is provided herein.

SUMMARY OF THE INVENTION

An antenna system having circular symmetry is employed to provide automatic retrodirective performance. This type of antenna includes N symmetrically spaced elements such as dipoles, or spaced slots disposed around a metallic periphery.

The antenna is connected by way of appropriate transmission lines to a multimodal N .times. N Butler feed network. The network, when properly fed, provides N separate and distinct beams around the array. The feeding of the multimodal network takes place through K(.ltoreq.N) "mode terminals" by way of a second Butler matrix. Each "mode terminal" is connected to the output of the second Butler matrix by way of an interconnecting line. The N inputs to the second Butler matrix consists of N beam terminals and directly correspond to the separate N beams from the array.

Active circuitry may be connected to the beam terminals to provide active retrodirectivity. Also, by providing a processor to the active circuitry, unwanted signal characteristics may be inhibited. Finally, in both the active and passive systems, selective beam terminals may be terminated so as to inhibit response in unwanted directions.

OBJECTS OF THE INVENTION

An object of this invention is to provide an active or passive retrodirective circularly symmetric antenna system.

A further object is to provide a simple technique for controlling the properties of the reradiated beam.

Another object of this invention is to provide retrodirective performance by the use of a multiple beam circular array configuration.

A further object of the present invention is to provide a system which is capable of identifying the angle of incidence of a wavefront, upon retransmission, in the active configuration.

Other objects of the invention will be readily apparent to those skilled in the art by referring to the following detailed description in connection with accompanying drawings wherein:

THE DRAWINGS

FIG. 1 shows a passive circularly symmetric retrodirective system; and

FIG. 2 shows the components required for active retro-directive performance; and

FIG. 3 is a diagram of the active retrodirective system capable of inhibiting unwanted signal characteristics.

DETAILED DESCRIPTION

Referring to FIG. 1, the system includes circular antenna system 10, interconnecting lines 12, N .times. N Butler matrix 14, interconnecting lines 18 which connect mode terminals 16 to a second N .times. N Butler matrix 20. Beam terminals 22 depend from Butler matrix 20.

The antenna 10 may be of a multitude of types including an array of N discrete radiating elements having circular symmetry; for example, an array of dipoles equally spaced and arranged on a circle concentric with a conducting cylinder. The selection of any particular antenna system 10 largely depends upon the well known design considerations such as maximum or minimum size requirement, weight, gain, element spacing, and the other usual requirements.

The N number of interconnecting transmission lines 12 are employed as a means of connecting the N element array 10 to the N .times.N Butler matrix 14. The lines could be made of coax cable or the like, and the selection of any particular interconnecting line 12 is unimportant to the operation of the entire system as long as the interconnecting lines 12 are capable of accurately maintaining amplitude and phase relationship information between the antenna system 10 and the N .times. N Butler matrix 14.

The N inputs of the N .times. N Butler matrix 14 provide the means for creating a phase progression at the antenna array 10 which results in N separate and distinct beams. Although, all N number of mode terminals may be connected to a second Butler matrix 20, this is not a necessary restriction. As shown in FIG. 1, K number of connections are effected between the output of the matrix 20 and the input of matrix 14 by way of K mode terminals 16. The N .times. N Butler matrix 20 has N beam terminal inputs 22. When a unit voltage is applied to anyone of the N beam terminals (say the nth terminal) a beam corresponding to that particular terminal is generated in the .phi..sub.n direction.

Connecting signal sources 25 to the set of beam terminals 22 results in a set of N beams which cover the entire azimuth angle from .phi. = 0.degree. to .phi. = 360.degree. such as shown in FIG. 2. If, however, the N beam terminals 22 are short circuited (or open circuited) a passive retrodirective array will result.

To understand the principle of operation, consider a plane wave incident upon the array 10 from the .phi. = .phi..sub.o direction. This plane wave results in voltages appearing at the set of beam terminals 22 which are proportional in amplitude and having the same relative phase that would have been required from the original set of signal sources to produce a beam in the .phi. = .phi..sub.o direction. Thus, if unity reflection coefficient terminations are provided at the set of beam terminals 22, incident energy is retransmitted with correct amplitude and phase relationship to cause a beam to reradiate in the .phi. = .phi..sub.o direction. If it is desirable to inhibit response in a particular sector, the sequence of beam terminals which correspond to that sector may be terminated in matched loads.

Instead of simply terminating the set of beam terminals to provide a passive response as shown in FIG. 1, circuitry may be fitted to these terminals to provide an active retrodirective system, with great flexibility. This system is shown in FIG. 2. Although active circuitry 25 is only shown to be connected to selected beam terminals, it should be understood that an individual active element 25 may be connected to each of the N beam terminals.

Typically, the N active circuits 25 may be a circulator 24 connected to an amplifier 23 having a particular gain Gn. This arrangement provides amplification before reradiation so as to provide an enhanced response. Furthermore, the gains of the individual networks may be individually adjusted to give a tailored response with far field angle .phi.. Also, amplifier circuitry 23 could also include a frequency translator or other similar device. Since the identification of particular beam terminals corresponds to the value of the far field angle .phi., a change in reradiated signal characteristics yields a ready means of identifying the wavefronts angle of incidence. This information may be included in the retransmitted signal as bearing information or the like.

Referring to FIG. 3, it should be noted that the array configuration provides a number of far field radiation patterns which are constant in amplitude but vary linearly with phase with far field angle .phi.. These patterns may be termed mode patterns and are available at the K "mode" terminals 16. A processor 26 is shown to be connected to one of the omnidirectional "mode" terminals by way of line 30. One application of this configuration is in inhibiting the response of the retrodirective system for unwanted signal characteristics, (as determined by the processor) by means of cross-connection 32 to the active circuit 28.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, the array shown in the figures may be arranged axially to form linear array of these circular subarrays. If corresponding beam terminals in such an axial array are interconnected, in the manner of a Van Atta, two dimensional retrodirective performance will result. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A circularly symmetric retrodirective antenna system comprising:

circular symmetric antenna means having N radiators; and

a first N .times. N Butler feed network means having N inputs and N outputs wherein the N outputs are sequentially connected to said antenna means; and

a second similar N .times. N Butler feed network means having N inputs and N outputs wherein at least a portion of the N outputs of said second feed network means are connected to inputs of the first feed network; and

unity reflection coefficient means connected to a plurality of said N inputs of said second N .times. N feed network means with the remaining unconnected inputs being connected to matched loads such that retrodirectivity results from said antenna means in selected directions.

2. The device as claimed in claim 1 wherein circulator means are connected to a plurality of said input terminals of said second network means; and

amplification means being coupled to said circulator means whereby active retrodirectivity results from said antenna means.