Abstract:
A three-dimensional holographic array of radio-frequency (RF) diffraction gratings, each of which has lengths of conductive and insulating fluid that are selected and adjusted to provide a desired diffraction effect on incident RF radiation. The three-dimensional array functions analogously to an optical hologram, and is programmable to provide desired refraction and focusing effects on multiple RF incident beams, which may be selectively directed to receivers or, if interferers, ignored. Because the gratings employ conductive and insulating fluids, the array can be reprogrammed in near real time to adapt to changes in the incident RF radiation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to radio-frequency (RF) antennas and, more particularly, to RF lens type antennas. Optical lens principles have been applied to RF antennas, either as an alternative to or in conjunction with reflective antennas. In simple terms, an RF lens comprises an array of elements that function as waveguides and provide regions of different propagation velocity for incident RF radiation. Therefore, an RF beam is refracted by the lens and can be selectively focused and/or steered as desired. RF lens type antennas have long been recognized for their advantages of high gain and very good interference rejection, but are also known to suffer from significant drawbacks. Specifically, RF lens antennas typically have a narrow instantaneous bandwidth, are limited to a handling single beam, and have a direction and frequency that are fixed by the specific hardware implementation of the lens. Unfortunately, these drawbacks are extremely significant in many fields of application, such as signals intelligence (SIGINT) and electronic warfare (EW) systems, which involve processing signals from multiple beams of different frequencies, scattered over a wide field of view and susceptible to the effects of jamming and interfering signals. 
     Therefore, there is a need for an RF lens type antenna that does not suffer from these drawbacks. In particular, there is a need for an RF lens type antenna that can handle multiple beams simultaneously, is not as limited in bandwidth, and can steer beams as desired, without being limited to particular hardware configurations. Ideally, it would be highly desirable to provide an RF lens type antenna that was adjustable in as near as possible to real time, to adapt rapidly to changing signal conditions. The present invention is directed to these ends. 
     SUMMARY OF THE INVENTION 
     The present invention resides in an RF lens structure that applies well known optical principles of holography and diffraction grating theory to the RF domain. Briefly, and in general terms the present invention may be defined as a three-dimensional RF holographic transformer, comprising an array of elemental RF diffraction gratings, each of which comprises selected lengths of electrically conductive material and electrically insulating material arranged in a selected alternating sequence; and means for programming each of the elemental gratings independently, to vary the lengths of conductive and insulating material contained in each of the elemental gratings in order to achieve a desired effect on one or more RF beams incident on the array of gratings. The array of gratings is programmable to perform different desired refractive operations on the incident RF beams, with each such operation being performed independently of and simultaneously with the others. 
     More specifically, each elemental RF diffraction grating in the three-dimensional holographic transformer comprises an elongated tube; and means for filling the tube with fluid selected from a supply of conductive fluid and a supply of insulating fluid. The means for filling the tube further comprises an electrically actuated fluid valve for selecting a supply of fluid. 
     In a preferred embodiment of the invention, the elemental gratings are formed across a continuous substrate ribbon, which comprises, in addition to the elongated tubes, an electrically actuated fluid valve coupled to each of the elongated tubes, a first plenum extending along the substrate ribbon to supply conductive fluid to the elongated tubes, a second plenum extending along the substrate ribbon to supply insulating fluid to the elongated tubes, a third plenum extending along the substrate ribbon to provide a discharge path for fluids removed from the elongated tubes during reprogramming of the tubes&#39; contents, and at least one electrical conductor extending along the substrate ribbon and coupled to the electrically controlled fluid valves to provide controlled selection of fluid for each elongated tube either from the first plenum, the second plenum, or a blend of the first and second plenums. In this embodiment, the substrate ribbon is rolled to form a three-dimensional bundle of the elemental gratings, although other possible structures can be envisioned in which the elemental gratings are arrayed in a different manner. 
     More specifically, each elemental grating comprises, in addition to the elongated tube, a feed tube supplied from a selected fluid through its fluid valve. The feed tube is smaller in cross section than the elongated tube, and extends though the elongated tube through substantially all of its length. The feed tube is open at its distal end and the elongated tube is closed at its distal end. The elemental grating is programmed by drawing selected amounts of conducting and insulating fluid through the feed tube and into the elongated tube, such that the latter becomes filled with selected lengths of conductive fluid, insulating fluid and, if selected, blended proportions of both conductive and insulating fluids. 
     The invention may also be defined in terms of a method for processing radio-frequency (RF) beams in a three-dimensional holographic transformer having an array of independently programmable RF grating elements. Briefly, the method comprises the steps of determining by computation the grating configuration needed to effect a transformation of each of one or more RF signals incident on the array; configuring each elemental grating in an array of such gratings, to comprise selected lengths of conductive and insulating fluids, as arrived at in the determining step; and adjusting and reconfiguring each elemental grating as necessary to adapt to changing configurations of incident RF signals. 
     More specifically, in accordance with this method the step of configuring each elemental grating comprises selecting fluid from a supply of conductive fluid, a supply of insulating fluid, or a combination of the two supplies; pumping selected fluid supplies into an elongated tube, to form therein layers of conductive fluid, insulating fluid and, if selected, a blend of conductive and insulating fluids; and simultaneously with the pumping step, discharging unwanted fluid from the elongated tube. 
     It will be appreciated from the foregoing that the present invention represents a significant advance in RF lens type antennas. In particular, the invention provides an RF hologram that can be rapidly and conveniently programmed to perform a desired refraction effect on multiple incident RF beams, which may emanate from different directions and have different frequency bandwidths. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is diagrammatic view depicting the general functions performed by a three-dimensional holographic transformer in accordance with the present invention. 
         FIG. 2  is a diagrammatic view of an elemental RF grating in accordance with the invention. 
         FIG. 2A  is an enlarged cross-sectional view the elemental grating of  FIG. 2 . 
         FIGS. 3A ,  3 B and  3 C are additional views of the elemental grating of  FIG. 2 , showing the grating in an off configuration, a conductor fluid selection configuration and an insulating fluid selection configuration, respectively. 
         FIG. 4  depicts a continuous tape assembly of multiple gratings of the type shown in  FIG. 2 . 
         FIG. 5  depicts how the tape assembly of  FIG. 4  is rolled into a cylindrical shape to form the three-dimensional holographic transformer of the invention. 
         FIG. 6  is a diagrammatic representation of four independent holograms recorded in the three dimensional holographic transformer of the invention. 
         FIG. 7  a diagrammatic view of a single grating for use in a test bed. 
         FIG. 8  is an exploded view of a unit assembly of a single grating of the type shown in  FIG. 7 . 
         FIG. 9  is a schematic diagram of a test bed for evaluating the performance characteristics of a single grating of the type shown in  FIGS. 7 and 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for purposes of illustration, the present invention pertains to the application of holography and diffraction grating principles to radio-frequency (RF) lens type antennas. Basically, an RF lens comprises an array of elements that have the effect of refracting an RF beam, in a manner analogous to the refraction of optical radiation by an optical lens. Although RF lens type antennas have been known for some years, their use in practice has been limited by their known disadvantages of a narrow instantaneous bandwidth, and frequency and directional characteristics that are fixed by the specifics of hardware implementation. 
     In accordance with the invention, optical principles of holography and diffractions gratings are applied in the context of RF radiation. As will become apparent from the following description, a holographic device applying the principles of the invention can be conveniently configured to handle multiple RF beams, to null out interfering RF sources, and to provide excellent linearity and high dynamic range of operation. 
       FIG. 1  depicts the invention in diagrammatic form. A three-dimensional holographic transformer is indicated by a cube shape and reference numeral  10 . By way of example, three RF beams are shown as being received by the transformer  10 : A first signal of interest F 1  received from an azimuth Az 1  and an elevation El 1 , an interferer F 2  received from an azimuth Az 2  and an elevation El 2 , and another signal of interest F n  received from an azimuth Az n  and an elevation El n . Also depicted in  FIG. 1  are two receivers  12 A and  12 B to which the signals of interest are to be directed. Ideally, the holographic transformer  10  should be capable of performing the functions of: (1) beam pointing in azimuth and elevation, simultaneously for the multiple received beams, (2) beam shaping for a specified center frequency and bandwidth, (3) cancellation of undesirable interference by superimposing nulls for the interfering sources, (4) focusing transformed received beams to one or more of the receivers  12 A and  12 B, and (5) performing these functions passively to provide a wide bandwidth, such as 20 GHz, and a wide dynamic range, such as 120 dB, with minimal insertion losses. 
     Holograms have been generated for decades by photographic processes in which an optically coherent reference is interfered with its own reflection from an object, onto a photographically sensitive plate or volume emulsion. The result is a hologram that, when illuminated by the same reference, creates a virtual image that is equivalent to the original object, including perspective change. Changing the position of the real image or the reference will result in different holograms and, therefore, new and different virtual images. The present invention generates a synthesis of three-dimensional holographic solutions in a computer, thereby removing the need for a reference, since it is common to the coordinate system of each three-dimensional matrix solution. Additionally, the ability to process multiple beams, multiple frequencies, and multiple focusing solutions requires that the holograms be constructed with diffractive media that retain amplitude control over the transfer functions, since the entire vector expression includes coefficients and cannot be accomplished successfully with laminations of binary phase planes. 
     An elemental grating in a three-dimensional volume hologram is shown diagrammatically in  FIG. 2 , in which the grating element is indicated at  20 . The element  20  is a rod-shaped grating construction channel, which is “programmed” or constructed to comprise alternating regions of electrically conductive fluid, indicated at  22 , and electrically insulating fluid, indicated at  24 , of selected lengths. During programming of a grating element  20 , conductive fluid is drawn up through a central feed tube  20 A in the element, from a supply of conductive fluid  26 , and insulating fluid is drawn up through the same tube from a supply of insulator fluid  28 . Selection of either the conductive fluid  26  or the insulator fluid  28 , or a blend of both, is controlled by a fluid switch  30 , which may, for example be a piezoelectric switch. The selected fluid volumes are drawn up the central feed tube  20 A and then flow back down the construction channel around the outside of the feed tube. The feed tube  20 A is open at its top but the outer grating construction channel  20  is closed at the top. Programming of the grating element  20  is complete when the desired pattern of conductive and insulator fluids is present in the grating element. Whenever the grating element  20  is programmed in this manner, fluid layers previously present in the element are ejected into a discharge plenum  32 . 
     Programming of a single grating element  20  is further depicted in  FIGS. 3A-3C , from which it will be apparent that the central feed tube  20 A is of a relatively small diameter and operates at a relatively high flow rate to provide grating solutions to the top of the tube. If desired, the tube  20 A may be connectable to a “benign” liquid supply (not shown) containing a dielectric, or may be totally filled with the insulator fluid  28  to return the grating element  20  to a benign starting configuration.  FIG. 3A  depicts an empty grating element  20 , with the switch  30  in an ‘off’ position, not connected to either the conductor fluid plenum  26  or the insulator fluid plenum  28 .  FIG. 3B  shows the grating element  20  with its switch  30  in the ‘conductor’ position, which connects the feed tube  20 A with the conductor fluid plenum  26 .  FIG. 3C  shows the grating element  20  with its switch  30  in the ‘insulator’ position, which connects the feed tube  20 A with the insulator fluid plenum  28 . 
     The grating construction of the invention is well suited to facilitate manufacturing, for reasons that become apparent from consideration of  FIGS. 4 and 5 . First, a volume hologram comprising multiple grating elements  20  can be produced with multiple printed circuit laminations, where each lamination is cut and/or thermally stamped with the required patterns and cavities controlled by computer aided design and manufacturing (CADAM) techniques. Second, an array of gratings is inherently adaptive, which can create a calibration equalizer matrix that is applied to each volume hologram solution computed thereafter over time and temperature changes. Errors in grating spacing may be induced by environmental changes, such as temperature or materials aging, as well as by inaccuracies in the manufacturing process. Compensation for such errors can be easily superimposed onto the generated grating dimensions in order to achieve optimal performance and stability. A third advantage of the invention is that the array is inherently its own test set, since a pilot tone can be generated and then adaptively tuned. Reference drive or pilot tones are useful as a training signal for correlative recognition algorithms. Recognition of a desired signal can be achieved by adapting the volume gratings to a transformer maximum power output while being irradiated with a synthesized version of that desired signal or reference waveform prior to online operation. This is easily performed with closed loop adaptation and is called training. Conversely, if the inversion of the reference is used as the training signal, then nullification of an undesired signal, such as an interferer, may be achieved. 
       FIG. 4  depicts a linear array of grating elements  20  embedded in a polyvinyl substrate carrier  40  in the form of a continuous tape. The structure also includes embedded switches  30  for grating control, embedded plenums  26 ,  28  and  32  for conductor fluid, insulator fluid and discharge, respectively, and embedded cavities for the supply lines connecting the two supply plenums to the switches  30 . Also embedded in the substrate carrier  40  is a grating control signal and control feedback line  42 , chained between the switches  30  to carry control signals that are uniquely recognizable by the switches, and to carry status information back to a controller (not shown). The Tape assembly also contains a power distribution line and a ground plane. The substrate carrier  40  may be perforated with multiple openings  44  to reduce the total mass of the structure. In the illustrative embodiment of the invention, the tape assembly of gratings depicted in  FIG. 4  may be of any desired width, such as 100 mm to 1000 mm. 
       FIG. 5  shows how the linear array of  FIG. 4  may be configured to form a three-dimensional array of grating elements. In this embodiment of the invention, a holographic transformer is formed by winding the tape assembly of  FIG. 4  into a cylindrical form. The tape assembly is wound onto itself repeatedly until a cylinder of approximately one meter diameter is formed, as shown in  FIG. 5 . 
       FIG. 6  diagrammatically illustrates four holograms superposed in a holographic transformer in accordance with the invention. Each of the four holograms in general has a unique frequency band, azimuth, elevation and beam shape, and each of the separated beam solutions is shown as a set of diffractive functions achieved in a single volume. In effect, the four curves define the equivalent reflector that would result from the unrecognizable three-dimensional grating. They are shown superimposed in a single view in an attempt to depict a translation from Fourier space into reflector surfaces. The horizontal axis in  FIG. 6  is the length dimension of the tape assembly when in an unrolled condition. The vertical axis is distance along the height or width of the tape assembly. 
     The vertical lines in  FIG. 6  represent transparent transport ducts that are used to form the individual grating elements  20 , and the small rectangles located on the transport ducts diagrammatically represent conductive nodes positioned to perform the diffraction grating effect. The overall confetti-like appearance of the  FIG. 6  representation is intended to depict what the three-dimensional diffraction volume would look like if it was injected with gratings everywhere along the tape length, as viewed from the side of three-dimensional tape assembly. Thus,  FIG. 6  depicts two aspects of the tape assembly. First it depicts by way of example four reflector characteristics as viewed for a portion of the unrolled tape assembly, and second it depicts the nature of a typical three-dimensional diffraction volume. 
     The principles of operation of the three-dimensional fluidic grating described above can be verified in a test bed having a single programmed grating element, as shown in  FIGS. 7 and 8 . a grating element  50  is formed on a plastic substrate  52  as an elongated tube. Each end of the tube is connectable to one of two conductor liquid plenums  54  and to one of two dielectric liquid plenums  56 . Each end of the tube  50  has an installed fluid type detector  58  to determine the type of fluid entering or leaving the tube. As best shown in  FIG. 8 , a voice coil pump  60  is installed over each plenum to force fluid into the grating  50  under computer control. The structure may be conveniently constructed as three plastic sheets  62 ,  64  and  66 , as shown in  FIG. 8 . The top layer  62  is a thin, transparent cover sheet, for example 0.25 mm in thickness. The middle sheet  64  may be, for example, 1 mm thick and has the plenums  54  and  56  and the tube  50  formed in it by processing on a conventional laser cutting table. The third layer  66  is a transparent support base of suitable thickness, such as 8 mm, to provide rigid support for the entire assembly. 
     As shown in  FIG. 9  the grating unit assembly of  FIG. 8  may be usefully tested by directing signals toward the grating element  50  from an RF source  70  and associated transmitting antenna  72 , and receiving signals received on the opposite side of the grating element, at a receiving antenna  74  and spectrum analyzer  76 . A conventional digital computer  78  controls the RF source  70  and the spectrum analyzer  76 , and receives data from the spectrum analyzer. The computer  78  also generates four analog signals to activate the four voice coil pumps  60 , and receives two analog signals from the fluid type detectors  58  at the ends of the grating tube  50 . The computer  78  can be a conventional desktop computer equipped with a data acquisition card, a waveform synthesis card and a suitable control interface, such as an IEEE-488 interface bus (or HP-IB), for connecting to the RF source  70  and the spectrum analyzer  76 . Performance criteria of the single grating element  50  can be completely characterized using this arrangement, by varying the grating physical characteristics and measuring the effect of the grating on RF beams of various frequencies and angles of incidence. Performance characteristics of the single grating element  50  can then be used to predict the performance of the three-dimensional hologram described above and to determine the grating parameters of each grating element needed to achieve a desired effect in the three-dimensional hologram. 
     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of RF lens type antennas. In particular, the invention provides a three-dimensional holographic transformer that can be programmed to refract multiple received RF beams toward desired receiver locations, even when the multiple received beams are at different frequencies and are incident from different angles of azimuth and elevation. It will also be appreciated that, although a specific embodiment of the invention has been illustrated and described in detail, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.