Abstract:
An electronically-controlled steerable beam antenna with suppressed parasitic scattering includes a feed line defining an axis x; and first and second arrays of electronically-controlled switchable scatters distributed along the axis x, each of the scatterers in the first and second arrays being switchable between a high state and a low state to scatter an electromagnetic wave propagating through the transmission line so as to form a steerable antenna beam. Each of the scatters of the second array is configured to be 180°-phase-shifted relative to a corresponding scatter of the first array. The switchable scatterers of the first and second arrays are configured into high states and low states relative to each other so as to suppress parasitic scattering of the electromagnetic wave without suppressing the steerable antenna beam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND 
       [0003]    The present disclosure relates to directional or steerable beam antennas, of the type employed in such applications as radar and communications. More specifically, it relates to leaky-waveguide antennas, of the type including a dielectric feed line (i.e., a potentially leaky waveguide) loaded with scatterers (antenna elements), where coupling between the scatterers and the feed line can be altered by switches, whereby the antenna&#39;s beam shape and direction are determined by the pattern of the switches that are respectively turned on and off. 
         [0004]    Steerable antennas, particularly leaky-wave antennas, are capable of sending electromagnetic signals in, and receiving electromagnetic signals from, desired directions. Such antennas are used, for example, in various types of radar, such as surveillance radar and collision avoidance radar. In such antennas, the receiving or transmitting beam is generated by a set of scatterers coupled to the feed line or waveguide. Interacting with the feed line, the scatterers create leaky waves propagating outside of the feed line. If the scatterers are properly phased, they create a coherent beam propagating in a specific direction. The leakage strength and phase caused by each scatterer depend on the geometry and location of the scatterer relative to the feed line or waveguide. The coupling strength can be controlled by changing the geometry of the scattering elements. Correspondingly, the shape and direction of the scattered beam can be controlled by varying the scatterer geometry or topology. The geometry (topology) of the scatterers can be electronically altered by using microwave (or other suitable) switches connecting parts of the scatterers. Thus, the shape and direction of the antenna beam can be controlled electronically by changing the state of the switches. Different ON/OFF switch patterns result in different beam shapes and/or directions. 
         [0005]    Any of several types of switches integrated into the structure of the antenna elements or scatterers may be used for this purpose, such as semiconductor switches (e.g., PIN diodes, bipolar and MOSFET transistors, varactors, photo-diodes and photo-transistors, semiconductor-plasma switches, phase-change switches), MEMS switches, piezoelectric switches, ferro-electric switches, gas-plasma switches, electromagnetic relays, thermal switches, etc. For example, semiconductor plasma switches have been used in antennas described in U.S. Pat. No. 7,151,499, the disclosure of which is incorporated herein by reference in its entirety. A specific example of an antenna in which the geometry of the scattering elements is controllably varied by semiconductor plasma switches is disclosed and claimed in U.S. Pat. No. 7,777,286, the disclosure of which is incorporated herein in its entirety. Another example of a currently-available electronically-controlled steerable beam antenna using switchable antenna elements (scatterers) is disclosed in U.S. Pat. No. 7,995,000, the disclosure of which is incorporated herein its entirety. 
         [0006]      FIG. 1  schematically illustrates a conventional steerable-beam antenna  10  comprising a single array  12  of switchable scatterers  14  coupled to a feed line or waveguide  16  extending along an axis x. Each of the scatterers  14  is switchable between an open state or state of low scattering L, and a closed state or state of high scattering H. Typically, in operation, the scatterers  14  will be selectively switched to low and high states to create a diffraction grating with P scatterers in each repetitive period Pd, where P includes N low-state scatterers and M high-state scatterers, and where d is the spacing between adjacent scatterers  14 . In the illustrated example, the period P=4, comprising three L-scatterers and one H-scatterer. The resultant beam angle α will thereby be given by the equation (1): 
         [0000]      sinα=β/ k−λ/Pd    (1)
       where β is the wave propagation constant in the feed line  16 , k is the propagation wave vector in a vacuum, and λ is the wavelength in vacuum. It will thus be seen that, by selectively switching the scatterers  14  between a high state and a low state, the grating period Pd can be controllably varied, thereby controllably changing the beam angle α of the electromagnetic radiation emanating from the feed line  16 .       
 
         [0008]    The above-described antenna  10  may be viewed as a single array  12  of switchable scatterers  14  and a feed line  16  that feeds an electromagnetic signal to, or receives an electromagnetic signal from, the array  12 . Each of the scatterers  14  is switchable between a low state L and a high state H. A specific pattern of H-state and L-state scatterers  14  represents a hologram that forms a coherent “leakage” (coupling between the free space and the feed line  16 ). By changing the pattern of H-states and L-states by means (for example) of a control signal source (not shown), the beam can be steered or manipulated in different ways, such as beam-steering, tracking, control of side lobes, multi-beam creation, control of beam width, etc. 
         [0009]    In theory, in an ideal antenna, the L-state scatterers would not scatter electromagnetic power at all. In practice, however, real L-state scatterers still scatter a small amount of power. This so-called “parasitic” scattering degrades the desired steerable antenna beam, and may result in compromised radar resolution, detection of non-existing targets, etc. A beam pattern affected by parasitic scattering is illustrated graphically in  FIG. 2 , which charts relative antenna gain versus angle in a single array antenna. The steerable beam is labeled “A,” and the accompanied parasitic edge scattering is labeled “B.” The power level of the parasitic scattering is lower than that of the steerable beam A (about −20 dB relative to the peak gain of the steerable beam A in the illustrated example), but it still may degrade the antenna operation. 
         [0010]    It would therefore be desirable to provide a mechanism for reducing the parasitic scattering in an electronically-controlled steerable beam antenna without measurably reducing the amplitude of the steerable beam. 
       SUMMARY 
       [0011]    In one aspect, this disclosure relates to an electronically-controlled steerable beam antenna with suppressed parasitic scattering, comprising a feed line defining an axis x; and first and second arrays of electronically-controlled switchable scatters distributed along the axis x, each of the scatterers in the first and second arrays being switchable between a high state and a low state to scatter an electromagnetic wave propagating through the transmission line so as to form a steerable antenna beam; wherein each of the scatters of the second array is configured to be 180°-phase-shifted relative to a corresponding scatter of the first array; and wherein the switchable scatterers of the first and second arrays are configured into high states and low states relative to each other so as to suppress parasitic scattering of the electromagnetic wave without suppressing the steerable antenna beam. 
         [0012]    In another aspect, this disclosure relates to a method of scattering an electromagnetic wave into a steerable antenna beam, in an electronically controllable steerable beam antenna including a feed line defining an axis x and a first array of electronically controlled scatterers arranged along a first side of the axis x, each of the scatterers in the first array being switchable between a high state and a low state, the method comprising providing a second array of electronically-controlled switchable scatters arranged along the opposite side of the axis x from the first array, the scatterers in the second array being switchable between a high state and a low state; phase-shifting the scatters of the second array 180° relative to the scatterers in the first array; and switchably configuring the scatterers in the first and second arrays into high states and low states relative to each other so as to suppress parasitic scattering of the electromagnetic wave without suppressing the steerable antenna beam. 
         [0013]    More specifically, the disclosure relates to an electronically-controlled steerable beam antenna with suppressed parasitic scattering, comprising a feed or transmission line defining an axis x (which may be linear or curved), and first and second linear arrays of electronically-controlled switchable scatterers parallel to, and on opposite sides of, the axis x, wherein the scatterers of the first array are configured to scatter an electromagnetic wave propagating through the transmission line in given phases φ(x), wherein the scatterers of the second array are configured to scatter the propagating wave in phases opposite to the given phases (i.e., λ(x)+π radians, or 180° out of phase with respect to the given phases), and wherein the H-state scatterers in the first array follow the periodic or quasi-periodic pattern H 1 (x) with a period Pd (where d is the spacing between the scatterers along the axis x, and P is the number of scatterers per period, as in Equation (1) above), and the H-state scatterers in the second array follow the pattern H 2 (x)=H 1 (x±Pd/2), i.e., a pattern that is shifted by one-half period (180°) along the x axis relative to the H-state scatterers in the first array. The parasitic scattering created by the L-state scatterers in the second array destructively interferes with, and thus suppresses, the parasitic scattering created by the L-state scatterers in the first array. The half-period shift of the H-state scatterers in the second array gives the H-state scatterers in the second array an additional 180° phase shift, so that the H-state scatterers in the second array scatter the propagated wave in phase with H-state scatterers in the first array, thereby avoiding the suppression of the steerable beam, and creating a constructive interference in the direction given by the desired angle a of the steerable beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a semi-schematic representation of a prior art electronically-controlled steerable beam antenna, employing a single linear array of scatterers. 
           [0015]      FIG. 2  is a graphical representation of an antenna beam-pattern for a prior art antenna of the type shown in  FIG. 1 , with the steerable beam designated as A and parasitic scattering as B. 
           [0016]      FIG. 3  is a diagrammatic representation of an electronically-controlled steerable beam antenna in accordance with the present disclosure, showing first and second scatterer arrays on either side of a transmission line defining an x axis, with the H-state pattern of the second array shifted relative to that of the first array by one half-period along the x axis. 
           [0017]      FIG. 4  is a simplified schematic view of an electronically-controlled steerable beam antenna, of the type represented in  FIG. 3 , in accordance with the present disclosure. 
           [0018]      FIG. 5  is a simplified plan view of a physical embodiment of the antenna shown schematically in  FIG. 4 . 
           [0019]      FIG. 6  is a simplified cross-sectional view of the physical embodiment shown in  FIG. 5 . 
           [0020]      FIGS. 7A and 7B  are graphical representations, respectively, of hologram patterns for the first and second scatterer arrays in simulations of the operation of an antenna in accordance with the present disclosure. 
           [0021]      FIG. 8  is the antenna beam pattern for an antenna of the type shown in  FIGS. 3-6 , demonstrating suppression of the parasitic scattering, with the steerable beam designated as A′. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 3  diagrammatically illustrates an electronically-controllable steerable beam antenna  50  in accordance with the present disclosure. The antenna  50  comprises a first plurality of scatterers  52  in a first linear array  54  and a second plurality of scatterers  56  in a second, complementary linear array  58 . The first array  54  and the second array  58  are disposed parallel to, and on opposite sides of, a longitudinal feed line or transmission line  60  extending along an axis x. The scatterers  52 ,  56 , as will be more specifically described below, are functionally similar to the scatterers  14  of the prior art antenna  10  described above and illustrated in  FIG. 1 , and thus are switchable between an L-state and an H-state. The scatterers  52  in the first array  54  are configured to scatter an electromagnetic wave propagating through the transmission line  60  in a given phase φ(x), while the scatterers  56  of the second array  58  are configured to scatter the propagating wave in a phase opposite to the given phase, i.e., φ(x)+π radians (180°). Thus configured, the parasitic scattering created by the L-state scatterers  56  in the second array  58  destructively interferes with, and thereby suppresses, the parasitic scattering created by the L-state scatterers  52  in the first array  54 . 
         [0023]    In operation, some of the scatterers  52  in the first array  54  will be switched to the H-state, as will the complementary scatterers  56  in the second array  58 . To avoid destructive interference among the H-state scatterers, the H-state pattern H 2 (x) of the second array  58  of scatterers is shifted relative to the H-state pattern H 1 (x) of the first array  54  of scatterers by a distance equal to Pd/2 along the x axis. In the illustrated example, P=4; therefore, the shift of Pd/2 equals the distance of two scatterer separation distances. Thus, the H-state pattern in the second array may be expressed as H 2 (x)=H 1 (x±Pd/2). This H-state pattern shift produces an additional phase shift of π radians (180°) for the H-state scatters only in the direction of the steerable beam, and thus avoids destructive interference between the H-state scatterers in the first array  54  and the H-state scatterers in the second array  58  (and, in fact, may produce constructive interference between the H-state scatterers  52 ,  56  in each complementary pair). The result is that the antenna  50  produces the steerable beam in the desired direction and/or shape, but with strongly suppressed parasitic scattering. 
         [0024]      FIG. 4  shows an exemplary implementation of the antenna  50  shown diagrammatically in  FIG. 3 , in which the antenna components are disposed on a dielectric substrate (see  FIGS. 5 and 6 ). The first scatterer array  54  is arranged longitudinally along one side of the feed or transmission line  60 , and the second scatterer array  58  is arranged longitudinally along the opposite side of the feed or transmission line  60 . Each of the first plurality of scatterers (i.e., those in the first array  54 ) comprises a conductive scatterer element  62 , preferably configured as a U-shaped metallization area forming an open loop. Each of the first scatterer elements  62  has a first end connected to a first ground line  64  through a first grounding capacitor  66 , and a second end connectable to the first ground line  64  through a first electronically controlled switch, represented, in this embodiment, by a PIN diode  68 . Each of the second plurality of scatterers (i.e., those in the second array  58 ) likewise comprises a conductive scatterer element  72 , preferably configured as a U-shaped metallization area forming an open loop. Each of the second scatterer elements  72  has a first end connected to a second ground line  74  through a second grounding capacitor  76 , and a second end connectable to the second ground line  74  through a second electronically controlled switch, represented, in this embodiment, by a PIN diode  78 . Other types of electronically controllable switching elements may be used instead of the PIN diodes  68 ,  78 , as discussed above. Operation of the switches represented by the PIN diodes  68 ,  78  is controlled by a control signal circuit or bias circuit  80  under the control of an appropriately programmed processor or computer (not shown), as is well known in the art, e.g., U.S. Pat. No. 7,995,000, supra. 
         [0025]    The arrangement of the components of the antenna part comprising the second array  58  of scatterers  72  is a mirror image of the arrangement of the components of the antenna part comprising the first array  54  of scatterers  62 . Specifically, the conductive scatterer elements  62  in the first array  54  and the conductive scatterer elements  72  in the second array  58  are disposed back-to-back (mirror symmetry with respect to each other relative to the axis x); that is, the closed portion of each of the scatterer elements  62  in the first array  54  faces the closed portion of a corresponding scatterer element  72  in the second array  58  across the feed or transmission line  60 , with the open ends of the scatterer elements  62 ,  72 , in the first and second arrays  54 ,  58 , respectively, facing away from the feed or transmission line  60 . This arrangement creates the 180° phase shift between the scatterers  62  in the first array  54  and the scatterers  72  in the second array  58 , which, as discussed above, results in the suppression of parasitic scattering. For transmission/reception of an electromagnetic wave having a wavelength λ, the total length of each conductive scatterer element  62 ,  72  is advantageously about λ/2, as corrected for the substrate material and the particular scatterer geometry. 
         [0026]    The direction and the shape of the steerable antenna beam is controlled by switching the appropriate scatterers  62 ,  72  between the L-state and the H-state by means of the control signal circuit or bias circuit  80 , as noted above. In  FIG. 4 , the L-state scatterers are indicated as those having PIN diodes  68 ,  78  represented by solid black symbols (filled triangles), while the H-state scatterers are indicated as those having PIN diodes  68 ,  78  shown in outline (unfilled triangles). If each of the H-state scatterers in the second array  58  were to be directly opposed to the corresponding H-state scatterer in the first array  54 , the above-described 180° phase differential between the first and second arrays would tend to suppress the major beam along with the parasitic scattering as a result of destructive interference, thereby greatly attenuating the amplitude of the steerable beam. Therefore, as described above, and as shown in  FIG. 4 , the pattern of H-state and L-state scatterers in the second array  58  is shifted along the x-axis defined by the feed or transmission line  60  relative to the pattern of the first array  54  by half-period (Pd/2), so that each of the H-state scatterers in the second array  58  acquires an additional phase shift of π radians (180°) and thus scatters in phase with the H-state scatterers of the first array  54 . 
         [0027]      FIGS. 5 and 6  show that the first and second scatterer arrays  54 ,  58 , the feed or transmission line  60 , and the first and second ground lines  64 ,  74  of the antenna  50  described above with reference to  FIG. 4  may be formed or disposed on a dielectric substrate  82 , by suitable means well-known in the art. For example, the conductive feed or transmission line  60 , the ground lines  64 ,  74 , and scatterer loops  62 ,  72  can be fabricated using printed circuit techniques. The capacitors  66  and  76  can be implemented as constructive elements or lumped components. An optional backing ground plate  84  may be provided (e.g., by printing or plating) to block backward antenna scattering. 
         [0028]    The antenna  50  is reciprocal: it can operate in both transmitting mode and receiving mode. In the former case the feed line  60  is coupled to a transmitter (not shown); in the latter case the feed line is coupled to a receiver (not shown), as is well-known in the art. 
         [0029]    The performance of the antenna  50  shown in  FIGS. 3-6 , as described above, is illustrated in  FIGS. 7A ,  7 B, and  8 .  FIGS. 7A and 7B  respectively show the relative scattered power of the H-scatterers and the L-scatterers in the first and second arrays, as distributed along the axis x. As shown in  FIGS. 7A and 7B , the scattered power from the L-scatterers of the first array is identical to the scattered power from the L-scatterers of the second array. The power pattern of the H-scatterers of the second array, however, is shifted one half-period relative to the pattern of H-scatterers of the first array.  FIG. 8  shows that, contrasted with the performance of the prior art antenna ( FIG. 2 ), the antenna structure of  FIG. 4  exhibits a steerable antenna beam A′ that is as strong as the steerable antenna beam A of  FIG. 2 . The parasitic beams, by contrast, are significantly attenuated relative to the parasitic beams of the prior art antenna (region B in  FIG. 2 ).