Patent Publication Number: US-2011074646-A1

Title: Antenna array

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present U.S. patent application is a continuation-in-part application and claims priority to commonly owned U.S. patent application Ser. No. 12/571,175 filed Sep. 30, 2009, titled “Aperiodic Antenna Array,” which is expressly incorporated by reference herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the United States Government for any governmental purpose without payment of any royalties thereon. 
     FIELD OF THE INVENTION 
     The invention relates generally to antenna arrays. In particular, the invention relates to antenna arrays having radiating elements of varying characteristics. 
     BACKGROUND 
     An antenna array comprises a multitude of elements coupled to produce a directive radiation pattern which is the composite of the patterns radiated by each element. The spatial relationship of the elements contributes to the directivity of the antenna. A beam former may use variable phase or time-delay control at each radiating element to create a pattern of constructive and destructive interference in the wave front to achieve a desired radiation pattern. 
     Phase control is used to steer a main beam. The antenna array size may be increased to narrow the main lobe of the radiation pattern. Side lobes of various sizes may develop. As the number of elements in the array increases, the sizes of the side lobes may reduce. Combined amplitude tapering and phase controls may be used to adjust side lobe levels and steer nulls better than can be achieved by phase control alone. Feed networks and element-level electronics such as filters and amplifiers are generally included to enable the beam former to steer the main beam. The nulls between side lobes occur when the radiation patterns pass through the origin in the complex plain. Thus, adjacent side lobes are generally 180 degrees out of phase to each other. Grating lobes may be formed depending on the main beam steering angle and the spacing of the elements. 
     Antenna arrays may suffer from bandwidth limitations and mutual coupling between closely-spaced elements. Another disadvantage is that closely-spaced elements may lack sufficient spacing for the insertion of electronic components associated with the element feed network and element modules (element-level electronics). Improvements are needed to reduce the effect of grating lobes to increase gain and directivity of the antenna arrays. 
     SUMMARY 
     Antenna arrays and methods of making and using them are disclosed herein. In one embodiment of an antenna array according to the invention, the antenna array comprises a plurality of steering elements. Each steering element includes two radiating elements overlaid so as to have a common phase center and radiating electric fields orthogonal to each other. Each steering element also includes two phase altering portions separately and electronically coupling the two radiating elements and being adapted to electronically couple a signaling device to establish separate communication paths between the signaling device and the two radiating elements. Each phase altering portion is electronically coupled to a radiating element. The two phase altering portions are engagable in one of two positions defining four configurations of the steering element. The configurations are selectable to generate or detect a radiation pattern comprising one of linear, elliptical and circular polarization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other disclosed features, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of disclosed embodiments taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a graph of a periodic antenna array pattern with element spacing smaller than λ/2; 
         FIG. 2  is a graph illustrating a pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in the pattern shown in  FIG. 1 ; 
         FIG. 3  is a graph of a periodic antenna array pattern with element spacing larger than λ/2; 
         FIG. 4  is a graph illustrating a pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in the pattern shown in  FIG. 3 ; 
         FIG. 5  is a conceptual diagram illustrating a periodic antenna array pattern; 
         FIGS. 6 to 11  are conceptual diagrams illustrating antenna array patterns according to various embodiments of the invention; 
         FIG. 12  is a graph illustrating an embodiment of an aperiodic antenna array pattern; 
         FIG. 13  is a graph illustrating a pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in the pattern shown in  FIG. 12 ; 
         FIG. 14  is a graph illustrating an aperiodic antenna array pattern according to another embodiment of the invention; 
         FIGS. 15 and 16  are schematic representations of steering elements according to yet another embodiment of the invention; and 
         FIG. 17  is a graph illustrating radiation patterns of first and second elements. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings, which are described below. The embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise form disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings. It will be understood that no limitation of the scope of the invention is thereby intended. The invention includes any alterations and further modifications in the illustrated devices and described methods and further applications of the principles of the invention which would normally occur to one skilled in the art to which the invention relates. 
     Embodiments according to the invention of a method for designing and operating antenna arrays, and antenna arrays resulting therefrom, are disclosed herein. In one embodiment, one or two dimensional aperiodic antenna arrays are provided wherein the spacing between radiating elements vary depending on the position of each array element in relation to the center of the array. By “aperiodic” it is meant that the element spacings are not uniform although the non-uniformity may be regulated. In other words, the variations in element spacing may be determined according to a regulated pattern. The regulated pattern is illustrated herein with reference to a pattern center, element distance and element spacing. The pattern center is an illustrative point of reference and may be chosen in any known manner. The pattern center may coincide with the center of the array although it does not have to. Element distance is the distance between an element and the pattern center. Element spacing is the distance between one element and another element, where the other element is the element nearest the one element. Element spacing may increase in a linear, logarithmic, or any other relationship. 
     In another embodiment of an array comprising a first pattern of first elements, the pattern center is defined based on the closest element spacing, and element spacing increases in relation to the element distance. Thus, element spacing varies. The first elements may be controlled to transmit or reflect energy in a first radiation pattern. The first elements comprise elements which effectively radiate at a particular frequency and bandwidth. For example, the first elements may radiate effectively within a 10% band, e.g. 10.0 +/−0.5 Ghz frequency. In the present embodiment, elements located further away from the pattern center have greater element spacings and elements located closer to the pattern center have smaller element spacings. The first pattern may be regulated in any known manner. Elements may be arranged in rows and columns in a planar array. Aperiodicity may be provided by spreading the rows, or the columns, or both. Thus, in another embodiment the first element spacings increase relative to element distance in one axis but not the other, or increase in one axis more than in the other. In an alternative embodiment, the first elements are disposed in a growing Archimedean spiral. In a further embodiment, the first elements are disposed in concentric circles of increasing diameter. Furthermore, in an additional embodiment the first elements are disposed in a conformal array where the elements are attached to a substrate which conforms to the shape of a supporting structure, e.g., a fuselage, turret, and the like. 
     Grating lobes are undesired sidelobes that are of the same magnitude as the main beam. Grating lobes are not generated when: 
         d/λ&lt; 1/(1+sinθ)
 
     Where d is the spacing between elements, λ is the wavelength and θ is the angle from normal or perpendicular to the array. So at the greatest possible steering angle, 90 degrees, d/λ=½ and with the main beam at the least steering angle, normal to the array, d/λ=1. Element spacing greater than half the wavelength may cause grating lobes depending on the main beam steering angle, and element spacing greater than a wavelength will generally generate grating lobes. 
     Advantageously, the aperiodic patterns reduce the intensity of grating lobes and enable array modifications which further improve directivity and reduce grating lobes. One modification entails the addition of second elements such as steering elements and wideband elements. Whereas the first elements generate a first radiation pattern, the second elements generate a second radiation pattern, and the first and second radiation patterns produce a composite radiation pattern for the hybrid array which results from the radiation of the first and second patterns and the constructive and destructive interference between them. Increases in element spacing enable addition of wideband elements and steering elements with associated control circuitry. 
     In a further embodiment, the second elements comprise wideband elements. In a preferred embodiment, a wideband element radiates within +/−10% of a selected frequency without substantial losses where a first element transmitting at the same frequency radiates inefficiently if the frequency changes by more than +/−5%. In a more preferred embodiment, the wideband element radiates in a +/−15% range without substantial losses. The combination of a majority of elements having a particular bandwidth with a minority of elements having wider bandwidths may enable generation of improved radiation patterns. Furthermore, the second elements may enable generation of the composite radiation pattern over a wider range of frequencies as compared to the range of frequencies over which the first radiation may be produced. As the driving frequency is lowered below the low end of the range of frequencies operable with the first elements, the efficiency of the first elements rapidly decays. However, the efficiency of the wideband elements, or second elements, does not decay since their frequency range is wider. Thus, the ratio of the directivity of the second radiation pattern to the first radiation pattern increases as the efficiency of the first elements decays, thereby increasing the effect of the second radiation pattern on the composite radiation pattern. 
     In yet another embodiment, the second elements comprise steering elements. Steering elements comprise two or more commonly driven elements which are disposed within a group of first elements. As described with reference to  FIGS. 15-17 , an amplifier may drive the steering element and element circuits introduce phase-shifts or time-delays to the driving signal from the amplifier to generate a second radiation pattern. Multiple steering elements may be provided to produce a stronger second radiation pattern and an even more improved composite pattern. In a preferred embodiment, the group of first elements within which the steering element is placed comprises at least eight elements and is characterized by element spacings greater than ½λ. A ninth element may be disposed within the group and the steering element as shown with reference to element  42  in  FIG. 8 , although as shown in  FIG. 7 , the ninth element may also be absent from the first pattern. In further embodiments of the invention, first and second elements may be combined in different ways based upon the first pattern, element spacings and array frequency. The radiating elements of the steering element are referred to as third elements and may comprise base elements, wideband elements, or other elements. 
     In a further embodiment, an antenna array comprises a first plurality of first elements and a second plurality of first elements. The first and second pluralities of first elements are arranged in the regulated pattern described hereinabove. The first plurality of first elements is driven to generate a first radiation pattern. The second plurality of first elements is commonly driven similarly to steering elements to generate a second radiation pattern. A third, fourth and fifth plurality of first elements may be driven like steering elements in combination with the second plurality of first elements to form the second radiation pattern. In a preferred embodiment, the second, third, fourth and fifth plurality of first elements form first, second, third and fourth steering elements which are distributed evenly around the pattern center. 
     Periodic and aperiodic patterns will now be described conceptually with reference to  FIGS. 1 to 11 .  FIG. 1  is a graph of a periodic antenna array pattern with element spacing smaller than λ/2 and  FIG. 2  is a graph illustrating a radiation pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in the pattern shown in  FIG. 1 . The periodic element pattern, denoted by numeral  10 , illustrates a 16×16 element array. The horizontal and vertical axes represent a number of wavelengths. As illustrated, sixteen elements are located in a spacing of approximately seven wavelengths resulting in an element spacing of about 7/15λ which is less than a half wavelength (½λ). The radiation pattern shows main lobe  12  resulting from steering the main beam to 22 degrees and a plurality of side lobes  14 . 
       FIG. 3  is a graph of a periodic antenna array pattern with element spacing greater than λ/2 and  FIG. 4  is a graph illustrating a radiation pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in the pattern shown in  FIG. 3 . The periodic element pattern, denoted by numeral  20 , illustrates a 16×16 element array. As illustrated, sixteen elements are located in a spacing of approximately twenty-two wavelengths resulting in an element spacing of about 22/15λ which is greater than a wavelength (1λ). The radiation pattern includes main lobe  22  and grating lobe  24  as well as a plurality of side lobes  26  located right and left of main lobe  22 . Main lobe  22  results from steering the main beam to 22 degrees from boresight. Grating lobe  24  results from the uniform expansion of element spacings. 
       FIG. 5  is a graph of a portion of a periodic antenna array pattern. The portion, denoted by numeral  30 , comprises nine equally spaced elements. A dashed circle denotes the position of an element and its bandwidth. The position is at the center of the dashed circle, and the diameter of the circle represents the size of the radiating pattern of the element which is proportional to the largest wavelength or lowest frequency. Portion  30  represents nine elements positioned in close proximity. Elements  31 ,  32 ,  33  and  34  form pattern  36 . In contrast, aperiodic pattern  40 , shown in  FIG. 6 , illustrates the position of nine elements located northwest of the pattern center as evidenced by the increased element spacing V 2  compared to V 1  and element spacing H 2  compared to H 1 . Stated differently, element spacing above and left of element  42  is greater than element spacing right and below element  42 .  FIG. 7  shows pattern  50  which is a modified pattern  40 . Four elements were added and arranged in pattern  36  such that the center of pattern  36  overlaps the position of element  42 , which has been removed. In one embodiment, the four elements of pattern  36  comprise a steering element. The other elements, those which comprise the majority of elements in the array, will be referred to as the first, or base, elements.  FIG. 8  shows pattern  60  which is a modified pattern  40 . Elements  61 ,  62 ,  64  and  65 , comprising a steering element, were added. Element  42  may be included with the steering element or may be controlled with the base elements.  FIG. 9  shows pattern  70  comprising seven base elements  72  and nine elements  74  having a bandwidth larger than the bandwidth of the base elements. As with steering elements, the aperiodic pattern enables the replacement of base elements  72  with elements  74 . 
     As shown in  FIGS. 10 and 11 , hybrid patterns may also be formed to improve uniform array patterns.  FIG. 10  is a graph of periodic antenna array patterns  80  and  86 . Pattern  80  comprises nine equally spaced base elements and pattern  86  comprises a steering element formed with elements  81 ,  82 ,  83  and  84 .  FIG. 11  is a graph of periodic antenna array pattern  90 . Pattern  90  comprises nine equally spaced elements wherein four elements  94  are wideband elements and five elements  92  are base elements. Obviously, the patterns illustrate relationships between first and second elements and are not intended to limit the invention to the precise number of elements described herein. 
     Having described various embodiments of the invention comprising periodic and aperiodic patterns and modifications thereto, further embodiments of the invention will now be described with reference to  FIGS. 12 to 17 .  FIG. 12  is a graph of first pattern  100  including base elements, represented by solid-line circles, at the intersections of sixteen rows and sixteen columns. A group of four elements in pattern  36  is shown at the intersection of columns  102 ,  103  and rows  106 ,  107 . A pattern  40  of base elements is shown at the intersections of columns  112 ,  113 ,  114  and rows  116 ,  117 ,  118 . Specific elements are pointed out with reference to their coordinates (column, row) including elements E 1,16 , E 2,16 , E 2,15 , E 3,15  and E 2,14 .  FIG. 13  is a graph illustrating a first radiation pattern obtained by simulating transmission from a two-dimensional antenna array having elements disposed in first pattern  100 . The first radiation pattern shows main lobe  120 , a plurality of side lobes  122  and  124 , null  132  at negative 22 degrees from boresight, and sidelobes  130  on either side of null  132 . The grating lobe previously located at negative 22 degrees has been attenuated as a result of the aperiodicity of the first pattern  100 . 
       FIG. 14  illustrates another embodiment according to the invention wherein a number of base elements in first pattern  100  were replaced with steering elements to form aperiodic array pattern  200 . Groups of four elements comprise steering elements located at C 2,2 , C 2,3 , C 3,2 , C 14,2 , C 15,2 , C 15,3 , C 2,14 , C 2,15 , C 3,15 , C 15,14 , C 15,15  and C 14,15 . The letter C indicates a composite element, e.g., a steering element. Each of these steering elements may be controlled independently or combined with other steering elements to produce a signal which reduces the amplitude of any particular side lobe to thereby increase the directivity of the pattern. In another embodiment, wide bandwidth elements, denoted by the letter W, may replace base elements, for example at W 2,2 , W 2,3 , W 3,2 , W 14,2 , W 15,2 , W 15,3 , W 2,14 , W 2,15 , W 3,15 , W 15,14 , W 15,15  and W 14,15 . The pattern center is located between rows 8-9 and columns 8-9 which is where the element spacing is at a minimum. Elements in the steering elements may be the same as the first elements or may be different. 
     Digital beam forming techniques can be used to overcome the deficiencies of the higher side lobes. For example, amplitude tapering or weighting, typical on uniform spaced arrays, may be applied to further distinguish the main lobe from side lobes. By comparing signal strength versus beam position a computer can determine where the target, or signal emitter, as the case might be, is located. Increased spacing between elements allows greater freedom in design of wider band radiating elements, especially for flat panel antennas, i.e., antennas built on a single or multilayer circuit board. Increased spacing between elements allows room for both vertical and horizontal polarization and wider-band radiating elements. Polarization diversity and wider-band can be very expensive to achieve with tighter spacing between elements. Flat panel antennas are made possible because of the increase in element spacing, for example going from 0.5 wavelength spacing to 1.0 wavelength spacing increases the available circuit board area by at least 300 percent at the center of the array. For elements that are further away from the center, the available circuit board space increases more. The greater circuit board area per element allows a single or multilayer circuit board antenna array, greatly reducing cost versus the conventional technique of stacking modules side-by-side. Cooling may be simple forced air versus liquid due to greater element spacing. 
       FIG. 15  is a schematic representation of an embodiment of a steering element according to the invention. Steering element  210  may be steered to produce any of a plurality of patterns having directivity  212 . Four elements  220  of steering element  210  are shown equally spaced by distance  214 . A signal is transmitted by amplifier  240  through lines  230  to phase shifting components  222  which provide a phase-shifted signal to elements  220 . The input to amplifier  240  may also be phase-shifted relative to the signals provided to the group of first elements surrounding steering element  210 . Phase shifting component  222  may comprise, for example, two or more signal paths of different lengths to delay signals to element  220 . The four signals provided by phase shifting components may be shifted together or individually so that the combined pattern of the four radiating elements  220  is stronger in the direction of the main beam and weaker in the direction of the grating lobe or strong side lobe thereby improving the overall or composite array pattern. Amplifier  240  may weigh the signal it provides to phase shifting components  222  to magnify or deemphasize the contribution of steering element  210  to the composite radiation pattern. 
       FIG. 16  is a schematic representation of another embodiment of a steering element according to the invention. Although shown side-by-side for clarity, four elements  220  are disposed in a square pattern like the pattern shown with reference to  FIG. 15  forming steering element  250 . Alternatively, the four elements  220  may be disposed in a rectangular pattern. Depending on their size, additional or fewer elements  220  may be provided. Each phase shifting component  223  comprises a switch  256  and connectors  252  and  254  having different lengths and electronically communicating a signal provided by amplifier  240  to elements  220  through either connector  252  or  254 . A time delay is introduced by transmitting an incoming signal through switch  256  and connector  254  compared to transmission through switch  256  and connector  252  due to the longer length of connector  254 . A multi-pole switch and a plurality of connectors having a plurality of lengths may be provided to introduce a plurality of signal paths of different lengths to delay signals to elements  220 . The four signals provided by phase shifting components may be shifted together or individually so that the second pattern generated by elements  220  is stronger in the direction of the main beam and weaker in the direction of the grating lobe or strong side lobe thereby improving the composite array pattern. 
     In a further embodiment according to the invention, a steering element adapted to provide enhanced polarization diversity is disclosed. The steering element includes a pair of radiating elements oriented perpendicularly to each other so as to have a common phase center. Horizontal and vertical radiating elements may be provided having separate feed points which are driven with separately set one bit phase shifter or time delays. The amount of phase shift may be different for each horizontal or vertical portion to provide different polarizations of the steering element. For example, the horizontal portions phase shifter may provide 0 or 180 degrees of shift and the vertical may provide 0 or 90 degrees of phase shift. So for the exemplary steering element described herein there are four possible combinations of phase shifter settings. Setting both at 0 degrees provides linear polarization at +45 degrees from vertical. Setting one at 180 degrees and the other at 0 degrees provides linear polarization at −45 degrees from vertical. Setting one at 0 degree and the other at 90 degrees provides right hand circular polarization. Setting one at 180 degree and the other at 90 degrees provides left hand circular polarization. Control components, e.g., components  222  or switches, are provided for each horizontal and vertical portion to generate independent feed signals and the desired polarization. The steering element may also comprise pairs of horizontal and vertical elements or patch antennas with two feed points. Of course, the angles are relative to the vertical and horizontal orientation of the radiating portions. If the radiating portions are set at other than vertical or horizontal, then the polarization angles will change accordingly. For example, setting the radiating elements at 45 degrees while still orthogonal to each other provides vertical or horizontal polarization. 
     Polarization diversity may be further enhanced by adapting the enhanced polarization steering element as disclosed herein. Rather than setting the time delays or phase shifts as described in the paragraph above, the time delays or phase shifts may be set to generate polarization at angles different than 45 degrees. Linear, circular and elliptical polarization may be achieved. 
     In an embodiment of an antenna array according to the invention, the antenna array comprises a plurality of steering elements. Each steering element includes two radiating elements overlaid so as to have a common phase center and radiating electric fields orthogonal to each other. Each steering element also includes two phase altering portions, or control components as described above, separately and electronically coupling the two radiating elements and being adapted to electronically couple a signaling device to establish separate communication paths between the signaling device and the two radiating elements. The signaling device can be an amplifier that provides a signal to feed both radiating elements. The signal can be passed out of the signaling device through one connector which then splits into two connectors separately coupling each phase altering portion. Alternatively, the signal can be provided through separate connection paths to/from the signaling device. Then, each phase altering portion is electronically coupled to a radiating element. Since radiating elements can transmit and also receive electromagnetic signals, the signal flow can also be reversed to pass a signal received by the radiating elements through the phase altering portions to the signaling device. The two phase altering portions are engagable in one of two positions defining four configurations of the steering element. The configurations are selectable to generate or detect a radiation pattern comprising one of linear, elliptical and circular polarization. 
     In an additional embodiment of an antenna array according to the invention, the antenna array comprises a plurality of base radiating elements in addition to the plurality of steerable elements. The base radiating elements radiate within a predetermined frequency band to generate a first radiation pattern or a signal representative of a first radiation pattern received by them. Advantageously, the first radiation pattern may be used to provide a calibration reference for a radiation pattern received by a plurality of steering elements configured as described above. In another embodiment, the base radiating elements and the steering elements receive a radiation pattern and produces corresponding signals. The signals are compared to detect whether the radiation pattern is circular. Generally, an amount of power approximating 3 db is lost, or not detected, by linearly polarized elements receiving a circularly polarized radiation pattern. The phase shift altering portions can then be set to maximize the signal, e.g., to regain the 3 db by properly tuning the steering elements to the radiating pattern. 
       FIG. 17  is a graph illustrating radiation patterns of base and steering elements. Pattern  280  results from steering a base element to 0 degrees. Patterns  290  and  292  result from steering a steering element to 0 and 45 degrees, respectively. The steering elements have higher gain than the single base element due to the four radiating portions of the steering element adding together. Advantageously, the modules associated with elements  220 , which include components  222  and  223  and may, additionally, include filters and amplifiers, may be of simple construction, e.g., one-bit phase shifters and bipolar switches, which, while having limited steering capability nonetheless add another control lever to reduce the amplitude and directivity of selected side lobes. For example, components  222  and  223  may steer to one of four quadrants rather than to precise angles. Multiple steering elements  210 ,  250  may be provided which may be steered together to form the second radiation pattern and improved composite pattern. 
     While this disclosure has been described as having exemplary designs, the present disclosure can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this disclosure pertains and which fall within the limits of the appended claims.