Abstract:
An antenna array of “n” individual parabolic dishes is arranged about a center in subarrays extending in North, East, South and West rows. Signals received by the antennas are combined into representations of the electromagnetic environment as viewed with various simultaneous beam geometries, through a combination of analog signal summation, and digital implementation of aperture synthesis techniques. All antennas monitor the same portion of sky. The array is able to scan the sky without physically moving the antennas. The angular resolution of the array greatly exceeds that of the individual antennas. A variety of different research objectives, such as targeted searches, all-sky surveys, sky mapping, interference studies, the study of a variety of natural astrophysical phenomena, and the search for intelligently generated microwave emissions of possible extraterrestrial origin, may be realized simultaneously.

Description:
Cross Reference to Related Application  
       [0001]    This application is based upon, and claims the benefit of the filing date of prior U.S. Provisional Patent Application No. 60/224,967, entitled Adaptive Microwave Antenna Array, filed Aug. 11, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    I. Field of the Invention  
           [0003]    The present invention relates generally to radio astronomy, and the microwave antenna arrays and systems utilized in such installations. More particularly, the present invention relates to multi-element antenna arrays primarily adapted for astrophysical research and the Search for Extra-Terrestrial Intelligence (SETI), and methods for electrically aiming them.  
           [0004]    II. Description of the Prior Art  
           [0005]    It has long been recognized by those skilled in the art that multiple antennas can be combined together for increased receiver performance. The advantages are numerous and well-known. Various forms of prior art technology exist for combining the antennas.  
           [0006]    For example, in astrophysical research and the electromagnetic Search for Extra-Terrestrial Intelligence (SETI), it has been the common practice to combine multiple dish antennas into an array, optimized to produce a specific beam geometry. Beam geometries tend to be highly application-specific. For example, drift-scan SETI receiving stations are best served by an antenna pattern that is somewhat broader in the declination axis than it is in right ascension. Such a beam pattern was implemented by the late Ohio State University “Big Ear” radio telescope, circa 1964-1997, which was one of the great pioneers in SETI. Total power studies of the galactic core favor an opposite antenna pattern (that is, a geometry which is broader in right ascension than it is in declination). Targeted searches of individual stars and quasi-stellar objects require a spot beam, narrow in both planes.  
           [0007]    At this time, in order to change from one beam pattern to another, different physical array geometries are used. Obviously this approach has limitations, especially when funds are limited, and thus only a single array geometry may be practicable. Alternatively, antennas may be physically relocated. This approach is obviously difficult, and often impractical. For example, the twenty-seven dish antennas at the multi-million dollar Very Large Array (VLA) in Socorro N. Mex. each weigh two hundred and thirty tons. to change this array between operating configurations, each of its dishes is moved along approximately thirty miles of railroad track. However, multiple diverse beam geometries often tend to be mutually exclusive. An adaptive antenna array, one that can operate in multiple geometric modes simultaneously, would be highly advantageous.  
           [0008]    The advantages gained by combining multiple antennas into an array are well known, and fall into two broad categories: (a) improving sensitivity, and (b) improving resolution. The two most common ways of connecting multiple antennas into an array are (a) as a radiometer, and (b) into correlation detectors. (Burke and Graham-Smith, 1997). In the case of the radiometer connection, a single detector is connected to all of the antennas in the array via a branched feedline, which maximizes sensitivity by producing a single beam. The best known (though never implemented) example of this configuration is Project Cyclops (Oliver et. al., 1973).  
           [0009]    With interferometers (Ryle, 1952), resolution is improved by combining the signals of two antennas which are separated by a specified distance (called the baseline). With dish antennas, the resulting gain is simply that which would be achieved by a single dish with a surface area equal to the sum of that of the two antennas. However, the angular resolution of such an interferometer is equivalent to that of a single dish with a diameter equal to the baseline. Thus, interferometers provide a modest improvement in sensitivity with a much greater increase in resolution. A multiple-antenna interferometer array may be constructed using a technique known as aperture synthesis. Each possible pairing of antennas in the array is accomplished by applying the outputs of the antennas to a multitude of correlator circuits. The correlator outputs may be combined to produce multiple beams, making it possible to image distant astrophysical objects with high levels of detail. Well-known multiple-antenna interferometers include the Very Large Array (Napier et. al., 1983) and the Giant Meter-Wave Radio Telescope (Swarup et. al., 1991). Both of these arrays arrange their antennas (27 in the case of the VLA; 30 at the GMRT) in a “Y” configuration with extremely wide baselines, and use digital correlators to combine the signals from the multiple dishes.  
           [0010]    The Mills Cross arrangement (Mills, 1963) consists of two line-type antennas, one oriented North-South and the other East-West. The former antenna produces a beam pattern which is narrow in declination and broad in right ascension. The latter produces a beam pattern which is broad in declination and narrow in right ascension. When signals from the two antennas are combined, a beam is produced which is narrow in both axes.  
           [0011]    Bracewell and Swarup (1961) produced an array of thirty-two small parabolic dish antennas, oriented in a Mills Cross arrangement, to produce a pencil-beam interferometer with micro-steradian resolution. All of the antenna arrays described above achieve stated design goals of high sensitivity or high angular resolution. In each case, one and only one of these design objectives can be achieved, often at the expense of the other.  
           [0012]    Known prior art concepts are discussed in the following references:  
           [0013]    Bracewell, R. N., and G. Swarup,  The Stanford Microwave Spectroheliograph Antenna: A Pencil Beam Interferometer , IRE Trans. Antennas and Propagation, vol. AP-9, pp. 22-30, January 1961.  
           [0014]    Mills, B. Y.,  Cross - type Radio Telescopes , Proc. IRE Australia, vol. 24, pp. 132-140, 1963.  
           [0015]    Ryle, M.,  A New Radio Interferometer and Its Application to the Observation of Weak Radio Stars , Proc. Royal Soc. London Ser. A, vol. 211, pp. 351-375, 1952.  
           [0016]    Burke, B. F., and F. Graham-Smith,  An Introduction to Radio Astronomy , Cambridge University Press, 1997.  
           [0017]    Swarup, G., S. Ananthakrishnan, V. K. Kapahi, A. P. Rao, C. R. Subrahmanya, and V. K. Kulkarni,  The Giant Meter - wave Radio Telescope , Current Science, vol. 60 no. 2 pp. 95-105, Jan. 25, 1961.  
           [0018]    Napier, P. J., A. R. Thompson and R. D. Eckers,  The Very Large Array, Design and Performance of a Modern Synthesis Radio Telescope , Proc. IEEE, vol. 71 no. 11 pp. 1295-1320, November 1983.  
           [0019]    Oliver, B. M., and J. Billingham, eds.,  Project Cyclops, A Design Study of a System for Detecting Extraterrestrial Intelligent Life , NASA CR 114445, 1973.  
         SUMMARY OF THE INVENTION  
         [0020]    The invention presents an array of small, dish antennas all united to accomplish specific beam patterning. Preferably the array comprises n individual antennas. Four subarrays, each with (n/4) individual antennas, are established in a cross-like formation, with a subarray running north, south, west and east. The array resembles the Bracewell and Swarup array in physical configuration. Unique circuitry is added to allow it to operate both as a total-power radiometer, and as a correlated interferometer, simultaneously. These multiple operating modes allow the array to achieve both high sensitivity and high angular resolution, fulfilling a variety of research objectives.  
           [0021]    A solution is provided for electronically changing a complex, multiple-antenna array into different configurations yielding different beam patterns. In other words, radio signals derived from the four subarrays can be electronically processed and combined into a variety of beam patterns. These multiple patterns are synthesized through a combination of analog in-phase combining means, analog phase-quadrature signal combining means, and digital conversion and software correlation means. As different individual antennas forming each subarray all monitor the same broad portions of sky, such analog and digital processing of signals derived from the individual antennas can be processed not only to yield the composite observed target sought by the radio telescope, but can produce high angular resolution beam patterns subtending selected portions of the overall sky coverage.  
           [0022]    Thus a basic object is to provide an adaptive antenna array system which can operate in multiple geometric modes simultaneously.  
           [0023]    Fundamentally, it is desired to be able to electronically convert a radiotelescope and switch it between beam patterns.  
           [0024]    Another fundamental object is to electronically aim an antenna system comprising multiple subarrays of multiple antennas.  
           [0025]    A related object is to provide an adaptive antenna array of the character described that can achieve a beam pattern that is broader in the elevation axis than in the azimuth axis.  
           [0026]    Conversely, another object is to provide an adaptive antenna array of the character described that can achieve a beam pattern that is broader in azimuth than in elevation.  
           [0027]    A related object is to provide an electronic means of creating a beam geometry narrower in both azimuth and elevation that those of the individual antennas, so as to improve angular resolution, to aid in the study of individual stars, quasi-stellar objects, and other deep space targets.  
           [0028]    Another object is to provide an array that can produce multiple, simultaneous spot beams, allowing detailed sky maps to be developed.  
           [0029]    A major object is to simplify the changing from one beam pattern to another.  
           [0030]    A still further object is to maximize sky coverage while minimize the necessity of physically moving antennas.  
           [0031]    Another basic object is to provide a highly versatile multiple antenna array system suitable for use by universities or layman, public and private owners, and/or professional and amateur observers.  
           [0032]    Recognizing that some beam patterns in microwave antenna arrays are best achieved by digital processing, and that different beam patterns are better achieved through analog processing, a final object is to provide multiple signal outputs from each antenna in the array, to allow simultaneous analog and digital processing of the available signals. 
       
    
    
       [0033]    These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent in the course of the following descriptive sections.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    In the following drawings, which form a part of the specification and which are to be construed in conjunction therewith, and in which like reference numerals have been employed throughout wherever possible to indicate like parts in the various views:  
         [0035]    [0035]FIG. 1 is a pictorial view of a preferred embodiment of a complete antenna array comprising sixteen individual dish antenna assemblies;  
         [0036]    [0036]FIG. 2 is block diagram of the preferred antenna feed assembly hardware, which is associated with each individual antenna;  
         [0037]    [0037]FIG. 3 is a block diagram of the single, preferred digital signal correlator assembly that performs aperture synthesis on the signals from all “n” individual antennas in the entire array;  
         [0038]    [0038]FIG. 4 is a block diagram of a preferred in-phase, analog coupler assembly associated with each subarray that combines signals from “n/4” antennas in each individual subarray of antennas;  
         [0039]    [0039]FIG. 5 is a block diagram of a preferred quadrature, analog coupler subassembly that combines the signals from all four sub-arrays into a single beam pattern;  
         [0040]    [0040]FIG. 6 is a block diagram of a preferred in-phase, analog coupler subassembly that combines the signals from pairs of sub-arrays into three additional beam patterns; and,  
         [0041]    [0041]FIG. 7 is a diagrammatic view showing five distinct antenna beam patterns, plotted in elevation vs. azimuth, all simultaneously obtainable with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0042]    Turning now to FIGS.  1 - 7  of the appended drawings, the multiple antenna array  22  constructed in accordance with the best mode of the invention, electronically switches between the beam patterns A-E (collectively represented by the reference numeral  21 ) that are represented schematically in FIG. 7. Switching is accomplished through a combination of analog and digital signal combining techniques. Array  22  (FIG. 1) simultaneously produces beam patterns A-E (FIG. 7). Pattern A (FIG. 7) is broad in both azimuth and elevation, providing maximum sky coverage with minimum resolution. Beam pattern B, which is narrow in azimuth and broad in elevation, is optimized for meridian transit (‘drift-scan’) sky surveys. Beam pattern C, which is broad in azimuth and narrow in elevation, maximizes sky coverage in the equatorial plane while providing high resolution in the elevation axis. Beam pattern D comprises a narrow spot-beam formed by the intersection of patterns B and C, for higher resolution follow-up scrutiny of candidate signals detected in the earlier modes. Beam pattern E is a narrow spot-beam which is steerable electronically and instantaneously anywhere within the beamwidth of the individual antennas comprising the array. Pattern E is most useful for sky-mapping studies and, in the preferred implementation, will produce a multiplicity of narrow and overlapping spot beams, collectively filling the search space depicted in Pattern A.  
         [0043]    The multi-antenna array  22  (FIG. 1), constructed in accordance with the teachings of the invention, preferably comprises four subarrays each having “n/4” individual antenna assemblies  23 . In the best mode, the total number of antennas (i.e., normally a plurality of ground-based, parabolic dish assemblies  23 ) is designated by the variable number “n” that is an integer multiple of four. The antenna assemblies  23  are physically arranged in orderly rows that emanate outwardly from a specified, central location  25  (known as the array phase center). Location  25  actually comprises a structural building sheltering computers and other components of the array  22 . In the best mode, the array comprises four subarrays oriented along baselines radiating outwardly from the array center  25 . Ideally each subarray comprises n/4 individual, equally spaced-apart antenna assemblies  23 . In the best mode each antenna comprises a conventional dish antenna with a parabolic reflector. However, other individual high-gain antennas including, but not limited to, helices, Yagis, and loop designs are acceptable.  
         [0044]    An East Subarray  30  (FIG. 1) comprises n/4 individual antenna assemblies  23  oriented equidistant along a baseline radiating from the array phase center  25  in a direction to the true East. A West Subarray  40  comprises n/4 antennas oriented equidistant along a baseline radiating westerly from the array phase center  25 . Similarly, (n/4) antenna assemblies  23  oriented equidistant along a baseline radiating from the array phase center in a direction to the true North constitute a North Subarray  36 . The n/4 antennas oriented equidistant along a baseline radiating from the array phase center in a direction to the true South constitutes a South Subarray  38 .  
         [0045]    The array comprises n antenna assemblies, where n is an integer multiple of four. Each of the sixteen illustrated antenna assemblies  23  comprises an identical feed assembly,  41  (FIG. 2), that is mechanically disposed over the parabolic reflector  42  at the focal point to collect signals reflected by the dish. FIG. 2 shows the prime-focus feed assembly mounted to each of the sixteen dishes in the illustrated array. Note that each feed assembly consists of a cylindrical wave guide feedhorn  43  (preferably including a conventional choke ring, not shown) operational over the array&#39;s preferred 1.3 to 1.7 GHz bandwidth. The feedhorns  43  are each fitted with two conventional, orthogonally polarized monopole probes, schematically indicated as  46  and  49  (FIG. 2). The probes provide dual linear polarization, and they physically project from and are mounted to the center pin of type N coaxial connectors fitted to the feedhorn  43  in radially spaced apart relation. Gain and phase-matched low noise amplifiers  50 ,  52  (i.e., LNAs) are attached to the two probe connectors via identical short lengths  47 ,  48  of low-loss coaxial cable. Using matched, low-loss coaxial cables  53 ,  54 , the outputs of each LNA preamplifier  50 ,  52  are connected to a microstrip quadrature coupler and dual in-phase power splitter assembly  55  (FIG. 2), to produce two RHCP signals (referred to here as Phased and Combined), and two LHCP signals, similarly labeled. The phased outputs will be used downstream for digital correlation, and the combined signals drive analog combiner circuitry, as described hereinafter.  
         [0046]    It is important to note that the success of this array depends upon precise phase and gain matching of all thirty-two (i.e., 2n) LNAs in the system, not only across the operating frequency spectrum, but also over temperature and with changes in applied operating potential. The use in all LNAs of high electron mobility field effect transistors (i.e., HEMTS), and monolithic microwave integrated circuits (i.e., MMICs) from the same production wafer, and microstrip circuit boards etched from the same physical substrate stock, is preferred. For consistency, it is expected that any spare LNAs likely to be needed over the life of the array will have to be manufactured in the same production run as those initially placed into service.  
         [0047]    Preferably each feedhorn  43  is spaced-apart from reflector  42  (FIG. 2) such that proper illumination is achieved, according to well known standards of the art. Probes  46 ,  49  extending within the feedhorn  43 , at ninety-degree, radially spaced apart locations, provide outputs on coaxial lines (i.e., adapters)  47 ,  48 , that are respectively delivered to sensitive, Low Noise Preamplifiers (LNA&#39;s)  50 ,  52 , that drive quadrature coupler  56  (FIG. 3). The signals from LNA&#39;s  50 ,  52  are equal in amplitude and phase, but are orthogonally polarized relative to one another. The quadrature coupler  56  (FIG. 2) receives the orthogonal linear outputs of each LNA  50 ,  52 . Each of quadrature couplers  56  (i.e., associated with each of the n antenna assemblies  23 ) produces two simultaneous outputs, equal in amplitude and in phase, but of two orthogonal circular polarizations. A Right Hand Circular Polarization (RHCP) signal appears on coaxial line  62 . A Left Hand Circular Polarization signal (LHCP) is outputted by the quadrature coupler  56  on coaxial line  63 .  
         [0048]    The twin, in-phase, two-way power splitters  70 ,  72  (FIG. 2) receive the quadrature coupler&#39;s output signals on lines  62  and  63  respectively. Each power splitter outputs an analog “Combined Signal” for subsequent analog processing, and an analog “Phased Signal” for subsequent digital processing. For example, power splitter  70  outputs an RHCP (i.e., right hand circular polarization) phased signal on line  78 , and an RHCP combined signal on line  79 . Splitter  72  outputs an LHCP Phased signal on line  81  and an LHCP combined signal on line  83 . The phased and combined signals emanating from each power splitter  70  or  72  are equal in amplitude and phase, and are effectively isolated from one another.  
         [0049]    Thus each of the n antenna assemblies  23  provides the following four output signals: RHCP Phased, RHCP Combined, LHCP Phased, and LHCP Combined. FIG. 3 shows how we will perform digital processing of the quantity “2n” RHCP and LHCP phased signals derived from all n antennas in the array. The object is to produce by aperture synthesis a pair of detailed amplitude gradient maps, one in each of the two orthogonal circular polarizations, to resolution Pattern E (FIG. 7), of that total sky coverage represented by Pattern A. (Analog processing of the quantity “2n” RHCP and LHCP Phased Signals, to produce beam patterns A, B, C, and D of FIG. 7, will be discussed subsequently.)  
         [0050]    The complete array  22  outputs to a correlator assembly  86  (FIG. 3) that comprises two correlators  90 ,  92 , one to process the n RHCP correlated signals and one to process the n LHCP correlated signals ultimately derived from the n feed assemblies (FIG. 2). In other words, the digital correlator assembly  86  (FIG. 3) processes phased RHCP and LHCP signals from all n individual antennas. It will thus be appreciated, for example, that RHCP phased output line  78  (FIG. 2) corresponds to A/D input line  78  of FIG. 3. Phased RHCP signals from other antennas appear on lines  68  (FIG. 3). Likewise, LHCP phased output line  81  (FIG. 2) corresponds to A/D input line  81  of FIG. 3. Phased LHCP signals from the other antennas enter correlator assembly  86  via lines  69 . Thus, a quantity of “2n” phased signals is delivered to individual corresponding Analog-to-Digital Converters  88  (i.e., ADC&#39;s). Each ADC  88  (FIG. 3) digitizes the analog signals delivered thereto, and preserves amplitude and phase information, thus producing simultaneous RHCP and LHCP digital signal representations, and outputting them on lines  89  and  91  respectively (FIG. 3.)  
         [0051]    A pair of n-input digital signal correlators  90 ,  92  (FIG. 3) is driven, one by the n digitized RHCP digitized signals and the other by the n digitized LHCP signals respectively. Each correlator  90 ,  92  is controlled by and provides its data output to a beam steering and data analysis computer  95 . Computer  95  instructs each correlator  90 ,  92  to look at its n inputted signals, analyzing data from two antenna assemblies  23  at a time, in every possible combination. For each possible pair of signals a voltage sum is computed, one of the signals is digitally phase shifted a selected increment, i.e., one degree, and they are added again. This process is repeated independently for the RHCP and LHCP digitized signals, in their respective correlators  90 ,  92 , until voltage sums have been obtained for each possible pair of signal samples over a phase range of zero to 180 degrees.  
         [0052]    The above process generates massive quantities of data. The computer  95  outputs this data to a conventional mass data storage means  97 , as depicted in FIG. 3, which comprises a bank of hard drives, read-write CDROMS, a network connection, or the like. The computer then analyzes the data thus stored, for each of the possible signal pairs in all of the possible phase relationships, looking for voltage maxima. Each voltage maximum observed for each antenna pair analyzed is a representation of the incident signal falling on the array from a particular direction on the sky (one possible Pattern E in FIG. 7, out of a plethora of such patterns) within the total spatial coverage (Pattern A in FIG. 7) of the entire array. Thus, the data analysis computer creates from the aggregate of fixed antennas a plethora of simultaneous spot beams on the sky, each of size and resolution corresponding to Pattern E in FIG. 7, and collectively filling the entire coverage area Pattern A in FIG. 7.  
         [0053]    From these spot beams, the computer is able to create in software an amplitude contour map, to the resolution E from FIG. 7, of that sky coverage encompassed by pattern A of FIG. 7. This total sky coverage is inversely proportional to the physical size of the antennas used in the array; the smaller the antennas, the greater the area of sky observed at any given time. The resolution achieved is directly proportional to the number of antennas used in the array; the more individual antennas, the smaller the area of the multiple spot beams (E in FIG. 7) produced by the correlator system.  
         [0054]    In the foregoing paragraphs, we have disclosed how aperture synthesis is accomplished by digital processing of the quantity “2n” phased signals from the n antenna feed assemblies. Concurrently, analog circuitry is applied to the quantity “2n” Combined Signals from the n antenna feed assemblies, to produce four simultaneous, independent beam patterns on the sky. These multiple beam patterns (i.e., patterns B-E in FIG. 7) serve the needs of various astrophysical and SETI research projects, including sky surveys and targeted searches of natural and artificial electromagnetic phenomena. It is a feature of the present invention that the two separate analysis techniques, analog and digital, can be accomplished simultaneously, effectively doubling the search space for any given radio astronomical study.  
         [0055]    The RHCP and LHCP “Combined Signals” (FIG. 2) generated by each antenna  23  (FIG. 1) are delivered to the analog power combiner assembly  100  (FIG. 4). Each subarray of antennas requires its own power combiner  100 . The power combiner  100  as illustrated is actually wired to the East subarray, but the three other required power combiners associated with each other subarray of antennas, are preferably identical. In the left side of FIG. 4 it will be noted that (n/4) LHCP Combined Signals are delivered to an (n/4)-input analog power combiner  110 . A typical LHCP combined signal from one antenna in the array appears on line  83  in FIGS. 2 and 4. Additional LHCP combined signals derived from other antennas in the subarray are delivered to lines  94  of power combiner  110  (FIG. 4). Concurrently, FIG. 4 shows that (n/4) RHCP Combined Signals are delivered to an (n/4)-input analog power combiner  112 . A typical RHCP combined signal from one antenna appears on line  79  of FIGS. 2 and 4. The RHCP combined signals from other antennas in the East subarray are delivered to lines  96  of power combiner  112  (FIG. 4). It will thus be appreciated, for example, that an RHCP signal outputted on line  79  (FIG. 2) derived from one of the antennas in the East subarray  30  corresponds to line  79  in the East array&#39;s power combiner assembly  100  (FIG. 4). Likewise, LHCP combined signal outputted on line  83  (FIG. 2) corresponds to input line  83  of FIG. 3.  
         [0056]    Thus, the antennas can be grouped into four distinct sub-arrays, each radiating in directions to the true North, South, East and West of the array center. These groups of multiple antennas are called the North, South, East and West subarrays, respectively. A total of four RHCP and four LHCP signal samples are thus produced by implementing, for each polarity and for each subarray, the linear power combiner circuitry depicted in FIG. 4.  
         [0057]    The outputs from each of the power combiners  110 ,  112  (FIG. 4) drive two-port, in-phase, analog power dividers  113 ,  114  (FIG. 4) respectively. For each subarray of antennas, this produces two identical, analog RHCP signal components, and an additional two identical analog LHCP signal components. Thus the EAST subarray power combiner  100  (FIG. 4) produces and outputs four distinct signals, comprising LHCP outputs A, and B, on lines  115 ,  117  respectively, and RHCP outputs A, B appearing on lines  118 ,  119  (FIG. 4) respectively. The two identical signal components A and B available for each of the four available subarrays, in each of the two available polarizations, will be further processed as discussed in the following paragraphs. In general, outputs A will be combined in analog circuitry to produce an angularly broad beam pattern, while outputs B will be combined in analog circuitry to produce multiple, angularly narrow beam patterns.  
         [0058]    Thus, for the entire array of n antennas, comprising four subarrays each having n/4 antennas, sixteen analog signal components result. The available analog signal components outputted by the power combiners associated with each subarray, are as indicated in the following Table 1:  
                                                               TABLE 1                           Total 2n Signal Outputs                RHCP       LHCP                                East (FIG. 4)   A   B   A   B           West   A   B   A   B           North   A   B   A   B           South   A   B   A   B                      
 
         [0059]    Note that only the circuitry associated with the East subarray is depicted in FIG. 4. Identical circuitry is used to produce the two RHCP and two LHCP signal components for each of the three remaining subarrays.  
         [0060]    The net effect is that the combiner assembly  100  (FIG. 4) for each of the four antenna subarrays (i.e., North, East, West, and South) produces four outputs. Four sets of LHCP Outputs A and B are produced by the four power combiners  100  used with the north, south, east and west subarrays. Four sets of RHCP Outputs A and B are similarly produced. These sixteen signals will be further processed by the circuitry depicted in FIGS. 4 and 5, thus producing beam antenna patterns A, B, C, and D illustrated in FIG. 7.  
         [0061]    Referring jointly now to FIGS. 4 and 5, Output A and Output B signals are applied to Quadrature Antenna Coupler assembly  155  (FIG. 5) that comprises identical LHCP and RHCP subassemblies  156 ,  157  respectively. LHCP OUTPUT A signals (i.e., from power divider  113  in FIG. 4) from each of the four subarrays NORTH, SOUTH, EAST and WEST are applied to inputs of quadrature subassembly  156 . The LHCP Quadrature Antenna Coupler Subassembly  156  comprises three interconnected quadrature hybrids  160 ,  162 , and  164  with three ports terminated in resistive loads  170 ,  166  and  168  respectively shown in (FIG. 5), to produce a single composite signal which is angularly broad in both azimuth and elevation. Quad hybrid  160  receives an input on line  115  corresponding to the East subarray LHCP output A on line  115  of FIG. 4. The south LHCP output A signal is applied to line  154 . Similarly, the West subarray LHCP output A signal is applied to quad hybrid  162  on line  158  and the North LHCP output A signal is applied to hybrid  162  via line  161 . The output of quadrature hybrid  164  (FIG. 5) on line  159  (i.e., the output of the LHCP Quadrature Antenna Coupler Subassembly  156 ) produces an LHCP antenna pattern as depicted in FIG. 7, Beam Pattern A.  
         [0062]    Similarly, an RHCP OUTPUT A signal is derived from the four subarrays NORTH, SOUTH, EAST and WEST. Each subarray power divider, such as divider  114  in FIG. 4, outputs to Quadrature Antenna Coupler Subassembly  157  (FIG. 5). The RHCP Quadrature Antenna Coupler Subassembly  157  consists of three interconnected quadrature hybrids  140 ,  142 , and  144  with three ports terminated in resistive loads  146 ,  150  and  148  respectively as shown in FIG. 5, to produce a single composite signal which is angularly broad in both azimuth and elevation. Line  118  (FIGS. 4, 5) delivers the East subarray RHCP output A signal to quad hybrid  140 . An RHCP output A signal from the south subarray reaches quad hybrid  140  (FIG. 5) on line  141 . Similarly, lines  143  and  145  respectively deliver West RHCP Output A and North RHCP output A signals to quad hybrid  142 . The output from the RHCP Quadrature Antenna Coupler Subassembly  157  outputs on line  169  and produces an RHCP antenna pattern as depicted in FIG. 7, Beam Pattern A. Lines  159 , and/or  169 , for example, may be delivered to the antenna input terminals of a modern microwave receiver.  
         [0063]    Thus, for each of the two available orthogonal polarizations, we have summed the outputs of multiple antennas so as to produce beams which maximize sky coverage.  
         [0064]    Referring jointly now to FIGS. 4 and 6, the OUTPUT B signals (i.e., from power dividers  113 ,  114  in FIG. 4) from each subarray are applied to a Linear Antenna Coupler Assembly  125  (FIG. 6). The coupler assembly  125  comprises an LHCP Linear Antenna Coupler Subassembly  127 , and an identical RHCP Linear Antenna Coupler Subassembly  129 . The RHCP Linear Antenna Coupler Subassembly  129  comprises five, two-way, in-phase power dividers  120 ,  122 ,  124 ,  126 , and  128  interconnected as shown in FIG. 6. Similarly, the LHCP subassembly  127  (FIG. 6) comprises five, two-way, in-phase power dividers  130 ,  132 ,  134 ,  136 , and  138 . East and West RHCP output B signals are respectively applied to subassembly  129  (i.e., divider  120 ) on lines  117  and  121 . North and South RHCP output B signals are respectively applied to divider  124  via lines  123  and  131  (FIG. 6). East and West LHCP output B signals are respectively applied via lines  119  and  135  to subassembly  127  (i.e., divider  130 ). Finally, suitable North and South LHCP output B signals derived from dividers similar to power divider  100  (FIG. 4) are respectively applied to divider  134  (FIG. 6) on lines  133  and  137 .  
         [0065]    The above interconnections will allow us to produce three simultaneous and independent signal representations for each of the two available orthogonal polarizations. One of these signal representations is that of a line of individual antennas oriented from East to West. The second such signal representation is that of a line of individual antennas oriented from North to South. The third such signal representation is an algebraic sum of the previous two signals.  
         [0066]    The first pair of outputs from Linear Antenna Coupler assemblies  125  appear on output lines  180  and  183 , producing beam Pattern B (FIG. 7) with RHCP and LHCP polarizations, respectively. This results because the signals appearing at output lines  180  and  183  are derived solely from the East and West subarrays, thus from antennas which are oriented along an East-West baseline.  
         [0067]    The second pair of outputs from Linear Antenna Coupler assemblies  125  appear on output lines  182  and  185 , producing beam Pattern C (FIG. 7) with RHCP and LHCP polarizations, respectively. This results because the signals appearing at output lines  182  and  185  are derived solely from the North and South subarrays, thus from antennas which are oriented along a North-South baseline.  
         [0068]    The third pair of outputs from the Linear Antenna Coupler assemblies  125  appear on output lines  181  and  184 , producing Beam Pattern D (FIG. 7) with RHCP and LHCP polarizations, respectively. This results because the signals appearing at output lines  181  and  184  are derived by summing the outputs of a group of antennas oriented along a North-South baseline, with the outputs of a similar group of antennas oriented along an East-West baseline. Lines  180 - 185  can be connected to the input terminals of a suitable microwave receiver.  
         [0069]    As is common astronomical practice, the term Baseline as used above implies a specific, equidistant linear distribution of multiple antenna elements, oriented in a particular specified direction.  
         [0070]    Specifically, the described antenna array produces five independent and simultaneous output signals for each of two available orthogonal circular polarizations, with individual beam patterns corresponding to each of the five geometries. Four of these geometries (A, B, C, and D in FIG. 7) are accomplished by analog processing of the available signals from the individual antennas. Pattern E, the fifth such geometry (FIG. 7) results from digital processing of the available signals from the individual antennas.  
         [0071]    Referring to FIG. 7, the beam patterns thus produced are further described as follows:  
         [0072]    A. An aggregate pattern of equal broad azimuth and elevation beamwidths w, such pattern A corresponding to the beam pattern of any single antenna in the array. This pattern is suitable for studies in which it is desired to maximize spatial sky coverage in both the right ascension and declination planes, and for which low angular resolution is sufficient. Such studies include, but are not limited to, broad sky surveillance for natural or artificial electromagnetic phenomena.  
         [0073]    B. An elongated pattern of elevation beamwidth w and azimuth beamwidth (2w/n), where n is an integer multiple of four and equals the total number of antennas in the array, such pattern B corresponding to the beam pattern of all n/2 antennas in an East-West baseline, combined linearly in phase. This pattern is suitable for studies in which it is desired to maximize spatial sky coverage in the declination plane, while maximizing angular resolution in the right ascension plane. Such studies include, but are not limited to, drift-scan all-sky surveys for natural or artificial electromagnetic phenomena.  
         [0074]    C. An elongated pattern of elevation beamwidth (2w/n) and azimuth beamwidth n, where n is an integer multiple of four and equals the total number of antennas in the array, such pattern C corresponding to the beam pattern of all n/2 antennas in a North-South baseline, combined linearly in phase. This pattern is suitable for studies in which it is desired to maximize spatial sky coverage in the right ascension plane, while maximizing angular resolution in the declination plane. Such studies include, but are not limited to, edge-on surveys of the ecliptic for natural or artificial electromagnetic phenomena.  
         [0075]    D. A fixed spot pattern of equal azimuth and elevation beamwidths (2w/n), where n is an integer multiple of four and equals the total number of antennas in the array, such pattern D corresponding to the beam pattern of all n/2 antennas in an North-South baseline, plus all n/2 antennas in an East-West baseline, combined linearly in phase. This pattern is suitable for studies in which it is desired to maximize angular resolution in both the right ascension and the declination planes. Such studies include, but are not limited to, targeted searches for natural or artificial electromagnetic phenomena.  
         [0076]    E. A steerable spot pattern of equal, narrow azimuth and elevation beamwidths (2w/n), where n is an integer multiple of four and equals the total number of antennas in the array, such pattern E corresponding to the beam pattern of all n/2 antennas in an North-South baseline, plus all n/2 antennas in an East-West baseline, combined in varying phases. This pattern is suitable for studies in which it is desired to maximize spatial sky coverage in both the right ascension and declination planes through aperture synthesis, while achieving high angular resolution in both the right ascension and the declination planes. Such studies include, but are not limited to, high-resolution sky mapping of natural or artificial electromagnetic phenomena.  
         [0077]    As is common astronomical practice, in the foregoing discussions, Right Ascension refers to the aiming of the antennas with respect to the plane of the ecliptic, generally along an East-West line, and is dominated by the date and time of observation, and the observer&#39;s longitude, and changes according to the Earth&#39;s rotation on its axis. Right Ascension is generally measured in hours, minutes and seconds, with 24 hours of elapsed Right Ascension corresponding to one sidereal day. Thus, Right Ascension beamwidth most closely relates to the azimuth pattern of the antennas.  
         [0078]    Similarly, as is common astronomical practice, in the foregoing discussion Declination refers to the aiming of the antennas with respect to the plane of the ecliptic, generally along a North-South line, and is dominated by the observer&#39;s latitude, as well as the antenna&#39;s elevation angle with respect to the Earth&#39;s horizon. Declination is generally measured in degrees, minutes and seconds North or South of the ecliptic. Thus, Declination beamwidth most closely relates to the elevation pattern of the antennas.  
         [0079]    From the foregoing, it will be seen that this invention is one well adapted to obtain all the ends and objects herein set forth, together with other advantages which are inherent to the structure.  
         [0080]    As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.