Abstract:
A wide bandwidth, wide scan, antenna array element provides an active element impedance close to 350 ohms over a bandwidth approaching one octave in a periodic equilateral triangular array lattice. The two balanced feed inputs to each element may be phased to produce any desired polarization. The interelement spacing is the maximum possible for grating lobe free operation. This antenna element combines features of a crossed grid dipole with a folded dipole. The antenna structure consists of conductors lying in two planes which are parallel to a ground plane. These are joined by a limited number of perpendicular conductors. The VSWR remains below 2:1 in a 300 or 350 ohm system for scan in any direction out to 30 degrees off of broadside over a bandwidth approaching one octave.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to antenna elements, generally, and particularly to an antenna element which provides arbitrary polarization and can be used to form a scanning array with a minimum number of elements while maintaining relatively constant active element input impedance over bandwidths approaching one octave. 
     2. Related Art 
     Crossed dipole (or turnstile antennas), folded dipoles and wire biconical antennas have been used alone and in arrays in a variety of communications and radar applications. C. Balanis in Antenna Theory Analysis and Design (1982) discloses at page 330 that biconical antennas have broadband characteristics useful in the VHF and UHF frequency ranges, but that the size of the solid shell biconical structure limits many practical applications. As a compromise, multielement intersecting wire bow tie antennas have been employed to approximate biconical antennas. Johnson and Jasik in the Antenna Engineering Handbook (1984) disclose crossed dipole antennas at page 28-10. Such antennas are used for producing circular polarization. Johnson at Jasik at page 4-12 also disclose biconical dipoles and, beginning at page 4-13, disclose the formation of folded dipoles by joining cylindrical dipoles at their ends and driving them by a pair of transmission lines at the center of one arm. 
     To date, however, there has been no disclosure of an antenna element that combines the desirable features of the biconical, crossed dipole and folded dipole antenna elements. 
     SUMMARY OF THE INVENTION 
     In view of the above described related art limitations, and others, it is an object of the invention to provide an antenna which minimizes the number of elements for grating lobe free operation over a conical scan volume. 
     It is another object of the invention to maintain a wide impedance bandwidth. 
     It is still a further object of the invention to provide an array antenna element which provides arbitrary polarization and permits the minimum number of array elements for a scanning array, while maintaining a relatively consistent active element impedance over a wide bandwidth, approaching one octave. 
     It is another object of the invention to provide an antenna element formed as a crossed grid dipole element from a pair of folded grid dipoles. 
     It is still another object of the invention to combine in a single antenna the features of a crossed dipole or turnstile antenna, the folded dipole and the wire biconical antenna, with improved bandwidth performance. 
     The above and other objects of the invention are accomplished with an antenna element having a two tier construction, with conductors in each tier being parallel to an X-Y plane. The element is formed from four grid dipoles (two crossed grid dipoles). Each tier has two dipoles (one crossed dipole) formed from a grid of conductors. Each grid dipole has an axial conductor with additional peripheral conductors around a perimeter producing a wide grid dipole shape. All the conductors on the top tier converge at a center and are connected to improve performance, which is another novel feature of this element. Each dipole is 0.612λ long at the reference frequency. Typically, each element has four arms with each arm being shaped as a quadrilateral. In this configuration, the lower and upper tiers are connected at 12 points on the periphery of the element. The arms may be shaped as polygons other than a quadrilateral. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood in accordance with the description of the embodiments herein with reference to the drawings in which: 
     FIG. 1 is a top view of an upper tier of an antenna element according to the invention; 
     FIG. 2 is a top view of a lower tier of an antenna element according to the invention; 
     FIG. 3 is a side view of the antenna element of the invention; 
     FIG. 4a shows a seven element array lattice employing antenna elements of the invention; 
     FIG. 4b identifies the center points of the antenna elements of the seven element array of FIG. 4a; 
     FIGS. 5-12 are Smith charts showing performance of the antenna elements under various conditions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An antenna element according to the invention has a ground plane and a first crossed grid dipole, arranged in an X-Y plane corresponding to a first tier, the first tier being vertically separated from a second tier and the ground plane. The first crossed grid dipole has an interconnected plurality of arms. The antenna element also has a second crossed grid dipole, arranged in an X-Y plane corresponding to a second tier, the second crossed grid dipole having a plurality of non-interconnected arms, each of the non-interconnected arms having a feed input. Each arm has a central conductor and periphery conductors forming a perimeter that surrounds at least a portion of the central conductor. The first and second crossed grid dipoles are interconnected at corners on their peripheries but are unconnected at a central point. On the upper tier, each of the arms is connected at the central point, while on the lower tier the arms are connected to feeds at the center point. An array of such elements can be formed. 
     All dimensions and distances given herein are in the wavelength (λ) at exactly the highest frequency at which an array of elements can be scanned to 30 degrees off broadside in any direction without the formation of visible grating lobe peaks in the array factor. This frequency would usually be very close to the highest frequency of desired operation while scanning the array to 30 degrees off broadside. This frequency is referred to herein as the reference frequency. The dimensions given for the element described herein were optimized for a given set of requirements, which include no grating lobes with a 30 degree conical scan, circular polarization, independent control over two feed ports, octave bandwidth, a seven element array with air dielectric and thin conductors over a very highly conductive ground plane. Those of ordinary skill will recognize that a different set of requirements might advise somewhat different dimensions, but the fundamental geometric concept of the claimed antenna element would be the same. 
     The antenna element disclosed herein is designed to be used, typically, in an array of 7 or more identical elements. The center points and orientations of the elements for the given requirements are shown in FIGS. 4a and 4b. Additional elements may be used by adding to the periodic element lattice shown in FIGS. 4a and 4b. Arrays with 7 elements and with 19 elements have been investigated. The 7 element array is described herein, although it will be known to those of ordinary skill that the scope of the claims herein includes arrays with other numbers of elements. 
     Since all elements in the array are identical, only one element need be described in detail. An antenna element according to the invention has two crossed grid dipoles. Each crossed grid dipole lies upon a planar surface, referred to as a tier. One of the crossed dipoles is arranged in a first tier and the second crossed dipole in the second tier. The two tiers are separated vertically and lie one above the other, parallel to each other, and parallel to a ground plane. The first tier is uppermost. The second tier lies between the first tier and the ground plane. The first tier may be referred to as the upper tier, and the second tier as the lower tier. The tiers are separated by air or non-conducting dielectric material. 
     The two tiers are laid out as shown in FIGS. 1 and 2 when viewed from the top. Each line drawn in the figures represents the location of a conductor such as a wire. All the wires or conductors in each tier lie in a plane parallel to the X-Y plane. The ground plane is in the X-Y plane and the Z axis is vertical. The side view of FIG. 3 shows the two tier construction of the element. 
     The element is comprised of two crossed grid dipoles, one per tier. Each crossed grid dipole is comprised of two grid dipoles. Each grid dipole is comprised of two arms, typically quadrilateral arms. Each quadrilateral arm is formed from four perimeter or peripheral conductors and one axial conductor. The axial conductors of each arm are positioned along the axis of the dipole. As shown in FIGS. 1 and 2, in each tier one dipole axis is oriented parallel to the X axis and a second dipole axis is oriented parallel to the Y axis. 
     The conductors may be identified as follows in FIGS. 1 and 2. On the upper tier, quadrilateral conducting grid arms 9a and 9b form one of the dipoles and quadrilateral arms 9c and 9d form the other one of the dipoles. All four arms 9a,9b,9c and 9d form the crossed dipole. Similarly, on the lower tier, conducting grid arms 11a and 11b form one of the dipoles and conducting grid arms 11c and 11d form the other one of the dipoles. Arms 11a,11b,11c, and 11d form the crossed grid dipole. The two dipoles on the upper tier have axial conductors 10a-10b or 10c-10d. The two dipoles on the lower tier have axial conductors 12a-12b or 12c-12d. The dipoles have additional conductors around a perimeter to produce the wide grid dipole shapes shown. Each of the dipoles is 0.612λ long at the reference frequency. 
     The first or upper tier is located 0.33λ above the ground plane and the lower tier is 0.23λ above the ground. Each element is symmetric about the X and Y axes, so the coordinates of all points may be deduced from the coordinates given. 
     In the first or upper tier the conductor grid arm 9a forms a quadrilateral having four sides with its furthest perimeter or periphery corner 13a along its respective axis 10a at a distance of 0.306λ from a common center 8 or interior corner. The remaining perimeter or periphery corners of quadrilateral arm 9a are shown at 14a and 15a and are located a distance of 0.133λ away from the axis and 0.173λ from the common center 8. 
     Similarly the conductor grid arm 9b forms a quadrilateral having four sides with its furthest perimeter or periphery corner 13b along its respective axis 10b at a distance of 0.306λ from a common center 8 or interior corner. Thus, the two quadrilateral arms 9a and 9b along the same axis 10a-10b form a dipole of 0.612λ long. The second dipole in FIG. 1 is formed from quadrilateral arms 9c and 9d, and has identical dimensions to dipole 9a-9b except that it lies along axis 10c-10d. 
     The lower tier (FIG. 2) has identical coordinates to the upper tier with two exceptions. The lower tier is located in a plane closer to the ground plane (lower Z coordinate) as shown in FIG. 3. Also, the lower tier is fed by a transmission line, so there is a small gap 15 between the quadrilateral arms on the lower tier to permit feeding from a balanced transmission line, in the manner of a turnstile antenna. FIG. 2 shows these feed points. One dipole on the lower tier formed by quadrilateral grids 11c and 11d is fed between points a and a&#39;, and the second dipole formed by quadrilateral grids 11a and 11b on the lower tier is fed between points b and b&#39;; a-a&#39; is one balanced input and b--b&#39; is a second balanced input. To transmit circular polarization from the element the two balanced inputs are fed in quadrature phase. 
     The dipoles on the upper tier are not fed. Instead, all the 12 conductors converging at the center 8 are electrically connected together at the center 8 as shown in FIG. 1. This unconventional connection on the upper tier at the center of both dipoles has not previously been documented and is one of several novel features which differentiates this antenna element from existing designs. 
     With the quadrilateral arms shown, the top and bottom tiers are connected at 12 points on the periphery of the element by 12 vertical conductors 17a-17d located at each of the 12 perimeter corners (conductor junctions) on the periphery of the dipoles. Each quadrilateral arm is therefore connected at 3 points to the quadrilateral arm directly above or below it on the other tier. Each vertical conductor is 0.10λ long. 
     FIG. 3 shows a side view of one dipole on each tier and the vertical connections between them. The axis of the second dipole on each tier is orthogonal or out of the page. FIG. 3 also illustrates that the vertical connection between the tiers provides some features of a folded dipole, since the upper tier forms the folded portion of the folded dipole. A novel and unique feature of this element is that it combines the concept and operation of a crossed grid dipole with that of a folded dipole. 
     The antenna array element of the invention provides circular polarization and permits the minimum number of array elements for a scanning array while maintaining a relatively constant active element input impedance over a wide bandwidth approaching one octave. 
     FIG. 4 shows an equilateral triangular array lattice which has been shown to require the minimum number of array elements for grating lobe free scanning over a conical scan volume. See, Johnson and Jasik, Antenna Engineering Handbook, 1984, page 20-17, incorporated herein by reference. The array could be larger than the 7 elements shown in FIG. 4. 
     As shown in FIGS. 4a and 4b, the array is a repeating pattern of rows of antenna elements in an equilateral triangular lattice. The centers of the elements are separated in the X direction by 0.7698λ, where λ is the wavelength at the reference frequency. In the Y direction, the centers of the antenna elements are separated by 0.66λ. In alternating rows the elements are shifted in the x direction by 0.3846λ. Thus, if element 25 is centered at a particular location, elements 26-29 are centered at a location defined by 0.3846λ away in the X direction and 0.66λ away in the Y direction, since they are in rows adjacent to the row containing element 25. Elements 30 and 31 are located 0.7698λ away in the X direction and at the same coordinate as element 25 in the Y direction. 
     FIGS. 5 through 12 illustrates one of the main advantages provided by this array element, which is that the driving input impedance stays relatively constant over a wide bandwidth even in the scanned array environment. FIGS. 5 through 12 are Smith chart plots of input impedances (normalized to 350 Ohms) for each of the two orthogonal dipoles comprising the center element of the 7 element array. This impedance is taken at the antenna input to each individual dipole, as shown by the point marked &#34;Zin&#34; in FIG. 3. No matching components are used to obtain the impedances plotted. 
     The impedances plotted in FIGS. 5 through 12 are the impedance seen for the center dipoles with all 14 dipoles excited (both dipoles in all 7 elements) for circular polarization and for scan to some beam angle. This is sometimes known as the &#34;active input impedance&#34;. Since the relative phase of each element is different for different beam scan angles, the input impedance of the center dipoles is also different for each beam scan angle due to mutual coupling effects. 
     Scan angles are measured from the Z axis, so zero degree scan is when the array beam is formed in the direction normal to the plane of the array (beam in the Z axis direction). This is also known as &#34;broadside&#34; scan. 
     Each dipole of the pair of dipoles in an element sees a different array environment. This can be seen in FIGS. 4a and 4b. The center dipole along the X axis sees adjacent elements at 0, 60, 120, 180 degrees relative to its axis. The center dipole oriented along the Y axis sees the adjacent elements at 30, 90, and 150 degrees off its axis. Due to these different locations of the surrounding elements relative to the two center dipoles, the two center dipoles have somewhat different active input impedances. Table I correlates the plots in FIGS. 5 through 12 to center dipole. The angle Phi listed in Table I is measured as shown in FIG. 4. Phi is used to identify which of the two center dipoles is being plotted, and also identifies the plane of scan for the given figure. 
     
                       TABLE I______________________________________Identification of FIGS. 5-12*.Figure # Dipole orientation (Phi)                      Plane of scan (Phi)______________________________________5        0                 0       or   1806        0                 45      or  -1357        0                 90      or   -908        0                 135     or   -459        90                0       or   18010       90                45      or  -13511       90                90      or   -9012       90                135     or   -45______________________________________ *The direction of the vector from the origin through the point in the XY plane defined by angle Phi identifies dipoles and planes of scan. Phi is in degrees from X axis as shown in FIG. 4 
    
     Each figure shows scan results from broadside to 30 degrees scan off broadside at each frequency. The legend identifies the frequencies plotted, where λ is the reference frequency defined above. The reference frequency is the highest frequency plotted. The lowest frequency plotted is 0.59 of the reference frequency. The other three frequencies are intermediate frequencies. The broadside scan point is marked on FIGS. 5 and 9 by the solid line connecting the broadside points across the frequency band. As shown, as the frequency is swept across the band from low to high frequency the broadside impedance follows a clockwise rotation about the center of the chart. FIG. 5 is for the dipole parallel to the X axis and FIG. 9 is for the dipole parallel to the Y axis. FIGS. 6, 7, and 8 have the same broadside impedance as FIG. 5 since they are for the same dipole, and FIGS. 10, 11, and 12 have the same broadside points as FIG. 9. The other points (scanned impedance points) differ on each figure, since the planes of scan are different. The farthest point from broadside at each frequency is the 30 degree scan point. 
     The results shown in FIGS. 5 through 12 were obtained from an accurate computer model using the Lawrence Livermore NEC-2 Method of Moments computer code. The NEC-2 computer code is widely used to computer model electromagnetic phenomena including a wide variety of antenna types, and it has been extensively verified as accurate for structures comprised of wires surrounded by air. The NEC-2 model includes mutual coupling effects between the array elements. 
     The antenna according to the invention is a synthesis of features found in three known antennas, with some novel and unique features added which are not found in any known antenna. The three precursors are: the crossed dipole (or turnstile antenna), the folded dipole, and the wire biconical antenna. These three known antennas are described in antenna texts and handbooks as discussed above. These precursors to the present invention are used for a variety of communication and radar applications, both singly and in an array. The instant invention combines some features similar to those of the above three precursors, with the novel and unique feature that the top (folded) arms of the pair of dipoles are joined together electrically at the center. The design is also simplified for ease of mechanical construction to the extent possible by placing the conductors in two planar tiers. The resulting element is unique and provides greatly improved bandwidth in the array environment. 
     The computer modeling has shown that an array of these elements permits the use of the minimum number of elements for grating lobe free operation over a conical scan volume while also maintaining an unusually wide impedance bandwidth. 
     While specific embodiments of the invention have been described and illustrated, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims.