Abstract:
A folded crossed grid dipole antenna element includes one or more partially or fully conductive areas in the antenna element. The conductive areas are defined by a group of conductors of the antenna element. In a further configuration, upper and lower tiers of a conventional antenna element are essentially switched and a transmission feed line is provided to the upper tier via an opening in the lower tier.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to antenna elements. More specifically, the invention relates to antenna elements that provide 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. 
     BACKGROUND 
     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. Biconical antennas have broadband characteristics that are useful in the VHF and UHF frequency ranges, but the size of the solid shell biconical structure limits many practical applications. As a compromise, multi-element intersecting wire bow tie antennas have been employed to approximate biconical antennas. 
     U.S. Pat. No. 5,293,176, the contents of which are incorporated herein by reference, discloses a folded cross grid dipole antenna element that attempts to combine the desirable features of biconical, crossed dipole and folded dipole antenna elements. It is an object of the invention to provide improvements to the folded crossed grid dipole antenna element disclosed in U.S. Pat. No. 5,293,176. 
     SUMMARY OF THE INVENTION 
     The invention provides improvements in folded cross grid dipole antenna elements by incorporating one or more partially or fully conductive areas in the antenna element. A further improvement is provided by essentially reversing the upper and lower tiers of the conventional antenna element and providing a transmission feed line to the upper tier via an opening in the lower tier. 
     More specifically, an antenna element is provided that includes a ground plane, a first crossed grid dipole including a plurality of perimeter conductors, where the first crossed grid dipole is arranged in an X-Y plane corresponding to a first tier and includes an interconnected plurality of arms, and a second crossed grid dipole including a plurality of perimeter conductors, where the second crossed grid dipole is arranged in an X-Y plane corresponding to a second tier and includes a plurality of non-interconnected arms. Each of the non-interconnected arms of the second crossed grid dipole includes a feed input. The first tier is vertically separated from the second tier and the ground plane, and the second tier is located between the ground plane and the first tier. The first crossed grid dipole is connected to the second crossed grid dipole by vertical conductors. An area defined by at least one of a group of the perimeter conductors of the first crossed grid dipole, a group of the perimeter conductors of the second crossed grid dipole, and a group of the perimeter conductors of the first and second crossed gird dipoles and the vertical conductors, is at least partially conductive. 
     Still further, an antenna element is provided that includes a ground plane, a first crossed grid dipole including a plurality of perimeter conductors, where the first crossed grid dipole is arranged in an X-Y plane corresponding to a first tier and includes a plurality of non-interconnected arms, and where each of the non-interconnected arms of the first crossed grid dipole includes a feed input, and a second crossed grid dipole includes a plurality of perimeter conductors, where second crossed grid dipole is arranged in an X-Y plane corresponding to a second tier and including a plurality of interconnected arms. The first tier is vertically separated from the second tier and the ground plane, and the second tier is located between the ground plane and the first tier. The first crossed grid dipole is connected to the second crossed grid dipole by vertical conductors. Center corners of the arms of the second tier are interconnected to permit a transmission feed line to pass through the second tier and to the feed inputs of the arms of the first crossed grid dipole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail with reference the accompanying drawings, wherein: 
     FIG. 1 is a top view of a first tier of an antenna element in accordance with a first embodiment of the invention; 
     FIG. 2 is a top view of a second tier of an antenna element in accordance with a first embodiment of the invention; 
     FIG. 3 is a side view of the first and second tiers illustrated in FIGS. 1 and 2; 
     FIG. 4 is a top view of a first tier of an antenna element in accordance with a second embodiment of the invention; 
     FIG. 5 is a top view of a second tier of an antenna element in accordance with a second embodiment of the invention; and 
     FIG. 6 is a side view of the first and second tiers illustrated in FIGS. 4 and 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1-3, an antenna element 1 in accordance with a first embodiment of the invention, and generally of the type disclosed in U.S. Pat. No. 5,293,176, is shown as including two tiers comprising two crossed grid dipoles. Each crossed grid dipole lies upon a planar surface. The two tiers are separated vertically and lie one above the other, parallel to each other, and parallel to a ground plane. The first or upper tier is uppermost. The second or lower tier lies between the first tier and the ground plane. The tiers are separated by air or a non-conducting dielectric material. Each line illustrated in FIGS. 1 and 2 represents the location of a conductor, such as a wire or other conductive element, in the upper and lower tiers. FIG. 3 illustrates a side view of the two tier construction of the antenna element 1. 
     Each crossed dipole includes two grid dipoles. Each grid dipole includes 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. 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. 
     In the first or upper tier the conductor grid arm 9a forms a quadrilateral having four sides with its furthers perimeter or periphery corner 13a along its respective axis 10a at a distance 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 away from the axis and 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 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. 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 illustrated in FIG. 2 has identical coordinates to the upper tier with two exceptions. First, the lower tier is located in a plane closer to the ground plane (lower Z coordinate) as shown in FIG. 3. Secondly, 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 illustrates 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. The two balanced inputs are fed in quadrature phase to transmit circular polarization from the antenna element. The dipoles on the upper tier are not fed. Instead, all twelve of the conductors converging at the center 8 are electrically connected together at the center 8 as shown in FIG. 1. 
     With the quadrilateral arms shown, the top and bottom tiers are preferably connected at twelve points on the periphery of the antenna element 1 by twelve 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 three points to the quadrilateral arm directly above or below it on the other tier. 
     FIG. 3 illustrates 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. Further details of the basic construction, including preferred element lengths, can be found in U.S. Pat. No. 5,293,176. 
     The first embodiment of an improvement to the basic structure of the antenna element 1 includes filling in, either partially or completely, some or all of the area or areas defined by a group or groups of the perimeter conductors described above with conductive material. For example, as illustrated in FIG. 1, at least one area between a group of the perimeter conductor elements on the upper tier is either partially or completely metallized. In addition, as illustrated in FIG. 2, at least one area between a group of the perimeter conductors on the lower tier can be either partially or completely filled with a conductor. Still further, as indicated in FIG. 3, at least one area between a group of the conductor elements of the upper and lower tiers and the vertical conductors at an outward facing end of the antenna element is either partially or completely filled with a conductive material. Any combination of partially or completely filled areas between the upper and lower tiers and sides is possible. Further, any desired pattern of conductive material can be utilized in the partially conductive areas. 
     In a second embodiment of an improvement to the basic structure of the antenna element, the upper and lower tiers of FIGS. 1 and 2 are reversed. In the second embodiment illustrated in FIGS. 4-6, the upper tier of FIG. 4 is the same as the lower tier of FIG. 2. The lower tier of FIG. 5 is substantially the same as the upper tier of FIG. 1, with the exception that the connection of the four arms at the center (for example a circular, square or polygonal connection) leaves an opening through which the transmission feed line can extend to the upper tier as shown in FIG. 6. The feeding of the antenna element from the upper tier instead of the bottom tier may be more economical in certain applications. 
     The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modifications and variations are possible within the scope of the appended claim. For example, the areas between conductors in the embodiment illustrated in FIGS. 4-6 can also be partially or complete filled with conductive material. Further, if an area is completely filled with a conductive material, for example a metallized layer, it will be understood that the edges of the conductive material can be defined as perimeter conductors for the purposes of this disclosure. Accordingly, a completely filled area is an area defined by a group of conductors for the purposes of interpreting the appended claims.