Patent Publication Number: US-8988303-B1

Title: Extended performance SATCOM-ORIAN antenna

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/446,138, filed on Feb. 24, 2011. The entire teachings of the above application(s) are incorporated herein by reference. 
    
    
     BACKGROUND 
     In certain applications of radio communications it is important to be able to robustly communicate without knowing the relative orientation of the transmit and receive antennas in advance. For example, in the case of communication from a satellite to a terrestrial vehicle, as the vehicle moves about the terrain (or even within a building), signals arrive at the antenna on the vehicle with a variety of different polarizations from different directions. If the vehicle uses, for example, a simple vertical dipole, one obtains 360° coverage but only for vertically polarized signals. Such a vertical dipole is relatively insensitive to horizontally polarized signals. 
     Many antennas mounted on vehicles also take the form of a mast that may be purposely flexible so that if the antenna hits an object it will bend and not snap or break. Antennas formed with flexible masts thus have their vertical and/or horizontal orientation direction altered by the flexibility of the mast, meaning that reliable communication cannot always be established if the polarization direction of the antenna is not exactly aligned with that of the transmitter. In short, it is often the case that as the vehicle moves throughout an environment its antenna may tilt at various angles and therefore compromise communications with either a terrestrial base station or a satellite. 
     It is known that an Orientation-Independent Antennas (ORIAN) can be formed from crossed vertical loops in combination with a horizontal loop. This arrangement may provide circular polarization in a hemisphere surrounding the antenna such that signals are robustly received regardless of their polarization or angle of arrival. The antenna can be a free standing antenna. 
     One such ORIAN antenna is in the form of a cube with the various loops implemented as triangular shaped antenna elements disposed on the surfaces of the cube. Such antennas are described in further detail in U.S. Pat. No. 7,852,276 by Apostolos, et al., entitled “Orientation-Independent Antenna (ORIAN)” issued Dec. 14, 2010, and U.S. Pat. No. 7,623,075 by Apostolos, et al. entitled “Ultra Compact UHF SATCOM Antenna” issued Nov. 24, 2009, the entire contents of each of which are hereby incorporated in their entirety. 
     SUMMARY 
     In one embodiment herein a compact orientation independent (ORIAN) antenna is provided having four triangular shaped elements positioned on or formed in at least one surface of a cube or other six-sided structure. The other surfaces of the cube may be formed of metal plates which may themselves have other types of triangular or other conductive elements. In a preferred arrangement, the triangular antenna elements are formed on a bottom face of the cube and fed at the intersection of the four triangles using a phasing network. The phasing network combines the four elements to provide Right Hand (and/or Left Hand), circularly polarized outputs as well as a vertically polarized (V-POL) output. 
     As a result, at low angles of arrival such as from satellites near the horizon, when the horizontal component of the circularly polarized wave is diminished in amplitude, then the vertically polarized line of sight mode is chosen. A switch may provided to select the mode that is best to use at periodic intervals. One mode or the other can therefore be determined by receiver circuits for example, that detect signal power in modes that operate the switch to select the mode that provides the better performance under current conditions. 
     The phasing network may itself take several different forms. In one implementation, this can be a pair of 180° combiners. The first pair of 180° combiners is coupled to a first selected pair of the triangular elements; and a second 180° combiner couples to the other pair of triangular elements. The plus or positive (in phase) outputs of the combiners are each fed to a summing network to provide the vertical polarization (V-POL) output. The negative or out of phase outputs of the 180° combiners are fed to a quadature combiner to provide the Right Hand (RH) and Left Hand (LH) circularly polarized (C-POL) outputs. 
     In yet another embodiment, the phasing network may take the form of a centrally located vertical coupling element and a ferrite core. In this embodiment, the centrally located vertical coupling element is disposed inside the center of the cube is located at an electric field null of the right hand (RH) and/or left hand (LH) feeds. cables are coupled to the triangular elements such that a first coaxial cable feeds one of the triangular elements at a center conductor and a shield of the coaxial cable feeds the opposite triangular element. A second coaxial cable is similarly fed from opposing triangular elements at respective center and shield conductors. The coaxial cables are wrapped around the ferrite core; the result is electrically equivalent to the pair of 180° combiners in the earlier described embodiment but at a much lower cost and compact size. The two coaxial cables then feed a quadrature combiner to provide the Right Hand and Left Hand circularly polarized feeds. 
     The vertically polarized signal is provided by the centrally located vertical coupling element which excites currents in the metal plates on the other side of the cubes. In this mode, the metal sides of the cube operate as parasitic elements to provide the vertically polarized signal. 
     In yet another arrangement suitable for vertical polarization cases only, low angle of arrival or line of sight communications can be optimized the providing all four quadrant beams simultaneously. In this arrangement, the triangular elements are again fed to a pair of 180° combiners. The sum and difference outputs of the 180° combiners are fed to a respective pair of 90° quadrature combiners. These then provide the four independent quadrant beams simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings where like reference characters refer to the same parts throughout different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating example embodiments. 
         FIG. 1  illustrates a general structure of a cube having four triangular shaped antenna elements on a bottom surface thereof, one metallic elements on four sides thereof, and optional triangular elements on a top surface thereof. 
         FIG. 2  illustrates a phasing network that may be used to combine the four bottom triangular elements to provide a Right Hand (RH) circularly polarized (V-POL) output and a vertically polarized (V-POL) output. 
         FIG. 3  is another implementation providing a similar function as the circuit of  FIG. 2  but with a centrally located vertical coupling element and ferrite core. 
         FIG. 4A  and  FIG. 4B  are an electrical diagram corresponding to the embodiment of  FIG. 3  showing the connection of the triangular elements to the ferrite and how the vertically polarized output is provided to the centrally located coupling element. 
         FIG. 5  is a cross sectional view of the implementation of  FIG. 3  showing the location of the centrally located vertical coupling element ferrite and quadrature combiner in more detail. 
         FIG. 6  is a general diagram illustrating how simultaneous quadrant line of sight (LOS) beams can be provided with the same antenna structure. 
         FIG. 7  is a simultaneous quadrant implementation using a ferrite and quadrature combiners. 
     
    
    
     DETAILED DESCRIPTION 
     The general structure of a compact ORIAN antenna (Suitable for Satellite Communications) is shown in  FIG. 1 . Four triangular elements  102  are positioned on or formed in at least the bottom surface  101  of a six-sided structure which may be a cube  100 . In a preferred embodiment, the triangular antennas are fed at the intersection of the four triangular elements as will be described in more detail below. 
     More specifically, the four triangular elements may be considered to have (for the sake of identification only) an east  102 -E, south  102 -S, west  102 -W and north  102 -N position on the bottom face of cube  100 . These elements are formed from a conductive material on a dielectric substrate. The four corresponding vertical faces of cube  100  (faces  104 -E,  104 -S,  104 -W,  104 -N) are also formed of conductive material on a substrate. The substrates may physically isolate the conductive surfaces on the six sides from one another such that a dielectric gap is formed between and along the corners and the edges of the cube  100 . In specific arrangements discussed, herein the top face of the cube  106  may also have additional parasitic triangular elements  108  formed thereon. A pair of the elements  102 -E and  102 -W are disposed opposite and orthogonal to each other, and a second pair  102 -N,  102 -S are similarly opposite and orthogonal to each other. 
     It should be understood that reference to the letters (“E”), (“S”), (“W”), and (“N”) herein are meant herein to merely identify particular surfaces and/or antenna elements and are not meant to imply that the cube must be oriented with respect to the terrain in any particular way; in fact, it is a fundamental aspect that the cube  100  provides an orientation independent (ORIAN) antenna. It should be understood that the term “cube” is used herein to generally refer to structures with six faces, and that all faces of cube  100  need not be exactly the same size. 
     Corresponding feed points  110  are associated with each of the triangular elements  102 . These feed points are preferably located adjacent a point in the middle of the cube bottom surface  101  of cube  100  where the triangular elements come together. 
       FIG. 2  is one example of a phasing network combining the  102 -E,  102 -S,  102 -W, and  102 -N elements to provide both a right hand circularly polarized (RH, C-POL) output and a vertically polarized (V-POL) output. As shown in  FIG. 2 , a first pair of triangular elements such as  102 -W and  102 -E are coupled to a first 180° combiner  202 - 1 ; the other pair of triangular elements,  102 -S and  102 -N, are coupled to a second 180° combiner  202 - 2 . The plus (or sum) outputs of each of the 180° combiners  202  are connected to a summing network  204  to provide the vertically polarized (V-POL) output. The minus or difference outputs of the 180° combiners  202  are connected to a quadrature combiner  206 . The outputs of the quadrature combiner  206  then provides respective Right Hand (RH) and Left Hand (LH) circularly polarized (C-POL) feed points. As illustrated, it may be the case that for example, only a Right Hand (RH) polarized feed is of interest and thus only it is fed to switch  210 . The Left Hand (LH) output is thus fed to a dummy load in this instance. 
     The switch  210  thus allows selection of the VPOL or RH-CPOL mode depending upon the desired mode of operation. At low angles of arrival (AOA), such as from satellites located near the horizon, the horizontal component of the RH C-POL mode is diminished in amplitude due to earth losses. Better signal to noise ratio (SNR) may be possible if all the power is switched to select the vertically polarized (V-POL) line of sight (LOS) mode in this condition. However, for overhead satellites, the RH C-POL mode is preferred. 
     Decision logic can be used to pick the best mode and set the switch  210  at periodic intervals. One mode or the other can be determined by another circuit (not shown) that detects receiver/signal power in each of the two modes, and then operates the switch  210  to select the mode that provides the better performance. 
       FIG. 3  is another implementation that achieves the same results, that is the ability to provide both vertical polarization (V-POL) and right hand circular polarization (RH-C-POL) outputs using the same six-sided structure  100 . This implementation uses a centrally located vertical coupling element  300 , ferrite core  310  and ground plane  202 . The view of  FIG. 3  is of the cube  100  with one of the faces (that is, the front face  102 -S) removed so that the interior of the cube  100  can be seen in more detail. The bottom face  101  of the cube  100  is arranged as in  FIG. 1  with a set of four triangular elements. The metal sides  104  are also provided as in the previous explained embodiment of  FIG. 1 . 
     However, in the implementation of  FIG. 3  there is provided a centrally located vertical coupling element  300  which may take the form of a tube having conductive surface. The vertical coupling element  300  is disposed in the center of the cube  100 . This location, in center of the cube  100 , is at or near an electric field null of the circularly polarized triangular elements  102 . The result is that operation of the centrally located vertical coupling element  300  does not effect operation of the right hand/left hand circular polarization modes. 
     The center fed vertical coupling element  300  excites currents in the metal sides  104  to produce the V-POL pattern. This excitation is therefore parasitic; in other words, the vertical coupling element  300  acts to excite currents on all four sides  104  of the cube  100  being physically connected to the sides  104 . 
     In specific embodiments herein, patches  312  may provide an impedance between the each of the sides  104  and a respective one of the adjacent sides  104 . The impedances  312  may include matching and/or balancing impedances, such as through various combinations of capacitive and/or inductive elements. Selection of impedances can result in improving match across a particular radio frequency band of interest and relative immunity of input impedances from proximity to the vehicle platform. 
     As further illustrated herein, if the top surface  106  is also provided with triangular elements  108 , additional conductive patches  312  provide conductivity between each top triangular element and its respective adjacent one of the sides. In this way, the top triangular elements also become part of the structure parasitically fed by the centrally located vertical coupling element  300 . 
       FIG. 4A  is an electrical diagram showing the connection of elements corresponding to the structure of  FIG. 3 . The top part of the figure is a schematic view of the cube  100  looking from above, showing the centrally located vertical coupling element  300  and the four triangular elements  102 -E,  102 -S,  102 -W and  102 -N formed on the bottom surface thereof, as well as ground plane  302 . An opposite pair of the triangular elements, such as elements  102 -E and  102 -W, are coupled to a first coaxial cable  401 - 1 . One of these elements  102 -E is coupled to the center conductor of coaxial cable  401 - 1  and the other element  102 -W is coupled to the shield of coaxial cable  401 - 1 . A second coaxial cable  401 - 2  feeds the other two opposing triangular elements  102 -N,  102 -S. One of the elements  102 -N is coupled to a center conductor of cable  401 - 2  with the other element  102 -S coupled to the shield. 
     The coaxial cables  401 - 1  and  401 - 2  are each wrapped around a ferrite core  310  as shown. Typically, only a few windings are required around the ferrite  310 . The center conductor of coaxial cables  401 - 1  and  401 - 2  are then fed to respective inputs of a hybrid 90° combiner  306 , and the respective shields are grounded nearby or on combiner  306 . The hybrid 90° combiner  306  provides the Right Hand (RH) and Left Hand (LH) circularly polarized (C-POL) feeds. 
     The vertical polarization feed is directly from the centrally located vertical coupling element  300 . The detail shown in  FIG. 4B  illustrates the element  300  embodied as a metallic surface tube, with for example, the center conductor of the cable  410  being coupled to the metal surface of the tube and the shield of cable  410  being coupled to the ground plane  302 . The cable  410  then directly provides the vertical polarized (V-POL) feed. In preferred embodiments, this coaxial cable  410  can be routed straight through the center of the ferrite  310  core or in other ways. 
       FIG. 5  is a cross sectional view of the embodiments of  FIGS. 3 ,  4 A and  4 B taken in a plane through the center thereof. A cover or radome  500  protects the cube  100  from the elements is shown. The centrally located vertical coupling element  300  is shown in orientation with respect to the east face  104 -E and west face  104 -W of the cube  100 . The ferrite  310  may be mechanically supported beneath the bottom surface  101  of the cube, and a stand-off may also support the hybrid combiner  106  in a convenient location. The Right Hand Circularly Polarized (RH C-POL)  502  and V-POL  504  feeds are via BNC connectors. Other stand offs  510  may support the cube  100  above ground plane  302 . 
     The phasing network of  FIG. 6  can be used with the same cube  100  to simultaneously generate four quadrant beams. This arrangement provides 4-5 dB more gain than the omni-directional Line of Sight (LOS) (V-POL) mode. 
     This arrangement of combiners provides simultaneous reception of all four directions. The opposite triangular elements such as  102 -N,  102 -S are coupled to respective ports of the first 180° combiner  610 - 1 . A second 180° combiner  610 - 2  feeds the other two opposing triangular elements,  102 -E,  102 -W. The sum (or positive) port of 180° combiner  610 - 2  and negative (or difference) port of 180° combiner  610 - 1  are coupled to respective inputs of quadrant combiner  620 - 1 . Quadrature combiner  620 - 1  thus provides a respective west oriented beam  620 -W and an east oriented beam  620 -E. Similarly, quadrature combiner  620 - 2  is fed from the sum port of 180° combiner  610 - 1  and difference port of 180° combiner  610 - 2  to provide north facing beam  630 -N and south facing beam  630 -S. The respective beams can be fed, for example, to and from a transceiver or simultaneous and/or combined in various ways to provide orientation independent operation. 
       FIG. 7  illustrates how four simultaneous beams can be generated using a ferrite  310  and coaxial cables  401 - 1 ,  401 - 2 . The connection between the antenna elements  102 , center vertical element  300  and ferrite  310  are similar to that of  FIG. 4A , but here the center conductors of coaxial cables  401 - 1 ,  401 - 2  are each fed to a respective 90° combiner  601 - 1 ,  601 - 2 . The centrally located vertical element  300  couples to a splitter  603  to provide the other input to each combiner  601 - 1 ,  601 - 2 . The combiners provide the W, E and N, S beams independently. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.