Abstract:
An antenna comprising a pair of bent monopole elements (a doublet) that are fed in a manner that results in elevation coverage from the horizon to horizon and dual polarization. Two orthogonal bent monopole doublets provide hemispherical coverage with horizontal and vertical polarization. Combining the doublet terminals through a processing circuit will provide polarization diversity and/or angle diversity capability.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to an antenna and, more particularly, to an antenna for transmitting and receiving electromagnetic radiation signals to and from fixed or mobile communication platforms. 
     BACKGROUND OF THE INVENTION 
     The signal fading problems associated with fixed and mobile communications platforms in a multipath environment have been and continue to be studied to determine antenna and data processing designs that solve the problems in a cost-effective manner. From an antenna standpoint, previous designs have included the use of adaptive arrays and space diversity antennas. In recent years, studies have shown that frequency diversity techniques that utilize antennas with orthogonal polarization ports result in performance at least comparable to systems using space diversity. 
     The angular coverage desired from communications antennas, other than fixed point-to-point systems, is very large, typically equal to or approaching instantaneous hemispherical coverage. Earlier antenna designs that best achieved hemispherical coverage utilize a turnstile antenna plus a monopole antenna switched to achieve high or low angle coverage. The height of these designs range from about 0.4 to 0.5 wavelengths at the system&#39;s operating frequency. The Rodal design is a modified version of the turnstile antenna that uses curved dipole elements and provides nearly hemispherical coverage without switching. See Rodal et al., U.S. Pat. No. 5,173,715. Rodal et al. is incorporated herein by this reference. The Rodal design is still too large for many applications with heights greater than or equal to one quarter wavelength and is a single port single polarization design. 
     Recently Altshuler described a simpler, non-switching design that provides hemispherical coverage. This however is not a low profile design and does not provide dual polarization outputs. See Edward E. Altshuler. Derek S. Linden, “Design of a Vehicular Antenna for GPS/Iridium Using a Genetic Algorithm.” 
     Diversity antenna designs using crossed loop conductors have been used to combat multipath interference. See Lee, U.S. Pat. No. 4,611,212, and Johnston, et al., U.S. Pat. No. 5,784,032. Lee and Johnston et al. are incorporated herein by this reference. Both designs are narrow band designs. The Lee design is a receive antenna design. The Johnston design requires impedance matching with reactive components and does not offer the possibility of combining antenna signals to generate instantaneous hemispherical coverage. 
     What is needed is a low profile transmit and receive antenna design that provides (a) circular polarized hemispherical coverage using a single port output, and/or (b) orthogonal linear or circular polarized coverage using a two port output. A simple antenna design that can have an operating bandwidth &gt;25% and that provides one or both of these modes of operation would be an improvement over the present state of the art. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a novel, inexpensive and highly effective low-profile antenna that is useful in both heavy multipath and minimal multipath environments. It is a further object of this invention to provide an improved, low-profile circular polarized antenna that has instantaneous coverage over a hemisphere of solid angle. 
     It is a further object of this invention to provide an improved, low-profile circular polarized antenna that has essentially uniform gain over a hemisphere of solid angle. 
     It is a further object of this invention to provide an improved, low-profile antenna that can generate a scannable, directive dual linear polarized pattern with coverage down to the horizon with scannable or switchable peaks and nulls in the azimuth plane. 
     It is a further object of this invention to provide an improved, low-profile two port antenna that generates dual linear polarized hemispherical coverage. 
     It is a further object of this invention to provide an improved, low-profile antenna with typical design dimensions of between 0.05 to 0.15 wavelengths in height by less than or equal to one-half wavelength in diameter at the desired operating frequency. 
     This invention results from the realization that pairs of appropriately shaped bent monopole elements that are properly oriented and properly fed form a bent monopole doublet that will provide horizon to zenith to horizon coverage. When two of these element pairs are orthogonally located and fed in phase quadrature, the result is a circular polarized antenna with hemispherical coverage. Moreover, the gain over the hemisphere can be tailored to have higher gain at low or high angles or to have uniform gain over the entire hemisphere. The two orthogonal bent monopole doublets formed provide orthogonal polarized and orthogonal angular patterns that can be processed for polarization diversity or angle diversity to mitigate multipath. If the bent monopoles are designed to be self-resonant the need for frequency bandwidth limiting reactive tuning is eliminated. 
     This invention most basically features an antenna comprising a ground plane having a first surface; a first pair of spaced antenna elements extending from the first surface of the ground plane; and a second pair of spaced antenna elements orthogonal to the first pair of elements and extending from the first surface of the ground plane, such that the centerpoint between each pair of antenna elements is identical. The antenna elements are preferably designed to be self-resonant. This is readily achieved by selecting the appropriate element length and geometry. Where reduced size is of greater importance than bandwidth smaller, non-resonant elements may be used with reactive tuning elements added to achieve good impedance match. At least one antenna element extends from the first surface of the ground plane and bends towards the antenna centerline that extends along an axis normal to the ground plane such that a vector representative of the element has both horizontal and vertical components as viewed against the ground plane. 
     The bent element, in some implementations, can be described as an asymmetric top loaded monopole with the greatest amount of top loading directed towards the antenna centerline that extends along an axis normal to the ground plane. 
     The first pair of antenna elements comprise first and second antenna elements, having first and second feed points respectively. The second pair of elements comprises third and fourth antenna elements having third and fourth feed points respectively. The feed points supply electrical signals to and receive electrical signals from the antenna elements. The feed points for each antenna element pair are a distance of up to and including approximately one-half wavelength apart at an operating frequency of the antenna. There may be at least one splitter/combiner having at least two input terminals and at least one output terminal. The input terminals are electrically coupled to separate antenna feed points, and the splitter/combiner is used for splitting and combining electrical signals when transmitting and receiving signals respectively. 
     The output ports can be combined using passive or active circuitry to achieve the desired coverage and polarization diversity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as other features and advantages thereof, will be best understood by reference to the description which follows, read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic showing an arrangement of antenna elements according to one aspect of the invention; 
     FIG. 2 is another schematic illustrating an arrangement of one pair of antenna elements according to one aspect of the invention; 
     FIG. 3 is another schematic showing an arrangement of bent antenna elements according to one aspect of the invention; 
     FIG. 4 is another schematic showing an isotropic radiator arrangement of antenna elements according to one aspect of the invention; 
     FIG. 5 is a plan view showing two possible arrangements of the antenna elements and microstrip feed lines according to one aspect of the invention; 
     FIG. 6 is a schematic showing an embodiment of signal processors according to one aspect of the invention; 
     FIG. 7 is a schematic showing another embodiment of signal processors according to one aspect of the invention; 
     FIG. 8 is a schematic depicting an embodiment of the element pair and microstrip feed lines according to one aspect of the invention; 
     FIG. 9 is a schematic showing the azimuthal patterns generated by one embodiment of the present invention with an infinite ground plane; and 
     FIG. 10 is a schematic illustrating the azimuth and elevation patterns of another embodiment of the present invention with an infinite ground plane. 
     FIG. 11 is a schematic illustrating the elevation patterns of one embodiment of the present invention with a small ground plane. 
     FIG. 12 is a schematic showing a configuration of the invention with four (4) output ports that are selectable via a 4PST switch. 
     FIG. 13 illustrates the port  3  and port  4  azimuth patterns of the FIG. 12 configuration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There is shown in FIG. 1 a schematic showing an embodiment of antenna  10  comprising ground plane  12 , which has first surface  14 . Antenna  10  also comprises first pair  16  of spaced, antenna elements  18 , which extend from first surface  14  of ground plane  12 . In this embodiment each element in the first pair  16  of antenna elements  18  is resonant. Additionally, second pair of spaced, self-resonant antenna elements  18  extend from first surface  14  of ground plane  12 . Second pair  20  of spaced antenna elements  18  are orthogonal to first pair  16  of elements  18 . 
     As shown in FIG. 1, antenna  10  has an identical centerpoint  22  between pairs  16  and  20  of antenna elements  18 . This identical centerpoint  22  allows both pairs  16  and  20  of elements  18  to have common phase centers. 
     In this embodiment, antenna  10  has all antenna elements  18  as L-shaped, or asymmetrically top-loaded monopoles. FIG. 2 provides a better view of top loaded section  24  of antenna element  18 . The cross-section of top loading section  24  may be rectangular in shape, but other shapes such as triangles, cylinders and cones, as well as other shapes known in the art, are contemplated by this invention. 
     Antenna  10  has the greatest amount of top loading on each asymmetrically top-loaded antenna element  18  directed towards antenna centerline  26 , which extends along an axis normal to ground plane  12 . A printed circuit board fabrication may also be used in the implementation of antenna element  18 . 
     As shown in FIGS. 1 and 2, a preferred embodiment of antenna  10  comprises separate feed points  58  for supplying electrical signals to, and receiving electrical signals from, antenna elements  18 . First pair  16  of antenna elements  18  comprise first antenna element  38  and second antenna element  40 , which have first feed point  42  and second feed point  44 , respectively. 
     Second pair  20  of elements  18  comprise third antenna element  46  and fourth antenna element  48 , which have third feed point  50  and fourth feed point  52 , respectively. Antenna feed points  58  may pass through vias  59  in ground plane  12 , but can also remain above first surface  14  of ground plane  12 . Antenna feed points  58  receive and transmit electrical signals along electrical coupling  68 . Electrical coupling  68  may comprise microstrip transmission line, coaxial cable, waveguide or other signal transmission devices known to those skilled in the art. 
     As partially depicted in FIG. 2, one embodiment of the present invention uses feed points  58  for each antenna element pair  16  that are distance  54  apart. Distance  54  equals up to and includes distances of approximately one-half signal wavelength at a predetermined operating frequency of antenna  10 . 
     Antenna  10  further comprises splitter/combiners  69  having at least two input terminals and at least one output terminal, the input terminals electrically coupled to separate antenna feed points through electrical coupling  68 . Splitter/combiners split and combine electrical signals when transmitting and receiving signals respectively. In the embodiment represented in FIG. 1, splitter/combiners  69  are “T” splitter/combiners. In this embodiment, the length of electrical coupling  68  between separate antenna feed points  58  and the splitter/combiner input terminal differs by approximately one-half wavelength at an operating frequency of the antenna. The output of splitter/combiners  69  connect to two main ports  71  of the antenna. 
     FIG. 3 illustrates another embodiment of the invention. In this embodiment, the antenna elements  18  extend from first surface  14  of ground plane  12  and bend toward antenna centerline  26 . Centerline  26  extends along an axis normal to ground plane  12 . Element  18  bends such that a vector representative of the element has at least some horizontal components viewed against ground plane  12 . 
     As shown in FIG. 3, one preferred embodiment of the invention comprises four antenna elements  18  that extend from first surface  14  of ground plane  12  and bend toward antenna centerline  26 . This design is in contrast to the design in FIGS. 1 and 2 where the bends in antenna elements  18  are 90 degree bends. 
     FIGS. 4 a  and b show an alternative embodiment of the present invention. As FIG. 4 a  illustrates, antenna  10  has elements  18  arranged about an imaginary ground plane  13  with first surface  15  and second surface  33 . Antenna  10  comprises a first pair  16  of spaced antenna elements  18  extending from first surface  15  of imaginary ground plane  13 . Antenna  10  further comprises second pair  20  of spaced, self-resonant antenna elements  18  orthogonal to first pair  16  of elements  18 , which also extending from first surface  15  of imaginary ground plane  13 . These two pairs of elements are arranged such that the centerpoint between the first and second pairs of antenna elements is identical. Beyond these components, antenna  10  further comprises third pair  34  of spaced antenna elements  18  extending from the second surface  33  of imaginary ground plane  13  and in line with second pair  20  of spaced antenna elements. Finally, antenna  10  comprises fourth pair  36  of spaced, self-resonant antenna elements  18 , which are orthogonal to third pair  34  of elements  18  extending from the second surface  33  of imaginary ground plane  13  and in line with first pair  16  of spaced antenna elements, such that the centerpoint between the third  34  and fourth pair  36  of antenna elements  18  is identical. Whereas the antenna feed points  58  in FIG. 3 are coupled to a splitter/combiner  69  through unbalanced transmission line such as coaxial cable or microstrip, in this embodiment the feed points  58  are connected to a splitter/combiner  69  via balanced transmission lines as illustrated in FIG. 4 b . The design can now be described as consisting of asymmetrically top loaded dipoles or U shaped dipoles. The physical presence of the balanced transmission line feed  68  and splitter/combiner  69  does not effect the radiation pattern as long as the are contained and lie in the plane of the U shaped dipoles and the output line runs along the vertical centerline of the antenna. This embodiment simultaneously generates hemispherical patterns away from first surface  15  and second surface  33  of imaginary ground plane  13 , resulting in complete isotropic coverage. 
     FIGS. 5 a  and b show two views of contemplated embodiments of the present invention with respect to microstrip transmission line. As shown in previous drawings, electrical coupling  68  may be used to transfer electrical signals to and from antenna feed points  58 . In one embodiment of the present invention, antenna  10  includes at least one microstrip transmission line  56  used as electrical coupling  68 , which is electrically coupled to at least one element feed point  58 . 
     FIG. 5 a  shows an embodiment wherein microstrip transmission line  56  is mounted on the opposite side of ground plane  12  from antenna element  18 . In this embodiment, antenna element  18  is positioned above first surface  14  of ground plane  12 . Microstrip transmission line  56  is mounted on dielectric substrate  60 , which is, in turn, mounted to second surface  32  of ground plane  12 . Antenna feed points  58  pass through vias  59  in ground plane  12 . 
     FIG. 5 b  shows another embodiment of the current invention wherein microstrip transmission line  56  and antenna elements  18  are both located on the same side of ground plane  12 . In this embodiment, microstrip transmission line  56  is coupled to dielectric substrate  60 , which is in turn coupled to first surface  14  of ground plane  12 . Also shown in FIG. 5 b  is low density dielectric spacer  61  mounted between microstrip transmission line  56  and antenna element  18 . Low density dielectric spacer  61  may be printed circuit board substrate if a printed circuit board fabrication is used as antenna element  18 . This low density dielectric spacer may also be used in the configuration in FIG. 5 a.    
     In the embodiment in FIG. 6, splitter/combiners  69  split or combine electrical signals when antenna  10  is transmitting or receiving signals respectively. In accordance with this embodiment, antenna  10  comprises at least one splitter/combiner  69  having at least two input terminals  64  and at least one output port  71 . Input terminals  64  of splitter/combiner  69  are electrically coupled to antenna feed points  58  through electrical coupling  68 . The splitter/combiners  69  and electrical coupling  68  are designed to produce a nominal 180 degree phase difference to, or from, signals at feed points  58 . This can be accomplished in a number of ways including a “T” splitter-combiner and differential transmission line lengths, a 180 degree splitter/combiner, a balun, or other means known to those skilled in the art. 
     The electrical signals from output ports  71  can be combined in many different ways well known in the art. They can be combined using a 90 degree combiner to produce circular polarized hemispherical coverage or through other processing circuitry or to achieve polarization and/or angle diversity patterns through the use of other processing circuitry known to those skilled in the art. 
     A preferred embodiment utilizes the 90 degree combiner as the signal processor  62 . The quadrature combination of the signals generates circular polarization with full hemisphere coverage using a single antenna connection  86  or alternatively, the electrical signals from output ports  71  can be combined through a four port ninety (90) degree hybrid combiner  90  to generate hemispherical coverage with left and right hand circular polarization from two outputs  86 . 
     FIG. 7 illustrates another embodiment of the signal processing system according to one aspect of the invention. In this embodiment, output ports  71  of first and second splitter/combiners  70  and  76  are electrically coupled to input terminals  84  of signal processor  62 . Within signal processor  62  are pre-amplifiers  88  and signal splitter/combiner  69 . The individual ports  71  produce an orthogonal figure “8” shaped pattern as illustrated in FIG.  9 . When the port outputs are combined, the figure “8” shaped pattern rotates in the azimuth plane by an amount determined by the preamplifier weighting. This configuration linearly combines terminal outputs  71  of antenna  10 . The resulting pattern provides discrimination against multipath signals at orthogonal angles to the beam peak direction for a given polarization. Assuming a linear incoming polarization, signals arriving from other directions and/or polarizations will see a lower antenna gain. If an unwanted signal has the same polarization as a desired signal, the unwanted signal&#39;s gain reduces as the angular separation of the signals increases and would be completely rejected at 90 degrees separation where there is a pattern null. Signals with different linear polarizations have decreased gain due to polarization mismatch and pattern nulling at specific angles. Multipath and interference mitigation is therefore achieved via two means: polarization matching and pattern nulling. 
     FIG. 8 illustrates a portion of another embodiment of the present invention that achieves a similar phase shift result as previously described using splitter/combiner  69  with phase shifting capabilities between antenna feed points  58 . This embodiment is one in which the length of the electrical coupling  68  between separate antenna feed points  58  and two splitter/combiner input terminals differ by approximately one-half wavelength at the operating frequency of the antenna. This electrical coupling length difference combined with a “T” splitter/combiner results in combined signals with the desired phase shift properties as produced by splitter/combiner  69  with phase shifting abilities. 
     FIG. 9 illustrates the azimuthal patterns of the present invention. Antenna  10  generates independent orthogonal figure-eight-shaped, vertically polarized (“VP”) E θ  patterns and horizontally polarized (“HP”) E φ  patterns. These patterns are similar to those generated by crossed dipoles over a ground plane, with the added benefit of E θ  coverage down to the horizon. The signals from the output terminals  71  can be quadrature combined to generate circular polarization with full hemisphere coverage using a single antenna connection or alternatively the signals from the output terminals  71  can be combined through a four port 90 degree hybrid combiner to generate hemispherical coverage with left and right hand circular polarization outputs. 
     FIG. 10 illustrates the azimuth and elevation patterns achieved when quadrature combining the element pair outputs  71 . The elements in this simulation are mounted on an infinite ground plane. The average power gain over the entire hemisphere in this case is 3 dBi with a maximum variation of +/−1.25 dB. 
     FIG. 11 illustrates the gain by elevation of one embodiment of the present invention with a one (1) wavelength diameter circular ground plane for a predetermined operating frequency. The traces on the plot represent HP gain, VP gain and total gain. As shown in FIG. 10, the net result of the antenna design using a ground plane with a one wavelength diameter is some coverage below the horizon and the filling in of the horizontal E φ  pattern null at the horizon. If the antenna is mounted on a very large ground plane, even greater uniformity in pattern is achieved as illustrated in FIG.  10 . 
     FIG. 12 illustrates a configuration of the invention wherein the crossed doublets output ports  1  and  2  provide orthogonal radiation patterns and orthogonal polarizations. The port signals can be processed with signal processor  62  using techniques well known and published in the technical literature. These processing techniques include switching between ports, or combining the output ports with equal or system defined weights and/or phases to obtain the benefits of polarization and/or angle diversity. The embodiment in FIG. 12 is an example of a configuration of the present invention that could be utilized and does not require special processing circuitry, such as weighting amplifiers. Within signal processor  62  may be SPDT switches  92  and  94 , and sum/difference combiner  96 . In this case, four ports (two ports at any one time) can be made available to the user. The original ports  1  ( 100 ) and  2  ( 102 ) provide orthogonal figure eight radiation patterns in the azimuth plane and ports  3  and  4  provide figure eight patterns that are rotated + and −45 degrees from the port  1  pattern. The result is a rotated figure eight pattern as illustrated in FIG.  13 . 
     As shown in FIG. 13, the rotated patterns are obtained by switching the port  1  and  2  outputs via the SPDT switches  92  and  94  in FIG. 12 to a sum difference combiner  96  at ports  3  ( 104 ) and  4  ( 106 ). The system could be programmed to select the port that provides the best signal. Alternatively, the design may allow the user to manually activate the SPDT switches  92  and  94  and SP4T switch  98 , as shown in FIG. 12, to select any one of the four ports to obtain the desired signal.