Abstract:
An antipodal antenna has an active member arranged between two diverging ground elements. The active member and ground elements are shaped to provide a tapered slot. The ground elements may be planar or may be curved outwardly. In some embodiments the ground elements follow semi-parabolic conical sections. The active and ground elements may be separated by air.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the filing date of U.S. patent application No. 60/286,367 filed on 26 Apr. 2001. 

   TECHNICAL FIELD 
   This invention relates to antennas for transmitting and/or receiving electromagnetic radiation. 
   BACKGROUND 
   There are various applications for which wide band transmitting and receiving antennas are required. These include applications in fields such as medical imaging, radar, radio frequency crystallography and telecommunications. 
   One type of antenna which is used in such applications are microstrip antennas. A typical microstrip antenna is fabricated by forming a shaped metallized layer on a planar circuit board substrate. Another metallized layer on the substrate serves as a ground plane. U.S. Pat. No. 5,036,335 describes an example of a microstrip antenna. 
   A balanced stripline antenna is similar to a microstrip antenna except that it has a pair of ground planes, one on each side of the active element. Guillanton et al.  A new design tapered slot antenna for ultra - wideband applications  Microwave and Optical Technology Letters v. 19, No. 4, November 1998 discloses a balanced antipodal Vivaldi antenna made using stripline technology. 
   Microstrip and stripline antennas suffer from the disadvantage that the dielectric substrate materials on which the metallized layers are supported adversely affect the radiation characteristics of the antennas at certain frequencies. 
   There is a need for antennas capable of transmitting, receiving and/or receiving and transmitting over a wide frequency range. 
   SUMMARY OF THE INVENTION 
   This invention provides antennas for the transmission and/or reception of electromagnetic radiation. A first aspect of the invention provides an antipodal antenna comprising an active element located between a pair of matched, symmetrically diverging, ground elements. The active and ground elements may comprise sheets of electrically conductive material. In some embodiments, inside edge portions of the active element and ground elements at distal ends of the active and ground elements diverge from one another to provide a tapered slot. 
   In various embodiments of the invention the inside edge portions of the active element and ground elements follow convex exponential curves. The active element may comprise a broad distal portion supported at an end of a thinner member. The ground elements may also each comprise a broad distal portion supported at an end of a thinner member. Where the active and ground elements comprise broad distal portions the broad distal portion of the active element may be entirely on a first side of the centerline (i.e. on a first side of an imaginary transversely-extending plane which includes the centerline) and the broad distal portions of the ground elements may be entirely on a second side of the centerline (i.e. on a second side of the transversely-extending plane). 
   In various specific embodiments, the ground elements each follow: a semi-cubical parabolic curve; an arc; an exponential curve; a line (e.g. the ground elements are planar); or an elliptical curve. In some embodiments, the ground elements comprise resiliently flexible sheets and the antenna comprises a member holding each of the resiliently flexible sheets in a curved configuration. 
   Further features of the invention and specific embodiments of the invention are described below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In drawings which illustrate non-limiting embodiments of the invention: 
       FIG. 1  is a perspective view of an antenna according to one embodiment of the invention; 
       FIG. 2  is a top view of the antenna of  FIG. 1 ; 
       FIGS. 2A ,  2 B,  2 C,  2 D and  2 E are top plan view of antennas according to embodiments of the invention in which the ground elements have different curvatures; 
       FIGS. 2F and 2G  are top plan view of antennas according to embodiments of the invention in which the ground elements are held in curved configurations; 
       FIG. 3  is a detailed view of an antenna according to an embodiment of the invention in which the antenna incorporates a coaxial cable connector; 
       FIG. 4  is a side elevational view of the active element of the antenna of  FIG. 1 ; 
       FIG. 5  is a side elevational view of a ground element of the antenna of  FIG. 1 ; 
       FIG. 6  is a side elevational view of the antenna of  FIG. 1  with one ground element removed; 
       FIG. 7  shows a return loss curve for a prototype antenna; 
       FIGS. 8 and 9  show E and H plane radiation patterns for the prototype antenna at 9 GHz. 
   

   DESCRIPTION 
   Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     FIG. 1  shows an antenna  10  according to one embodiment of the invention. Antenna  10  has an active element  12  located symmetrically between a pair of ground elements  14 . Each of elements  12  and  14  may be formed from a sheet of an electrically conductive material. The electrically conductive material may be a metal. For example, elements  12  and  14  may be formed of copper sheets. Active element  12  is electrically isolated from ground elements  14 . 
   Active element  12  is separated on either side from ground elements  14  by an air gap  15 . Ground elements  14  are not parallel to active element  12  but diverge from one another. Ground elements  14  are symmetrical with respect to active element  12 . In a currently preferred embodiment of the invention, each of ground elements  14  follows a semi-cubical parabolic curve. A semi-cubical parabolic curve is a curve on which points (r, θ) satisfy the equation:
 
r=α tan 2  θsecθ  (1)
 
   In other embodiments of the invention, ground elements  14  may diverge in different manners. For example:
           FIG. 2A  shows a top view of an antenna  10 A wherein ground elements  14  are straight and diverge with an angle Φ.     FIG. 2B  shows a top view of an antenna  10 B wherein ground elements  14  follow an exponential profile given by the equation:
 y=e f(x)   (2)   in the example of  FIG. 2B , f(x)=x;     FIG. 2C  shows a top view of an antenna  10 C wherein ground elements  14  follow arcs;     FIG. 2D  shows a top view of an antenna  10 D wherein ground elements  14  follow an elliptical profile given by the equation: 
                   x   2       a   2       -       y   2       b   2         =     c   2             (   3   )                   FIG. 2E  shows a top view of an antenna  10 E wherein ground elements  14  follow irregular profiles.       

   The curved shapes of ground elements  14  may be provided in various ways including:
         making elements  14  from a flexible material, such as a metallic sheet, which can be bent to have the desired curve;   casting or molding elements  14  in the desired shapes from a castable or moldable material; or,   providing elements  14  made from a resiliently flexible material and holding elements  14  in a flexed configuration.       

     FIG. 2F  shows a top view of an antenna  10 F wherein ground elements  14  are made from a resiliently flexible material and are held in a curved configuration by non-conductive strings  16 . In the embodiment of  FIG. 2F  the curve of ground elements  14  is determined by the length of strings  16  and the bending characteristics of ground elements  14 .  FIG. 2G  shows a top view of an antenna  10 G wherein ground elements  14  are made from a flexible material and are shaped by forms  17 . Forms  17  may contact ground elements  14  only at a few points to minimize the amount of dielectric material near ground elements  14 . 
   As shown in  FIG. 3 , antenna  10  may be driven by a signal supplied through a coaxial cable  19 . Antenna  10  may incorporate a coaxial cable connector  20  having a center conductor  22 . Active element  12  may be affixed directly to center conductor  22 . Ground elements  14  may be attached to the ground conductor  23  of cable connector  20 . In alternative embodiments of the invention, active element  12  and ground elements  14  may be attached to a base comprising a printed circuit board. The elements of antenna  10  may be driven by signals provided by way of conductive elements of the printed circuit board. 
   As shown in  FIGS. 4 ,  5  and  6  active element  12  comprises a broad distal portion  30  supported at the end of a thinner member  32 . Distal portion  30  has curved corners. Ground elements  14  also each comprise broad distal portions  31  supported at the ends of thinner members  33 . Members  32  and  33  may be equal in width to one another and may extend along a centerline  37  of antenna  10  when viewed from the side. As shown in  FIG. 2D , members  32  and  33  may be substantially parallel to one another over most of their lengths as viewed from above. 
   Medial ends  14 A of ground elements  14  are flared. The edges of ground elements  14  follow suitable curves. For example, in portions  34  and  36  the edges of ground element  14  may follow elliptical or exponential curves. In one embodiment, portions  34  on edge of ground elements  14  follow elliptical curves and portions  36  follow exponential curves. The medial end of active element  12  is preferably not flared. 
   As shown best in  FIG. 6 , distal portion  30  of active element  12  has an inside edge portion  38  which, together with an inside edge portion  39  on ground elements  14  forms a tapered slot  40  when antenna  10  is viewed from the side. Inside edge portion  38  of active element  12  and inside edge portions  39  of ground elements  14  may diverge symmetrically from centerline  37 . Inside edge portion  38  may follow an exponential curve. Inside edge portions  39  may follow exponential curves. 
   Distal portion  30  of active element  12  may have flats  42  and  44  on its outer and end edges. Distal portions  31  of ground elements  14  may also have flats  43  and  45  on their outer and end edges. 
   Antennas according to the invention may have particular application in receiving and transmitting signals having frequencies in the range of 20 MHz to 100 GHz. 
   Antennas according to some embodiments of the invention are characterized by a return loss of less than −3 dB and a deviation about the mean return loss of less than 10 dB over a bandwidth of 5 GHz. 
   EXAMPLE 
   An antenna according to a prototype embodiment of the invention, has the dimensions:
     L1=10 cm;   L2=3.3 cm;   L4=1.7 cm;   L5=2.4 cm;   D1=00.5 cm;   D2=9.0 cm;   H1=7.4 cm;   H2=2 cm;   H3=5.0 cm; and,   H4=0.5 cm.
 
The active and ground elements of the prototype antenna were fabricated from copper sheet having a thickness of approximately 0.675 mm.
   

   In the prototype antenna, edges of active element  12  followed the following curves:
         in portion  50 —concave circular arc;   a in portion  51 —convex circular arc; and,   in portion  38 —convex exponential curve.
 
In the prototype antenna, edges of ground elements  14  followed the following curves:
   in portion  34 —concave elliptical curve;   in portion  36 —concave exponential curve;   in portion  39 —convex exponential curve;   in portion  52 —concave circular arc; and,   in portion  53 —convex circular arc.
 
The ground elements of the prototype antenna followed exponential curves, as shown in FIG.  2 B.
       

   The prototype antenna demonstrated a 10 dB bandwidth of 2.2 GHz to 13.5 GHz.  FIG. 7  shows a S 11  return loss curve for the prototype antenna.  FIGS. 8 and 9  show respectively E and H plane radiation patterns for the prototype antenna at 9 GHz. In  FIGS. 8 and 9 , co-polarization is indicated by solid curves and cross polarization is indicated by dashed curves. The level of cross-polarization in the E plane is below 18 dB at 0°. The level of cross-polarization in the H plane is approximately −21 dB at 0°. The gain at 9 GHz is 6 dB. 
   As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
         Active element  12  and ground elements  14  do not need to be made entirely of the same conductive material. These elements could comprise a core of some other material coated or plated with an electrically conductive material.   The dielectric surrounding the elements of antenna  10  may be air, a gas, a liquid, vacuum, or a solid material (solid materials include mixed-phase materials such as foams). Antenna  10  may be mounted within a suitable radome (i.e. an enclosure). The atmosphere within the enclosure may be varied to change the dielectric properties of the material surrounding antenna  10 .   Additional active elements or ground elements may be added to refine the properties of an antenna  10 .   The dimensions of an antenna according to the invention may be scaled for operation in different frequency ranges.   While it is generally not preferred, small dielectric spacers could be provided between the active element and the ground elements to maintain a desired shape of the ground elements by holding the ground elements away from the active element.
 
Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.