Abstract:
A flat, ultra wideband, unidirectional antenna is disclosed, the antenna may comprise a pair of active elements having the shape of substantially half-circles or half-ellipsoids made of thin conductive material and a ground element made of thin conductive material placed parallel and against to the active electrodes and spaced from them, the antenna having a nominal gain of at least 6 dbi and variations of gain in that range of +/−1.5 dbi at its bore sight.

Description:
BACKGROUND OF THE INVENTION 
   Several ultra wide band (UWB) antennas are known in the art, such as flat spiral, conical spiral, log periodic, Vivaldi-type, “horn”-type and dipole ‘bow tie’ antennas. These types of UWB flat antennas suffer from various drawbacks such as having an omni-directional radiation patterns, a low gain, or having a low-quality time response or combinations of the above. There is an ongoing demand for small dimensioned, relatively flat antenna with UWB response curve, a directional radiation pattern, a high gain and good time response over a wide angle of coverage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which: 
       FIGS. 1A and 1B  are schematic top and side views respectively of an antenna made according to some embodiments of the present invention; 
       FIGS. 2A-2C  are a schematic top view with blow-up view, a positional view and partial side cross-section view respectively of a flat balun according to some embodiments of the present invention; 
       FIGS. 3A and 3B  are response diagrams of an antenna according to some embodiments of the present invention; 
       FIG. 4  is a graph depicting electrical gain of antenna according to the present invention; and 
       FIGS. 5A and 5B  are graphs depicting the radiation curve of an antenna according to some embodiments of the present invention. 
   

   It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed, description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
   It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the antenna design disclosed herein may be used in many apparatuses in vide band or pulse type applications, such as wide band radar for ground penetration or looking through walls and the like. 
   Reference is made now to  FIGS. 1A and 1B , which are schematic top and side views, respectively, of antenna  10  according to some embodiments of the present invention. Antenna  10  may be comprised of two co-planar flat elements  12  made of conductive material, a ground conductive plane  14 , an insulating layer  15 , feeding ports  16 , two resistors  18  and two auxiliary conductive planar elements  19 . For the sake of clarity the description of antenna  10  will be aided by the use of two symmetry lines A and B as in  FIG. 1A . Elements  12  may each have a planar shape having a perimeter including a straight edge  13  and a remainder, which is typically shaped, as shown in shaped edge  23  so that the shaped edges of each of flat elements  12  are facing each other and arranged symmetrically with respect to symmetry line B. Planar elements  12  are further arranged so that their symmetry line coincides with symmetry line A. The straight edges  13  of the two elements  12  may be parallel to each other and the shaped edges  23  may be facing each other. 
   Shaped edges  23  may have at least one vertex, which may be for example, one or more points or a line, where the distance between the elements is at a minimum. Shaped edge  23  may have any shape, including a curve or a polygon or a combination of the two. Typically, the shape may be such that the length of cross-sections of each element transverse to the line of symmetry A decrease as the distance from the straight edge increases, until the vertex or vertices are reached. In some embodiments, the shape of shaped edge may be such that its cross-section tapers continuously, for example, in accordance with an equation or formula. Shaped edge  23  may be or include, for example but is not limited to, an arc, semi-circle, or other circular section, a semi-ellipsoid or other ellipsoid section, a polygon, or the like. For purposes of obtaining wide bandwidth, good VSWR, and fairly constant gain and beam width over a very wide band shaped edge  23  may preferably have the shape of a smoothly or continuously curved line such as a perimeter of a semi-circle or a semi-ellipsoid. In some embodiments, the contour of the shaped edge may include a notch, by which the contour of the notch section of the shaped edge is curved concave inwards towards the straight edge, for example, in order to filter out a sub-band frequency. 
   The points on the curved edges  23  most distal from the straight edges  13 , i.e., the vertices, may be proximal to each other with a small gap there between. Feeding port  16  may be placed symmetrically close to said small gap at or near the respective vertices of active elements  12 , to allow feeding of RF energy to active elements  12 . Ground conductive plane  14  may be mounted substantially parallel to the plane containing two active elements  12 , in a different plane, with a small gap between the planes. 
   In some embodiments of the invention, the typical size of the gap between the planes may be approximately 1/10 (one tenth) of the wavelength at low frequency end, yet this size may vary according to various engineering considerations, such as bandwidth or beamwidth requirements. Elements  12  may be co-planar, i.e., on the same flat plane, for example, both may be printed on the same single substrate board. An insulating layer  15  may be placed between the plane of the two active elements  12  and ground plane  14 . Insulation layer  15  may be realized using any kind of insulation material and preferably air, which may give better efficiency and bandwidth. Elements  12 ,  18  and  19  may be supported by or installed on a substrate layer (not shown), which may be made of materials such as teflonglass, epoxyglass, polyesterene, polypropylene and materials for printed circuit board (PCB), etc. 
   The size and position of ground conductive plane  14  with respect to active elements  12  may vary according to engineering considerations. In the example depicted in  FIGS. 1A-1B  ground conductive plane  14  may be larger than that of a rectangle inscribing active elements  12  and it may be placed with its center point substantially opposite to the center point between two feeding ports  16  and to the cross of symmetry lines A and B. In another embodiment active elements  12  and ground plane  14  may be printed on two separate insulating boards spaced from each other with any kind of method to space between them. 
   The two main axes of antenna  10  are commonly marked H for the vertical axis and E for the horizontal axis, as marked by the respective double-headed arrows in  FIG. 1A . Main axis E coincides with symmetry line A and main axis H coincides with symmetry line B. Antenna  10  has a boresight axis which is substantially perpendicular to the plane of the page of  FIG. 1A  and crosses substantially in the cross point of symmetry axes A and B. Reference planes H and E are defined so that they comprise the antenna boresight and either main axis H or E respectively. 
   Auxiliary conductive planar elements  19  may have substantially rectangular, circular, elliptical or other shapes, which substantially may be enclosed in a rectangle as depicted in  FIG. 1A . Auxiliary elements  19  may be positioned symmetrically with respect to symmetry line B along symmetry line, spaced on the side of primary elements  12  proximal to the straight edge and at distance d 4  from the straight edge  13  of the respective active element  12 . Auxiliary elements  19  may be called also auxiliary active elements  19 . Impedance elements such as resistors  18  may be electrically connected at one end to one of active elements  12  substantially at a point most distal from its vertex, on its bisector. Resistors  18  may further be connected at its other end to auxiliary active element  19 . Two auxiliary active elements  19  may be placed in the plane of active elements  14  with one of their symmetrical axis coinciding with axis E of antenna  20 . This arrangement may provide forward flow path for RF energy fed to two active elements  12  and by this substantially minimize and even eliminate back-flow of such energy, thus enhancing the dispersion of the impulse response signal (by eliminating the trailing rings) of antenna  10 . Active elements  12  and auxiliary active elements  19  may be realized on a common PCB layer. It will be noted that impedance element may be a resistor, a capacitor or an inductor, or any suitable combination thereof. 
   The various parts of antenna  10  may have dimensions d 1 -d 8  ( FIG. 1 ) as may be dictated by the performance required from it. Typical dimensions of the various parts of antenna  10 , which may allow the performances depicted in drawings  FIGS. 3A to 5B  may be, as a non-limiting example, in fractions or multiples of the wavelength λ of the low-end of the working frequency band width of antenna  10 : d 1 =0.008, d 2 =0.27, d 3 =0.36, d 4 =0.02, d 5 =0.08, d 6 =0.07, d 7 =0.93 and d 8 =0.93. It would be apparent to a person with ordinary skill in the art that these typical dimensions may be varied so as to satisfy various engineering requirements without departing from the concept of the invention. 
   Feeding ports  16  may feed two active elements  12  allowing a balanced feed. Feeding lines (not shown) may be realized by two parallel printed lines on the opposite sides of a PCB being the substrate layer. According to yet another embodiment of the present invention feeding ports  16  may be fed from an unbalanced feeding line (such a coax cable) using any kind of balanced-to-unbalanced (“balun”) adaptor device. 
   Baluns of the known art may be used in connection with the antenna of the present invention; however, such known baluns may typically quite large and bulky with respect to typical dimensions of a flat antenna. For purposes of providing an antenna with a very low profile, a flat UWB balun is presented that may be used in connection with the antenna of the present invention. Attention is made now to  FIGS. 2A-2C , which are a schematic top view with blow-up view, a positional view and partial side cross-section view respectively of a flat balun  60  according to some embodiments of the present invention. Flat balun  60  according to an embodiment of the present invention may be realized by removing part of conductive ground plane  14 , substantially shaped as an “H”, having two side legs and a middle leg, and centered at the crosspoint of symmetry lines A and B and placed with respect to active elements  12  as shown in  FIG. 2B . Flat balun  60  may be achieved, for example, by removing a rectangle  62  having width e 1  and height h 1 +h 2 + h 3  centered at the cross point of symmetry lines A and B, but leaving two non-removed strips  63  and  64  protruding from two opposite sides of perimeter of rectangular  62  into its center along symmetry line A, symmetrically with respect to both symmetry lines A and B, leaving a space e 2  between them. 
   Flat balun  60  may have balanced and unbalanced ports. The unbalanced port may be located at  61  and be between microstrip line  66 , which is a conducting strip on the underside of the ground plane substrate and ground plane  14 . Microstrip  66  may begin at a side of ground substrate proximal to strip  63  and on a side opposite the conducting side, extend underneath strip  63 , across the gap separating strips  63  and  64  and have its terminus at port  68 . The balanced port may be at edges  67  and  68 . The connection between the balanced side and unbalanced side may be via feed-through hole  68 . Thus, the ground plane may be common to both balanced and unbalanced ports. 
   RF energy emitted from the output of flat balun  60  may be conveyed to feeding ports  16  of antenna  10  by means of conductors  69 ,  70  (shown in  FIG. 2C ), in a plane perpendicular to the plane shown in  FIG. 2A . Conductors  69 ,  70  may be printed on substrate. Accordingly, unbalanced RF energy may be provided to the system of antenna  10  via connector  61  and strip line  66  and converted to balanced energy to antenna  10 . 
   Installation of flat balun  60  made according to embodiments of the present invention may comprise feeding of RF energy in an unbalanced line  66  to unbalanced port  68  and feeding of RF energy to active elements  12  in balanced conductors  69 ,  70 , where ground element  14  is realized on the top side of PCB  65  and strip line  66  on the lower side of it. 
   Typical dimensions of balun  60  that may provide for the performances described in this application may be, as a non-limiting example, in fractions of the wavelength λ of the low-end of the working frequency band width of antenna  10 : h 1 =h 3 =0.05, h 2 =0.04, e 1 =0.14 and e 2 =0.008. 
   Reference is made now to  FIGS. 3A ,  3 B,  4 ,  5 A and  5 B which are diagrams of the electrical performance of antenna  10  according to some embodiments of the present invention. 
   An antenna made according to the present invention may have a UWB performance profile, a very low physical profile, high gain, low dispersion, high quality of impulse response and time response. 
   Reference is made now to  FIGS. 3A and 3B , which are normalized impulse response diagrams of antenna  10  according to some embodiments of the present invention, given for seven different angles, substantially equally distributed off the bore sight from −30 degrees to +30 degrees, plotted on same graph.  FIG. 3A  depicts normalized impulse response of antenna  10  for 0, +/−10, +/−20 and +/−30 degrees off bore sight line in the E plane and  FIG. 3B  depicts normalized impulse response of antenna  10  for 0, +/−10, +/−20 and +/−30 degrees off bore sight line in the H plane. As may be seen in  FIGS. 3A and 3B , impulse response of antenna  10  exemplifies very low dispersion across the various angle of deviation from the bore sight line. The dispersion may be measured as the standard deviation between the graphs at every given point along the horizontal axis (time), averaged over time required for reception of 98% of the pulse energy. This mean deviation at any time taken over all time required for reception of 98% of the pulse energy may be denoted A rel     —     div     —     avg . 
   Preferably, in embodiments of the invention having the flat balun described above, A rel     —     div     —     avg  may be less than 4×10 −4  for each of the E and H planes. The graphs of  FIGS. 3A and 3B  show the deviation in time domain for an antenna with the flat balun described herein with a 2 mm thick radome, having values 2.5×10 −4  and 3.7×10 −4  respectively for the E and H planes. It will be apparent to person with ordinary skill in the art that these values of A rel     —     div     —     avg  indicate a very low dispersion in the angle of interest of antenna  10 . In another embodiment of the invention using a conventional or mechanical balun, A rel     —     div     —     avg  may be less than 3×10 −4  or more preferably less than 2.5×10 −4 . In one embodiment (graph not shown), A rel     —     div     —     avg  may have values of 1.4×10 −4  and 2.4×10 −4  respectively for the E and H planes. 
   Attention is made now to  FIG. 4 , which depicts the electrical gain of antenna  10  in varying frequencies at the boresight of the antenna.  FIG. 4  depicts results received in both E and H planes (also known as azimuth and elevation planes respectively). In one embodiment of the present invention, the antenna may have gain variation within limits of +/−1.5 dbi (decibels referenced to isotropic radiator) over a frequency range having a ratio of high end-to-low end higher than 3 and preferably 3.4 or higher, for example, from 3.1 to 10.6 GHz. The absolute nominal gain may generally be better than 6 dbi over the band 3.1 to 10.6 GHz, which is much higher than that of prior art UWB flat antennas. It would be noted that the gain of antenna  10  as depicted in graph of  FIG. 4  complies with the definitions of an ultra wide band (UWB) antenna, as defined, for example, by the US Federal Communications Commission (FCC). 
   Attention is made now to  FIGS. 5A and 5B , which depict normalized radiation curves of antenna  10  according to the spatial inclination angle from the boresight of the antenna  FIG. 5A  depicts measurements taken in E plane and  FIG. 5B  depicts measurements taken in H plane, both with respect to boresight axis for 10 different frequencies in the range of 3.1 to 10.6 GHz.  FIGS. 5A and 5B  exhibit the performance of antenna  10  with respect to beam width versus frequency exemplifying that its beam width is substantially constant over the bandwidth for beam angles in the range of −/+30° from boresight. 
   It will be appreciated by persons of ordinary skill in the art that according to some embodiments of the present invention other designs of flat antenna with substantially two circle-like conductive planes and a ground planes according to the principles of the present invention are possible and are in the scope of this application. 
   While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.