Abstract:
A compact dual narrow band microstrip antenna particularly suited for remote wireless metering includes a dielectric, a directly fed radiating element, a parasitic radiating element gap coupled to the directly fed radiating element and a truncated ground plane. Each radiating element is partially shorted to the ground plane and each radiating element has a reactance window to lengthen the current path. The reactance window and partial shorting reduce the size of the each radiating element. The antenna is compact with good isotropic characteristics and sensitivity to two perpendicular polarizations.

Description:
This application claims the benefit under 35 U.S.C. § 119(e) of the U.S. provisional patent application No. 60/158,767 filed Oct. 12, 1999. 
    
    
     TECHNICAL FIELD 
     The present invention relates to antennas and more particularly to a compact dual narrow band microstrip antenna that is particularly suited for wireless meter applications. 
     1. Background Art 
     Systems for wireless meter applications are often positioned beside buildings in urban areas and are subject to multipath reflections from other buildings. The portable reading equipment for such systems may be randomly oriented causing rotation of polarization. An antenna for a wireless meter application should be sensitive to two perpendicular orientations with good isotropic characteristics. 
     2. Disclosure of the Invention 
     A compact dual band microstrip antenna having a dielectric with a directly fed first radiating element and a spaced, coplanar, parasitic second radiating element on a first side of the dielectric, and a ground plane on an opposite second side of the dielectric is disclosed. Each radiating element has a reactance window and each radiating element is partially shorted to the ground plane by a plurality of spaced shorting posts extending through the dielectric. A uniform gap of a selected width separates the radiating elements. A feed point in the form of a hole extending through the dielectric connects to the directly fed radiating element near the gap. An opening is provided around the feed point on the side of the dielectric with the ground plane to electrically isolate the feed point from the ground plane. The ground plane is truncated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of this invention are described in connection with the accompanying drawings that bear similar reference numerals in which: 
     FIG. 1 is a perspective view of an antenna embodying features of the present invention. 
     FIG. 2 is a top plan view of the antenna of FIG. 1 with an alternative arrangement of the second radiating element. 
     FIG. 3 is a bottom plan view of the antenna of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1,  2  and  3  the compact antenna embodying features of the present invention includes a dielectric  10 , a directly fed first radiating element  11 , a parasitic second radiating element  12  and a ground plane  13 . The dielectric  10  is in the form of a thin, flat substrate that is generally planar with a rectangular outline, and includes a planar first side  15  and a spaced, oppositely facing planar second side  16 . Preferably the dielectric  10  has a square outline. The ground plane  13  is disposed over the entire second side  16  of the dielectric  10  and is truncated, having an area approximately equal to the combined area of the first and second radiating elements  11  and  12 . 
     The first and second radiating elements  11  and  12  are made of conductive material and are each generally rectangular. The first and second radiating elements  11  and  12  are disposed in a side by side arrangement on the first side  15  of the dielectric  10  and are separated by a uniform gap  18 . The first radiating element  11  has a first edge  20  extending transverse to the gap  18  and the second radiating element has a first edge  21  extending transverse to the gap  18 , with the first edges  20  and  21  of the first and second radiating elements  11  and  12  being substantially co-linear. 
     The first edge  20  of the first radiating element  11  is partially shorted to the ground plane  13  by a first means that includes, in the illustrated embodiment, a plurality of uniformly spaced first shorting posts  23 . The first edge  20  of the first radiating element  11  is considered to be partially shorted because only a portion of the first edge  20  of the first radiating element  11  is connected to the ground plane  13  instead of the entire first edge  20  of the first radiating element  11 . Each first shorting post  23  is a plated though hole extending through the dielectric  10  and connected to the ground plane  13  and the first radiating element  11 . The first shorting posts  23  are arranged linearly along and as close as possible or in close proximity to the first edge  20  of the first radiating element  11 . The first shorting post  23  that is nearest to the gap  18  is spaced away the gap  18  by a selected distance so that the plurality of first shorting posts  23  is substantially centered along the first edge  20  of the first radiating element  11 . The first means for partially shorting may alternatively include conductive tape or a tab extending around the dielectric  10  from the first edge  20  of the first radiating element  11  to the ground plane  13 . 
     The first radiating element  11  has a second edge  24 , connected to the first edge  20  and extending transverse therefrom, opposite and parallel to the gap  18 . The first radiating element  11  has a third edge  25 , connected to the first edge  20  and extending transverse therefrom, adjacent to the gap  18  and parallel to the second edge  24 . A reactance window  26 , in the form of a narrow rectangular strip cut into the first radiating element  11 , extends parallel to and spaced from the first edge  20 , opening through and extending inward from the second edge  24  towards the third edge  25  of the first radiating element  11 . The first radiating element  11  includes a feed point  27  between the first edge  20  and the reactance window  26  spaced from the first edge  20  a selected distance and spaced a selected distance from the third edge  25 . 
     The feed point  27  is preferably located at a selected distance from the first edge  20  that is about half the distance from the first edge  20  to the reactance window  26 . The feed point  27  is located at a distance from the third edge  25  that is selected provide an impedance match and is preferably is less than half the distance from the third edge  25  to the second edge  24 . In the illustrated embodiment the feed point  27  is located a selected distance from the third edge  25  that is about one fourth the distance from the third edge  25  to the second edge  24 . In the illustrated embodiment the feed point  27  includes a plated through hole through the dielectric  10 . The feed point  27  may include a non-plated hole instead of the plated through hole. The ground plane  13  has an opening  28  around the feed point  27  on the second side  16  of the dielectric  10  so that the feed point  27  is electrically isolated from the ground plane  13 . A coaxial cable (not shown) may be attached to the antenna with the center conductor of the coaxial cable connecting to the feed point  27  at the second side  16  of the dielectric  10  and the outer conductor of the coaxial cable connecting to the ground plane  13 . 
     The first edge  21  of the second radiating element  12  is partially shorted to the ground plane  13  by a second means that includes, in the illustrated embodiment, a plurality of uniformly spaced second shorting posts  30 . Each second shorting post  30  is a plated though hole extending through the dielectric  10  and connected to the ground plane  13  and the second radiating element  12 . The second shorting posts  30  are arranged linearly along and as close as possible or in close proximity to the first edge  21  of the second radiating element  12 . The second shorting post  30  that is nearest to the gap  18  is adjacent the gap  18  with the remaining second shorting posts  30  spaced at intervals therefrom. The second means for partially shorting may alternatively include conductive tape or a tab extending around the dielectric  10  from the first edge  21  of the second radiating element  12  to the ground plane  13 . 
     The second radiating element  12  includes a second edge  31  connected to the first edge  21  and extending transverse therefrom, and a reactance window  32  opening through and extending inward from the second edge  31 , parallel to and spaced from the first edge  21  of the second radiating element. As shown in FIG. 1, the second edge  31  of the second radiating element  12  may be opposite the gap  18  or as shown in FIG. 2, the second edge  31 A of the second radiating element  12  may be adjacent the gap  18 . 
     By way of example, and not a limitation, an antenna as described above can be dimensioned as follows for the cellular frequency bands of 834-836 MHz and 879-881 MHz. The dielectric  10  and the ground plane  13  each have a length of 48 mm and a width of 48 mm. The dielectric is 3 mm thick. The first radiating element  11  is 22 mm wide and the second radiating element  12  is 20 mm wide, and the first and second radiating element are both either 43 or 44 mm long. The gap  18  is 2 mm wide. There are five first shorting posts  23 , each 1 mm in diameter, with the first shorting post  23  nearest the gap  18  being spaced 6 mm from the gap  18  and the remaining first shorting posts being spaced at intervals of about 3 mm. The reactance window  26  in the first radiating element  11  is spaced 12 mm from the first edge  20 , extends inward 12 mm from the second edge  24  and is 1 mm wide. The feed point  27  is 1.4 mm in diameter, and is spaced 5 or 6 mm from the first edge  20  and 5 mm from the third edge  25  of the first radiating element  11 . There are five second shorting posts  30 , each 1 mm in diameter, spaced at intervals of about 2.5 mm. The reactance window  32  in the second radiating element  12  is spaced 12 mm from the first edge  21 , extends inward 10 mm from the second edge  31  and is 1 mm wide. The opening  28  in the ground plane  13 , around the feed point  27  is 2.8 mm in diameter. 
     The antenna may be constructed of FR4 or any other adequate substrate material. An exemplary material is MC3D Medium Frequency Laminate from GIL technologies, Collierville, Ten., with a dielectric constant of about 3.86. 
     The first radiating element  11  is directly fed or driven by the feed point  27  and the second radiating element  12  is gap coupled to the first radiating element  11  and thereby parasitically fed. The partial shorting of the first and second radiating elements  11  and  12  to the ground plane  13  reduces the size of each element. The reactance windows  26  and  32  in the first and second radiating elements  11  and  12  each lengthen the current path and thereby reduce the size of each element. The reactance windows  26  and  32  in the first and second radiating elements  11  and  12  also increase the amount of diffracted waves, which improves the isotropic characteristics of the antenna and helps make the antenna sensitive to two perpendicular linear polarizations. The truncation of the ground plane  13  reduces the antenna size and improves the isotropic characteristics. The location of the feed point  27  near the gap  18 , the location of the first shorting posts  23  spaced away from the gap  18  and the location of the second shorting posts  30  adjacent the gap  18  increase the antenna efficiency. 
     Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that changes in details of structure may be made without departing from the spirit thereof.