Abstract:
A crossed-slot antenna is fabricated using rectangular sheet of metal. A crossed-slot is etched or stamped in the sheet of metal. A feed structure is similarly formed in the sheet of metal. Sidewalls are integrally formed with the sheet of metal and are bent to form a rectangular box. A circuit board is attached to the rectangular box. An air-filled cavity is defined by the sheet of metal, the sidewalls, and the circuit board. Alternatively, a crossed-slot antenna with a solid cavity is fabricated using a sheet of plastic. Ridges are formed in a cross pattern on the sheet of plastic. The sheet of plastic is plated with metal. The metal is removed from a surface of the sheet of plastic, exposing the ridges.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to antennas, and more particularly to crossed-slot antennas for mobile satellite and terrestrial reception.  
         BACKGROUND OF THE INVENTION  
         [0002]    Smaller, less visible antennas are an increasing trend in vehicle design. One approach for providing these antennas employs a crossed-slot antenna. The crossed-slot antenna can receive signals from satellite radio broadcasting systems such as satellite digital audio radio system (SDARS). Crossed-slot antennas can be as thin as a small fraction of one wavelength tall when combined with a resonant cavity. The reception characteristics and the relatively small size of crossed-slot antennas are ideal for mobile receiver applications.  
           [0003]    Conventional fabrication techniques for crossed-slot antennas require the use of low-loss dielectric materials such as Teflon or Duroid. These materials may be prohibitively expensive for commercial applications such as high-volume automobile manufacturing. Absent these specialized low-loss materials, the internal dielectric loss of the crossed-slot antenna is unacceptably high.  
           [0004]    Conventional fabrication methods for the crossed-slot antenna employ printed circuit boards. A circuit board is initially plated with a suitable metal, such as copper, which acts as the antenna. Typically, slots are made in the antenna using standard photolithography techniques. The printed circuit board is formed with a suitable dielectric material and acts as a cavity for the antenna.  
         SUMMARY OF THE INVENTION  
         [0005]    A crossed-slot antenna according to the present invention is fabricated by forming a feed structure and a crossed-slot in a sheet of metal Sidewalls are formed on the sheet of metal. The sidewalls are attached to a circuit board to form an air-filled cavity defined by the sheet of metal, the sidewalls, and the circuit board.  
           [0006]    In another embodiment, a crossed-slot antenna with a solid cavity is fabricated. First and second intersecting ridges are created on one side of a sheet of plastic. A feed aperture is formed in the sheet of plastic. The sheet of plastic is plated with metal. The metal plating is removed from one side of the sheet of plastic to expose a first ridge and a second ridge. The sheet of plastic is attached to a circuit board.  
           [0007]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1A is a plan view of an exemplary crossed-slot antenna according to the present invention;  
         [0010]    [0010]FIG. 1B is a side cross-sectional view of the exemplary crossed-slot antenna of FIG. 1A;  
         [0011]    [0011]FIG. 2 is a plan view of a crossed-slot antenna according to the present invention;  
         [0012]    [0012]FIG. 3 is a side cross-sectional assembly view of the crossed-slot antenna of FIG. 2;  
         [0013]    [0013]FIG. 4 is a side cross-sectional view of the crossed-slot antenna of FIG. 2;  
         [0014]    [0014]FIG. 5 is a plan view of an alternative crossed-slot antenna;  
         [0015]    [0015]FIG. 6 is a side cross-sectional assembly view of the crossed-slot antenna of FIG. 5;  
         [0016]    [0016]FIG. 7 is a cross-sectional view of the alternative crossed-slot antenna of FIG. 5;  
         [0017]    [0017]FIG. 8A is a plan view of a crossed-slot antenna formed with injection molding;  
         [0018]    [0018]FIG. 8B is a cross-sectional view of the crossed-slot antenna of FIG. 8A;  
         [0019]    [0019]FIG. 9A is a cross-sectional view of a metal plated crossed-slot antenna; and  
         [0020]    [0020]FIG. 9B is a cross-sectional view of a planed crossed-slot antenna. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.  
         [0022]    An exemplary crossed-slot antenna according to the present invention is shown in FIGS. 1A and 1B. The crossed-slot antenna  10  receives radio frequency (RF) waves having left hand circular polarization and RF waves having vertical linear polarization. A first slot  12  crosses a second slot  14  to form a crossed-slot pattern in an antenna plane  16 . The first slot  12  is slightly detuned from the second slot  14  and has a different resonant frequency.  
         [0023]    The crossed-slot antenna  10  is suited for satellite radio broadcasting systems such as SDARS. Satellites in satellite radio systems broadcast information to a terrestrial repeater network, which subsequently rebroadcasts the information to a mobile receiver. Satellites typically broadcast in circular polarization, wherein the orientation of the receiver is not important. Terrestrial broadcasters, however, use vertical linear polarization. The crossed-slot antenna  10  is able to receive transmissions from both satellite and terrestrial broadcasters as will be described below.  
         [0024]    The first slot  12  is shorter than the second slot  14 . Consequently, the first slot  12  has slightly higher resonant frequency than the second slot  14 . An antenna feed point  18  is positioned along a line  20  lying at a forty-five degree angle between the first slot  12  and the second slot  14 . The position of the feed point  18  causes the first slot  12  and the second slot  14  to be excited equally.  
         [0025]    The antenna  10  is designed so that the first slot  12  is out of phase with the second slot  14  by approximately ninety degrees when both slots are excited simultaneously. The antenna arrangement results in circular polarization for angles near zenith and for angles within the upper hemisphere. For angles near the horizon, the effective cross section of one of the slots approaches an infinitesimal point. The resulting radiation from the opposing slot is linearly polarized in the vertical direction.  
         [0026]    Referring now to FIG. 2, a crossed-slot antenna  20   a  includes a crossed-slot  22  that is etched or stamped into a sheet of metal  24 . The sheet of metal  24  may be constructed of any suitable conducting material. The conducting material can be copper, brass, or steel, although other conducting materials can be used. Sidewalls  26  are integrally formed with the sheet of metal  24 . Additionally, mounting tabs  28  are integrally formed with the sidewalls  26 . A three-sided feed structure  30  is etched or stamped in the surrounding metal sheet  24 . The sidewalls  26  and feed structure  30  may be scored to facilitate bending. The antenna  20   a  may be plated with a suitable material, such as tin or solder to allow the antenna to be mated with a printed circuit board during fabrication.  
         [0027]    Referring now to FIG. 3, the sidewalls  26  are formed into a rectangular box  32 . The feed structure  30  extends in a perpendicular direction from the plane of the antenna  20   a . The antenna  20   a  is joined with a printed circuit board  34 . The perimeter of the circuit board  34  is lined with metal-plated mounting apertures  36  that align with the mounting tabs  28  of the sidewalls  26 . A feed point aperture  38  aligns with the feed structure  30 . The antenna  20   a  is aligned with the printed circuit board  34  due to the mating of the mounting tabs  28  and the feed structure  30  with the corresponding apertures on the circuit board  34 .  
         [0028]    A ground plane  40  made of a conducting material is attached to a side of the circuit board  34 . The metal forming the ground plane  40  is preferably interrupted only by the feed point aperture  38 . A receiver circuit  42  mounted on the circuit board  34  shares the ground plane  40  with the antenna  20   a . The feed structure  30 , which communicates with the circuit board  34  via the feed point aperture  38 , acts as the input from the antenna  20   a  to the receiver circuit  42 .  
         [0029]    Using the above-described method, the circuit board  34  may be a simple, two-layer circuit board that is constructed from high loss, low cost material. The amplifiers, filters, and other circuit elements of the receiver circuit  42  are attached to the underside of the circuit board  34  using surface mount techniques. The antenna  20   a  is attached to the circuit board  34  by soldering the mounting tabs  28  to mounting apertures  36 .  
         [0030]    Referring now to FIG. 4, the completed antenna structure  50  includes the circuit board  34 , the shared ground plane  40 , and the antenna  20   a . The antenna  20   a  and the ground plane  40  form an air-filled cavity  52 . This design has extremely low RF loss because the cavity  52  is filled with air. The only loss in the antenna structure  50  is due to ohmic losses in the metal of the sidewalls  26 . Thus, the antenna structure  50  does not necessitate the use of low-loss dielectric materials as conventional designs require. If the cavity  52  is filled with a dielectric material, the material must have extremely low loss due to the high electric fields that are present in the resonant cavity  52 . The thickness of the cavity  52  determines the bandwidth of the antenna  20   a . As can be appreciated, low loss is not required for the remainder of the receiver circuit  42  because a signal received by the antenna  20   a  is sufficiently strong. Additionally, the design requires only a single ground plane  40 , which serves as both one wall of the cavity  52  and a ground plane for the receiver circuit  42 . Therefore, a single layer, double-sided circuit board  34  may be used, rather than an expensive multilayer board.  
         [0031]    Referring now to FIG. 5, an alternative embodiment of the sheet of metal  24  used to construct the antenna  20   b  is shown. Additional bands of metal  60  are integrally formed with the sidewalls  26 . The bands  60  and sidewalls  26  may be formed as a uniform structure that is subsequently scored. The bands  60  replace the mounting tabs  28  in FIG. 2. The crossed-slot aperture  22  and the feed structure  30  are etched or stamped into the sheet of metal  24  as previously described.  
         [0032]    Referring now to FIG. 6, the sidewalls  26  are bent at a ninety-degree angle to the plane of the antenna  20   b . The bands  60  are bent at a ninety-degree angle to the sidewalls. The feed structure  30  is bent in a similar manner. This embodiment does not include mounting apertures  36  on the circuit board  34 . A feed point aperture  38  is provided for the feed structure  30  to supply the signal to the receiver circuit  42 .  
         [0033]    The antenna  20   b  is mounted to the circuit board  34  to form the completed alternative structure  70  as shown in FIG. 7. Because the circuit board  34  provides a single aperture  38  for attachment purposes, the alternative structure  70  is a simpler, less expensive design. The antenna  20   b  is aligned with the circuit board  34  using an alternative method due to the absence of mounting tabs  28  and mounting apertures  36 . While this complicates the fabrication process, alternative alignment methods are well known to those skilled in the art of surface mounting techniques. The antenna  20   b  may be attached to the circuit board  34  using soldering paste or other attachment methods.  
         [0034]    Referring now to FIG. 8, a crossed-slot antenna  20   c  that is produced using injection molding is shown. A rectangular plastic block  80  is formed with a feed aperture  82  provided as a feed point. A cross pattern  84  that is integrally formed on the surface of the plastic block  80  acts as a crossed-slot. The block  80  and cross pattern  84  may be formed of a variety of suitable, low-loss plastics, including but not limited to acrylic or lucite plastics.  
         [0035]    Referring now to FIG. 9, the plastic block  80  is plated with copper or other suitable metal to form the antenna  20   c . The structure may be further plated with tin for weather protection purposes. The antenna  20   c  is then passed through a planing machine or other device, which removes the top surface of the copper plating and leaves the crossed-slot pattern  84  exposed. The antenna  20   c  is attached to a circuit board containing a receiver circuit as described previously. The feed aperture  82  is filled with copper during the plating process, which acts as a feed structure for the receiver circuit.  
         [0036]    The plastic block  80  acts as the cavity as described in previous embodiments. The antenna  20   c  has a higher dielectric loss due to the plastic material filling the cavity. Additionally, antenna  20   c  is more expensive to produce than previous embodiments discussed herein. However, antenna  20   c  may be advantageous in applications wherein size is an important factor. Antenna  20   c  may be constructed smaller than embodiments with an air-filled cavity  52 . Nonetheless, antenna  20   c  is less expensive to produce than conventional methods.  
         [0037]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.