Abstract:
A window pane antenna suitable for radio communication, for automobiles, consists of a loop-shaped antenna conductor ( 6 ) and of a conductive area ( 8 ) disposed in the edge zone of the glass pane ( 1 ). The feed point ( 7 ) of the antenna conductor ( 6 ) is disposed in the immediate vicinity of the conductive area ( 8, 9, 10 ) without connection to it. The other end ( 6′ ) of the antenna conductor ( 6 ) is in electrically conducting connection with the conductive area ( 8, 9, 10 ). In comparison with known antenna structures for radio communication, the tuning behavior of this antenna structure is comparative insensitive to mechanical tolerances during installation of the antenna window pane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a pane antenna for automobiles, in which an antenna conductor is disposed on the surface of or inside an automobile glass pane. The antenna feed point is connected to the central conductor of a coaxial cable and is disposed in the vicinity of a conductive area located in the edge region of the glass pane and connected to the sheath of the coaxial cable. 
     2. Description of the Related Art 
     Antenna structures of this type are suitable as signal transmission antennas for radio communication, especially as antennas for digital radio telephone communication. For maximum transmission efficiency, such antennas must be precisely tuned. They require, for this purpose, a defined reference plane to which all electrical parameters are referred. As the reference plane, therefore, a strip-shaped, electrically conducting coating is disposed directly on the edge region of the glass pane. 
     An antenna structure of this type is known from WO 88/09569. In this known antenna, which is intended for use as a radio antenna, preferably for the frequency range from 890 to 960 MHz, the antenna conductor consists of a rectilinear conductor having a free end which projects from the edge of the glass pane into the viewing field of the glass pane. The antenna conductor having a conductive area and acting as a reference plane can be printed and baked onto the surface of the glass pane facing towards the interior of the vehicle, like heating conductors for heating panes. 
     The resonance frequency of this known antenna structure exhibits a certain dependence upon the materials of the glass pane and upon the geometrical tolerances necessary for installation of the glass pane. With these known antennas, therefore, it is still necessary to tune them again after installation if they are to possess an optimum efficiency. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a pane antenna of the initially named type, in which the dependence of the resonance frequency upon the materials used and upon the geometric and mechanical tolerances of the glass pane and of the window frame of the automobile body is further reduced. 
     According to the invention, this object is achieved by an antenna conductor having the form of a loop and connected at an end to a conductive area on the glass pane. 
     The antenna conductor of the antenna structure according to the invention is therefore formed as a folded unipole, one end of which is directly connected to a conductive area forming the reference potential. The antenna structure of this invention is substantially less sensitive to installation tolerance because, as a result of its direct connection to the conductive area, it is unambiguously earthed. It furthermore has the advantage that as a result of the ability to vary the width of the loop, it exhibits an additional degree of freedom of an impedance adaptation via the width of the loop. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a rear view of a monolithic safety glass with an antenna structure printed onto the surface and baked in; 
     FIG. 2 is an enlargement of a portion of FIG. 1; 
     FIG. 3 shows a windscreen (windshield) of laminated glass, with an antenna wire laid therein, in elevation; and 
     FIG. 4 is a section on the line IV—IV in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2, a monolithic, thermally toughened glass pane  1 , which is used as rear pane, is provided with a heating conductor structure. The heating conductor structure comprises a series of heating conductors  2  which extend transversely across the glass pane, and two collector conductors  3 ,  4  disposed laterally at the edge of the pane. Heating conductors  2  and collector conductors  3 ,  4  are of an electrically conducting bakable finish, so-call conducting silver, which is printed by the screen printing process onto the glass surface and is baked into the glass surface during the glass bending and/or toughening process. 
     In the upper region of the glass pane  1 , in addition to the heating conductor structure and unconnected with it, is an antenna structure suitable for radio operation at a frequency in the 900 MHz range. This antenna structure is of the same material as the heating conductors  2  and the collectors  3 ,  4  and is printed onto the glass surface in the same printing operation as them, and is also baked in at the glass bending or toughening temperature. The antenna structure consists of an antenna conductor  6 , approximately 0.5 mm wide and disposed in the shape of a loop, with the antenna feed point  7  in the form of a widened-out surface, to which the central conductor of a coaxial feed cable is connected. The length L of the conductor loop is, for example, 120 mm, while the width B of the conductor loop, that is to say the spacing of the two rectilinear conductor portions, disposed parallel to each other, is about 10 mm. The loop-shaped antenna conductor  6  is illustrated as being disposed in an upper corner zone of the glass pane  1  at an angle of about 45° to the horizontal, but it can be disposed at any other position on the glass pane and basically can be oriented at any angle to the horizontal. If conditions allow, the antenna conductor  6  is preferably orientated at a right angle to the horizontal. 
     The electrical counterbalance to the antenna conductor  6 , formed as a folded unipole, is formed by the conductive area  8 , which also is of the same electrically conducting baking finish as the heating conductors  2  and the antenna conductor  6 , and is also applied on the glass surface in the same process step as the other conducting structures. The conducting area  8  consists of a strip, 1 to 3 cm wide, along the edge of the pane. The length  1  of this strip-shaped area  8  is not critical, but should preferably be at least about 120 mm. Approximately in its middle, the conductive area  8  has a flat projection  9  pointing towards the center of the surface of the pane, which forms at its end an annular area  10 . The annular area  10  is provided with a radially orientated slit  11 , through which the antenna conductor  6  extends. The feed point  7  is located inside the annular area  10  and concentric to it. The end region  6 ′ of the loop-shaped antenna conductor  6  is connected to the projection  9  of the conducting area  8 . 
     The heating conductor structure and the antenna conductor structure are on the surface of the glass pane facing towards the passenger space. If the glass pane is installed in the window frame of the automobile body by the bonding technique generally employed today, the conductive area  8  is connected to the adhesive bonding cord and, via the adhesive cord, to the metallic frame flange of the automobile body via a large-area connection. Since the adhesive compounds commonly used today have a certain electrical conductivity, the conductive area  8  is electrically conductingly connected to the body by a conductive adhesive layer. In this manner the antenna is directly electrically attached to the bodywork. 
     In FIGS. 3 and 4 is illustrated a laminated glass pane  15  with a wire antenna constructed according to this invention. The laminated glass pane  15  consists of the single glass sheet  16 , facing outwards in the installed condition, of the glass sheet  17  facing towards the passenger compartment, and of the thermoplastic intermediate layer  18  connecting the two glass sheets  16 ,  17  together. The inner glass sheet  17  is provided, at its lower edge, with an edge recess  19 . In the thermoplastic intermediate layer  18  there is a metal wire  20  disposed in the form of a loop, that is in the shape of a folded unipole, the two ends of which project into the region of the edge recess  19 . The length L of the wire loop  20  once again depends upon the transmission frequency and, for example, is once again 120 mm, while the width B of the wire loop  20  is once again about 10 mm. 
     Along the lower edge of the pane, on the free surface  22  of the laminated glass pane, is disposed a strip-shaped, electrically conducting area  23 . This may be, for example, a coating of an electrically conducting paint or bakable electrically conducting finish. The electrically conducting area  23  may instead preferably be a metal foil which is firmly and permanently bonded to the glass surface  22 . The area dimensions of the electrically conducting area  23  are again not critical, but the length of the metal foil should be at least about 120 mm. 
     In the embodiment illustrated, the antenna is disposed at the lower edge of the pane, but it may instead be disposed at any other desired position of the laminated glass pane. It has been found that especially good results are achieved if the antenna is disposed at the upper edge zone of the laminated glass pane. 
     Within the edge recess  19  is disposed the end of an antenna feed cable  25  in the form of a coaxial cable. One end of the conductor loop  20 , i.e., the feed point of the antenna, is connected to the middle conductor  26  of the coaxial cable, while the other end  20 ′ is connected with both the sheath  27  of the coaxial cable  25  and the conducting area  23 . In order to make the electrical connection between the sheath  27  and the metal foil  23 , a metal foil strip  28  is connected by soldering to both the metal foil  23  and the sheath  27  of the coaxial cable  25 . The edge recess  19  is thereafter filled with a suitable age-hardening polymer and sealed in order to protect the junction point and to secure the coaxial cable  25  to the glass pane. 
     In the installed state of the laminated glass pane, the electrically conducting area  23  is again in electrically conducting connection over a large area, via the adhesive cord, with the metal frame of the automobile body, with the result that the antenna receives a clear reference potential. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.