Abstract:
An exterior rear view mirror for a motor vehicle comprises a case containing a reflective member and an antenna, consisting of an electrically conductive layer on a surface of the case, for transmitting and/or receiving radio frequency electromagnetic radiation. The electrically conductive layer has at least one zone having its periphery shaped in accordance with the frequency of said electromagnetic radiation.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. Ser. No. 09/404,101 filed Sep. 23, 1999. 
     
    
     
       FIELD  
         [0002]    This invention relates to an exterior rear view mirror for a motor vehicle comprising a case containing a mirror glass or other reflective member and an antenna for transmitting and/or receiving radio frequency electromagnetic radiation comprising an electrically conductive layer on a surface of the case.  
         RELATED ART  
         [0003]    An exterior mirror of this type is disclosed in GB-A-1590824. The case comprises a body moulded from plastics material having a bright copper layer deposited on substantially the whole of its outer surface for receiving a chromium-plated outer layer. The bright copper layer also serves as an antenna.  
         SUMMARY OF THE INVENTION  
         [0004]    According to the invention, in a rear-view mirror assembly of the type described above, the antenna comprises an electrically conductive layer on a surface of a rigid member forming part of the mirror assembly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a perspective view of a vehicle exterior mirror in accordance with the invention;  
         [0006]    [0006]FIG. 2 is a perspective view of the case of the mirror shown in FIG. 1;  
         [0007]    [0007]FIG. 3 is a perspective view of the mirror case shown in FIG. 2, from a different angle;  
         [0008]    [0008]FIG. 4 is a view into the mirror case of FIGS. 2 and 3 through the opening in which the mirror glass would be mounted;  
         [0009]    [0009]FIG. 5 is a cross-sectional view taken on the line  5 - 5  in FIG. 4;  
         [0010]    [0010]FIG. 6 is a cross-sectional view taken on the line  6 - 6  in FIG. 4; and  
         [0011]    [0011]FIG. 7 is a diagram illustration dimensions of a theoretical conical antenna. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    [0012]FIG. 1 shows a vehicle rear-view mirror comprising a reflective member  10  fitted in a case  11 , which is mounted on a bracket  12 , which is secured to one of the front doors  14  of a motor car  16 . As can be seen from FIG. 2, the case  11  has a clip  18  and guides  20  and  22  by means of which it is secured to an internal frame member (not shown) of the mirror assembly. In accordance with the invention, a first metallic foil element  30  is secured by adhesive to the inside surface of the outboard end of the case  11 . As can best be seen from FIG. 4, the element  30  is formed in two symmetrical halves  30   a  and  30   b  interconnected by a link part  32  which is connected by a coaxial cable  34  to a mobile telephone (not shown). The two symmetrical halves  30   a  and  30   b  approximate to a conical antenna. FIG. 7 shows a cone of half cone angle θ and length └. Theoretically └ should be equal to wavelength λ of the radio signals (i.e. the speed of light divided by the centre frequency). In practice, acceptable results are achieved if └ is three tenths of the theoretical value.  
         [0013]    The theoretical formula for the feed-point impedance (Zk) of a conical antenna is: 
           Zk= ( Zo /π)Ln cot  g (θ/2) 
         [0014]    where Zo is the free space impedance (377 ohms in air) and “Ln cot g” means “logarithmic cotangent”. 30° is a realistic practical value for θ.  
         [0015]    The relationship between the angle θ and the impedance of the antenna is linear. Good reception can be obtained if the actual feed-point impedance is between half and twice its optimum value. Provided this condition is met, it can be shown from FIGS.  8 - 15  on page 355 of John D. Kraus, “Antennas”, published by McGraw Hill, ISBN 0-07-0354-22-7, that, because the curve is practically linear in the area used, a practical optimised value Zo for the actual feed-point impedance is: 
           Zo= 1500(└/λ)−113-3 
         [0016]    It follows that the length └ of the foil elements  30   a  and  30   b  and the half-cone angle θ is: 
         ( Zo /π)Ln Cot  g θ/2=1500(└/λ)−113-3 
         [0017]    In practice, the length └ of foil element may be about three tenths of the wavelength λ.  
         [0018]    Two other foil elements  36  and  38  that are a mirror image of one another are secured by adhesive to the central and inboard parts of the interior surface of the case  11 . These foil elements  36  and  38  are connected by a cable  40  to a radio broadcast receiver, for example an FM radio receiver (not shown).  
         [0019]    In addition to the above two antennae, a third antenna  42  is connected by a cable  44  to a transponder (not shown) for an automatic road toll charging system; a fourth antenna  46  is connected by a cable  48  to a controller for the central door locking system for the car  16 . Another antenna  50 , positioned in the centre of the mirror case  11  is connected by a cable  52  to a digital radio receiver (not shown) while a further antenna  54  is connected by a cable  56  to a GPS receiver (also not shown).  
         [0020]    The antennae  42 ,  46 ,  50  and  54  are rigid antennae mounted within the case  11 . Although the various cables  34 ,  40 ,  44 ,  48 ,  52  and  56  are shown as parallel to one another, in practice they are gathered together so as to extend through the interior of the bracket  12  into the interior of the car  16 , where they are connected to their respective transmitters and/or receivers.  
         [0021]    Antennae comprising metal inserts moulded into the case  11  may replace the foil antennae  30 ,  36  and  38 . Another alternative is for the case or cover member to be formed as co-moulding of two different plastics material, only one of which will accept surface metallisation. A layer of metal is then deposited on this part to serve as the antennae.  
         [0022]    If the case has a separate decorative exterior cover member of the type described in European Patent Application No. 98302674.1, the electrically conductive layer may be formed either on the inner surface of such cover member or on that part of the outer surface of the mirror case which is enclosed by the cover member.  
         [0023]    Alternatively or additionally one or more antennae may take the form of a conductive layer on part of the mounting for the mirror glass or other reflective member.  
         [0024]    The mirror assembly may in addition incorporate other antennae for receiving GPS signals, remote operation of the car door locks and transponders for automatic road toll accounting systems. Some of these additional antennae may be formed as self-supporting rigid members.