BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a window antenna for a motor vehicle having a thin electrically conductive layer which is transparent and reduces heat transmission. 
     2. The Prior Art 
     Antennas of this type are known from German Patent Application No. P 197 35 395. If disposed in the front windshield they suffer from interference due to digitally operating vehicle equipment. In the case of a front engine, interference results from the ignition system. For this reason, rear window antennas have been used in the past. The heating elements of the window heater were used containing a conductive layer which reduces heat transmission. To prevent the unfavorable impedance conditions due to the supply of heating current, the heating current for rear window antennas must always be supplied through an RF choke circuit. This choke circuit is particularly complex, especially for frequencies in the LMS region. For this reason, flat antenna conductors are used to receive LMS signals, which in many cases are offset from the heating surface, as are known from U.S. Pat. No. 4,791,426. In the case of a continuous conductive coating however, the antenna cannot be used without additional measures. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an antenna having good reception characteristics both in the USW and TV region and in the LMS region while minimizing its complexity. 
     The antenna according to the invention provides special advantages compared with known antennas of the type which are based on heating areas with complex RF choke circuits in the region of long, medium, and short wave reception (LMS frequency region). By virtue of the long wavelength, the operating principle of an antenna according to the invention can be described by its capacitive effects, while the inductive effects can be disregarded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an antenna according to the invention on the lowerable window of a vehicle door. 
     FIG. 2 shows an antenna according to the invention with an electrode-in the form of an elongated narrow strip or wire. 
     FIG. 3 shows an antenna according to the invention with reduced capacitance between an area of limited conductivity and the metal window frame. 
     FIG. 4 shows an antenna according to the invention with an electrode coupled capacitively to an area of limited conductivity. 
     FIG. 5 shows an antenna according to the invention for the LMS frequency region as part of an antenna system with a USW antenna array having a USW/LMS antenna unit in the vicinity of a heated rear window pane of the motor vehicle. 
     FIG. 6 a  shows a circuit having electrical active components of an antenna with an electrode capacitively coupled to an area of limited conductivity. 
     FIG. 6 b  shows an electrical equivalent circuit diagram with an internal noise source of antenna, an amplifier and capacitance which is effective for providing an improved signal-to-noise ratio. 
     FIG. 6 c  shows a circuit having signal source transformed to the internal signal source which is effective at the location of the noise source of antenna amplifier, for determination of S/N in the form of the internal effective height. 
     FIG. 7 a  shows an antenna as in FIG. 6 a , having an electrode grounded to a layer of limited conductivity, and an antenna amplifier connected directly to the antenna connection point. 
     FIG. 7 b  shows an equivalent electrical circuit diagram, analogous to the circuit of FIG. 6 b.    
     FIG. 8 a  shows an antenna as in FIG. 6 a , but with an RF transformer with the smallest possible winding capacitance and an optimal step-up ratio disposed at the input of the antenna amplifier. 
     FIG. 8 b  shows an equivalent electrical circuit diagram analogous to the circuit of FIG. 6 b.    
     FIG. 9 shows a diversity antenna array according to an additional embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1-5, there is shown a vehicle door  17  with its window frame  2 , which is shaded in the diagram. By means of a window lifter  14 , window  1  can be lowered into bottom part  18  of vehicle door  17 . The window covered with the conductive layer is electrically insulated all around, from window frame  2  and from window lifter  14 . The normal position of the window is the closed condition (see FIG.  1 ). An antenna according to the invention has an electrode  6  on the lowerable window of a vehicle door  17 , formed in a free area  11  of the window. Electrode  6  is connected via electrode lead  7  to an antenna lead  8  on the conductive layer. Antenna cable  15  is connected with its first lead to antenna lead  8 , and with its second lead to a ground point  3  of the vehicle body. Antenna cable  15  is usually brought out of vehicle door  17  through a rubber grommet  23 , and routed to receiver  25 , which can be mounted in a region such as the dashboard. Vehicle door  17  is attached with hinges  22  to the vehicle body. 
     FIG. 9 shows a diversity antenna array according to an additional embodiment of the invention. In this embodiment, at least two electrodes  6  with a sufficiently large distance between one another are formed in the free area  11  to form a diversity type array of at least two antennas on the windowpane. 
     By virtue of the layer which reduces heat transmission in combination with the air-conditioning system, the windows are opened very infrequently during driving. Reception is often very adequate even with the window three quarters open. By combining the signals of a plurality of window panes such as rear window  21  in the LMS region, or by using a plurality of inventive antennas in different door windows of a vehicle in a USW or TV diversity-type antenna system, the probability of reception loss is very small. Therefore, a very high-performance and inexpensive antenna system can be designed. Rear window  21  shown in FIG. 5 is a heated rear window pane that also serves as an antenna. 
     FIG. 6 a  schematically illustrates the no-load voltage Eh eff A  across area  4  of limited conductivity at a received field strength E, the space capacitance C A  of area  4  of limited conductivity, the capacitance C R  between area  4  of limited conductivity and window frame  2  as well as the coupling capacitance C K  between capacitive electrode  6  and area  4  of limited conductivity. The space capacitance C A  is designated for the time being as the capacitance that can be measured at an area  4  of limited conductivity when the window is closed, assuming the coating of window pane  1  does not cover a window border which is sufficient in window frame  2 . A customary value of C A  is approximately 60 pF to 120 pF. The no-load voltage measured under these conditions per unit of received field strength E is the effective antenna height h eff A  with typical values of between 3 and 4 cm. The capacitance C R  for the coating of the border region is typically 100 to 250 pF depending on the construction of the window seal. 
     The received signals are conducted through antenna cable  15  to antenna amplifier  10  connected at its end. Amplifier  10  is contained, for example, in a USW antenna unit  12  as shown in FIG.  5 . The capacitance of antenna cable  15  is denoted by C L  and the active capacitance at the input of antenna amplifier  10  with C V . Typical values for C V  range between 5 and 20 pF and those for the cable capacitance range between 100 and 150 pF. The effective value of the noise voltage active at the “internal” amplifier element with an equivalent noise resistance R a  is expressed for a bandwidth B by u r , where: 
     
       
         u r ={square root over (4+L ·k·T·B·R a +L )}  (1) 
       
     
     where k=Boltzmann constant, T=temperature in K. 
     For a simple and inexpensive embodiment of the invention, an electrode  6  having a form such as a conductive film, is adhesively bonded to one of the outer surfaces of the glass sandwich structure in order to create a capacitive connection between area  4  of limited conductivity and antenna connection point  8 . In the process, electrode width  9  (FIG. 1) and electrode length  5  (meaning the electrode area) are given values sufficiently large to create an adequate coupling capacitance C K . The electrical equivalent circuit diagram for determination of the signal-to-noise ratio S/N is illustrated in the reception situation in FIG. 6 b  and in FIG. 6 c  with the excitation Eh eff IV  transformed at the point of action of the noise voltage. As a measure of the sensitivity, there can be used the limit field strength E g  for S/N=1, and so: 
     
       
         E g =u r /h eff IV   (2). 
       
     
     In the interests of adequate sensitivity, the internal effective height h eff IV  should not be smaller than 1 cm at the available values of R a  of modern low-noise amplifier elements. A standard rod antenna of 90 cm geometric length in the rear region of a car corresponds, for example, to an internal effective height h eff  of about 3 to 4 cm, allowing for the cable capacitance at the amplifier input of a car radio. In the antenna, therefore, substantial importance is attached to transformation of the excitation Eh eff A  achieved by area  4  of limited conductivity. 
     From FIG. 6 b  there can be derived the following relationship for h eff IV :                h     eff                 IV       =       h     eff                 A         1   +         C   R     +     C   L         C   A       +       (     1   +       C   R       C   A         )     ·       C   L       C   K         +         C   V       C   A       ·     (     1   +         C   A     +     C   R         C   K         )                   (   3   )                                
     By virtue of the capacitive load C L  due to antenna cable  15 , the coupling capacitance C K  should have values on the order of several 100 pF for a few meters of cable length, in order that h eff IV  will not have too small a value as a result of too high a value of C L /C K . This leads to a relatively large electrode area which, assuming a glass thickness of about 2 mm and a dielectric constant of 7, yields approximately 
     
       
         A E =0,32 cm 2 /pF  (4) 
       
     
     In an advantageous embodiment of the invention, it is therefore practical (see FIG. 7) to avoid the sensitivity-reducing effect of antenna cable  15  by connecting antenna amplifier  10  directly to capacitively coupled electrode  6 . As also illustrated in FIG. 4, antenna amplifier  10  is then connected directly to electrode  6 . Coupling capacitance C K  should then be large only compared with the sum of the space capacitance C A  and the capacitance C R  of area  4  of limited conductivity relative to window frame  2 . As regards the sensitivity of the antenna, an internal effective length h eff IV  is given by:                h     eff                 IV       =       h     eff                 A         1   +       C   R       C   A       +         C   V       C   A       ·     (     1   +         C   A     +     C   R         C   K         )                   (   5   )                                
     If capacitively coupled electrode  6  is replaced by an electrode  6  coupled galvanically to area  4  of limited conductivity, this can be accomplished by laying a narrow strip-like or wire-like electrical conductor in the glass sandwich structure of the laminated glass pane so that electrode  6  is in contact with the conductive layer over a sufficient electrode length  5 . This is advantageous, in particular when free area  11  (of FIG. 1) provided for attachment of an electrode is very narrow, especially in its vertical extent. If antenna amplifier  10  is connected directly to electrode  6 , as illustrated in FIG.  4  and FIG. 7 a , the following relationship is obtained instead of equation (5) for the internal effective height h eff IV :                h     eff                 IV       =       h     eff                 A         1   +       C   R       C   A       +       C   V       C   A                   (   6   )                                
     However, in practice, it may often be less complex for antenna amplifier  10  to be connected to the end of antenna cable  15 , as in FIG. 6 a , and not to the movable window. In this case, considering the sensitivity of the receiving antenna, the absence of coupling capacitance C K  is particularly favorable. In order to minimize the influence of the contact resistance between electrode  6  and area  4  of limited conductivity, it may be preferable to choose an electrode length  5  corresponding substantially to the entire length extent of the window, as illustrated in FIG.  3 . This is particularly important where antenna amplifier  10  is connected at the end of antenna cable  15 , since electrode  6  is additionally loaded by the capacitance C V  of antenna amplifier  10 . The operating principle of such an antenna according to the invention with galvanic coupling of electrode  6  to area  4  of limited conductivity has the following internal effective height h eff IV :                h     eff                 IV       =       h     eff                 A         1   +         C   R     +     C   L     +     C   V         C   A                   (   7   )                                
     In a-advantageous embodiment, antenna amplifier  10  is, as illustrated in FIG. 8 a , connected inexpensively to the end of antenna cable  15 , and there is provided at the input of antenna amplifier  10  a low-capacitance transformer  24  with optimal step-up ratio ü opt  in order to reduce the sensitivity-reducing effect of load capacitances C R  and C L . 
     The source which at the end of antenna cable  15  (see FIG. 8) energizes antenna amplifier  10  has, in the case of a capacitively coupled electrode  6 , a capacitance C III , where                C   III     =       C   L     +         C   A     +     C   R         1   +         C   A     +     C   R         C   K                     (   8   )                                
     The EMF active at the input terminals III—III′ of antenna amplifier  10  is expressed by the height h eff III  as follows:                h     eff                 III       =       h     eff                 A         1   +         C   R     +     C   L         C   A       +       (     1   +       C   R       C   A         )     ·       C   L       C   K                     (   9   )                                
     If the winding capacitance of transformer  24  which is active on the secondary side is given by C T  and the capacitance of the antenna amplifier which is representative of the signal-to-noise ratio is given by C V , then the internal effective height h eff IV  relative to the signal-to-noise ratio at the amplifier output can be described as follows:                      h     eff                 IV       =                    h     eff                 III       ·       u   ..     opt       2                 =                    h     eff                 A       2     ·         C   A         C   V     +     C   T           ·                                1       1   +       C   R       C   A       +         C   L       C   A       ·     (     1   +         C   A     +     C   R         C   K         )             ·     1       1   +         C   A     +     C   R         C   K                             (   10   )                                
     where                  u   ..     opt     =         C   III         C   V     +     C   T                   (   11   )                                
     Equation (10) provides that, in the case of inadequate coupling capacitance C K , or in other words when the coupling capacitance C K  cannot be made noticeably larger than C A +C R , especially at large cable capacitance C L , galvanic coupling of electrode  6  to area  4  of limited conductivity is preferable to provide the largest possible internal effective height h eff IV . Instead of equations (8), (9) and (10), the following relationships for C III , h eff III  and h eff IV  are obtained for the galvanic coupling: 
     
       
         C III =C L +C A +C R   (12) 
       
     
     
       
         
           
             
               
                 
                   
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     The optimal step-up ratio ü opt  of the transformer, even for the galvanic type of coupling, is in this case given by:                  u   ..     opt     =         C   III         C   V     +     C   T                   (   15   )                                
     Special importance is attached to the effect of capacitance C R  between window frame  2  and area  4  of limited conductivity. Both in capacitive and galvanic coupling, capacitance C R  acts to reduce the internal effective height h eff IV  of the antenna. It is therefore advantageous to make this capacitance as small as possible. If a border clearance  20  is provided between area  4  of limited conductivity and window frame  2  (FIGS.  3 - 5 ), then h eff A  becomes larger in all of the above equations in question, whereas C A  becomes smaller, and so at values of several centimeters, there are obtained larger values of h eff IV  than in the case of the initially mentioned definition of a small border clearance  20  from window frame  2 . 
     For stylistic reasons, the introduction of a border clearance  20  is somewhat more complicated in terms of vehicle engineering, since in practice, different tints are applied in border region  20  of window pane  1  and in the adjoining region of area  4  of limited conductivity. These color differences can be avoided, however, by providing the glass in border region  20  with an electrically neutral tint which corresponds to the color of area  4  of limited conductivity, or by interposing in the border region  20  of the glass sandwich structure, a plastic film which is electrically neutral but which also simulates the tint of area  4  of limited conductivity. 
     If the no-load voltage measured with adequate border clearance  20  (≧0.5 cm) from window frame  2  to area  4  of limited conductivity is represented by Eh eff A , the effect of the border is included in this measurement, and the border capacitance can be inserted as C R =0. If in addition, antenna amplifier  10  is then connected directly to antenna lead  8 , meaning that C L →0, albeit by means of high-frequency transformer  24  such that the step-up ratio is still ü opt , the internal effective height h eff IV  from equation 14 becomes                h     eff                 IV       =         h     eff                 A       2     ·         C   A         C   V     +     C   T                     (   16   )                                
     and therefore usually exceeds the transformed internal effective height h eff  of a rear rod antenna with a length of about 90 cm that would be calculated taking into consideration the cable capacitance C L  at the receiver input. 
     The window pane ( 1 ) may preferably be constructed of two thinner glass panels joined together and having a transparent plastic film ( 4 ) disposed between them of limited conductivity. 
     Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.
The invention relates to an antenna on the window of a motor vehicle having a thin electrically conductive layer which is transparent for light, but reduces heat transmission. The window is formed by a window pane which can be lowered into the bottom part of a vehicle door, and can be moved by a window lifter. The pane is covered with an area having a limited conductivity formed by a layer of limited conductivity. An antenna connection point is formed between a horizontal sealing strip placed at the lower border of the window aperture and the window lifter in a free area which exists when the window is closed. The connection point is connected by high-frequency, low-loss means to the area of limited conductivity.