Abstract:
An antenna disposed in a windowpane of a motor vehicle having an electrically conductive motor vehicle body having a direct current heating source. Disposed on the windowpane of the car is at least one heating field having at least one bus bar disposed on one side of the heating field. Connected to the bus bar at a connection point is a feeding network for feeding heating current into the bus bar. The feeding network is installed adjacent to the windowpane and comprises at least one magnetic core. Mounted on the at least one magnetic core is a primary winding which has a sufficient number of turns to transfer the high frequency, high impedance connection of the heating field. In addition, there is also a field compensation winding mounted on the at least one magnetic core, and is connected to a compensating current source so that this connection has no substantial effect in reducing inductive high resistence of this feed network and thus the high frequency reception of the antenna.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an antenna disposed on a windowpane of a motor vehicle having an electrically conductive motor vehicle body. The windowpane has a substantially rectangular or trapezoidal heating field that is provided on each side with a bus-bar and has bus-bar connections for feeding heating current on both sides. A heating direct current source is connected to the heating field and is electrically connected to the electrically conductive body of the motor vehicle. The current is fed on each side via an inductively high-resistance current feed network which is installed within proximity of the side edges of the windshield. The heating field is largely high-frequency insulated against the body of the motor vehicle with the help of current feed networks due to their high impedance so that the heating field can conduct high-frequency voltage, that is insulated from the body of the motor vehicle. 
     2. Description of the Prior Art 
     A heating field inductively connected in such a way can thus be designed as an antenna with the help of the current feed networks, as shown, for example in FIG. 1 of German Patent DE 36 18 452. The high-frequency coupling to a heating field conducting the high-frequency voltage, to form the antenna, can be accomplished, for example, by a connection to a bus-bar of the heating field. 
     It is found in automobile construction that interference signals frequently caused by the electronic noise of the automobile are coupled in via the longer current feed lines connected to the bus-bars without any HF-effective filter means. These interference signals disturb the reception in undesirable ways. The advantage offered by the current feed networks installed on the two sides near the bus-bar lies in the possibility of a high-frequency connection of the heating current feeds to the auto body on each side of the respective feed network, facing away from the heating field, without requiring the current to be conducted by longer lines on both sides of the heating field. 
     Furthermore, high-frequency impedance conditions can be defined on the bus bars. These conditions are not dependent upon the way in which the heating current lines are configured. However, the problems connected with this arrangement are in providing a inductance value for heating currents, with intensities of up to 30 A, particularly within the range of AM radio transmission. The required inductance cannot be realized in the conventional way with small antennas and with light-weight feed networks. The invention is based upon designing feed networks of high inductance that are constructed as small as possible. Also, at low frequencies, the feed networks should have efficient RF insulation, and have adequately low high-frequency losses and filament wattage losses. 
     SUMMARY OF THE INVENTION 
     The invention relates to an antenna disposed on a windshield of a motor vehicle having an electrically conductive body. The windshield antenna comprises a direct current heating source electrically connected to the motor vehicle body. Disposed on the windshield of the car is at least one heating field having a bus-bar disposed on one side of each heating field. Connected to the bus-bar at a connection point is a network for feeding heating current to the bus-bar. The network is installed adjacent to the windowpane and comprises at least one magnetic core. Mounted on the magnetic core is a primary winding which has a sufficient number of turns to transfer the high-frequency, high impedance connection of the heating field to the antenna. In addition, there is also a field compensation winding, mounted on the one magnetic core, and connected to a compensating current source so that this connection has no substantial effect in reducing the inductive high-impedance of this feed network. 
     In this case, the field compensation winding receives a flow of direct current so that the magnetic fields, resulting from the number of turns, their winding direction and the primary winding receiving the flow of heating current, act in an opposite direction relative to one another in the magnetic core. In addition, the magnetic fields are compensated for in the magnetic core, so that there is no interfering saturation effect, so that the antenna is formed either by the heating field itself or by a wire-shaped or flat conductor on the windowpane adjacent to the heating field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose several embodiments of the invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. 
     In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
     FIG. 1 a  shows a windowpane antenna having a feed network on each side of the heating field; 
     FIG. 1 b  shows the same arrangement as FIG. 1 a  except having a divided heating field with a T-fed network; 
     FIG. 2 a  shows a similar arrangement as FIG. 1 a  with a controller for setting the correct compensating current in each current feed network; 
     FIG. 2 b  shows a similar arrangement as FIG. 2 a  but with the same magnet cores disposed on both sides of the windowpane; 
     FIG. 2 c  shows a similar arrangement as FIG. 2 b  with a compensating direct current that is fixed with the help of a compensating resistor; 
     FIG. 2 d  shows a similar arrangement as FIG. 2 c  except that the compensating direct current flows in a connecting conductor in the opposite direction of the flow of heating current, from one side to the other side of the windowpane; 
     FIG. 2 e  shows a similar arrangement as FIG. 2 d  except that both sides of the magnetic cores are grounded; 
     FIG. 3 shows an arrangement similar to FIG. 2 e  except that the heating field is divided into first and second partial heating fields; 
     FIG. 4 a  shows the same arrangement as in FIG. 3 with a third partial heating field; 
     FIG. 4 b  shows an arrangement similar to FIG. 4 a  except that it provides a decoupling of the antenna signal by connecting the third antenna circuit to a bus-bar; 
     FIG. 5 shows an electric substitute circuit diagram of the arrangement shown in FIG. 4 b  for receiving low-frequency signals; and 
     FIG. 6 is a plot of the signal to noise ratio in dB, and frequency in MHz of an antenna receiving a medium wave radio transmission. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 a  shows the windowpane antenna of the invention, with feed networks  19  and  20  disposed on each side of a heating field  2 . Feed networks  19  and  20  have magnetic cores  9  and  10 , respectively, with primary windings  5  and  6  respectively, through which a heating current  24  flows. Field compensation windings  13  and  14  are mounted on cores  9  and  10 , respectively, with the compensating direct currents  17  and  18  flowing through the compensation winding for generating compensating magnetic fields  17   a  and  18   a  that adequately compensate the primary magnetic field  24   a  of the heating field (see FIG. 4 b ). 
     The use of magnetic cores on both sides of the heating field is necessary in order to reduce the size of the antenna. The extremely high heating current  24  flowing in primary windings  5  and  6  of feed networks  19  and  20  leads to a saturation phenomena in magnetic cores  9  and  10  that must be avoided. As shown in FIG. 1 a,  this is accomplished with a field compensation winding  13 ,  14 , through which the compensating dc current  17 ,  18  flows. This compensating direct current is adjusted so that the dc field in the magnetic cores  9  and  10  is compensated for by a set number of turns of field compensation windings  13  and  14 . Compensating current source  15  and  16  must be designed, in this connection, with a high resistance, so that the inductance of primary windings  5  and  6  are not substantially reduced when compensating current sources  15  and  16  are switched on. Magnetic cores  9  and  10 , designed without an air gap, are preferred so that primary windings  5  and  6  that are as small as possible, and with as little copper used as possible. Field compensation windings  13  and  14  can be designed in this connection as a winding with a thin wire, and a large number of turns, so that the product of compensating dc currents  17  and  18 , and the number of turns, corresponds with the product of heating current  24  and the number of turns of primary windings  5  and  6 . In FIG. 1 a,  a field  2   a  located closest to antenna  1  is needed, having hating current fed via feed networks  19  and  20 . 
     FIG. 1 b  shows the same arrangement as FIG. 1 a,  but with a divided heating field, the first partial heating field  2   a  being fed via feed networks  19  and  20 , and whose further partial heating field  2   c  is grounded in terms of high frequency to vehicle body  21 . 
     The embodiments of FIGS. 2 a  to  2   e  show different variations for adjusting the correct compensating dc currents  17  and  18  in field compensation windings  13  and  14 , so that the magnetic fields are adequately compensated for. 
     FIG. 2 a  shows an arrangement similar to FIG. 1 a,  with a controller for setting the correct compensating dc current  17  and  18  in current feed networks  19  and  20 . FIG. 2 a  has a measuring resistor  29  on each side of the circuit. The voltage across each resistor  29 , which is generated by heating current  24 , is compared with the voltage of a rated-value emitter  30  on controller  31 , and the output of controller  31  adjusts the controllable direct-current source  22 . The direct current source is highly resistive at high frequency so that the required field of compensation is obtained with the preset field compensation windings  13  and  14 , and primary windings  5  and  6 . On the left-hand side of FIG. 2 a , direct-current source  22  is controlled by a three-contact amplifier  26 . High resistance at high frequencies is provided by the height resistance of the source-sink path  27  of the controllable three-contact amplifier  26 . 
     FIG. 2 b  shows an arrangement similar to FIG. 2 a , with the same magnetic cores  9  and  10 , primary windings  5  and  6 , compensation windings  13  and  14 , and with a controller  31  being present only on one side. 
     The two field compensation windings  13  and  14  here are connected via a connecting conductor  41 , so that these windings are connected in series, with the same compensating dc current  17  and  18  flowing through both windings. In FIG. 2 b , the heating current  24  is supplied from voltage connection  11  of dc heating source  25  to heating field  2 . Heating field  2  is connected on the left-hand side to ground connection  12 . With this type of heating current feed, heating current  24  in heating field  2  and compensating dc current  17  and  18  in cross-connecting conductor  41  flow in the same direction, from one side of windowpane  23  to the other. The compensating effect of the magnetic fields in magnetic cores  9  and  10  produces in the windings the effect so that when voltage Ua is developing on primary windings  5  and  6  in the direction shown, the secondary voltages ü1*Ua, ü2*Ua each develop on field compensation windings  13  and  14  in the opposite direction. Compensating dc currents  17  and  18  are usefully selected based on a high number of turns in field compensation windings  13  and  14  so that it is substantially smaller than heating current  24 , and thus ü1 and ü2 are substantially greater than 1. 
     FIG. 2 c  shows an arrangement similar to FIG. 2 b , with compensating direct-currents  17  and  18  being fixed with the help of a compensating resistor  40 . The controllable three-pole amplifier  26  is thus replaced by compensating resistor  40 . This is possible when the voltages on field compensation windings  13  and  14  are equal, and occurs when the ratios of turns in feed networks  19  and  20  have identical values (ü1=ü2). In this case, the high resistance dc source can be replaced by a low-resistance source. 
     FIG. 2 d  shows an arrangement similar to FIG. 2 c , wherein the compensating direct currents  17  and  18  flow in connecting conductor  41  in the opposite direction of the flow of heating current  24 , from one to the other side of windowpane  23 , and the number of windings and the direction of the windings in field compensation windings  13  and  14  are selected so that the required compensation of the magnetic excitation caused by heating current  24  is effected in magnetic cores  9  and  10 . Connecting conductor  41  is imprinted on the windowpane and installed with adequate spacing from heating field  2 . Compensating direct currents  17  and  18  flow through connecting conductor  41  in the same direction as heating current  24  in heating field  2 . Connecting conductor  41  conducts high-frequency voltage which, as compared to heating field  2 , is oppositely directed as against auto body  21 . For this reason, the capacitive coupling between connecting conductor  41  and heating field  2  should be kept as low as possible. Thus, the physical spacing between connecting conductor  41  and heating field  2  should be adequately large. 
     If voltage connection  11  and ground connection  12  are made available on each of the two sides of the heating field, a type of connection as shown in FIG. 2 e  is possible. 
     FIG. 2 e  shows an arrangement similar to FIG. 2 d , wherein compensating dc sources  17  and  18  flow in connecting conductor  41  in an opposite direction as heating current  24 , and the number of windings and the direction of the windings in field compensation windings  13  and  14  are in each case selected so that the required compensation is adjusted, or set. Connecting conductor  41  is imprinted on the windowpane and located with adequate spacing from the conducting frame of the window. Thus, the associated fields in the magnetic cores  9  and  10  compensate each other if the correct winding direction is selected for primary windings  5  and  6  and field compensation windings  13  and  14 . The voltages developing on primary windings  5  and  6  and on field compensation windings  13  and  14  will then have the same direction, as shown in FIG. 2 e . In this case, the capacitance between connecting conductor  41  and heating field  2  will be less damaging. 
     The invention is of special importance in connection with radio transmission services at where the dimensions of windowpane  23  are smaller than the received wavelengths by at least one order of magnitude. The inductive effects of heating field  2  are then negligible, and the heating field will serve as a quasi-potential surface. In a particularly advantageous embodiment of the invention, the connecting conductor  41  is designed in the form of a partial heating field, for example in the form of the second partial heating field  2   b  as shown in FIG.  3 . 
     FIG. 3 shows an arrangement similar to that of FIG. 2 e , with the heating field  2  divided into a first partial heating field  2   a  and a second partial heating field  2   b . The compensating direct current  17 ,  18  is conducted in the opposite direction of the flow of heating current  24  in the first partial heating field  2   a  by the suitably poled connection of partial heating field  2   b  to the heating dc current source  25 . For this purpose, ground connection  12  and voltage connection  11  of heating dc source  25  are required on both sides of the windowpane. The number of turns and the direction of the windings of primary windings  5  and  6  and field compensation windings  13  and  14  are selected so that the heating current primary magnetic field  24   a , generated by the heating current, and the compensating magnetic fields  17   a  and  18   a , generated by the compensating dc current, largely compensate one another in magnetic cores  9  and  20 . The magnetic effects of the inductive HF-current of the first partial heating fields  35  and  37 , and of the inductive HF-current of the second partial heating fields  36  and  38 , the latter HF-current being directed in the same direction as the former, support each other in magnetic cores  9  and  10 . 
     Particularly favorable dimensioning is obtained if the heating field is divided in two approximately equal sized partial areas so that the ratio of turns ü1, ü2 between primary windings  5  and  6  and field compensation windings  13  and  14  have the value ü1=ü2=1. Then, the compensating direct current  17 ,  18  in the second partial heating field  2   b  will have about the same value as the heating current  24  in the first partial heating field  2   a . In this arrangement, it is necessary that both voltage connection  11  and ground connection  12  are available on both sides of the windowpane. In the circuit shown in FIG. 3, heating current  24  and compensating direct current  17 ,  18  in the two adjacent partial heating fields flow in opposite directions relative to each other. If primary windings  5  and  6  and field compensation windings  13  and  14  are identically designed on the two sides of windowpane  23 , the magnetic fields in the magnetic cores  13 ,  14  will cancel each other. With equally sized partial heating fields and the same type of design of feed networks  19  and  20  on both sides of the windowpane, the capacitance Ck between the first partial heating field  2   a  and the second partial heating field  2   b  will not affect the HF-voltage developing on primary windings  5  and  6  and field compensation windings  13  and  14 . 
     FIGS. 4 a  and  4   b  show different ways of decoupling the antenna voltages. 
     FIG. 4 a  shows an arrangement similar to FIG. 3, with a first partial heating field  2   a , a second partial heating field  2   b , and with an additional partial heating field  2   c  which is grounded in terms of high frequency. The connections to voltage connection  11  are made in each case via a filter reactor or coil  34   b , and the high frequency grounding is made via a filter capacitor  34   a . The antenna signal is decoupled via a decoupling winding  39  located on the magnetic core  9  or  10  in the further-conducting antenna circuit  32 . 
     In FIG. 4 a , a transmitter, located between the primary winding  5  and the field compensation winding  13  on the common magnetic core  9 , is supplemented by the decoupling winding  39 . Decoupling winding  39  is loaded with the effective capacitance Cv of amplifying electronic circuit  42  in the further-conducting antenna circuit  32 . The amplified antenna signals are available in antenna connection line  33 . To explain the mode of operation, the inductive HF-current of the first partial heating field  35 ,  37 , and the inductive HF-current of the second partial heating fields  36  and  38  are shown on both sides of windowpane  23 . These currents flow through the primary windings  5  and  6  and field compensation windings  13  and  14 , and they generate in magnetic cores  9  and  10  the HF primary magnetic field  35   a ,  37   a , and, respectively, the HF secondary magnetic field  36   a ,  38   a . The HF primary magnetic field  35   a ,  37   a  and the HF secondary magnetic field  36   a ,  38   a  each are equally directed in magnetic cores  9  and  10 . These fields support each other in forming the inductance for the high-frequency insulation of the two partial heating fields against body  21  of the motor vehicle. This type of connection for the heating current has voltage connection  11  and ground connection  12  available on both sides. The heating currents  24  and  17  are directed opposite each other in the two partial heating fields  2   a  and  2   b . In addition, the associated heating-current primary magnetic field  24   a  and the compensating magnetic field  17   a  and, respectively,  18   a , are then directly opposing each other, and cancel one another out. In view of electromagnetic compatibility, voltage connections  11  in FIG. 4 a  each are supplied with filtered voltage by the filter choke  34   b  in association with filter capacitor  34   a . This applies also to further partial heating field  2   c , which becomes grounded at high frequency, and connected on one side to ground connection  12 , and supplied with filtered voltage on the other side of voltage connection  11 . Mounting the filter capacitors  34   a  and voltage connections  11  near the bus-bars of the heating fields is advantageous in view of preventing interference from being coupled in via the on-board network. 
     FIG. 4 b  shows an arrangement similar to FIG. 4 a , except there is a decoupling of the antenna signal by connecting the further-conducting antenna circuit  32  to a bus-bar  3   a  of the first partial heating field  2   a  with the help of a transmitting element with a suitable ration of windings üv. 
     In FIG. 4 b , the antenna signals are decoupled from a first partial heating field  2   a —which is insulated in terms of high frequency—via the primary windings  5  and  6 , with the help of a transmitter with ratio of windings üv, and transmitted to the further-conducting antenna circuit  32 . Decoupling takes place between the bus-bar of the first partial heating field  3   a  or  4   a , and body  21  of the vehicle. Again, with the same number of turns of primary windings  5  and  6  and field compensation windings  13  and  14 , the HF-voltages on the first partial heating field  2   a  have to be equal to those on the second partial heating field  2   b . Thus, the transmitter located in the further-conducting antenna circuit  32  could also be connected to one of the bus-bars  3   b ,  4   b  of the second partial heating field  2   b.    
     FIG. 5 shows an electrical equivalent circuit diagram of the arrangement shown in FIG. 4 b  for low-frequency received signals (e.g., in the AM frequency range). Coils L 1   a  and L 2   a  form the inductances based on primary winding  5  and, respectively, primary winding  6 , with field compensations windings  13  and  14  being on open-circuit. The ratios of windings ü1 and ü2 each result from the ratios of the numbers of turns of field compensation winding  13  and, respectively,  14  to primary windings  5  and  6 , respectively. Rigid coupling with negligible scatter is assumed between the two windings in each case. The first partial heating field  2   a  and the second partial heating field  2   b  each are shown by the thick lines, which show that the received voltage of the heating fields is the same on the left-hand and right-hand sides of windowpane  23 . The voltage Ua of the first partial heating field  2   a  and the voltage Ub of the second partial heating field  2   b  are determined via the ratio of windings ü1. The ratio of windings is given by the ratio of the number of turns of primary windings  5  and  6  to the number of turns of field compensation windings  13  and  14  on the right-hand side, and by the excitation E*heffa for the first partial heating field  2   a  with its self-capacitance Ca, and by excitation E*heffb for the second partial heating field  2   b  with its self-capacitance Cb. Furthermore, capacitance Ck is effective as a coupling capacitance between the two heating fields. The connection of transmitter uv for decoupling the antenna signals Uv via decoupling winding  39  is connected in parallel with the first partial heating field  2   a . As the received signals are flowing in, such an inflow being effected by the electromagnetic field intensity E, the self-inductance L 1   a  of the primary winding  5  and its loss factor δ 1   a  are important on the right-hand side of windowpane  23 . In addition, this also depends upon the self-inductance L 2   a  of primary winding  6 , and its loss factor  62   a  on the left-hand side. 
     In the special case where the first and second partial heating fields  2   a  and  2   b  are equally sized, and identical primary windings  5  and  6  are present on both sides of windowpane  23 , field compensation windings  13  and  14  can also be designed the same way as primary windings  5  and  6 . The following applies approximated in the application of such a particularly important case: 
     Ca=Cb=C, ü1=ü2=1, L 1   a =L 2   a =La=L, δ 1   a =δ 2   a =δa=δ, and heffa=heffb=heff. 
     With inclusion of a suitable value for uv, particularly favorable signal/noise ratios can be obtained under real conditions. This occurs at the output of amplifying electronic element  42  if the available total surface area for the first and second partial heating fields  2   a  and  2   b  is preset. In this case, Ua=Ub, and Ck has practically no effect. The system is optimized under such preconditions by creating an adequately high inductance L with a loss factor δ as low as possible. This is important particularly at the lower end of the frequency band for which the arrangement is conceived. With each of the two inductance, the loss factor represents a conductance loss factor of δ/(ωL), whose flow of noise into the parallel circuit substantially co-determines the signal-to-noise ratio obtained, especially at low frequencies. 
     In the following, the signal/noise ratio is determined on the output of the amplifying electronic element  42  in FIG.  5 . This is in the case that is to be preferred in practical use, where identically designed primary windings  5  and  6  and identical field compensation windings  13  and  14  are present on both sides of windowpane  23 . However, the second partial heating field  2   b  has to be designed differently from the first partial heating field  2   a . Therefore, the variables are as follows: 
     Ca; heffa; Cb; heffb; ü1=ü2=ü, L 1   a =L 2   a =L, δ 1   a =δ 2   a =δ. RT is the equivalent noise resistance of amplifying electronic element  42  with its effective capacitance Cv, and üv is the transmission ratio of the coupling. Resonance frequency fr results from the antenna capacities and capacitance Cv with inclusion of the winding capacitances and the two inductances L.          2      π                 fr     =       2     L   ·     [     Ca   +         u   ¨     2     ·   Cb     +       (     1   -       u   ¨     2       )     ·   Ck     +       u   ¨            v   2     ·   Cv         ]                                  
     The relative signal/noise ratio, as compared to an active antenna with a received structure with capacity CA, an effective height h, and with an identical electronic amplifying element  42  with an effective capacitance Cv, and thus with an equivalent noise resistance RT, follows from the following equation:        SNRrel   =               [         2   ·   π   ·   fr   ·       (       heffa   ·   Ca     +       u   ¨     ·   heffb   ·   Cb       )     2         Ca   +         u   ¨     2     ·   Cb     +       (     1   -       u   ¨     2       )     ·   Ck     +       u   ¨            v   2     ·   Cv           ·     (       1   δ     +   δ     )     ·       (     f   fr     )     3       ]     ·                 RT     h   2       ·       (     1   +     Cv   CA       )     2                     1   +       RT       u   ¨          v   2         ·     [           (     1   -       fr   2       f   2         )     2     δ     +   δ     ]     ·       (     f   fr     )     3     ·   2   ·   π   ·   fr   ·                 [     Ca   +         u   ¨     2     ·   Cb     +       (     1   -       u   ¨     2       )     ·   Ck     +       u   ¨            v   2     ·   Cv         ]                                    
     FIG. 6 shows, by way of example, the curve of the relative signal/noise ratio in dB. Optimal values can be obtained in this example with üv=3 and ü=1. It was assumed in this example that the values for the effective heights heffa=heffb=10 cm, and CA=120 pF, was put equal to (Ca+Cb)=120 pF. The curve shows that with a sufficiently high quality (δ=0.045) of the inductance being chosen for fr=0.5 MHz, the S/N ratio can be enhanced versus the test arrangement by feeding the heating current as defined by the invention with the help of transformative coupling of electronic amplifier  42  of FIG. 5, with an equivalent noise resistance of RT=50 ohms and an input capacitance Cv=10 pF. 
     Magnetic cores ( 9 ,  10 ) are preferably made from a highly permeable, low-loss material (δ=0.045) at high frequencies with a closed iron path without any air gap. For example ferrite material Fi 262 (by Vogt). 
     The two primary windings ( 5  and  6 ) and the two field compensation windings ( 13  and  14 ) can each be designed as bifilar windings with wires extending parallel to each other. 
     The further conducting antenna circuit ( 32 ) can be designed to receive a plurality of frequency ranges in the long, medium and short wave and ultra short wave ranges, and in the television range. 
     Accordingly, while several embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims.