Patent Publication Number: US-6215450-B1

Title: Glass window antenna system for motor vehicles

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a glass window antenna system for motor vehicles, particularly to increase the sensitivity for an AM band in the system which may receive both AM and FM bands. 
     2. Description of the Prior Art 
     As a conventional glass window antenna system for motor vehicles which may receive both AM and FM bands, there is a glass window antenna system in which antenna patterns are provided in proximity to a defogging device (hereinafter referred to as “defogger”) so as to be capacitively coupled thereto. The defogger is consisted of heater wires and bus-bars provided on a rear window, the bus-bars applying a current to the heater wires. A choke coil is provided between the bus-bars and a DC power supply for the defogger. 
     This type of conventional glass window antenna system has various problems such as the decrease of S/N ratio due to an engine noise for the defogger, the decrease of a sensitivity for a FM band due to an interference between the antenna patterns and the defogger, and the difficulty for making the antenna system compact due to the big and heavy choke coil. 
     In order to dissolve these problems, there is provided a glass window antenna system for motor vehicles which may receive both AM and FM bands in Japanese laid-open publication No. 9-181513. There is shown in FIG. 1 the antenna system disclosed in this publication. The antenna system comprises a glass plate of a rear window  1 , a circuit mounting component  2 , an antenna terminal/power supply terminal box  3 , a junction box for a second AM antenna  4 , a bus-bar  5 , a FM antenna  6 , a first AM antenna  7 , a second AM antenna  8 , a receiver  9 , a heater wire  10 , a flexible circuit board  11 , and a DC power supply  12 . 
     FIG. 2 shows a circuitry of the circuit mounting component  2 . The circuitry comprises an AM resonance inductor  13 , a damping resistor  14 , a high frequency inductor  15  to compensate the decrease of an impedance in an AM band wherein the AM resonance inductor  13  becomes capacitive, a damping resistor  16 , an AM resonance inductor  17 , a coupling capacitor  18 , a connection line  19  on the flexible circuit board  11 . 
     According to this glass window antenna system for motor vehicles, the first and second AM antennas  7 ,  8  provided on the glass plate  1  are connected together by the line  19  on the flexible circuit board  11 . As a result, a series resonance is caused by the stray capacitance for the AM antennas  7 ,  8  and the inductance of the AM resonance inductor  13 , and a parallel resonance is caused by the stray capacitance for AM antennas  7 ,  8  and the inductance of the AM resonance inductor  17 . By these two kinds of resonance, i.e. the series resonance and parallel resonances, a flat sensitivity characteristic is achieved for one received frequency band. Therefore, both the AM antennas  7 ,  8  and the FM antenna  6  may be used for receiving an AM broadcast while increasing the sensitivity thereto. 
     The conventional glass window antenna system disclosed in the Japanese laid-open publication No. 9-181513 has utilized both series and parallel resonances for receiving an AM band, so that it is difficult to set appropriately the inductance values of the resonance inductors  15 ,  17  in order to establish both series and parallel resonance conditions, respectively. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to increase the sensitivity for an AM band in a glass window antenna system for motor vehicles by means of an extremely simple structure in a limited space other than the defogger on a rear window. 
     Another object of the present invention is to increase the sensitivity for an AM band in a very simple structure by using not only a rear window but also a side window(s). 
     According to the present invention, a glass window antenna system for motor vehicles comprises one AM/FM antenna pattern provided on a rear window which mainly receives a FM band while maintaining an AM receiving characteristic, and one or more AM antenna patterns provided on the rear window. The total occupied area of both the AM/FM antenna pattern and the AM antenna patterns has at least 0.2 m 2 . An AM voltage received by the AM/FM antenna pattern and an AM voltage received by the AM antenna patterns are synthesized by superimposing the AM received voltage of the AM antenna patterns to the AM received voltage of the AM/FM antenna pattern through a low-pass filter. 
     The receiving sensitivity may be increased by means of a very simple structure, because at least one AM/FM antenna pattern and one or more AM antenna patterns are provided to synthesize the AM voltages received by these antenna patterns. The sensitivity may be further enhanced by increasing the magnitude of synthesized voltages using a resonance circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional antenna system. 
     FIG. 2 shows a circuitry of a circuit mounting component. 
     FIG. 3 shows a first embodiment of the present invention. 
     FIG. 4 shows occupied areas of the antenna patterns. 
     FIG. 5 shows a graph designating an enhancement of a sensitivity characteristic. 
     FIG. 6 shows a second embodiment of the present invention. 
     FIG. 7 shows a third embodiment of the present invention. 
     FIG. 8 shows a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     FIG. 3 shows a glass window antenna system of the first embodiment according to the present invention. This antenna system comprises one AM/FM antenna pattern  23  on an upper space to an area occupied by a plurality of defogging heater wires  22  provided at the center area of a glass plate  21  of a rear window, and an AM antenna pattern  24  on a lower space to the area occupied by the defogging heater wires  22 , as viewed in the figure. It should be noted that the word “AM/FM antenna pattern” means the antenna pattern which is capable of receiving both AM and FM bands. The AM/FM antenna pattern  23  mainly receives a FM band while maintaining an AM receiving characteristic. In this case, the antenna pattern has a fork-shape. The distance x between the bottommost element of the AM/FM antenna pattern  23  and the topmost element of the heater wires  22 , and the distance between the topmost element of the AM antenna pattern  24  and the bottommost element of the heater wires  22  are both in the range of 10-30 mm. 
     The total occupied area of these antenna patterns  23 ,  24  must be larger than 0.2 m 2 . It should be noted that the word “occupied area” means area to be enveloped by the antenna pattern. In FIG. 4, there is shown each area occupied by the antenna patterns  23 ,  24  in a shaded manner by oblique lines, respectively. 
     While it is desirable for an AM receiving characteristic that the occupied area of antenna patterns is as large as possible, the occupied area is naturally limited because the antenna patterns are provided in a small space. If the occupied area has at least 0.2 m 2  as stated above, then a desired receiving characteristic may be obtained. It should be noted that the occupied area of antenna patterns can not exceed the area of the space other than the defogger on the rear window. 
     According to this embodiment, the occupied area of the AM/FM antenna pattern  23  is 0.13 m 2  and that of the AM antenna pattern  24  is 0.17 m 2 , resulting in the total area of 0.30 m 2 . 
     The received voltage in an AM band is obtained by synthesizing both voltages received by these antenna patterns, respectively. The synthesize is carried out by superimposing these received voltages. In this case, only the received voltage in AM band is derived from the AM antenna pattern  24  through a low-pass filter L 3  consisting of an inductor, and then is superimposed to the voltage received by the AM/FM antenna pattern  23 . The low-pass filter L 3  causes the received voltage of AM antenna pattern  24  not to affect the high-frequency voltage of the AM/FM antenna pattern  23 . The synthesized voltage is sent to a tuner through a coaxial feeder (not shown). 
     When the AM broadcast is received in the above-described glass window antenna system for motor vehicles, the AM received voltage from the AM/FM antenna pattern  23  and the AM received voltage from the AM antenna pattern  24  are synthesized. The synthesized voltage becomes larger than respective received voltages of the AM/FM antenna pattern  23  and the AM antenna pattern  24 . The graph in FIG. 5 shows a sensitivity characteristic prior to and after the synthesis, in the FIG. the ordinate showing the received voltages in dB and the abscissa a frequency in Hz. It is understood that the receiving sensitivity after the synthesis has been increased by 5 dB in the AM band of 522-1629 kHz. 
     Where the total of occupied area of the AM/FM antenna pattern  23  and the AM antenna pattern  24  is varied, the difference between the resulting sensitivity and a target sensitivity is shown in Table 1. It is apparent from Table 1 that the total occupied area is required to be larger than 0.2 m 2 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Area occupied by antenna 
                 Difference 
               
               
                   
                   
               
             
            
               
                   
                 0.1 m 2   
                 −3 dB  
               
               
                   
                 0.2 m 2   
                 0 dB 
               
               
                   
                 0.3 m 2   
                 3 dB 
               
               
                   
                 0.4 m 2   
                 6 dB 
               
               
                   
                   
               
            
           
         
       
     
     According to this embodiment, a desired AM receiving characteristic may be obtained by the synthesis of the AM received voltage of the AM/FM antenna pattern  23  and the AM received voltage of the AM antenna pattern  24 . 
     In order to increase the received voltage, a resonance circuit may by added. In FIG. 6, there is shown a second embodiment in which a resonance circuit  25  is added. The resonance circuit  25  in FIG. 6 comprises two resistors R 1 , R 2 , two inductors L 1 , L 2  and one capacitor C 1 . The resistors R 1 , R 2  are damping resistors for decreasing the Q of resonance point. The capacitor C 1  is a high-pass filter for passing the FM voltage received by the AM/FM antenna pattern  23  to the tuner. 
     The values of these inductors, capacitor and resistors are, by way of example, L 1 =68 μH, L 2 =390 μH, C 1 =56 pF, R 1 =5.1 kΩ, and R 2 =5.1 kΩ, respectively. The value of the inductor L 3  is 4 μH. The AM received voltage after synthesis is amplified by the resonance circuit  25  and sent to the coaxial feeder  26 . Using such resonance circuit further increases the receiving sensitivity than that after synthesis shown in FIG.  5 . 
     While the number of AM antenna patterns is one in the first and second embodiments, further AM antenna patterns may be added. FIG. 7 shows a third embodiment of the present invention, in which an AM/FM antenna pattern  33  and a first AM antenna pattern  34  are provided on an upper space to the heater wires  32  provided at the center area of the rear window glass plate  31 , and a second AM antenna pattern  35  on a lower space to the heater wires  32 . In this case, the AM/FM antenna pattern  33  has a substantially reversed T-shape, the first AM antenna pattern  34  has a fell down squared U-shape, and the second AM antenna pattern  35  has a fork-shape. 
     The AM voltage received by the first AM antenna pattern  34  and the AM voltage received by the second AM antenna pattern  35  are synthesized to the AM voltage received by the AM/FM antenna pattern  33 . At this time, the AM received voltage of the first AM antenna pattern  34  passes through an inductor L 4  as a low-pass filter, and the AM received voltage of the second AM antenna pattern  35  passes through an inductor L 3  as a low-pass filter. According to this embodiment, a resonance circuit  36  is added so that the received voltage is further increased after synthesis. The structure of this resonance circuit  36  is the same as that of the resonance circuit  25  as shown in FIG.  6 . 
     In each embodiment described above, the antenna patterns are provided on the rear window of motor vehicles, but the place where the antenna patterns are provided is not limited to the rear window. A fourth embodiment is shown in FIG. 8 where antenna patterns are provided on side windows. 
     An AM/FM antenna pattern  42  is provided on a first side window  41 , and an AM antenna pattern  44  is provided on a second side window  43 . Each of these antenna patterns  42 ,  44  has a U-shape extended around the peripheral of each of the windows. The structure of the synthesis circuit and resonance circuit is the same as that in the second embodiment. The occupied area of the antenna pattern  44  is denoted by dotted oblique lines in the figure. It is noted that each area occupied by the AM/FM antenna pattern  42  or the AM antenna pattern  44  is limited within the area of respective side window  41  or  43 . 
     It is also possible to provide the antenna patterns on both a rear window and a side window. In this case, an AM/FM antenna pattern is provided on a rear window, while an AM antenna pattern is provided on a side window. Alternatively, it is possible to provide an AM/FM antenna pattern on a rear window, a first AM antenna pattern on a first side window, and a second AM antenna pattern on a second side window. 
     Each shape of the AM/FM antenna pattern and the AM antenna pattern in the embodiments described above is shown by way of example, so that the shape of an antenna pattern is not intended to restrict to that of these antenna patterns. As described hereinbefore, any shape of AM/FM antenna pattern is allowed in which AM sensitivity characteristic is ensured without degrading FM sensitivity characteristic significantly. 
     While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.