Window glass antenna for automobile

Abstract: An antenna for an automobile rear window uses a coil (14,15) connected directly to a bus (5,6) for defogging heaters (3) used as an FM subsidiary antenna and a T-shaped impedance matching circuit (13) to provide a monotonic change of impedance within the FM frequency band. An inverted T-shaped FM main antenna (4) is employed in conjunction with the FM subsidiary antenna (3) to provide a diversity system to enhance receiving sensitivity in all directions.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a glass antenna for an automobile in which defogging heater wires on a rear window glass of the automobile are used as antenna elements. Description of the Prior Art JP-A- 3 104 301 and DE-A1-3 910 031 both disclose a reversed T-shaped antenna provided on an inner surface of an inner glass plate of the laminated glass or window and an embedded antenna provided between inner and outer glass plates. Defogging heaters extending between bars are provided on the inner surface. Neither citation teaches or suggests the provision of coils directly connected to the bars and also mounted on the window glass. The inventors proposed with Japanese patent application laid open No. 3-49402 (1991) a glass antenna for an automobile in which defogging heater wires on a rear window of the automobile are used as antenna elements. Figure 1 shows this type of the glass antenna for the automobile. In Figure 1, a plurality of heaters or wires 3 is printed on the defogging area 2 of the rear glass 1, while an FM antenna 4 is printed on the upper portion of the most significant heater 26. Each heater 3 has an end connected to a power bus 5 or 6 and another end connected to a relay bus 7. The power bus 5 is connected to a power source +B having for example 12 volts through a power line 8 and a choke coil 9. The power bus 6 is connected to ground through another power line 10, another choke coil 11 and a switch 12. The choke coils 9 and 11 have high impedance characteristics against AM frequency band to be received and are wound by several turns around the toroidal core for high frequency use. A decoupling capacitor 25 is coupled between the main power source +B and ground to reduce a power source noise. When the heaters 3 is used as a defogging of the rear glass 1, the switch 12 is tumed on to short or connect the choke coil 11 to ground. The 12 volts' voltage is then supplied between the power buses 5 and 6 through the choke coils 9 and 11 and the power lines 8 and 10 respectively to heat the upper group of the heaters 3 between the power bus 5 and the relay bus 7 and the lower group of the heaters 3 between the relay bus 7 and the power bus 6. When the heaters 3 is used as the AM antenna, an AM signal is picked up through the power line 8 or 10. Figure 1 also shows an impedance matching circuit 13 connected to an FM antenna provided on the rear glass 1 as proposed with "A widow glass antenna for automobile" Japanese utility model application laid open No. 2-64219 (1990) invented by the inventors including the present inventors. The impedance matching circuit 13 comprises a reactance circuit consisting of a coil and a capacitance variable or varactor diode so as to match an impedance matching for a radio receiver viewed from the FM antenna through a cable 27 at a random frequency by using a frequency selective signal applied from the radio receiver through the cable 27. The sensitivity of the FM antenna 4 as shown in Figure 1 has a bidirectional characteristic. Particularly, the direction perpendicular to that of the maximum sensitivity has a poor sensitivity. The FM antenna 4 is not suitable itself as a vehicle antenna in which coming up direction of an electric wave to the antenna is often changed by 360 degrees along running the automobile. It is therefore necessary to construct a diversity antenna system by using FM main and subsidiary antennas to complement their directivities. The FM main antenna uses the FM antenna 4 as shown in Figure 1 while the FM subsidiary antenna uses the AM antenna in the defogging area 2. However, the resistance (real) and reactance (imaginary) components of the antenna impedance at the feeding point adjacent to the upper portion on the power bus 5 of Figure 1 are changed within the FM frequency band as shown in curves A of Figures 2 and 3 respectively when the heaters 3 are used as the FM subsidiary antenna. They show parallel resonance characteristics each having a peak within the FM frequency band. When such an FM antenna showing the peak impedance characteristic is employed with the conventional dynamic impedance matching circuit to match the impedance of the transmission cable 27, the matching circuit can not trace at the respective frequency the non-monotonic impedance changed non-monotonically with respect to the frequency because of applying a saw-toothed voltage sweep upon a channel selection of the FM program. The mean gain value of the antenna system with the matching circuit is lower than that of the antenna system without the matching circuit over the FM frequency band. Since the choke coils 9 and 11 generally have an inductance of 600 to 1300 micro-henry and a stray of several to several ten picofarads, they have sufficient high impedance enough to prevent the AM signal induced by the heaters from going through the body of the automobile in the AM band, but have low impedance in the FM band and result in the FM signal going through the body. Therefore, sufficient antenna voltage with open state can not be obtained. The gain of the antenna system is lowered. SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a glass antenna having a monotonic change or simply increased or decreased resistance and reactance components of the antenna impedance versus frequency. It is another object of the invention to provide heater antenna having high open voltage within the frequency range on or higher than the FM frequency band. A glass antenna for an automobile according to the present invention comprises a main antenna having reverse T-shape, a subsidiary antenna consisting of defogging heaters, and a coil directly connected to a power bus of the heaters to supply energy to said heaters through the coil and power bus. The above, and other objects, features and advantages of the invention will be apparent in the following detailed description of illustrative embodiments of the invention which is to be read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic view of a rear glass of a conventional automobile; Figure 2 is a graph indicating the frequency characteristics of the real or resistance component of the antenna impedance consisting of conventional heaters and of the heaters according to present invention; Figure 3 is a graph indicating the frequency characteristics of the imaginary or reactance component of the antenna impedance consisting of conventional heaters and of the heaters according to present invention; Figure 4 shows a first embodiment of the rear glass antenna for an automobile applied to the present invention; Figure 5 is a graph showing size, sensitivity and impedance characteristics of the FM main antenna of Figure 4; Figure 6 shows a second embodiment of the rear glass antenna for the automobile applied to the present invention; Figure 7 is perspective view of an embodiment of the coil applied to the present invention; Figure 8 is a circuit diagram showing an embodiment of the dynamic impedance matching circuit; and Figure 9 is a circuit diagram showing another embodiment of the dynamic impedance matching circuit. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Figure 4 shows a first embodiment of the glass antenna for an automobile according to the present invention. The same numerals are denoted in parts corresponding to the those of Figure 1. In Figure 4, a plurality of heaters 3 each serving as AM and FM antennas is printed or coated on the defogging area 2 of the rear glass 1. The reverse T-shaped FM main antenna 4 is also printed or coated on the above portion adjacent to the upper heater 26. The reverse T-shaped FM main antenna 4 has bidirectional characteristic in which right and left directions perpendicular to the running or drive direction of the automobile have the highest receiving sensitivity. The impedance of the FM main antenna 4 is changed monotonically or with the parallel resonance characteristic within the FM frequency band due to the distance between a horizontal element of the antenna and the heater and the length of the horizontal element. Assuming that the length of the horizontal element of the FM main antenna 4, that is, its antenna portion parallel to the most significant heater 26 is L1 and the distance between its parallel antenna portion and the most closed heater 26 is L2, the area that the antenna impedance is changed simply or monotonically within 76 to 90 megahertz is oblique regions S as shown in Figure 5. Figure 5 shows the relation among the pattern size in the FM main antenna 4, gain of the antenna system in which the antenna 4 is directly connected to the transmission cable to supply a signal to the radio receiver and the antenna impedance thereof. Considering the bandwidth of matching gain and the peak value of the system gain upon applying the dynamic impedance matching circuit 13 to the FM antenna 4, the dimension in point A of Figure 5 was found to be superior. In the present embodiment, the L1 is predetermined to be 500 millimeter while the L2 is predetermined to be 10 millimeter. The heaters 3 serving as both the AM and FM subsidiary antennas have the highest receiving sensitivity along back and forth directions of the vehicle and is identical to that of the Figure 1 except for comprising coils 14 and 15. The coil 14 has an end connected directly to the power bus 5 and another end connected to the power line 8. The coil 15 also has an end connected directly to the power bus 6 and another end connected to the power line 10. These coils 14 and 15 may be 0.5 to 2.5 microhenry respectively. The signals induced by the FM main antenna 4 and the AM/FM subsidiary antenna 3 are supplied to a dynamic impedance matching circuit 13 provided on the glass surface. The dynamic impedance matching circuit 13 has a function of impedance matching between the input impedance of the radio receiver viewed through the transmission cable 16 and the FM antenna impedance at random frequencies within the FM frequency band by changing the capacitance of the varactor diode in the circuit based on the frequency selection signal from the radio receiver through the cable. The circuit 13 also has another function of controlling the resonance between the AM antenna and the radio receiver including the transmission cable 17 within the AM frequency band. In Figures 2 and 3, the curvature A represents frequency change of resistance and reactance components of the impedance of the conventional antenna consisting of the heaters 3 without using the coils 14 and 15. The curvature B represents frequency change of resistance and reactance components of the impedance of the present antenna consisting of the heaters 3 with using the coils 14 and 15 (1.6 microhenry). As apparent from the drawings, it is noted that the conventional antenna impedance having non-monotonic change including peak or a parallel resonant point within the FM frequency band of 76 to 90 MHz, is changed simply or monotonically by only adding or inserting the coils 14 and 15 between the buses 5 and 6, and the power lines 8 and 10 respectively. By using the coils 14 and 15, the impedance of the FM subsidiary antenna 3 is assumed to have the monotone change as shown in Figure 2 and 3, its system gain is enhanced so that the impedance matching between the subsidiary antenna and the radio receiver viewed through the transmission cable 16 at the random frequency within the FM frequency band is performed completely. It is because the dynamic impedance matching circuit can have an adequate frequency tracking characteristic that a certain voltage of the saw-toothed sweep corresponds to one of the FM frequency. Further, the impedance of the coils 14 and 15 are high sufficient to the FM band. The FM signal leaked to the body by the stray capacitance of the choke coils 9 and 11 is reduced. The system gain of the subsidiary antenna is further enhanced to increase the open voltage of the subsidiary antenna. The following table 1 represents a measurement A of the relative system gain in the system having the transmission cable directly connected to the defogging heater antenna 3 without the coils 14 and 15, and a measurement B of the relative system gain in the system having the transmission cable directly connected to the present defogging heater antenna 3 comprising the coils 14 and 15. Relative system gains (db) Frequency 76 78 80 82 84 86 88 90 A 46.4 45.2 44.4 41.2 44.2 45.4 46.0 46.0 B 47.2 46.4 47.1 46.3 48.3 49.3 50.2 50.7 B-A 0.8 1.2 2.7 5.1 4.1 3.9 4.2 4.7 Frequency unit (MHz), Gain unit (db = decibel) As seen from the table 1, the gain of the basic system having the transmission cable directly connected to the defogging heater antenna 3 is totally increased over the entire FM frequency band by using the coils 14 and 15 and, particularly, has great increase to the higher frequency portion of the FM frequency band. The following table 2 represents a calculation value A of the system gain improvement by the matching circuit in the system having the dynamic impedance matching circuit 13 connected between the transmission cable and the defogging heater antenna 3 without the coils 14 and 15, and a calculation value B of the system gain improvement in the system having the dynamic impedance matching circuit 13 connected between the transmission cable and the defogging heater antenna 3 comprising the coils 14 and 15. Increase of the system gain by the dynamic impedance matching circuit Frequency 76 78 80 82 84 86 88 90 A 2.6 2.2 1.1 -0.1 -1.4 -0.2 0.5 1.1 B 4.6 4.1 3.6 2.5 1.7 1.7 1.3 1.0 Frequency unit (MHz), Gain unit (db = decibel) As seen from the table 2, the gain of the system without the coils 14 and 15 is reduced compared to that without the matching circuit 13 at partial FM band because frequency tracking operation by the dynamic impedance matching circuit 13 is not suitably performed. The increase of the system gain by the combination of the matching circuit 13 and coils 14 and 15 is higher than that of the matching circuit 13 alone. As described the above tables 1 and 2, the gain of the present system comprising the defogging heater antenna 3 having the coils 14 and 15 and dynamic impedance matching circuit 13 connected between the antenna 3 and the transmission cable is higher than that of the defogging heater antenna 3 without the coils 14 and 15 over the entire FM frequency band. Figure 6 shows a second embodiment of the rear glass antenna for the automobile. This glass antenna is different from that of Figure 4 in picking up method of FM subsidiary signal. The AM signal is supplied to the matching circuit 13 through the power bus 5 while the FM subsidiary signal is supplied to the matching circuit 13 through a lateral T-shaped FM subsidiary pattern 18 paralleled to the power buses 5 and 6 to provide a capacitance coupling. The subsidiary pattern 18 may have a distance of 5 millimeter to the power buses 5 and 6 and an effective length 125 millimeter or total length of 250 millimeter. A feeder line from the center of the lateral T-pattern 18 to the matching circuit 13 is isolated from the lateral T-pattern 18 by for example 7 millimeter to prevent the interference therebetween. The FM main antenna 4 is connected to the matching circuit 13 through another feeder line having 3 millimeter width. The another feeder cable is disposed so that its parallel portion to the FM main antenna 4 is far from the antenna 4 to prevent the interference therebetween. The distance between the feeder cable and an upper marginal of the rear glass 1 is for example 35 millimeter. Figure 7 shows the coil 14 or 15 suitable for use in the present invention and surface mounted on the rear glass 1. The coil 14 may be a leadless component comprising a cylindrical insulating ceramic, two metal caps 20 and 21 each covered on the one side of the ceramic and a wire 22 such as an enamel line wound on the cylindrical surface of the ceramic. The both ends of the wire 22 are connected to the metal caps 20 and 21 by soldering or welding method. The end of the coil 14 or 15, or the cap 20 is connected to the power bus 5 or 6 by soldering while another end or the cap 21 is soldered directly to a land 23 printed or coated on the rear glass 1. The land 23 is connected to the power line 8 or 10 to provide the current to the heaters 3. A toroidal coil wound around a toroidal core by several turns can be employed instead of the leadless coil. In this case, ends of the coil are soldered onto the power bus 5 or 6 and the land 23 respectively. Figure 8 shows an embodiment of the T-shaped dynamic matching circuit 13. In the drawing, an input terminal 31 is connected to the glass antenna 3 or 4 while an output terminal 32 is connected to the transmission cable 16 or 27 as shown in Figure 4 or 6. The matching circuit 13 comprises a central capacitor 33 connected between a first node 51 and ground, and first and second variable reactance circuits as right and left branch configurations. The first variable reactance circuit consists of a coupling capacitor 34 connected between the input terminal 31 and a second node 52, a DC cutoff capacitor 35 connected between the second node 52 and a third node 53, a capacitor 36 and a varactor diode 38 each connected between the first and third nodes 51 and 53, a coil 37 connected between the first and second nodes 51 and 52. The second variable reactance circuit consists of a coupling capacitor 39 connected between the output terminal 32 and a fourth node 54, a DC cutoff capacitor 40 connected between the fourth node 54 and a fifth node 55, another coil 41 connected between the first and fifth nodes 51 and 55 and another varactor diode 42 connected between the first and fourth nodes 51 and 54. Resistors 43 and 44 each connected to ground apply a bias voltage to the varactor diodes 36 and 42 through a resistor 45. The resistors 43, 44 and 45 have for example 100 kilo-ohms or more. In this embodiment, in order to set the impedance viewed from the transmission cable 16 to be 75 ohms, the saw-toothed voltage sweep having the voltage range corresponding to the predetermined FM frequency band is applied to cathodes of the varactor diodes 38 and 42 as well as the inner varactor diode in the FM receiver. When an appropriate FM channel at a given frequency is selected by the FM receiver, the appropriate voltage is applied to the matching circuit to match the FM antenna with the FM receiver through the transmission cable at the given frequency. Reactance component of the antenna at the given frequency is therefore cancelled with controlled capacitance of the varactor diodes. For example, the resistors 43, 44 and 45 have 100 kilo-ohms. The capacitors 34 and 39 may have 1 to 50 picofarads, preferably 6 picofarads, while the DC cutoff capacitor 35 and 40 have 1 to 500 nano-farads, preferably 100 nano-farads. The capacitor 33 may have 5 to 50 picofarads, preferably 10 picofarads, while the capacitor 36 have 0 to 50 picofarads, preferably 2 picofarads. The coils 37 and 41 may have 100 to 300 nanohenry, preferably 200 nanohenry. Figure 9 shows another embodiment of the T-shaped dynamic matching circuit 13. In the drawing, an input terminal 56 is connected to the glass antenna 3 or 4 while an output terminal 73 is connected to the transmission cable 16 or 27 as shown in Figure 4 or 6. The matching circuit 13 comprises a central capacitor 64 connected between a first node 81 and ground, and first and second variable reactance circuits as right and left branch configurations. The first variable reactance circuit comprises a coupling capacitor 57 connected between the input terminal 56 and a second node 82. A coil 58 and a capacitor 61 are connected between the first and second nodes 81 and 82. Anodes of cathode common varactor diodes 59 and 60 are connected to the second and first nodes 82 and 81 respectively. The common cathode of the varactor diodes 59 and 60 is connected to the output terminal 73 through a resistor 63. The second variable reactance circuit comprises a coupling capacitor 72 connected between the output terminal 73 and a third node 83. Another coil 65 and a capacitor 68 are connected between the first and third nodes 81 and 83. Anodes of cathode common varactor diodes 66 and 67 are connected to the first and third nodes 81 and 83 respectively. The common cathode of the varactor diodes 66 and 67 is connected to the output terminal 73 through a resistor 71. Resistors 69, 62 and 70 each connected to ground are connected to the first, second and third nodes 81, 82 and 83 respectively to apply a bias voltage to the varactor diodes 59, 60, 66 and 67 through the resistors 63 and 71. In this embodiment, to set the impedance viewed from the cable 16 to be 75 ohms, the saw-toothed voltage sweep is applied to the common cathodes of the varactor diodes 59, 60, 66 and 67 as well as the inner varactor diode in the FM receiver. The sweep has the voltage range corresponding to the predetermined FM frequency band. When an appropriate FM channel at a given frequency is selected by the FM receiver, the appropriate voltage is applied to the matching circuit to match the FM antenna with the FM receiver through the transmission cable at the given frequency. Reactance component of the antenna at the given frequency is therefore cancelled with controlled capacitance of the varactor diodes. For example, the resistors 62, 63, 69, 70 and 71 may have 100 kilo-ohms or more. The capacitor 57 may have 5 to 50 picofarads, preferably 30 picofarads, while the capacitor 72 have 5 to 50 picofarads, preferably 10 picofarads. The capacitor 64 may have 5 to 50 picofarads, preferably 10 picofarads, while the capacitors 61 and 68 have 0 to 50 picofarads, preferably 2 picofarads. The coils 58 and 65 may have 100 to 300 nanohenry, preferably 200 nanohenry. As described above, the present glass antenna for the automobile of the invention has an advantage to enhance the tracking characteristic of the dynamic impedance matching circuit with respect to the reverse T-shaped main antenna provided on the upper blank portion of the defogging heaters to improve the gain of the main antenna system. It is because the length and distance of its horizontal portion relative to the most significant heater are predetermined to have monotonic changes with respect to the resistance and reactance components of its impedance versus the frequency within the FM frequency band. When the defogging heaters are used the FM subsidiary antenna, the gain of the FM subsidiary antenna system is enhanced by improvements of increase of its basic antenna gain and enhancement of the tracking characteristic of the dynamic impedance matching circuit relative to subsidiary antenna. It is also because the antenna open voltage at the feeding point on the upper point of the bus is increased and its impedance versus frequency can be simplified monotonically within the FM frequency band. Because the main and subsidiary antennas each having intersecting directivity are disposed on the rear glass to complement each other, the total gain of the entire direction is further improved when the main and subsidiary antennas are connected to a diversity system.

Claims: A glass antenna for an automobile comprising: a first antenna (4) provided on a window glass (1) of said automobile and having a reverse T-shape; a second antenna (3) provided on said window glass and consisting of defogging heaters (3); a bus (5, 6) provided on said window glass and connected to said defogging heaters and connected to a choke coil (9, 11) through another coil (14, 15) to prevent an FM signal from leaking, characterised in that said coil (14, 15) is directly connected between said bus (5, 6) and a land (23) provided on said window glass to receive said defogging current through said choke coil, and said coil being a leadless component connected between said bus and said land on said window glass. A glass antenna according to claim 1, wherein said first antenna comprises a horizontal line arranged parallel to the uppermost heater of said heaters and having a predetermined distance thereto. A glass antenna according to claim 2, wherein said horizontal line has a length of 500 millimetres and said predetermined distance is 10 millimetres. A glass antenna according to claim 1, further comprising a lateral T pattern paralleled to said bus connected to said second antenna to provide a coupling capacitor. A glass antenna according to claim 4, wherein said lateral T pattern has a length of about 250 millimetres and includes a centre line having about 10 millimetres in length. A glass antenna according to claim 1 in which an impedance matching circuit (13) provided on said window glass is coupled to said second antenna through said bus and has its I/O port (16, 17) for supplying said FM signal to a receiver and receiving a control voltage from said receiver.

Inventors: ARISAWA MASATO, BABA YUJI, MURAKAMI HARUNORI, NATSUME HIROFUMI, NIIZAKI NOBUYA, URAKAMI KANTA, ARISAWA, MASATO, BABA, YUJI, MURAKAMI, HARUNORI, NATSUME, HIROFUMI, NIIZAKI, NOBUYA, URAKAMI, KANTA
Applicants: NIPPON SHEET GLASS CO LTD, SUMITOMO CHEMICAL CO, NIPPON SHEET GLASS CO., LTD., SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1992
Main classification: H01Q1
IPC codes: H01Q1