Antenna device for an automobile

Abstract: An antenna device for an automobile comprises an electric heating type defogger having heating strips (2) and an antenna conductor (6) having antenna strips arranged to form a pattern, which are fitted on the rear window glass fitted to a rear window opening formed in an automobile, wherein the defogger (4A) and the antenna conductor (6A) are spaced apart from each other with a predetermined small space in a capacitive coupling relation so that a high frequency current is caused to flow. A reactance circuit (8) is connected to the defogger to effect anti-resonance between the defogger and the rear window opening of the body of the automobile at the central frequency. A quality factor value is determined by the inductance of a chock coil (9) in the reactance circuit and the stray capacitance between the defogger and the rear window opening of the body of the automobile so that the ratio of the input impedance of the receiver to the impedance of the antenna conductor viewed from the side of a power feeding terminal (11) becomes 1; and a matching circuit (13) is inserted between the power feeding terminal (11) of the antenna conductor and the receiver (12).

Description: The present invention relates to an antenna device for an automobile for suppressing generation of noises. As an antenna device for receiving radio waves for an automobile, there has been known such ones having antenna conductors for AM broadcasts, FM broadcasts or AM/FM broadcasts which are provided on the surface or inside the rear window glass of the automobiles instead of rod antenna. The antennas provided on or in the rear window glass of the automobiles, having no part projecting from the car body unlike the rod antenna or the lid antenna, have many advantages that they are harmless to persons, are not easily broken, do not become rusty, hence there is no change of performance, and provide excellent appearance for the automobiles. In such type of antenna devices, it is possible to obtain desired performance, i.e. increasing a gain of antenna by suitably designing the pattern of an antenna conductor or antenna conductors provided on or in a glass plate. However, the surface area of the glass plate is not sufficiently large when the antenna device is installed in a window glass for an automobile. Accordingly, the distance between the antenna conductors and the body of an automobile can not be sufficiently large, whereby a leak current from the antenna conductor is large and a gain of antenna becomes insufficient. In case of the antenna device to be installed in or on the rear window glass of an automobile, an electric heating type defogger for preventing the rear window glass from being cloudy is provided together with the antenna conductor formed in a predetermined pattern. In particular, when the antenna conductor is placed reclose to a heater, a current in the antenna conductor leaks to the body of the automobile through the power feeding part of the electric heating type defogger thereby resulting in a loss of the gain. In order to eliminate the loss of gain, there has been proposed to attach a choke coil to a suitable position of a power feeding point of the electric heating type defogger. However, there is a tendency of reducing the antenna gain because the choke coil has not a sufficient high frequency current preventing function when the choke coil is used for an antenna device for an automobile. In a conventional antenna device ,there was practiced to insert a pre-amplifier 331 at a suitable position of a power feeding line between the power feeding terminal 311 of antenna conductors 306 and a radio wave receiver 312 in order to compensate a loss of antenna gain as shown in Figure 6. However, there occurred cross modulation in a strong electric field due to the presence of the preamplifier to thereby increase a noise level. Further, since it was necessary to connect the pre-amplifier in addition to the radio wave receiver in the above-mentioned system, the manufacturing cost is increased. Furthermore, the pre-amplifier to be disposed near the antenna device fairly restricts the condition of designing an automobile, e.g. in assuring a space for the pre-amplifier. Accordingly, it has been expected to develop an antenna device for an automobile capable of obtaining a high gain and reducing noises without the necessity of the pre-amplifier. It is an object of the present invention which is defined by claims 1 and 4 to provide an antenna device for an automobile capable of obtaining a high gain and reducing noises without the necessity of providing an expensive pre-amplifier, where by both a defogger and an antenna system are provided in close proximity on the window shield. The present invention relates to an antenna device for an automobile comprising an electric heating type defogger having heating strips and a bus bar for feeding a current to the heating strips and an antenna conductor having at least one antenna strip arranged to form a pattern, the defogger and the antenna conductor being in or on the rear window glass fitted to a rear window opening formed in an automobile, whereby the defogger and the antenna conductor are spaced apart from each other with a predetermined small space in a capacitive coupling relation so that a high frequency current is caused to flow but a direct current is not caused to flow between them such an antenna is disclosed in EP 0 065 263. The object of the present invention is achieved by an antenna device having in addition the features of the characterizing clause of claims 1 and 4, respectively. In the above-mentioned invention, it is most desirable that a choke coil is used for the reactance circuit connected to the defogger, and an anti-resonance frequency resulted by the inductance of the coil and the stray capacitance between the coil and the rear window opening of automobile is determined to be the center of a predetermined broadcast frequency band region, for instance, AM or FM band region. In the above-mentioned invention, it is also most desirable that the matching circuit Inserted between the power feeding terminal of the antenna conductor and the receiver is constituted by a combination of one or more coils, capacitors and resistors wherein the impedance of the matching circuit is substantially constant in the entire region of a broadcast frequency band region, and the sum of the impedance of the antenna conductor viewed from the input terminal of the receiver, the functional part as an antenna in the rear window glass and a part of an antenna feeder line in a broadcast frequency band region exhibits a little capacitive reactance. In accordance with the present invention, there is provided an antenna device for an automobile comprising an electric heating type defogger having heating strips and a bus bar for feeding a current to the heating strips and an antenna conductor having at least one antenna strip arranged to form a pattern, the defogger and the antenna conductor being in or on the rear window glass fitted to a rear window opening formed in an automobile, whereby the defogger and the antenna conductor are spaced apart from each other with a predetermined small space in a capacitive coupling relation so that a high frequency current is caused to flow but a direct current is not caused to flow between them; characterised by a reactance circuit inserted between a bus bar of the defogger and a d.c. power source for the defogger so that anti-resonance is effected, at the band region on a side of the central frequency in terms of a logarithmic scale of each FM or AM broadcast frequency band regions, between the impedance of the reactance circuit and the impedance of electric static capacitance, viewed from the connecting part of lead wires of the bus bar, in the antenna conductor, the defogger and the body of the automobile, wherein a quality factor value for the AM broadcast frequency band region is determined to be Q ≦ 1.2 and a quality factor value for the FM broadcast frequency band region is to be Q ≦ 2.4 so that the resonance characteristic of the quality factor for effecting the anti-resonance becomes relatively flat; and a matching circuit inserted between the power feeding point of the antenna conductor and a receiver so that resonance is effected, at the band region of the other side of the central frequency of each of the FM or AM broadcast frequency band regions, between the impedance of the matching circuit and the input impedance of the receiver and the impedance of the antenna conductor viewed from the matching circuit, wherein a quality factor for the AM broadcast frequency band region is determined to be Q ≦ 1.2 and a quality factor for the FM broadcast frequency band region is to be Q ≦ 2.4 so that the resonance characteristic of the quality factor for effecting the resonance becomes relatively flat. In drawings: Figure 1 is a diagram showing an embodiment of the antenna device for an automobile according to the present invention; Figure 2 is a frequency characteristic diagram in an AM broadcast frequency band region of the antenna device; Figure 3 is a diagram showing the S/N ratio in an AM broadcast frequency band region of the antenna device; Figure 4 is a frequency characteristic diagram in an FM broadcast frequency band region of the antenna device; Figure 5 is a diagram showing the directivity of FM broadcast frequency band region of the antenna device; and Figure 6 is a diagram showing a conventional antenna device for an automobile. In the following, a preferred embodiment of the present invention will be described with reference to the drawings. In Figure 1, a reference numeral 1 designates a transparent glass plate to be fitted to the rear window opening formed in an automobile wherein an electric heating type defogger 3 comprising a number of heater strips 2 and bus bars 5a, 5b, 5c opposing at both ends of the heater strips 2 are provided in a portion to be heated in the inside surface of the glass plate 1. Lead wires 222a, 222b are respectively connected to the bus bars of the defogger 3. Figure 1 shows an example of the defogger 3 wherein one of the opposing bus bars 5a, 5b, 5c is vertically separated at a desired position into two portions, i.e. a lower bus bar 5a and an upper bus bar 5b. The lower bus bar 5a is connected to a power source 210 through the lead wire 222a. A current flows through the lower bus bar 5a, the heater strips 2, the bus bar 5c, the heater strips 2 and the upper bus bar 5b. The defogger 3 as shown in the drawing is an electric heating type defogger formed by a printing method wherein a number of heater strips having a wire width of 0.5 mm - 2 mm are printed, by using a current conductive silver paste in the lateral direction of a glass plate in parallel to each other with intervals of 2 cm - 4 cm, followed by baking the silver paste. A numeral 6 designates an antenna conductor for AM/FM radio waves provided at the upper portion of the defogger 3 in a glass plate 1 for the rear window of an automobile. A part 6a of the antenna conductor 6 and a part 2b of the defogger 3 are closely positioned with a predetermined space in a capacitive coupling relation so that a high frequency current is caused to flow but a direct current is not caused to flow between them. For instance, the antenna conductor part 6a and the defogger part 2b are spaced apart with a distance of about 1 mm - 10 mm. Due to the capacitive coupling between the antenna conductor 6 and the defogger 3, the defogger 3 functions as if it is a part of the antenna conductor. Particularly, the defogger 3 functions as a part of the antenna device to AM frequency band regions for broadcasting, and the effective length of the antenna device for the AM band regions is elongated to increase sensitivity to be capable of receiving radio waves in a wide range. As described above, in order to connect the defogger 3 and the antenna conductor 6 in a capacitive coupling relation at at least their small portion, it is desirable to form the defogger 3 and the antenna conductor 6 on the same plane of the rear window glass, normally on the side of the cabin of the automobile. A patterned arrangement of an antenna conductor 6 or antenna conductors 6 can be selected as desired so as to obtain the optimum performance as an antenna device for AM band radio wave broadcast, FM band radio wave broadcast, AM/FM band radio wave broadcast or television broadcast depending on the shape of an automobile, the shape, the dimensions and the construction of a glass plate 1 for the automobile. The embodiment as illustrated in the drawing is provided with the antenna conductor 6 at the upper part of the defogger 3 in the rear window glass plate 1 of the automobile. However, the antenna conductor 6 may be provided at the lower part, or both upper and lower parts of the defogger 3, or may be provided at another marginal portion. The antenna conductor 6 is generally formed by using a printing method wherein electric conductive silver paste is printed in a form of wire in a predetermined pattern on a glass plate followed by baking the paste in the same manner as the heater strips of the defogger. However, the antenna conductor 6 may be formed by using a transparent electric conductive film or fine electric conductive wire arranged in a predetermined pattern. In the antenna device for an automobile as shown in Figure 1, a matching circuit 207 is inserted at a desired position between the power feeding point 204 of the antenna conductor 6 and a radio wave receiver 220. A high frequency current induced in the antenna conductor 6 is effected for resonance by the impedances of the matching circuit 207, the receiver 220 and the antenna conductor viewed from the matching circuit 207, and supplied to the receiver 220. The matching circuit 207 comprises a circuit constituted by coils 213, 214, 217 and 218, a capacitor 216 and a resistor 215. In an AM broadcast frequency band region, the electric characteristics can be determined by the coils 213, 214, 217, the capacitor 216 and the resistor 215. In an FM broadcast frequency band region, since the self-resonance frequency of the coils 213, 214, 217 is low, they show a capacitive reactance. The electric characteristics can be determined by using a core made of Ni - Zn ferrite or by inserting a solenoid or spirally wound coil 218. High frequency coils 212a, 212b in the reactance circuit 208 exhibit a high impedance in FM broadcast frequency band regions. Accordingly, a solenoid without having a magnetic core, a spirally wound coil or a lead wire having a suitable length is used for the coils 212a, 212b. Since the high frequency choke coil in a heater transformer 207 is low in self-resonance frequency in the broadcast frequency band regions, the inductance is lost. Accordingly, the high frequency coils 212a, 212b are used for the high frequency choke coil. In order to have the matching circuit 207 achieved good performance, it is necessary that the reactance circuit 208 is inserted between the bus bar of the defogger and a d.c. power source 210 so that anti-resonance is effected, at the band region on a side of the center frequency of each FM or AM broadcast frequency band region, by the impedance of the reactance circuit 208 and the impedance of electric static capacitance viewed from the connecting part of lead wires of the bus bar, in the antenna conductor 6, the defogger 3 and the body of the automobile, wherein a quality factor value Q for the AM broadcast frequency band region is determined to be Q ≦ 1.2 and a quality factor value Q for the FM broadcast frequency band region is to be Q ≦ 2.4 so that the resonance characteristic of the quality factor for effecting the anti-resonance becomes reatively flat; and the matching circuit 207 is inserted between the power feeding point of the antenna conductor 6 and a receiver 220 so that resonance is effected, at the band region of the other side of the center frequency of each of the FM or AM broadcast frequency band region, by the impedance of the matching circuit 207 and the input impedance of the receiver 220 and the impedance of the antenna conductor 6 viewed from the matching circuit 207, wherein a quality factor value Q for the AM broadcast band region is determined to be Q ≦ 1.2 and a quality factor value Q for the FM broadcast frequency band region is to be Q ≦ 2.4 so that the resonance characteristic of the quality factor for effecting the resonance becomes relatively flat. The frequency for causing the anti-resonance and the resonance is preferably determined in ranges of 550 KHz - 640 KHz and 1050 KHz - 1320 KHz in the AM broadcast frequency band regions, especially 580 KHz - 610 KHz and 1170 KHz - 1230 KHz in the Japanese broadcasting system. For the FM broadcast frequency band regions, it is desirable to use the ranges of 77.5 MHz - 80.5 MHz and 84 MHz - 88 MHz, more preferably, 78.5 MHz - 79.5 MHz and 85 MHz - 87 MHz, of course, foreign countries have their own frequency band regions for broadcasting. In order to satisfy the above-mentioned conditions, usually the circuit constant of the matching circuit 207 and the circuit constant of the reactance circuit 208 are suitably adjusted because the pattern of the antenna conductor 6 is fixed in a galss plate. It is difficult to manufacture the reactance circuit 208 having a high self-resonance frequency because a large current is passed therethrough. Therefore, it is desirable that the anti-resonance is effected at a low frequency region with respect to the center frequency in terms of logarithmic scale of each of the FM and AM broadcast frequency band regions and the resonance is effected at a higher region than the center frequency. In the matching circuit used for the antenna device of the present invention, the capacitor 219 of 560 pF - 1 µF, the coil 213 of 82 µH - 560 µH, the capacitor 216 of 5 pF - 220 pF, the coils 214, 217 of 82 µH - 700 µH and the resistor 215 of 200 Ω - 3 KΩ are preferably used in the AM broadcast frequency band regions. For the FM broadcast frequency band regions, it is preferable to use the coil 218 of 1 µH - 10 µH in addition to the above-mentioned elements. On the other hand, for the high frequency choke coil in the reactance circuit 208 connected to the defogger 3 is preferably determined to be 1.0 mH - 3 mH in the AM broadcast frequency band regions. On the other hand, for the coils 212a, 212b, they are preferably so determined as to be 1 µH - 5 µH in the FM broadcast frequency band regions. The capacitance of the capacitive coupling portion between the antenna conductor part 6a and a branch line 2b of the heating strip 2a is preferably determined to be 50 pF - 10,000 pF in both FM and AM broadcast frequency band regions. Further, a cable portion 225 extended between the power feeding point 204 of the antenna conductor 6 and the input terminal of the receiver 220 through the matching circuit 207 is adapted to transmit a high frequency current effectively. As a preferred example, a coaxial cable, a feeding line or the like is preferably used. The above-mentioned values for the coils, capacitors and resistors are raised as typical examples, and it is possible to change the values so as to obtain the optimum performance depending on an antenna device for an automobile. The length of an earth line 224a to be connected to the automobile body as a negative pole of the cable 225 and an earth line 224c connected to the automobile body as a negative pole of the d.c. power source 210 are preferably 30 cm or more, preferably 60 cm or more so as to reduce noises. In the antenna device for an automobile in accordance with the present invention, sensitivity for receiving radio waves can be maintained in the entire region of the radio wave broadcast frequency band regions by reducing a leak current from the defogger 3 by causing the anti-resonance in the band region at a side of the center frequency of a broadcast frequency band region in terms of logarithmic scale and by causing resonance by utilizing the matching circuit in the region other than the side of the center frequency. This is becuase sole use of the reactance circuit 208 or the matching circuit 207 can not cover the entire region of the FM and AM broadcast frequency band regions. For this purpose, the anti-resonance is effected at a low frequency region with respect to the center frequency of a selected broadcast frequency band region for the reactance circuit 208 and the resonance is effected at a higher region for the matching circuit 207. With respect to the reactance circuit 208, it is necessary to pass a large amount of current and a high degree of technique is required to manufacture the reactance circuit having a high self-resonance frequency. In the following, an example will be described as to the reactance circuit in which the anti-resonance is effected at a low frequency region in order to simplify explanation. The present invention derives from the technical concept described below. Anti-resonance is effected at a low frequency region by the impedance of a portion functioning as an antenna and the impedance of the reactance circuit 208 to thereby prevent a receiving current of broadcasting radio waves induced in the defogger 3 from flowing to the ground, and resonance is effected around the center frequency in terms of logarithmic scale in a high frequency region, which is caused by the impedance of the part functioning as the antenna and the matching circuit to thereby increase the sensitivity. In the reactance circuit 208, when one having a high self-resonance frequency is used, it is possible to obtain the anti-resonance at a high frequency region and to obtain the resonance at a low frequency region. In the antenna device according to the present invention, an anti-resonance phenomenon is produced at a low frequency region with respect to the center frequency in terms of logarithmic scale of each of the FM and AM broadcast frequency band regions between the impedance mainly composed of static capacitance formed by the cooperation of the three elements of the antenna conductor 6, the defogger 3 and the rear window opening and the impedance of the reactance circuit 208. When the center value in terms of logarithmic scale between the frequency f L of the lowest region and the frequency f H of the highest region in each of the F M and AM broadcast frequency band regions is expressed by f M determination is so made that the center value f LM in terms of logarithmic scale between f L and f M is a frequency which causes the anti-resonance. For instance, the impedance of the defogger 3 and the rear window opening becomes the largest at a frequency value f LM by determining the circuit constant of the reactance circuit 208 so that the center frequency f LM in the lower frequency region is about 600 KHz (f LM ≒ 600 KHz) as the anti-resonance frequency in the AM broadcast frequency band region when f M ≒ 900 KHz between f L ≒ 500 KHz and fH = 1600 KHz. Namely, a leak current from the defogger 3 to the rear window opening 24 becomes the smallest, and the leakage of a high frequency current induced in the defogger 3 to the automobile body can be prevented, hence reduction in the sensitivity can be prevented. Similarly, when f M ≒ 82 MHz, f L = 76 MHz, f H = 90 MHz and f LM ≒ 79 MHz in the FM broadcast frequency band regions, the same phenomenon takes place. In the reactance circuit 208, since the inductance of the coils 212a, 212b in the AM broadcast frequency band regions is sufficiently smaller than the inductance of the heater transformer 209, the inductance of the coils is negligible. Further, since the self-resonance frequency of the coil 209 is low in the FM broaccast frequency band regions, hence the coil 209 becomes of a capacitive reactance, the coils 212a, 212b function as inductive elements. In the case as described above, when the quality factor value Q is made small as possible in the both FM and AM broadcast frequency band regions, the above-mentioned impedance can be averaged in the respective FM and AM broadcast frequency band regions, whereby a leak current can be small in average. The defogger 3 and the antenna conductor 6 becomes such a state that they are connected in a high frequency range in both the FM and AM broadcast frequency band regions by the capacitive coupling between the adjacent portion 6a of the antenna conductor 6 and the branch line 2b of the heating strip 2a of the defogger 3. Further, they are isolated from the automobile body, whereby the defogger 3 functions as an antenna in the same manner as the antenna conductor 6. The quality factor value Q, which determines the circuit constant of the matching circuit 207 so as to cause the resonance at the center frequency value f HM ≒ 1200 KHz in terms of logarithmic scale of f M ≒ 900 KHz and f H = 1600 KHz in the AM broadcast frequency band regions and also determins the circuit constant of the matching circuit 207 so as to cause the resonance at f M ≒ 82 MHz, f H ≒ 90 MHz and f HM ≒ 86 MHz in the FM broadcast frequency band regions, should be small as possible so that the quality factor value Q is flat in areas f M and f H in the AM and FM broadcast frequency band regions. Theoretically, the quality factor value is determined to be nearly 0. Thus, power can be effectively transmitted from the antenna to the receiver 220 so that an induced current in the antenna due to radio waves coming thereto can be supplied to the receiver 220. Explanation will be made in detail as to the resonance in the AM and FM broadcast frequency band regions. The matching circuit 207 having reactance components constituted by the coils 213, 214, 217, 218, the capacitor 216 and the resistor 215 as shown in Figure 17 is inserted between the power feeding point 204 of the antenna conductor 6 and the receiver 220. In the case of the AM broadcast band regions, the resonance frequency obtained by the coils 213, 214, 217, the capacitor 216, all parts functioning as an antenna, and one receiver 220 is set to be the above-mentioned value f HM , and a quality factor value Q is rendered to be the optimum value by the coils 214, 217 and the resistor 215, whereby substantially averaged high sensitivity can be obtained in the AM broadcast frequency band regions. Further, in the case of the FM broadcast frequency band regions, the coils 213, 214, 217, the resistor 215 and the capacitor 216 provide a capacitive reactance due to the stray capacitance in each of the elements, which causes resonance at the above-mentioned frequency f HM in association of the coil 218, antenna elements and the input impedance or the receiver (in the matching circuit 207, only the coil 218 is effective in the FM broadcast frequency band regions), and a signal received by the antenna is transmitted to the receiver in the optimum form. Thus, high radio wave receiving sensitivity can be obtained in various frequency band regions. The matching circuit 207 causes the resonance at the above-mentioned value f HM with the all elements functioning as the antenna and the input impedance of the receiver 220. The capacitor 219 provides a nature of capacitive reactance to function as a low-pass filter so that a noiseless antenna for absorbing noises is obtainable. [EXAMPLE 1] An antenna device for an automobile as shown in Figure 1 was prepared. The values of each element in an AM broadcast frequency band region are as follows: a capacitor 219 is 0.01 µF, a coil 213 is 300 µH, a coil 214 is 150 µH, a resistor 215 is 680 Ω, a capacitor 216 is 27 pF, a coil 217 is 330 µH, a capacitive coupling portion of a part 6a of the antenna conductor and a part 2b of the defogger is 90 pF, the inductance of a heater transformer 209 is 400 µH, a capacitor 211 is 2.2 µF, and an antenna-cable portion extending between the feeding point 204 of the antenna conductor 6 and the input terminal of a receiver 220 is 30 pF/m. The characteristics of antenna gain and S/N ratio of the antenna device to the AM broadcast frequency band region as shown in Figures 2 and 3 were obtained. Figure 2 is a diagram showing antenna gains to the frequencies in the AM broadcast frequency band region obtained when the intensity of an electric field near the antenna device is 60 dBµV/m. As in Figure 2, the antenna gain is substantially the same as those of the conventional antenna device having a pre-amplifler. Figure 3 is a diagram showing a relation of S/N ratio at the time of non-modulation and at the time of modulation to electric field intensity when the frequency of carrier waves is 400 Hz. In Figure 3, the non-modulation means that the degree of modulation = 0, and the modulation means that the degree of modulation = 30%. The S/N ratio is not distinguishable from that of the conventional antenna device with an amplifier in a strong electric field. However, in a weak electric field, the antenna device of the present invention provides better results. Thus, the same high gain is obtainable by the antenna device of the present invention in the AM broadcast frequency band regions in the same manner as the conventional antenna device having an amplifier to increase antenna gain. Further, the antenna device of the present invention suppresses noises in the ordinary weak electric field. When a capacitor 219 of 0.01 µF, a coil 218 of 2 µH, coils 212a, 212b of each 2 µH and an antenna cable portion of 30 pF/m extended between a power feeding point 204 of the antenna conductor 6 and the input terminal of the receiver 220 are respectively used as elements effectively functioning in FM radio broadcast frequency band regions, the characteristics of antenna gain and directivity of the antenna device in the FM frequency band regions are shown in Figures 4 and 5. The antenna device is also applicable to an antenna device for FM radio waves of a high gain and of non-directivity. The quality factor Q in combination of the impedance of the heater strips 2 viewed from the bus bars 5a, 5b, 5c and the reactance circuit in the antenna device was 0.1 for AM radio waves and 0.5 for FM radio waves. The sum of the impedance of the antenna conductor viewed from the input terminal of the receiver and elements functioning as an antenna was 0.2 for AM and 0.4 for FM. In accordance with the present invention, an antenna device for an automobile having high radio wave receiving performance and obtaining a high gain at a low noise level without using a pre-amplifier can be provided. In particular, it is useful or receiving AM radio waves with a high gain and a low noise level. Further, it is applicable to receive FM radio waves and other radio waves. The manufacturing cost can be reduced by omitting the pre-amplifier. In the past, it was necessary to place the pre-amplifier near the antenna device, which restricts conditions of designing an automobile. In accordance with the present invention, however, such restriction can be eliminated because a simple matching circuit is only needed. In accordance with the antenna device for an automobile of the present invention, FM radio waves can be received with excellent directivity. The reactance two-terminal network circuit is a matching circuit having a simple circuit. Accordingly, it is seldom to cause reflection and reverse flow at structural elements in comparison with the conventional glass antenna having a pre-amplifier, hence it is seldom to cause the disturbance of the waveform of an electric signal entering in the radio wave receiver. This results in increasing the reproductivity of the radio waves entering in the receiver, and excellent stable radio wave-receiving performance can be obtained. The antenna device of the present invention can receive AM radio waves at a low noise level and it is also suitable not only to an antenna device for FM radio waves but also for AM/FM radio waves for an automobile. The reactance two-terminal network circuit used for the present invention provides a high noise suppressing effect in comparison with a four-terminal network circuit. Further, it provides sufficient matching effect. In addition, it does not require the pre-amplifier needed for the conventional antenna device, and it can be realized by using a simple reactance two-terminal network circuit. Accordingly, the manufacturing cost can be reduced and the fitting operation to an automobile and maintenance works can be easy. The configuration of the antenna device can be compact to thereby provide flexibility in designing an automobile. Further, since the antenna device is constituted by reactance elements, it is unnecessary to provide a d.c. power source for a pre-amplifier, and therefore the manuracturing cost can be remarkably reduced.

Claims: An antenna device for an automobile comprising an electric heating type defogger (3) having heating strips (2a) and a bus bar for feeding a current to the heating strips (2a) and an antenna conductor (6) having at least one antenna strip (6a) arranged to form a pattern, said defogger (3) and said antenna conductor (6) being in or on the rear window glass (1) fitted to a rear window opening formed in an automobile, whereby said defogger (3) and said antenna conductor (6) are spaced apart from each other with a predetermined small space in a capacitive coupling relation so that a high frequency current is caused to flow but a direct current is not caused to flow between them; characterized by (i) a reactance circuit (208) inserted between a bus bar of the defogger (3) and a d. c. power source (210) for said defogger (3) so that anti-resonance is effected, at the band region on one side of the central frequency in terms of a logarithmic scale of each FM or AM broadcast frequency band region, between the impedance of said reactance circuit (208) and the impedance of electric static capacitance, viewed from the connecting part of lead wires of said bus bar, in said antenna conductor (6), said defogger (3) and the body of the automobile, wherein a quality factor value for the AM broadcast frequency band region is determined to be Q ≦ 1.2 and a quality factor value for the FM broadcast frequency band region is to be Q ≦ 2.4 so that the resonance characteristic of the quality factor for effecting the anti-resonance becomes relatively flat; and (ii) a matching circuit (207) inserted between the power feeding point (204) of said antenna conductor (6) and a receiver (220) so that resonance is effected, at the band region of the other side of the central frequency of each of the FM or AM broadcast frequency band regions, between the impedance of said matching circuit (207) and the impedance of input of said receiver (220) and the antenna conductor (6) viewed from said matching circuit (207), wherein a quality factor for the AM broadcast frequency band region is determined to be Q≦1.2 and a quality factor for the FM broadcast frequency band region is to be Q≦2.4 so that the resonance characteristic of the quality factor for effecting the resonance becomes relatively flat. The antenna device for an automobile according to Claim 1, wherein said one side of the central frequency in terms of logarithmic scale of the FM and AM frequency band region is a low frequency band region, and the other side is a high frequency band region. The antenna device for an automobile according to Claim 1, wherein said reactance circuit (208) connected to said defogger (3) comprises a heater transformer (209) having a toroidal core of a magnetic substance which has a high magnetic permeability in the AM broadcast frequency band region, wherein one terminal of said heater transformer is connected to two-split bus bars of said defogger (3) and the other terminal is connected to a d. c. power source, and a high frequency coil usable in the FM broadcast frequency band region without residual magnetism which is inserted between said d. c. power source and said heater transformer (209), wherein the negative pole of said d. c. power source is connected to the body of the automobile at a position at least 30 cm apart from the negative pole of a cable. An antenna device for an automobile comprising an electric heating type defogger (3) having heating strips (2a) and a bus bar for feeding a current to the heating strips (2a) and an antenna conductor (6) having an antenna strip (6a) arranged to form a pattern, said defogger (3) and said antenna conductor (6) being in or on the rear window glass (1) in an automobile, whereby said defogger (3) and said antenna condutor (6) are spaced apart from each other with a predetermined small space in a capacitive coupling relation so that a high frequency current is caused to flow but a direct current is not caused to flow between them; characterized by (i) a reactance circuit (208) inserted between a bus bar of the defogger (3) and a d. c. power source (210) for said defogger (3) so that anti-resonance is effected, at the band region on one side of the central frequency in terms of a logarithmic scale of a desired radio wave region between the impedance of said reactance circuit (208) and the impedance of electric static capacitance, viewed from the connecting part of lead wires of said bus bar, in said antenna conductor (6), said defogger (3) and the body of the automobile, the resonance characteristic of the quality factor for effecting the anti-resonance being relatively flat; and (ii) a matching circuit (207) inserted between the power feeding point (204) of said antenna conductor (6) and a receiver (220) so that resonance is effected, at the band region of the other side of the central frequency of the radio wave region, between the impedance of said matching circuit (207) and the impedance of input of said receiver (220) and the antenna conductor (6) viewed from said matching circuit (207), the resonance characteristic of the quality factor for effecting the resonance being relatively flat. The antenna device for an automobile according to Claim 4, wherein said one side of the central frequency in terms of logarithmic scale of the radio wave region is a low frequency band region, and the other side is a high frequency band region.

Inventors: ISHII KENICHI, KUME MASAHARU, ODA KENJI, OGAWA MASANOBU C O ASAHI GLASS, SAITOU TOSHIHIKO, SHIBATA SHINYA, YAMASHITA TAKASHI, ISHII, KENICHI, KUME, MASAHARU, ODA, KENJI, OGAWA, MASANOBU C/O ASAHI GLASS COMPANY LTD., SAITOU, TOSHIHIKO, SHIBATA, SHINYA, YAMASHITA, TAKASHI
Applicants: ASAHI GLASS CO LTD, ASAHI GLASS COMPANY LTD.
Application year: 1989
Main classification: H01Q1
IPC codes: H01Q1