A glass window antenna for use in a motor vehicle

Abstract: A glass window antenna for use in a motor vehicle provides satis­ factory antenna characteristics while making best use of the space of a glass window (1) with a heating conductor (10) as the main antenna element which is heated by a pair of power supply lines (17, 18) through choke coils (16a, 16b). In a preferred embodiment, a capaci­ tor (27) is connected across the power supply lines at a position between the choke coils and the heating conductor to minimize the loss or leakage of radio signals from the heating conductor through the choke coils. A symmetrical pair of wires (23, 24) coupled with each other is formed on the glass window and connected to ends of the hea­ ting conductor to improve a radio wave reception gain.

Description: Background of the Invention The present invention relates to window glass antennas for motor vehicles and, more particularly to an antenna arrangement in which a defogging heater conductor formed on a window glass of a motor vehicle serves as an antenna for receiving radio signals. An antenna arrangement of this kind is known in the art. In order to assure that a sufficient amount of radio signals received in such a heating conductor be available for a radio receiver or tuner, a leakage of the received signals to a direct current (DC) heater power supply or ground must be minimized. To this end, it was proposed to connect choke coils in the heating power supply lines so that the heating conductor will be floated or isolated from the heater power supply in high frequencies while allowing the heating rower of DC current to pass through the choke coils. If the heating conductor antenna covers an FM broadcasting band only, choke coils having a relatively small inductance will suffice. However, such small coils will considerably decrease their impedance of radio signal leakage for an AM broadcasting band, thus reducing available antenna power in AM band. On the other hand, if a large inductance of choke coils is employed, this will give a satisfactory antenna gain for AM band but render a poor antenna reception sensitivity to FM band because of an increased parasitic capacitance in such high inductance choke coils. In this view, the prior art has adopted an arrangement in which choke coils associated with a defogging heating conductor is made to have a relatively large inductance so that the heating conductor will serve only as an AM antenna. Separate antenna conductor elements are formed on the same window as that with the heating conductor to provide an FM antenna. Fig. 1 shows an example of the prior art window glass antenna of this type. The antenna is arranged such that a large number of heater wires or strip conductors 2 constituting a defogging heater conductor 10 and serving as an AM antenna are formed in a defogging area of a rear glass window 1. The heater wires are subdivided into upper and lower wire groups which are powered from power supply buses 3 and 4 connected to first ends of the upper and lower wire groups, respectively. A connecting bus 5 is disposed along and commonly connects second or remote ends of the upper and lower wire groups to complete a heating current path. Since the heater wires 2 are used as an AM broadcast band reception antenna, a feeder cable 13 such as a coaxial cable is connected to a feeding point 12 provided in the connecting bus 5. The feeder cable 13 carries the received signal to an AM radio tuner mounted in the motor vehicle through a DC cut capacitor 14. A separate FM antenna 6 is formed in a blank portion of the window 1 above the heater wires 2. It comprises a main antenna element 6a in the form of two parallel wires connected to each other in a substantial U shape, an auxiliary antenna element 6b disposed above and connected to the upper wire of the main antenna element 6a, another auxiliary antenna element 6c connected to the lower wire of the main antenna element 6a and facing the heater wires 2, and a folded element 6d bent back from the upper wire of the main antenna element and having a portion adjacent to the auxiliary antenna 6b. A feeding point 7 is formed on the folded portion 6d and connected to a feeder cable 8 for supplying an FM reception signal to an FM radio tuner through a DC cut capacitor 9. A main DC power supply feeds a heating current to the power supply buses 3 and 4 through choke coils 16a and 16b magnetically coupled to each other. The choke coil 16a connected to a main power +B is negatively coupled to the choke coil 16b connected to a ground so that magnetic fluxes generated by the respective heating currents cancel each other within a core. Therefore, the core having a small volume can be operated in a nonsaturated state. A decoupling capacitor 20 is connected between a ground and a line connected to the main power +B to prevent power source noise from being superposed on the reception signal. Fig. 2 illustrates another example of the prior art window glass antenna arrangement. In Figs. 1 and 2, like references refer to like parts. In this example of Fig. 2, two FM antennas 36 and 38 are provided on spaces S1 and S2 in the window 1 above and below the heating conductor 10 as an AM antenna, respectively. The top FM antenna 36 includes a coupling element 37 along the uppermost heating wire 2 for coupling with the heater conductor 10, and a feeding pad 7 connected to a feeder cable 35 for carrying AM and FM signals to an AM/FM tuner. The bottom FM antenna 38 also includes a coupling element 39 along the lowermost heating wire for coupling with signals from the heater conductor 10 and a feeding pad 40 connected to another feeder cable 41 for supplying the AM/FM tuner with AM and FM signals received in the heater conductor 10 and the bottom FM antenna conductor 38. As noted from the foregoing, the prior art requires a separate antenna element or elements for receiving FM band signals in addition to the heater conductor because the heater conductor only serves as an AM antenna. Such separate antenna element thus requires a relatively wide space for its mounting, as wide as about 120 to 150 mm above and/or below the heater conductor. Therefore, the prior art antenna arrangement is applicable only to a vehicle window glass having a large space; in small-sized cars such as two-box car, the rear window glass is limited in size, and further limited when it is mounted relatively up-right in the vehicle so that there is only a small top or bottom margin left in available for mounting an FM antenna conductor, once the heating conductor is formed on such small vehicle window. Even in a motor vehicle having a relatively large window glass area and hence a large blank area, a TV broadcast reception antenna, an automobile telephone antenna, and the like are often formed in the blank portion. In this case, no further sufficient area is assured an FM antenna. Therefore, even if the glass window area is large, a glass antenna capable of receiving FM programs often cannot be formed. A glass window antenna having the features of the preamble of appended claim 1 is described in the document FR-A-2 250 329. It comprises a first antenna element connected to the upper end of a vertical power supply bus, and a second antenna element connected to the lower end of the same bus. It is the object of the invention to provide a glass window antenna for use in a motor vehicle in which a heating conductor means formed on a glass window can serve as an antenna means with an improved reception sensitivity to radio waves covering a relatively wide range of radio frequencies, e. g. both AM and FM broadcasting bands. The glass window antenna of this invention is defined by claim 1. In this antenna, a first and a second antenna element are connected to different power supply bus means. Further, a third antenna element extends between said first and second antenna elements. By this arrangement particularly the reception sensitivity to radio waves in the FM band is improved and the antenna gain is made flat in this band. The additional antenna element may be regarded as a coupling means for coupling the first and second antenna elements with each other with respect to signals generated or carried therein. Most preferably, the glass window antenna additionally comprises a choke coil in each power supply line, and a capacitive circuit connected to said power supply lines at a position between the choke coils and the power supply bus means. With this arrangement, improved antenna characteristics (e. g. antenna gain in the FM band) can be attained. It is expected that leakage of the signals generated in the heating conductor in response to radio waves is minimized because of the function of the capacitive circuit connected across the heating power supply lines. Therefore, the available radio signals for a radio receiver will be increased. Stated in another way, the combination of the capacitive circuit and the choke coils serves as an optimal impedance element which effectively prevents the radio signals received in the heating conductor means from passing or leaking through the choke coils while allowing passing the heating current to the heating conductor means through the choke coils and the power supply means. Moreover, the capacitive circuit serves to vary or adjust the effective length of the antenna because it is connected across the pair of power supply lines that are connected to the heating conductor receiving radio waves. The choke coils may have a relatively high impedance for a first band of frequencies (e. g. AM broadcasting band) and a relatively low impedance for a second band of frequencies (e. g. FM broadcasting bands), in which the frequencies are higher by at least one order of magnitude than in the first band. Brief Description of the Drawings The above and other objects, features and advantages of the invention will be more apparent from the following description taken in conjunction with the drawings in which: Fig. 1 shows a prior art vehicle glass window antenna together with associated electric circuitry; Fig. 2 shows another prior art vehicle glass window antenna with associated electric circuitry; Fig. 3 shows a vehicle glass window antenna together with associated electric circuitry in accordance with an embodiment of the invention; Fig. 4 is a graphic presentation of antenna reception sensitivity to FM band for the embodiment of Fig. 3 as compared with the sensitivity of an antenna arrangement without the capacitor 27 in Fig. 3; Fig. 5 is a graphic presentation of FM radio reception sensitivity for various values of the capacitor 27 in Fig. 3; Fig. 6 is a graphic presentation of relative antenna reception sensitivity to AM band with the embodiment of Fig. 3 as compared with a reference of the antenna arrangement without the capacitor 27 in Fig. 3; Fig. 7 is a graphic presentation of FM maximum reception sensitivity of the embodiment of Fig. 3 together with that of a pillar antenna and that of an antenna arrangement without the antenna elements 23 - 25 in Fig. 3; Fig. 8 is a graphic presentation of FM average reception sensitivity for the same antenna arrangements as those of Fig. 7; Fig. 9 is a graphic presentation of FM relative reception sensitivity for the embodiment of Fig. 3 and for the arrangement without the antenna elements 23 - 25 in Fig. 3, as normalized by the sensitivity of the pillar antenna; Fig. 10 is a graphic presentation of antenna directivity of the embodiment of Fig. 3 for radio frequencies in FM band with a horizontal polarity; Fig. 11 is a graphic presentation of antenna directivity of the embodiment of Fig. 3 for radio frequencies in FM band with a vertical polarity; Fig. 12 is a graphic presentation of FM maximum reception consisting of the embodiment of Fig. 3 as measured with vertically polarized radio waves in FM band; Fig. 13 is a graphic presentation of FM average reception sensitivity of the embodiment of Fig. 3 as measured with vertically polarized radio waves in FM band; Fig. 14 shows a glass window antenna in which the auxiliary antenna element 25 in Fig. 3 is omitted; Fig. 15 is a graphic presentation of FM reception sensitivity for the embodiment of Fig. 3 as compared with the sensitivity of the arrangement without the auxiliary antenna element 25 in Fig. 3 and the sensitivity of the arrangement without any of the antenna elements 23 - 25 in Fig. 3; Fig. 16 shows a modified location of the connection with the capacitor 27 in the choke coil assembly; Fig. 17 shows a glass window antenna arrangement in which the capacitor 27 is connected at a modified location of the power supply lines; Fig. 18 shows a modified connection of the capacitor 27 as applied to a glass window antenna with a one-sided heating power feeding arrangement; Fig. 19 shows a modified connection of the capacitor 27 as applied to a glass window antenna similar to that shown in Fig. 18; Figs. 20A - 20D show glass window antennas modified in respect of the antenna elements 23 - 25 in Fig. 3; Fig. 21 shows another glass window antenna modified with respect to the antenna elements 23 - 25 in Fig. 3; and Fig. 22 shows still another glass window antenna modified with respect to the antenna elements 23 - 25 in Fig. 3. Detailed Description of the Invention Referring first to Fig. 3, there is shown a front view of a rear glass window for a motor vehicle mounting a radio receiving antenna function in conjunction with associated circutry shown in a schematic diagram in accordance with an embodiment of the invention. A heating conductor 10 is arranged in a defogging area of a rear glass window 1. When a heating current is supplied to the heating conductor 10, it is heated to defog the glass surface. The heating conductor 10 has power supply buses 3 and 4 extending along opposite side edges of the rear glass window 1, and thirteen heater wires 2 spaced at equal intervals of a = 31 mm from one another and connected between the power supply buses 3 and 4. In accordance with the invention a pair of antenna elements 23 and 24 is formed on a top margin of the glass window 1 and bears a symmetrical relationship with each other with respect to a central axis of the heating conductor 10. The left-hand antenna elements 23 has a left and connected to the upper end of the power supply bus 3 of the heating conductor 10 while the right-hand antenna element 24 has a right end connected to the upper end of the power supply bus 4 of the heating conductor 10. These antenna elements 23, 24 are parallel to the uppermost heater wire 2 and spaced therefrom by an interval of 31 mm. The other or free ends of the antenna elements 23, 24 are spaced apart from each other by a distance b = 350 mm to oppose each other near a central axis of the rear glass window 1. An auxiliary antenna wire 25 is formed between the other ends of the antenna elements 23, 24, spaced therefrom by a distance c = 5 mm and in alignment with the wires 23, 24 so that the antenna wires 23 - 25 appear to be a single straight line. The antenna elements having free ends will change a state of a radio wave propagating on the heating conductor 10. Therefore, an appropriate choice of the dimensions of the antenna elements will optimize antenna reception sensitivity of the heating conductor 10 to an FM broadcasting band. Sizes d to g of the respective parts in Fig. 3 are given as follows: d = 10 mm; e = 28 mm; f = 895 mm; and g = 1,030 mm. A feed point or pad 11 is provided in the heating conductor 10, here, at the bottom of the right-hand power supply bus 4. The feed pad 11 gathers radio signals covering at least AM and FM broadcasting bands, as received in the heating conductor 10 and the symmetrical antenna elements 23 to 25. A feeder cable 22 is in the form of a coaxial cable is connected to the feeder pad 11 via a capacitor 21 to carry the signals to radio receiving circuitry such as AM and FM tuners (not shown). Choke coils 16a and 16b are respectively inserted into power supply lines 17 and 18. The choke coils are magnetically coupled with each other in a nonsaturated state. The opposite end of the choke coil 16a from the heater conductor 10 is connected to a direct current (DC) power supply at is positive terminal indicated by +B in Fig. 3 and it is also connected to a ground via a capacitor 20 for a surge or noise absorber while the opposite end of the choke coil 16 from the heater conductor 10 is grounded. The inductance of the choke coils 16a and 16b are preferably large to improve reception sensitivity to an AM broadcast band. However, this also makes large the capacitances in the choke coils 16a and 16b so that for higher frequencies such as FM broadcasting band, an impedance across each of the choke coil is decreased, thereby degrading antenna reception sensitivity. For example, with a toroidal core type choke oil having an inductance of 980 mH as each choke coil 16a, 16b, a test of the antenna reception sensitivity to an FM broadcasting band resulted in a low gain as indicated by a broken curve A in a reception sensitivity graph of Fig. 2. In this view and in accordance with the invention, there is provided a capacitor 27 connected across the power supply lines 17 and 18 between the choke coils 16a and 16b, and the heating conductor 10, as indicated within a block 26 in Fig. 3. The capacitor 27 serves to compensate for a change in the effective length of the heating conductor 10 resulting from the connection of the heating power supply lines 17 and 18 thereto. In addition, the capacitor 27 will compensate for an increase in the leakage of radio signals of higher frequencies such as FM broadcasting band through the choke coils 16a and 16b from the heating conductor 10 due to the capacitance in the choke coils which causes the impedance across each choke coil 16a, 16b to be decreased for higher frequencies. Therefore, with the capacitor 27, the arrangement of Fig. 3 can provide an increased antenna gain for a band of higher frequencies such as FM band covering at least 76 to 90 MHz while maintaining a satisfactory antenna gain for another band of relatively low radio frequencies such as AM broadcasting band covering at least 600 to 1600 KHz. A test of reception sensitivity of the antenna arrangement with the capacitor 27 has indicated a greatly improved antenna gain for the entire range of FM broadcasting band as shown in a solid curve B in Fig. 4 as compared with the curve A measured without the capacitor 27. Fig. 5 shows test results of reception sensitivity to the FM band using several different capacitive values of the capacitor 27. As noted, the peak of the antenna gain shifts to lower frequencies as the capacitance increases. The optimal capacitance of the capacitor 27 depends considerably on the electrical characteristics of the choke coils which is a function of their type, form, size and so an. According to tests, however, the capacitor with a capacitance between approximately 5 pico farads to 0.1 micro farads served to improve the reception sensitivity to FM band of 76 to 90 MHz. In particular, a capacitance between 30 to 200 pico farads yielded most desirable results. The addition of the capacitor 27 maintains reception sensitivity to AM broadcasting band of 600 to 1600 KHz as demonstrated in Fig. 6 which shows a relative reception sensitivity with the capacitor 27 as compared with that of no capacitor 27, normalized by zero decibel in Fig. 6. As described earlier, the symmetrical antenna elements 23 to 25 is expected to improve the radio signal condition in the heating conductor 10. According to tests, the antenna elements 23 to 25 did improved antenna characteristics for FM broadcasting band. In Fig. 7 to 9 showing reception sensitivity to FM band of 75 to 90 MHz, each curve A was obtained when the antenna elements 23 to 25 were omitted from the arrangement of Fig. 3. each curve B was obtained with the antenna elements 23 to 25, and each curve C was obtained with a pillar or rod antenna of 920 millimeters. Fig. 7 shows maximum reception sensitivity while Fig. 8 shows averaged reception sensitivity, and Fig. 9 shows a relative reception sensitivity with and without antenna elements 23 to 25 in relation to zero decibel reference of the pillar antenna. As noted from Figs. 7 to 9, the addition of the antenna elements 23 to 25 makes a significant improvement of reception sensitivity for FM band over the arrangement without the antenna elements 23 to 25. The reception sensitivity with the antenna elements 23 to 25 are noted as good as that obtained from pillar antennas. In addition, the antenna elements 23 - 25 can serve to improve antenna directivity. Fig. 10 shows antenna directivity measured with the arrangement of Fig. 3 with the antenna elements 23 - 25, using horizontally polarized radio waves at 76, 78, 80, 82, 84, 86, 88, 90 MHz, respectively. As noted, a substantially circular or omnidirectional antenna directivity was obtained i.e., the antenna gain was substantially constant irrespective of the direction of the radio wave in relation to the antenna. In some circumstances, such as in Japanese local radio stations and in U.S.A, vertically polarized radio waves are used in place of horizontally polarized radio waves. It has been formed that the antenna arrangement of Fig. 3 is also effective to such vertically polarized radio systems. Fig. 11 shows antenna directivity of the arrangement of Fig. 3 measured with vertically polarized radio waves in FM band. As noted, the resultant antenna directivity has indicated an ∞ curve with the minimum antenna gain when the radio wave is applied in forward and backward direction in relation to the antenna. This is in contrast to the prior art glass antenna FM antenna which provide antenna directivity of an 8 shape with the minimum antenna gain when the radio wave is applied in a direction horizontally and laterally crossing the antenna. Therefore, a combination of the prior art antenna with the arrangement of Fig. 3 will constitute a most useful diversity antenna system with optimized antenna directivity for vertically polarized radio waves. Fig. 12 shows maximum antenna reception sensitivity and Fig. 13 shows average antenna reception sensitivity, both measured with the arrangement of Fig. 3 using vertically polarized radio waves in FM band of 75 to 90 MHz. The resultant sensitivity is as good as that obtained with horizontally polarized radio waves (see Figs 7 and 8). Therefore, the addition of the antenna elements 23 to 25 serves to improve radio reception sensitivity to FM band of vertically polarized radio signals as well. In Fig. 14, there is shown a glass window antenna in which the auxiliary coupling antenna element 25 in Fig. 3 is omitted. Without the element 25, antenna reception sensitivity to FM band was somewhat concaved as indicated by a curve B in Fig. 15 in which the curve C was measured with the element 25 and the curve A was obtained without any of the antenna elements 23 to 25. As noted, the addition of the auxiliary antenna element 25 disposed between the antenna elements 23 and 24 has flattened the radio reception sensitivity over the substantially entire range of FM band. Figs. 16 to 19 show modifications of the embodiments of Fig. 3 in respect of the location and connection of the capacitor 27. In Fig. 16, the capacitor is connected across taps 28 and 29 provided in the respective choke coils 16a and 16b. In Fig. 17, the capacitor 27 is connected across the heating power supply lines 17 and 18 at a position between the choke coil assembly 26 to the heating conductor 10. An appropriate choice of the location of the power supply lines 17 and 18 where the capacitor 27 is connected thereacross may optimize the antenna characteristics. In the case of Fig. 18 in which a heating current is supplied to only one side of the rear window 1, here, power supply buses 3 and 4, the space between the power supply buses 3 and 4 is only a small distance. Thus, the capacitor 27 may readily be connected between the adjacent ends 30 and 31 of the power supply buses 3 and 4. Also, as shown in Fig. 19, auxiliary terminals or connecting pads 32 and 33 may be provided near the ends 30 and 31 of the buses 3 and 4 to connect the capacitor 27 between the pads 32 and 33. Fig. 20A - 20D, 21 and 22 show other modifications of the embodiment of Fig. 3 in respect of the antenna elements 23 - 25. In Fig. 20A, a plurality of parallel wires 23 - 25 are employed to increase the associated capacitance so that the arrangement of Fig. 20A will be useful for a broad band antenna. In Fig. 20B, a plurality of parallel and staggered or offset wires 23 - 25 are employed to improve the frequency characteristic. In Figs. 20C and 20D, in order to save conductive paste materials for the power supply buses 3 and 4, the length of each power supply bus 3, 4 is made short and some heating wires connected to the buses 3 and 4 extend obliquely at part hereof as shown. In Figs. 20C and 20D, an equivalent length of the buses 3 and 4 is indicated by ℓ. In Fig. 21, the antenna elements 23 - 25 are formed on the bottom margin of the window 1 while in Fig. 22, a first set of antenna elements 23 - 25 is formed on the top margin of the window 1 and a second set of antenna elements 23 and 24 is formed on the bottom margin of the window.

Claims: A glass window antenna for use in a motor vehicle, comprising: a heating conductor means formed on a glass window and comprising a left vertical power supply bus means (3; 3, 4), a right vertical power supply bus means (5) and heater wires (2) extending horizontally between said power supply bus means; and a first and a second antenna element (23, 24) connected to said heating conductor means; characterized in that said first antenna element (23) is connected to said left power supply bus means (3; 4); said second antenna element (24) is connected to said right power supply bus means (5); whereby said first and second antenna elements are arranged essentiallyl symmetrically in relation to the vertical symmetry axis of said heating conductor means; and a third antenna element (25) is axially aligned with and disposed between said first and said second antenna elements on said glass window and is spaced from said first and second antenna elements so as to be electrically disconnected therefrom. The glass window antenna of claim 1, further comprising a feeder (22) connected to said heating conductor means for gathering and carrying signals received in said heating conductor means and said antenna elements. The glass window antenna of claim 2, wherein said feeder (22) carries signals covering both AM and FM broadcasting bands available for radio receiving circuitry. The glass window antenna of claim 1, further comprising coupling means (27) for capacitively coupling said antenna elements (23, 24) with each other. The glass window antenna of claim 1, wherein said left vertical power supply bus means comprises a single bus connected to a power supply line (17), and said right vertical power supply bus means comprises a single bus (5) connected to another power supply line (18). The glass window antenna means of claim 1, wherein one of said left and right vertical power supply bus means comprises a lower bus (3) connected to a power supply line (17), and an upper bus (4) connected to another power supply line (18), and said other vertical power supply bus means comprises a single bus (5). The glass window antenna of claim 1, further comprising choke coils (16a, 16b) with a pair of power supply lines (17, 18), connecting said choke coils to said heating conductor means to supply a heating current to said heating conductor means, and a capacitor (27) connected across said power supply lines. The glass window antenna of claim 7, wherein said capacitor (27) has a capacitance which is between 5 pico farads and 0,1 micro farads. The glass window antenna of claim 8, wherein said capacitor (27) has a capacitance which is between 30 and 200 pico farads. The glass window antenna of claim 1, further comprising antenna elements extending between said left vertical power supply bus means (3) and said right vertical power supply bus means (5).

Inventors: IIJIMA HIROSHI, MAEDA MASARU, MURAKAMI HARUNORI, OHARA SHOUHEI, SAKURAI KAORU, IIJIMA, HIROSHI, MAEDA, MASARU, MURAKAMI, HARUNORI, OHARA, SHOUHEI, SAKURAI, KAORU
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 1989
Main classification: H01Q1
IPC codes: H01Q1