Window antenna

Abstract: A pair of antenna conductors (10, 11) are formed on upper and lower blank portions outside an area where defogging heater wires (2) are attached on a window glass (1) of a motor vehicle. One of the antenna conductors (11b) is RF-coupled with the defogging heater wires (2) and a feed terminal (11a) thereof is located at a lateral side of the glass. The other antenna conductor (10) has a feed terminal (10a) located at another lateral side of the glass. The two reception signals complement with each other in directivity due to asymmetric characteristic of the antenna conductors (10, 11) and are used in a diversity reception system for obtaining a nondirectional reception characteristic.

Description: This invention relates to a window glass antenna for a motor vehicle, comprising: heater wires formed in a heating area on the window glass; a first antenna conductor formed in an upper blank area outside the heating area; a second antenna conductor formed in a lower blank area outside the heating area; and first and second feed terminals arranged respectively at opposite sides of the window glass for the output of received signals from, respectively, said first and second antenna conductors. In a motor vehicle, diversity reception using a plurality of antennas may be employed for reducing changes in reception gain caused by changes in the travelling direction of the vehicle. For FM broadcast diversity reception, for example, a pole antenna attached on the body of the motor vehicle and an antenna comprising an antenna conductor on a rear glass window are used in a prior art. In carrying out diversity reception the highest level reception signal is selected (cf. Japanese laid open patent application No. 140301/1985). It is known to perform diversity reception with two or more antenna conductors and feed terminals on a rear window surface of the motor vehicle (Japanese utility model registration application laid open Nos. 138408/1988 and 29307/1988 and a patent application laid open No. 269625/1988). It is possible to tune the directivity of respective antennas when the antennas are formed by spaced conductors arranged on a surface of a window glass. However, since there is little space on the window for the antenna conductors, they show low average reception sensitivity, and preamplifiers with fixed gains must be inserted immediately after feed terminals of the antenna conductors. These fixed gain amplifiers often saturate at strong radio wave field so that voice reception is significantly degraded. US-A-4 439 771 describes a window glass antenna of the kind defined hereinbefore at the beginning in which the second antenna conductor is connected to the second feed terminal by a lead wire to the lowermost heater wire, and the second feed terminal serves as the output terminal to heater wires, which function as a subsidiary antenna having a different directivity from that of a main antenna constituted by the first antenna conductor together with a plurality of other conductors connected thereto in an asymmetrical pattern. JP-A-58 0706444 describes a window glass antenna in which, above a heating area with heating wires, two antennas with different patterns are provided to allow a choice of reception. JP-A-58 070647 describes a window glass antenna in which a pattern of antenna conductors provided below a heating area is connected to the centre of the lowermost heater wire and to a first feed terminal at the lower right hand corner of the glass, and another pattern of antenna element, arranged above and separate from the heater wires, is connected to a second feed terminal at the lower right corner of the glass by a feeder line provided along a metallic window frame in which the glass is mounted. It is an object of this invention to provide window glass antennas which have good complementary directivities and high reception sensitivities without using preamplifiers. According to the present invention, a window glass antenna of the kind defined hereinbefore at the beginning is characterised in that the first antenna conductor is a center feed antenna conductor having a central feed point which is coupled to the first feed terminal through a feed conductor and has a single horizontal element and a vertical element connected between the central feed point and the center of the horizontal element to form an inverse "T" shape, and in that the second antenna conductor has a main element capacitively RF-coupled to the lowermost heater wire. Because the wiring to the said feed terminals extends in opposite directions, the location of the first and second antenna conductors is asymmetrical with respect to a center line of the window glass. The directivities of the antenna conductors complement each other. A non-directional reception characteristic is thus achieved by diversity reception. The invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a front view of a rear window glass for a motor vehicle embodying a window glass antenna according to this invention; Fig. 2 is a graph showing directivities of upper and lower antenna conductors; Fig. 3 is a graph showing variation of reception sensitivity with respect to coupling capacity between heater wires and the second antenna conductor; Fig. 4 is a graph showing a gain of the first antenna conductor for various conductor width of feeder conductor connected thereto; Fig. 5 is a graph showing gain of the second antenna conductor in a case in which a horizontal element is added to a bus bar for heater wires and a case in which the horizontal element is not provided; Fig. 6 is a graph showing gain of antenna in a case in which a short-circuit conductor and a horizontal element are added and another case in which these are not provided; and Fig. 7 is a graph showing gain of antenna in a case in which a horizontal element is provided to capacitively couple to the first antenna conductor and another case in which the horizontal element is not provided. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 is shows a rear glass window 1 of a motor vehicle according to this invention. In the middle zone of the glass window, a plurality of defogging heater wires 2 is formed in parallel to each other through a printing and a baking processes. The heater wires 2 are divided into upper and lower groups. Power is fed through bus bars 3 and 4 through a return path via a common bus bar 5. Another power feed system may be arranged in which heater wires 2 are not divided into two groups so that a current flows from a side bus bar to another side bus bar. The heater wires 2 are used as an AM radio wave reception antenna. Magnetically coupled choke coils 8a and 8b are inserted into power supply lines 7a and 7b. The choke coils provide the heater wires 2 with large impedance to ground at radio frequencies. The power supply line 7a is coupled to a main power supply through a switch 6 and the power supply line 7b is coupled to ground. Antenna conductors 10 and 11 are arranged on upper and lower blank areas of the window glass 1. The upper antenna conductor 10 is exclusively used for FM broadcast reception. The lower antenna conductor 11 is used both for AM and FM broadcast receptions. A diversity reception is performed for FM broadcast reception with using the upper and lower antenna conductors 10 and 11. Asymmetrical arrangement of conductors is employed with respect to the center line of the glass window for satisfactory FM diversity reception. For securing asymmetrical arrangement of the conductors, a feed terminal 10a of the upper antenna conductor 10 is located at the right side (viewed from inside of a car) of the glass window 1 and a feed terminal 11a of the lower antenna conductor 11 is located at left side. The antenna conductor 10 comprises horizontal elements 10b - 10g. A main element 10b is a single element antenna of about 800 mm in length and is symmetrically arranged with respect to the center line of the glass window 1. At the center of the conductor 10b, a vertical coupling conductor 10h is connected to form an antenna of generally inverse letter-T configuration. The coupling conductor 10h is connected to a feeder conductor 10i arranged along an upper edge of the glass window 1. An end of the feeder conductor 10i is connected to the feed terminal 10a located at the side end of the glass window 1. The feeder conductor 10i is 3 mm in width and made wider than other conductors so that impedance matching is satisfied between a feeder cable of 50Ω to be connected to the feed terminal 10a and the inverse T-antenna comprising the main element 10b and the coupling conductor 10h. The feeder conductor 10i has a desired capacitance to ground which is produced by capacitive coupling to a window frame (ground potential) near the upper edge of the glass window. A desired impedance measured at the feed terminal 10a is obtained. Horizontal elements 10d and 10f are extended from the bus bar 3 of the heater wires 2 parallel with the main element 10b. These horizontal elements are capacitively coupled with the main element 10b so as to tune the antenna in directivity. Respective ends of the horizontal elements 10d and 10f are located at 300 mm and 400 mm away from the center line of the window 1. A short-circuiting conductor 10j of 4 mm in width is provided along the center line of the glass window 1 to short-circuit all of upper group of the heater wires 2. A horizontal element 10g is extended from an end of the short-circuit conductor 10j toward the bus bar 3 so as to couple capacitively with the element 10h for improving directivity. An end of the horizontal element 10g is located at 500 mm away from the center line of the glass window 1. The short-circuit conductor 10j connects equipotential points of the heater wires 2 so that no short-circuiting current actually flows through the short-circuit conductor 10j. In this embodiment, the spacing between lines 10d - 10b, 10b - 10f, 10f - 10g and 10g - heater wires 2 are respectively 10, 15, 7.5 and 7.5 mm. A horizontal auxiliary element 10e of 165 mm in length is extended from the feed terminal 10a so as to tune the antenna in directivity as well as high frequency characteristic. The above-mentioned elements 10d, 10h, 10g and 10e. improve reception directivity of the main element 10b on the left side thereof. These elements are arranged generally on the right side of the center line of the glass window 1. The antenna conductor 10 is thus arranged asymmetrically in general with respect to the center line of the glass window 1. The lower antenna conductor 11 has a horizontal main element 11b extended from the feed terminal 11a parallel with the heater wires 2 at a spacing of 2 mm. The main element 10b is capacitively coupled to the heater wires 2 to serve as an antenna for reception of both AM and FM waves induced on the heater wires 2. The lower group of the heater wires 2 serves as ground circuitry i.e. a return path of a heater current so that noises in a heater power supply do not propagate directly from the heater wires 2 to the main element 11b. No significant noise is perceptible in a received signal. An end of the main element 11b is located at 210 mm away from the center line of the glass window 1. Auxiliary horizontal elements 11c, 11d and 11e are extended from bus bars 3 and 5 of the heater wires 2 for improving directivity and frequency characteristics of the main element 11b. The heater wires 2 are thus tuned as an antenna. The element 11c is extended horizontally from the lower end of the common bus bar 5 to apoint 150 mm away from the center line of the glass window with a spacing of 30 mm from the heater wires 2. The element 11d is extended from the lower end of the bus bar 3 along the right side of the glass window 1 and then extended horizontally to a position 410 mm away from the center line of the glass window 1 with a spacing of 15 mm from the heater wires 2. The element 11e is extended from the upper end of the common bus bar 5 along a corner portion and upper side of the glass window 1 to a position 150 mm away from the center line. Fig. 2 shows directivity of antenna conductors 10 and 11 with respect to FM broadcast wave of 95 MHz. The directivity of dotted line D10 corresponds to the antenna conductor 10 and the directivity of a solid line D11 corresponds to the antenna conductor 11. As is apparent from the directivity chart, a dip in the gain of the lower antenna 11 appearing on the right side of the course of a travelling car is complemented by the gain of the upper antenna conductor 10. In the directivity of the antenna conductor 10, a dip appearing on the left side of the course of a travelling car is complemented by the gain of the lower antenna conductor 11. Diversity reception is achieved by selecting the received signal having the higher level from the complementary antenna conductors 10 and 11 on the basis of comparison of these signals. A stable reception signal is obtained regardless of changes of traveling direction of the car. The lower antenna conductor 11 functions as an AM reception antenna. In this radio band, the heater wires 2 are operable as an antenna conductor since the wires are relatively long. Fig. 3 is a graph showing a relation between AM reception sensitivity and coupling capacitance of the main antenna element 11 to the heater wires 2. Differences in sensitivity are plotted with reference to that (0dB) of a rear pole antenna 1200 mm long. The difference reaches the reference when the coupling capacitance exceeds 60 pF, and saturates at 70 pF or more. In the embodiment, the main element 11b and the heater wires 2 are spaced by 2 mm to give capacitive coupling not less than 70 pF so that an AM reception signal is obtained by the antenna conductor 11 with a sufficient gain. Fig. 4 shows reception gains of the upper antenna conductor 10 for FM broadcast frequencies in the range 80 - 110 MHz in cases where width of the feeder conductor 10i in Fig. 1 is 1 mm and 3 mm as respectively shown by a dotted line and a solid line. As is apparent from the graph, reception gains over the lower end and upper end of the FM broadcast band are respectively improved when the width of the feeder conductor 10i is 3 mm. Frequency characteristics are improved over a wide range. The feeder conductor 10i is extended from the center line of the window glass 1 toward right side to the feed terminal 10a and operates as an antenna element which serves to improve reception gain on the right side of the course of travel. Fig. 5 shows reception gains of the lower antenna conductor 11 in a range 80 - 110 MHz in the cases where the horizontal elements 11c and 11e are extended from the common bus 5 of the heater wires 2 and where these elements 11c and 11e are removed, respectively as shown by a solid line A and a dotted line B . The FM radio wave current induced in the heater wires 2 is changed by extending the elements 11c and 11e from the common bus bar 5 of the heater wires 2. Frequency bands over which a good sensitivity is obtained shift relative to each other as shown by the graph. These elements 11c and 11e are arranged on the left side of the glass window 1 so that it operates to improve reception gain of the lower antenna conductor 11 on the left side of the course of travel. Fig. 6 shows gain characteristics in the band 80 - 110 MHz for a case (solid line A ) where the short-circuiting conductor 10j is applied to the heater wires 2 in Fig. 1 and the horizontal element 10g is extended, from an end of the conductor 10j, parallel to the heater wires 2, and another case (dotted line B ) where these conductors 10j and 10g are not used. FM radio frequency current induced in the heater wires 2 is changed by attaching these conductors 10j and 10g. The gain characteristic is shifted as shown by the graph. The horizontal elements 10g is extended on the left side of the glass window 1 so that it operates to improve reception gain of the upper antenna conductor 10 on the left side of the course of travel. Fig. 7 shows reception gains in the band 80 - 110 MHz in a case (solid line A ) where the horizontal elements 10d and 10f are extended from the bus bar 3 of the heater wires 2 as shown in Fig. 1 to capacitively couple with the main element 10b on both sides (upper and lower sides) thereof and another case (dotted line B ) where the elements 10d and 10f are not used. The effect on the main element 10b of the heater wires 2 can be changed by capacitive coupling of horizontal elements 10d and 10f with the main element 10b at a distance within 15 mm. The gain characteristic is shifted as shown by the graph. These horizontal elements 10d and 10f operate to improve a gain of the upper antenna conductor 10 on the right side of the course of travel. Due to the asymmetrical arrangement of the first and second antenna conductors with respect to the center of the glass window, directivities complementing each other are obtained. Good reception characteristics are obtained with diversity reception regardless of traveling direction of car. In particular, a reception system in which no preamplifier is employed can be arranged so that a reception signal of high quality is received without distortion even in a strong radio wave field. According to other features of the antenna arrangement, reception gains of the first and second antenna conductors are improved. Complementary characteristics of the first and second antenna elements provide a diversity reception system with high performance.

Claims: A window glass antenna for a motor vehicle, comprising: heater wires (2) formed in a heating area on the window glass (1); a first antenna conductor (10) formed in an upper blank area outside the heating area; a second antenna conductor (11) formed in a lower blank area outside the heating area; and first and second feed terminals (10a,11a) arranged respectively at opposite sides of the window glass for the output of received signals from, respectively, said first and second antenna conductors (10,11), characterised in that the first antenna conductor (10) is a center feed antenna conductor having a central feed point which is coupled to the first feed terminal (10a) through a feed conductor (10i) and has a single horizontal element (10b) and a vertical element (10h) connected between the central feed point and the center of the horizontal element (10b) to form an inverse "T" shape, and in that the second antenna conductor (11) has a main element (11b) capacitively RF-coupled to the lowermost heater wire (2). A window glass antenna according to claim 1, characterised in that line conductors (10d,10f) are coupled capacitively to the said horizontal element (10b) and extend from a bus bar (3) of the heater wires (2) on the side where the first feed terminal (10a) is arranged. A window glass antenna according to claim 2, characterised in that the said line conductors consist of two elements (10d,10f) arranged along upper and lower sides of the said horizontal element (10b). A window glass antenna according to claim 1, characterised by a short-circuiting conductor (10j) which short-circuits a group of the heater wires (2) at the center thereof; and a line element (10g) extending substantially towards said first feed terminal (10a) from an upper end of the short-circuiting conductor (10j) parallel with said horizontal element (10b). A window glass antenna according to claim 1, characterised in that auxiliary horizontal elements (11c,11e) extend from a bus bar (5) of the heater wires (2) on the side of the second feed terminal (11a) in the upper and lower blank areas of the window glass to improve antenna characteristic of the heater wires (2). A window glass antenna according to claim 5, characterised in that an auxiliary horizontal element (11d) extends from a bus bar (3) of the heater wires (2) on the side of the first feed terminal (10a) in the lower blank area.

Inventors: MURAKAMI HARUNORI NIPPON SHEET, OKA HIDETOSHI NIPPON SHEET GLA, MURAKAMI, HARUNORI, NIPPON SHEET GLASS CO. LTD., OKA, HIDETOSHI, NIPPON SHEET GLASS CO. LTD.
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 1990
Main classification: H01Q1
IPC codes: H01Q1