WO2003026062A2

WO2003026062A2 - Film antenna, windshield, and film antenna grouding structure

Info

Publication number: WO2003026062A2
Application number: PCT/JP2002/009157
Authority: WO
Inventors: Noriyuki Mukai
Original assignee: Nippon Sheet Glass Co., Ltd.
Priority date: 2001-09-11
Filing date: 2002-09-09
Publication date: 2003-03-27
Also published as: WO2003026062A3

Abstract

The present invention provides a film antenna device that includes at least one film including a first film; and a first antenna, a second antenna and a third antenna that are formed on the at least one film. The first antenna and the second antenna are formed on the first film. At least one of the antennas can have a grounding structure in which a braided wire of a coaxial cable is connected electrically to an electrically conductive tape. The tape is adhered to a pillar of a vehicle body so as to form a capacitive coupling with the vehicle body.

Description

DESCRIPTION

FILM ANTENNA, WINDSHIELD, AND FILM ANTENNA GROUDING STRUCTURE

FIELD OF THE INVENTION The present invention relates to a film antenna that is retrofitted to an automotive window glass, and it relates in particular to a film antenna for receiving TV broadcasts.

BACKGROUND OF THE INVENTION

In recent years, more and more automobiles carry a TV device. When the TV device is retrofitted, an antenna also should be retrofitted.

Accordingly, a film antenna that can be attached to an automotive window glass is used frequently. Such a film antenna includes an antenna element and a lead pattern for the antenna element that are formed on a resin film.

The antenna element and a lead pattern may be formed on a laminated film. The film antenna is attached to a windshield glass with a double— sided adhesive tape or the like.

Since the area where these film antennas can be mounted has been limited by vision specifications of the windshield, a sufficient receiving sensitivity sometimes cannot be obtained. In the case of a glass antenna that is formed directly on a window glass surface, an antenna pattern is adjusted considering an influence of the automotive body, making it possible to achieve an excellent receiving sensitivity. On the other hand, in the case of the retrofitted film antenna, adjustment cannot be made considering the influence of the automotive body on which the film antenna is to be mounted, so that a sufficient receiving sensitivity cannot be obtained in some cases. For a signal line of such a film antenna, a coaxial cable is used in general. Its core wire is connected to a feeding portion of the above— entioned lead pattern, while its braided wire is connected to the vehicle body, thereby establishing a ground.

In order to perform a ground connection, an interior material first should be removed over a wide range in a peripheral portion of the windshield. Then, the grounding is screwed to the body or a grounding plate 701 made of rubber magnet is attached to the car body (see FIG. 14). Thus, the ground connection has involved many steps. Incidentally, such a film antenna often cannot achieve a sufficient sensitivity unless the ground is established.

The inventor has proposed a grounding structure of a film antenna in JP 7(1995)-122919 A for a single film antenna.

Also, JP 2000-13122 A and JP 2000-13125 A disclose vehicle glass antennas provided on windshield glasses. Furthermore, when a retrofitted film antenna is mounted on a windshield glass, its installation space is limited. Moreover, since the TV broadcast includes three receiving frequency bands of VHF-Lo, VHF— Hi and UHF, three antenna patterns for respective receiving frequency bands should be prepared. Accordingly, some of film antennas 211, 212, 213 often are attached around stickers such as a service sticker 5 and an automobile inspection sticker 6 with almost no space from them as shown in FIG. 14. In such cases, when the stickers are replaced at the time of an automobile inspection and periodic service, the film antennas have to be removed once and then attached again.

When there are many films to be attached, the work tends to become complicated. DISCLOSURE OF THE INVENTION

The present invention provides a film antenna device that includes a film component and antennas. The film component is composed of at least one film (e.g. one or two films) that includes a first film. The antennas include a first antenna, a second antenna and a third antenna that are formed on the at least one film. The first antenna and the second antenna are formed on the first film. To improve a receiving sensitivity, the first antenna and the second antenna preferably include antenna elements that are capacitively coupled.

The present invention further provides a windshield including a windshield glass and the above film antenna device that is attached to the windshield glass. The present invention further provides a film antenna grounding structure including the above film antenna device that is attached to a windshield glass, an electrically conductive tape, and coaxial cables (a first coaxial cable, a second coaxial cable and a third coaxial cable). The electrically conductive tape is adhered to a pillar of a vehicle body so as to form a capacitive coupling with the vehicle body. The core cables of the coaxial cables are connected electrically to the first antenna, the second antenna and the third antenna, respectively. The braided wire of at least one of the coaxial cables is connected electrically to the electrically conductive tape. Preferably, the braided wires of two or all of the coaxial cables are connected electrically to the tape. To simplify the grounding structure, it is preferable that the braided wires of at least two of the coaxial cables are connected electrically to the same electrically conductive tape. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an example of a film antenna device according to the present invention.

FIG. 2 is a plan view of an example of a film with first and second antennas.

FIG. 3 is a plan view of an example of a film with a third antenna.

FIG. 4 is a plan view of another example of the first and second antennas.

FIGs. 5A and 5B are schematic views for showing a connection of a coaxial cable or coaxial cables with connecting devices.

FIG. 6 is a sectional view of an example of a connector that is attached to a car body.

FIGs. 7A and 7B are graphs showing a receiving sensitivity in an example of the present invention.

FIG. 8 is a plan view of a reference example of a film antenna device in which no antennas are capacitively coupled.

FIG. 9A and 9B are graphs showing a receiving sensitivity in a reference example.

FIG. 10 is a view showing a result of VSWR for the first antenna ⁵ alone.

FIG. 11 a view showing a result of VSWR for the first antenna with which the second film antenna is capacitively coupled.

FIG. 12 a view showing a result of VSWR for the second antenna alone. ^ιυ FIG. 13 a view showing a result of VSWR for the second antenna with which the first antenna is capacitively coupled.

FIG. 14 is a view showing a retrofitted film antenna, which is a conventional example.

DETAILED DESCRIPTION OF THE INVENTION ¹⁵ In the film antenna device of the present invention, the first antenna and the second antenna are provided on the first film and include antenna elements that are capacitively coupled. Since the first and second antennas are provided on a common film, the number of films is reduced, thus easing the attaching work. Since these two antennas are capacitively ^^υ coupled, they have an excellent receiving performance in a wide frequency bands, for example from VHF-Lo, VHF-Hi to UHF. The third antenna may be formed on a second film, which is separate from the first film.

Since it is not easy for shorter antennas to keep a receiving sensitivity in a relatively low range of the above frequency bands in general, the antenna elements that are capacitively coupled preferably are the shorter two elements. The third antenna may include the longest antenna element of those in the film antenna device.

For laying elements extending from feeding portions so as to form a capacitive coupling, it is preferable that the first antenna includes an antenna element that extends counterclockwise and the second antenna includes an antenna element that extends clockwise. It is preferable that the antenna elements are capacitively coupled to each other. The first antenna and the second antenna can constitute a diversity antenna. The third antenna also preferably includes an antenna element that extends clockwise or counterclockwise, and may be included in the diversity antenna.

To secure a wide visible area for occupants of the vehicle, the first film and/or the second film may have an opening. The first antenna and the second antenna, or the third antenna may be arranged so as to surround or substantially surround the opening. The antenna elements that are capacitively coupled in the first film preferably extend along the peripheral edge of the opening. The antenna element of the third antenna also can extend along the peripheral edge of the opening of the second film. The first film and/or the second film can be of a substantially rectangular shape, or may be of a substantially rectangular frame shape.

In the film antenna grounding structure of the present invention, a capacitor is formed between the electrically conductive tape and the vehicle body, thus establishing body grounding. Although the frequency characteristics vary considerably depending on the kinds of an insulator, the impedance is 0.0015 Ω when electrode conductors are opposed to each other via an insulator in a FM/TV band (for example, at 100 MHz), forming a capacitance of about 1 μF. " For example, with respect to a coated metal plate as in an automotive body, electrodes are opposed to each other via a coating film (insulator). In such a structure, when the capacitance to be formed is about 1 μF, an alternating current signal can pass through and a direct current signal is blocked. Therefore, this capacitively coupled structure ^iυ can function as a grounding in a high-frequency band.

By reducing the spacing between the car body and the opposing electrodes or expanding the area of the electrodes, it is possible to reduce the impedance further.

The film antenna appropriately can be attached to an upper portion • of a windshield glass in front of a driver's seat and a passenger's seat. For example, the first film 21 appropriately is attached to the space between the service sticker 5 and the automobile inspection sticker 6 (see FIG. l), and it need not be attached so as to surround them. The second film 22 appropriately is arranged away from the automobile inspection sticker 6 on

^^υ and attached to the upper portion in front of the driver.

There are some regulation stickers that should be attached to a windshield glass. The film antenna device of two separate base films easily can be attached away from the stickers. The film antenna does not have to be removed when the service sticker and the automobile inspection sticker are replaced at the time of an automobile inspection and a periodic check. When a windshield includes a sticker that is required by government regulations, the at least one film of the film antenna device should be attached to the windshield glass so as to be away from the sticker. It is preferable that a shorter antenna dimension of an antenna is more than 100 mm. Here, the shorter antenna dimension of an antenna means a shorter side of the minimum rectangle that covers said antenna. The shorter antenna dimension usually corresponds to a distance between an upper side and a lower side of the antenna elongated clockwise or counterclockwise on a film base. This can secure an antenna performance. On the other hand, it is preferable that a lowermost portion of the film antenna is attached within 120 mm from an upper end of a visible region (an upper end of a visible opening) of the windshield glass. This is because the film antenna then will not come into the viewing field of a driver and a passenger very often, so that a view obstruction can be minimized.

Moreover, when the attaching position is within 110 mm from the upper end of the visible region, it becomes possible to reduce a view obstruction further. This limitation is particularly effective for a film antenna to be provided in front of a driver, in which securing the visual field is important.

A nontransparent portion in the upper portion of the windshield specifically includes a ceramic printed portion. Furthermore, when a periphery of this ceramic print is formed of a dotted pattern, the upper end appropriately refers to an outermost peripheral edge of the dotted pattern (a peripheral edge between the dotted pattern area and the unprinted area).

The following is a specific description of the present invention, with reference to accompanying drawings.

FIG. 1 schematically shows how a film antenna 1 is attached to a windshield glass 10, seen from the inside of a car. FIG. 2 is a plan view showing a first film 21 on which first and second antennas 11, 12 are formed, and FIG. 3 is a plan view showing a second film 22 on which a film antenna 13 is formed.

The first film 21 and the second film 22 have openings 23. The first antenna 11 includes a feeding portion 41 and an antenna element 31 that extends counterclockwise from the feeding portion 41 along the edge of opening 23. The second antenna 12 also includes a feeding portion 42 and ^lfc) an antenna element 32 that extends clockwise from a lead pattern 52 along the edge of opening 23. The two antenna elements 31, 32 are capacitively coupled on the film 21.

The third film has a single antenna 13 that includes a feeding portion 43 and an antenna element 33 that extends from a lead pattern 53. ^ΔU The element 33 extends along the frame of the film 22, surrounding the opening 23.

For example, each of these antennas 11, 12, 13 can be formed by printing a silver paste on a polyester film base (for example, having a thickness of 75 μm), followed by drying. Then, a clear coating (for example, "SG429B" manufactured by Seiko Advance Ltd.) further may be formed as a protective film.

To maintain electrical conductivity, the above-mentioned clear coating should not be formed on the feeding portions 41, 42, 43. A core ^δ wire of a coaxial cable may be connected to the non-coated feeding portion.

In FIGs. 2 and 3, each of the antenna patterns is illustrated as a rectangle for simplicity of drawing. However, upper and lower patterns of each antenna patterns can be formed to extend along a curve of an upper side of the windshield glass. -^ To improve a receiving performance, the first and second antennas

11, 12 should include antenna elements that form a capacitive coupling. A variation of the first and second antennas is shown in FIG. 4.

It is preferable that the length of the portion forming a capacitive coupling in the antenna elements 31, 32 should be at least (l/4λ) x k, where ^lδ λ is a designed wavelength and k is a shortening factor. The factor k is about 0.6, when taking dielectric constants of the glass and the film base into consideration. Thus, the length of the portion should be at least 200 mm when λ = 225 MHz and should be at least 270 mm (= (l/4λ) x k) when λ = 167 MHz. In general, the length of the portion forming a capacitive ^^u coupling preferably is in a range of 200 mm to 500 mm. In FIG. 2, the length of the coupling portion is 280 (185 + 95) mm. It is preferable that the spacing between the elements 31, 33 is not more than 5 mm.

The two films for the film antenna device should be attached to an upper portion of the windshield glass 10 in front of a driver's seat and passenger's seat, seen from the inside of the car (see FIG. l). An adhesive may be applied to the back surface of the film beforehand. In FIG. 1, this first film 21 is attached to the windshield glass between an automobile inspection sticker 6 and a service sticker 5, and the second film 22 is ° arranged away from the automobile inspection sticker 6. These film bases 21, 22 are away from the stickers 5, 6 that should be exchanged periodically. The film antenna that includes only two film bases is easy to be attached so as not to overlap the stickers that are required by the government regulations and may be exchanged. ^ιυ It is preferable that the line width of the antenna element is as small as possible considering their outward appearance. More specifically, it preferably is not more than 0.3 mm. In the example of FIG. 2, the line width was set to be 0.2 mm.

The dimensions of the film antenna should be determined

15 considering a required receiving sensitivity and an outward appearance. The antenna pattern preferably is as large as possible in order to improve the receiving sensitivity. Thus, each of the first antenna, the second antenna and the third antenna preferably has a shorter antenna dimension of more than 100 mm. On the other hand, it should not be excessively

20 large considering the outward appearance. Thus, the shorter antenna dimension is preferably not more than 120 mm.

In FIGs. 2 and 3, the shorter dimensions of the first antenna, the second antenna and the third antenna are 102 mm. 108 mm and 101 mm, respectively. The line width may be larger if an upper side portion of the antenna is covered with a ceramic print. However, if the upper side portion may not be covered with the ceramic print, the line width should be small. The antenna preferably includes a rectangular line antenna pattern as a lead pattern. The lead patterns 52, 53 in FIGs. 2 to 4 are combinations of elongated rectangular shapes by removing the internal of a rectangular painted pattern, or ladder-like shape patterns.

Next, a grounding structure will be described with reference to FIGs. 1, 5 and 6.

-^ First, an electrically conductive tape 9 having a back surface to which an electrically conductive adhesive is applied is attached to a coated car body 200 (see FIG. l). The electrically conductive tape 9 may have a dimension of 10 mm x 150 mm, width by length, for example. Incidentally, it has been confirmed by experiment that an effective grounding can be ^lδ achieved when the area is at least 10 mm x 100 mm, width by length (1000 mm²). The electrically conductive tape may be a tape obtained by attaching a double— sided adhesive tape to a metal foil.

It is appropriate to attach the electrically conductive tape 9 to a metal portion 202 of a pillar of the car body 200. The pillar is located " beside the windshield 10 (between the windshield and front door windows) and covered with an interior material. The metal portion 202 in this pillar can be exposed easily by removing a resin cover serving as the interior material. A capacitive coupling with a pillar is preferably in terms of work simplicity. In the present example, a metal foil 91 of the electrically conductive tape 9 was attached to each of the right and left pillars with a double-sided adhesive tape 92.

An electrically conductive tape for a grounding formation may be attached to one of the pillars when the cables can be arranged from the antennas to the pillar. In this case, the work becomes even simpler because only one of the resin covers should be removed.

Next, a braided wire 82 of the coaxial cable 8 and an electric wire 84 are connected via a pair of fittings 85, 86 while a core wire 81 of a coaxial cable 8 and an electric wire 83 are connected via another pair of fittings 85, 86 (see FIG. 5A). The electric wires 83, 84 are connected to the connectors 7, 87, respectively. Where two coaxial cables lies adjacent to each other, the braded wires 82, 82 of two coaxial cables 8, 8 can be twisted together and are connected electrically to the same connector 7 (see, FIG. 5B). In this case, the braded wire of the remainder of the three coaxial cables also can be connected electrically as in Fig. 5A. Each of the connectors 87 is attached to a feeding portion of one of the antennas, thereby connecting the core wire 81 to the antenna pattern.

On the backside of the connector 7, a metal foil 71 connected to the electric wire 84 is exposed (see FIG. 6). It is appropriate to apply a on double-sided adhesive tape 72 or an adhesive to the backside of the connector 7.

This connector 7 is attached to the electrically conductive tape 9, thereby connecting the braided wire 82 of the coaxial cable to the electrically conductive tape 9 (see FIG. 6). Then, this electrically conductive tape 9 and the metal portion 202 of the car body form a capacitor, establishing a grounding. The braided wire and the electrically conductive tape may be connected directly by soldering without using a connecting device.

Considering an antenna performance, the length of the exposed braided wire of the coaxial cable should be as small as possible within the bounds of not impairing the workability. In the present example, the third film antenna provided on the driver side had the exposed braided wire with a length of 50 mm, and the first and second film antennas provided on the ^υ passenger side had that with a length of 100 mm.

Since noise is caused easily in a core portion that is not shielded by the braided wire, it is preferable that an exposed core portion is as small as possible. The length of the exposed core portion preferably should be 200 mm at maximum. In terms of attaching work, the length generally should ^ be at least 100 mm. In the present example, it was 150 mm.

A total of three coaxial cables 8 to be connected to the three film antennas are provided on the periphery of the windshield (see FIG. l). These coaxial cables 8 are connected to a selector 30. This selector selects a signal from the above-described three antennas according to receiving

^ΔKJ frequency bands. This selected signal is sent to a receiver 40 via one coaxial cable. The selector 30 also may be provided with an amplifier. Furthermore, the signal from the receiver 40 is transmitted to a monitor 50 such as a liquid crystal display. (Antenna Performance) The receiving performance of the above-described film antenna device of the example was measured. The result of the measurement is shown in Table 1 and FIGs. 7A and 7B. For comparison, the receiving performance of a film antenna device including three antennas 111, 112, 113 shown in FIG. 8 in which the first and second antennas 111, 112 are not capacitively coupled also is shown in Table 1 and FIGs. 9A and 9B. For both of the film antenna devices, data shown here are measurement data obtained when three antennas 11, 12, 13 or 111, 112, 113 constitute a diversity antenna.

Furthermore, in the example, the antenna performance for the case where the first and second antennas are capacitively coupled and the case where they function as an individual antenna was evaluated by VSWR (Voltage Standing Wave Ratio). The results are shown in FIGs. 10 to 13. Moreover, for each antenna, the values of VSWR at typical frequencies are shown in Table 2.

When the first and second antennas are capacitively coupled, there are two kinds of antennas • one is for the case of capacitively coupling the second antenna to the first antenna, and the other is for the case of capacitively coupling the first antenna to the second antenna. In other words, there are two antennas when seen from the side of each terminal. They are expressed by "first antenna (coupled)" and "second antenna (coupled)."

Table 2

FIG. 10 shows the result of VSWR in the case of the first antenna alone. The vertical axis indicates VSWR, and the horizontal axis indicates the frequency. The antenna performance is better as the value of the vertical axis becomes smaller. As becomes clear form FIG. 10, the first antenna alone can achieve an effective antenna performance at frequencies of about 90 and 220 MHz.

FIG. 11 shows the result in the case of capacitively coupling the second antenna to the first antenna. It is indicated that, with the capacitive coupling, the antenna performance slightly improves at frequencies of about 90 and 220 MHz.

FIG. 12 shows the result of VSWR in the case of the second antenna alone. As becomes clear form FIG. 12, the second antenna alone can achieve an effective antenna performance only at a frequency of about 170 MHz. •

FIG. 13 shows the result in the case of capacitively coupling the first antenna to the second antenna. It is indicated that, with the ^δ capacitive coupling, an effective antenna performance can be achieved also at a frequency of about 108 MHz.

Moreover, the results of FIGs. 11 and 13 indicate that, when the first and second antennas are capacitively coupled, an effective antenna performance can be achieved at any frequencies of 90, 108, 170 or 220 MHz, ^ιυ which are typical frequencies

In other words, it was indicated that, by capacitively coupling the first and second antennas and suitably selecting an antenna with a good performance, an excellent antenna performance can be obtained in VHF— Lo and VHF-Hi bands.

15 The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes

20 which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. A film antenna device comprising^ at least one film including a first film, and a first antenna, a second antenna and a third antenna, the first antenna, the second antenna and the third antenna being formed on the at least one film, wherein the first antenna and the second antenna are formed on the first film.

2. The film antenna device according to claim 1, wherein the first antenna and the second antenna comprise antenna elements that are capacitively coupled.

3. The film antenna device according to claim 1, wherein the at least one film further comprises a second film, and the third antenna is formed on the second film.

4.. • - The film antenna device according to claim 1, wherein the first antenna comprises an antenna element that extends counterclockwise and the second antenna comprises an antenna element that extends clockwise, and wherein the first antenna and the second antenna constitute a diversity antenna.

5. The film antenna device according to claim 1, wherein the first film has an opening.

6. The film antenna device according to claim 6, wherein each of said antenna elements that are capacitively coupled extends along the peripheral edge of the opening.

7. The film antenna device according to claim 1, wherein each of the first antenna, the second antenna and the third antenna has a shorter antenna dimension of more than 100 mm, where the shorter antenna dimension of an antenna is a shorter side of the minimum rectangle that covers said antenna.

8. The film antenna device according to claim 7, wherein the shorter dimension is not more than 120 mm.

9. The film antenna device according to claim 1, wherein the length of a portion that are capacitively coupled in said antenna elements is a range of 200 mm to 500 mm.

10. A windshield comprising a windshield glass and a film antenna device as claimed in claim 1, wherein the film antenna device is attached to the windshield glass.

11. The windshield according to claim 10, wherein a lowermost portion of the film antenna device is attached within 120 mm from an upper end of a visible region of the windshield glass.

12. The windshield according to claim 10, wherein a lowermost portion of the film antenna device is attached within 110 mm from an upper end of a visible region of the windshield glass.

13. The windshield according to claim 10, further comprising a sticker that is required by regulations, wherein the sticker is attached to the windshield glass and the at least one film are attached so as to be away from the sticker.

14. A film antenna grounding structure comprising: a film antenna device as claimed in claim 1 that is attached to a windshield glass? an electrically conductive tape, the electrically conductive tape being adhered to a pillar of a vehicle body so as to form a capacitive coupling with the vehicle body: and a first coaxial cable, a second coaxial cable and a third coaxial cable, the core cables of the coaxial cables being connected electrically to the first antenna, the second antenna and the third antenna, respectively, wherein the braided wire of at least one of the coaxial cables is connected electrically to the electrically conductive tape.

15. The film antenna grounding structure according to claim 14, wherein the structure further comprises a connector, and the connector is attached to the electrically conductive tape, and wherein the braided wire is connected electrically to the electrically conductive tape via the connector.

16. The film antenna grounding structure according to claim 14, wherein the braided wires of at least two of the coaxial cables are connected electrically to the electrically conductive tape.

17. The film antenna grounding structure according to claim 16, wherein the braided wires of two of the coaxial cables are connected electrically to the electrically conductive tape, and wherein the structure further comprises a second electrically conductive tape that is adhered to a pillar of a vehicle body so as to form a capacitive coupling with the vehicle body, and wherein the braided wires of the remainder of the coaxial cables are connected electrically to the second electrically conductive tape.