WO2002075844A1

WO2002075844A1 - Grounding structure for vehicle glass antenna

Info

Publication number: WO2002075844A1
Application number: PCT/JP2002/002277
Authority: WO
Inventors: Hidetoshi Oka
Original assignee: Nippon Sheet Glass Co., Ltd.
Priority date: 2001-03-15
Filing date: 2002-03-12
Publication date: 2002-09-26
Also published as: JP2006173658A

Abstract

The present invention provides a grounding structure of a vehicle glass antenna that includes: a vehicle window glass; at least one antenna element provided on the vehicle window glass that receives at least very high frequency waves; an antenna module provided on the vehicle window glass that is in electrical contact with the antenna element; and a coaxial cable including an outer conductor that is connected to the antenna module. The grounding structure includes a connection that electrically connects the outer conductor to a vehicle body, thus making the vehicle body function as a part of the vehicle glass antenna.

Description

DESCRIPTION

GROUNDING STRUCTURE FOR VEHICLE GLASS ANTENNA

FIELD OF THE INVENTION

The present invention relates to vehicle glass antennas and grounding structures therefor, and in particular to glass antennas that are suitable for receiving at least very high frequency (VHF) band waves assigned to FM broadcast and TV broadcast.

BACKGROUND OF THE INVENTION

JP H02 (1990)-13311U discloses a "amplifier attaching structure for a vehicle glass antenna", wherein an amplifier 1212 is directly connected to the feeding terminal portion 1211 of antenna elements 1210, and a ground wire 1207 for the amplifier is grounded to the pillar of a vehicle body. This structure is shown in Fig. 2 of JP H02-13311U and Fig. 12 of the present application (Note that numbers have been reassigned in Fig. 12). It should be noted that the glass antenna of JP H02-13311U is for receiving AM

(Amplitude Modulation) broadcast and/or FM (Frequency Modulation) broadcast.

JP H08 (1996)-330833A discloses a vehicle glass antenna in which a grounding point is made by connecting the outer conductor of a coaxial cable to an aperture portion of a vehicle body on a peripheral portion of a side that is different from a peripheral portion of the side with the window glass plate that has been provided with a feeding point (refer to Fig. 1 of JP H08-330833A).

JP H06 (1994)-276008A discloses a window glass antenna device 1101 for a vehicle telephone in which the ground-side conductor of a feeding cable 1103, which connects a vehicle telephone antenna 1110 mounted on a vehicle window glass 1102 to a vehicle telephone receiver, is electrically connected to a vehicle body 1161, at an intermediate portion 1141 of that cable. This structure is shown in Figs. 1 and 2 of JP H06-276008A and Figs. 13 and 14 of the present application (Note that the numbers have been reassigned in Figs. 13 and 14). In recent years, automobiles are equipped with airbags as a safety measure. Furthermore, side airbags also are beginning to be installed. Side airbags for the rear seats in sedan-type vehicles often are installed on an upper portion of the so-called C-pillar.

In the vehicle glass antenna disclosed in JP H02-13311U, a ground 1204 grounds an amplifier 1212 to the vehicle body via an unshielded connector cable 1207. Because this connector cable functions as a part of the antenna, there has been the problem that the antenna performance changes depending on the position and length of the connector cable. In the vehicle glass antenna disclosed in JP H08-330833A, the outer conductor of the coaxial cable extends for a long way, and is connected to a grounding point on the side pillar. Therefore, for the same reason, there has been the problem of variations in antenna performance.

In the window glass antenna device 1101 for a vehicle telephone disclosed in JP H06-276008A, a ground-side conductor 1103a of a feeding cable 1103 is connected to a vehicle body 1161 at an intermediate portion 1141 of that feeding cable. This connection to the vehicle body ground is for reducing noise that is picked up by the antenna. The wavelength for vehicle telephones is so short that this connection to the vehicle body does not function as a part of the antenna.

This also is made clear by the following. In the window glass antenna device 1101 for a vehicle telephone, a grounding pattern 1112 is provided together with an antenna pattern 1111 on a window glass 1102. The ground— side conductor 1103a of the feeding cable 1103 is connected to the grounding pattern 1112 as shown in Fig. 2 of JP H06-276008A and Fig. 14 of the present application. DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a grounding structure for a vehicle glass antenna with stable antenna performance. In a grounding structure of a vehicle glass antenna according to the present invention, the vehicle glass antenna includes ^'■ a vehicle window glass; an antenna element provided on the vehicle window glass that receives at least very high frequency (VHF) band waves; an antenna module provided on the vehicle window glass that is in electrical contact with the antenna element; and a coaxial cable including an outer conductor that is connected to the antenna module. The ground structure includes a connection that electrically connects the outer conductor to a vehicle body, thus making the vehicle body function as a part of the vehicle glass antenna. The present invention also provides a vehicle glass antenna device that includes: a vehicle window glass; an antenna element provided on the vehicle window glass that receives at least VHF band waves; an antenna module provided on the vehicle window glass that is in electrical contact with the antenna element; and a coaxial cable including an outer conductor that is connected to the antenna module. The outer conductor is grounded to a vehicle body.

According to still another aspect of the present invention, a vehicle glass antenna device includes^ a vehicle window glass; an antenna element provided on the vehicle window glass that receives at least VHF band waves; an antenna module provided on the vehicle window glass that is in electrical contact with the antenna element; and a coaxial cable. One end of the coaxial cable is connected to the antenna module while the other end of the coaxial cable is connected to a connector. The length of the coaxial cable is not more than 400 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic drawing of an example of the vehicle glass antenna device according to the present invention.

Fig. 2 shows an example of the grounding structure of the present invention from inside the vehicle.

Fig. 3 is a partial magnified view illustrating the grounding structure in Fig. 2.

Fig. 4 illustrates another example of the grounding structure of the present invention. Fig. 5 illustrates Comparative Example 1 in accordance with the attachment structure disclosed in JP H02-13311U.

Fig. 6 is a graph that shows how the reception sensitivity in the AM band changes with the length of the cable. Fig. 7 is a graph that shows how the reception sensitivity in the FM band changes with the length of the connecting cable in Working Example 2.

Fig. 8 is a graph that shows how the reception sensitivity within FM bands changes with the length of the connecting cable in Comparative Example 1.

Figs. 9A and 9B illustrate the position of the connecting cable for the measurements of the sensitivity.

Fig. 10 is a graph that shows how the reception sensitivity in the Working Example 2 changes with the position (path) of the connecting cable.

Fig. 11 is a graph that shows how the reception sensitivity in the Comparative Example 1 changes with the position (path) of the connecting cable.

Fig. 12 illustrates the amplifier attaching structure for a vehicle glass antenna disclosed in JP H02-13311U.

Fig. 13 illustrates the window glass antenna device for a vehicle telephone disclosed in JP H06-276008A.

Fig. 14 is a detailed illustration of the antenna of the window glass antenna device for a vehicle telephone disclosed in JP H06-276008A. DETAILED DESCRIPTION OF THE INVENTION

In the aforementioned structure, the outside conductor of the coaxial cable preferably is connected to the body of the vehicle. Thus, there is little variation in antenna performance, because unshielded connector wiring is not used.

It should be noted that the wavelength band of the antenna device disclosed in JP H06-276008Ais for mobile communication (for vehicle telephones), and the wavelengths are so short that it is possible to ground it by providing a grounding pattern on the window glass. The vehicle glass antenna device according to the present invention is an antenna for receiving e.g. medium frequency (MF) band waves (AM broadcast) and very high frequency (VHF) band waves (FM broadcast and/or TV broadcast), which have relatively long wavelengths. Therefore, providing a grounding pattern on the glass and connecting the outer conductor of the coaxial cable is not sufficient to function as a ground. There is no need to provide a grounding pattern in the antenna element of the present invention. Working Example 1

The present invention is explained below with reference to the drawings. As shown in Fig. 1, in this antenna device 1, a heating line group 21 is disposed on the surface on a rear glass 2, and antenna elements 10, which do not include a grounding pattern, are disposed on a margin portion of the glass above the heating line group 21. An antenna module 12 is connected directly to feeding points 11 of the antenna elements 10. A coaxial cable 3, which has one end connected to the antenna module 12, has the other end connected to a receiver 6 and transmits the receiving waves thereto. In other words, the antenna module 12 is connected to the receiver 6 by a path that includes the coaxial cable 3. The antenna elements 10 of the present invention should be able to receive, as a whole, at least VHF band waves. In this Working Example 1, an antenna pattern 101 is for receiving MF band waves (AM broadcast), and an antenna pattern 102 is for receiving VHF band waves (FM broadcast and/or TV broadcast). The antenna device can be made of a single element that receives both MF band waves and VHF band waves. The antenna device also may include three antenna elements, for example one dedicated to MF with VHF and two or more elements dedicated to VHF only, and may include four or more elements. The antenna module 12 can include at least one selected from an amplification circuit, a switching circuit, and a filter circuit. The amplification circuit may be included when the receiving intensity is not expected to be sufficiently strong. The switching circuit may be included when the antenna device has two or more elements and is used for switching according to the received wavelength band or for switching in order to perform diversity reception. The filter circuit may be included when removing noise is preferable.

When the antenna module 12 includes an active element such as an amplification circuit, the active element can be driven by electrical power supplied by a power source wiring 13. The antenna module 12 in the Working Example 1 includes an amplification circuit.

Fig. 2 illustrates an example showing how a so-called C-pillar 61 of the vehicle is provided with a grounding point 4, and the coaxial cable 3 is grounded at its intermediate portion to the vehicle. Fig. 3 is a partial magnified view illustrating the grounding structure. A portion of the jacket 32 of the coaxial cable 3 is removed to expose an outer conductor (metal shield) 31. At this portion, the coaxial cable 3 is screwed down and fastened to the C-pillar 61 by a grounding metal fitting 41 with a screw 42. Thus, the outer conductor 31 is electrically connected to the vehicle body via the grounding metal fitting 41, and is grounded to the veliicle body. The spot screwed down by the screw 42 serves the grounding point 4.

In Working Example 1, the length of the coaxial cable from the antenna module to the grounding position was set at 200 mm. Working Example 2

In Working Example 2, a pair of connectors is used for the electrical connection between the coaxial cable and the vehicle body as shown in Fig. 4.

The coaxial cable 3a with a length of 200 mm has one end (not shown) connected to the antenna module. The other end of the coaxial cable 3a is attached to a male connector 51. A coaxial cable 3b that is connected to a receiver (not shown) has one end attached to a female connector 52 that includes a fastening metal fitting 53. The female connector 52 is fastened to the vehicle body by screwing the fastening metal fitting 53 down to the C-pillar portion 61 with the screw 42.

When inserting the male connector 51 into the female connector 52, the antenna module 12 and the receiver are connected to each other via the coaxial cables 3a and 3b and a pair of connectors 5 (51 and 52). That is, the inner conductors and the outer conductors are in electrical contact with each other, respectively. The path from the antenna module to the receiver includes the connectors 5 as well as the connected coaxial cable. The connected coaxial cable includes a first portion 3a and a second portion 3b. A path from the antenna module to the connectors includes the first portion and a path from the connectors to the receiver includes the second portion. The length of the first portion 3a, which is the length of the coaxial cable from the antenna module to the position where outer conductor is grounded to the vehicle body, is preferably not more than 400 mm or not more than λ/8. Here, λ is a wavelength of VHF band waves, for example FM band waves.

The part of the female connector 52 that is connected to the outer conductor of the coaxial cable 3b also is in electrical contact with the fastening metal fitting 53. Thus, the outer conductors of the coaxial cables 3a and 3b are electrically connected to the C-pillar portion 61 via the female connector 52 and the fastening metal fitting 53, and is grounded to the vehicle body. It is preferable that the outer conductor is electrically connected to the vehicle body via at least one of the connectors.

In this way, when the coaxial cable is partitioned at an intermediate portion between an antenna module and a receiver, there is hardly any impediment in handling a rear window glass, and no unnecessary efforts occur during assembly of the vehicle.

In the case of at least one of the connectors includes a metal fitting that electrically connects to the outer conductor, it is preferable that the outer conductor is grounded to the vehicle body via the metal fitting. Comparative Example 1

In this Comparative Example, a vehicle glass antenna device includes the same antenna elements and an antenna module as those of Working Example 1. On the other hand, the device includes a different grounding structure as shown in JP H02-13311U. That is to say, as is shown in Fig. 5, the device is grounded to the C-pillar portion 61 of the vehicle body with a connecting cable 7 instead of a coaxial cable 3 from the antenna module 12. The following is a comparison of receptive properties. Influence of Cable Length First, the influence of the cable length was studied in the antennas of the above-mentioned Working Example 2 and the Comparative Example 1.

In the Working Example 2, the length of the coaxial cable from the antenna module to the grounding location was varied from 100 mm to 500 mm, and the reception sensitivity was measured. In the Comparative Example 1 as well, the length of the connecting cable was varied over the same range, and its receptive properties were measured.

It should be noted that in both cases the path of the cable from the antenna module to the grounding position was kept constant. The result of the measurements in the MF band is shown in Fig. 6. With the Working Example 2 there was no change in the reception sensitivity despite the length of the coaxial cable. On the other hand, with the Comparative Example 1, the reception sensitivity slightly decreased as the length of the cable increased. Moreover, the sensitivity itself was better in the Working Example 2.

In the VHF band, with the Working Example 2 there was a small change in reception sensitivity depending on the length of the coaxial cable as shown in Fig. 7. On the other hand, in the case of the Comparative Example 1, there was a significant change in reception sensitivity depending on the length of the cable as shown in Fig. 8.

It can be seen in Figs. 7 and 8, the reception sensitivity tends to decrease when the length of the cable from the antenna module to the grounding position is increased. That tendency is especially conspicuous in the Comparative Example 1.

Moreover, there were fluctuations in the reception sensitivity depending on the wavelength as well, which tended to be particularly noticeable in the Comparative Example 1.

From the results above, it is apparent that when trying to maintain reception sensitivity in the grounding structure of the comparative example, the length of the connecting wire from the antenna module to the grounding position is significantly limited, and that the grounding structure is no longer practical when the wire length exceeds 100 mm.

It should be noted that with the grounding structure of the present invention, for very high frequency bands, it is advantageous that the length of the coaxial cable from the antenna module to the grounding position is short.

Influence of Cable Position Next, the influence of the position (course) of the cable was compared.

The path of the coaxial cable 3 from the antenna module to the grounding position in the Working Example 2 was varied into three courses, as shown in Fig. 9A, and the reception sensitivity was measured. Furthermore, the path of the connecting cable 7 in the Comparative

Example 1 was also varied, as shown in Fig. 9B, and its receptive properties were measured. In both cases, the length of the cable from the antenna module 12 to the grounding position 4 was set to 200 mm.

With the Working Example 2, the reception sensitivity stayed substantially constant as shown in Fig. 10, regardless of the path of the coaxial cable. On the other hand, it was found that with the Comparative Example 1, the reception sensitivity significantly varied depending on the bend of the cable as shown in Fig. 11.

Thus, in Comparative Example 1, limitations are placed on the length of the connecting wire from the antenna module to the grounding position in order to maintain reception sensitivity. There are also limitations as to where the grounding position can be set.

Moreover, the glass antenna itself is often provided on the upper portion of the rear glass. Consequently, in the case of a sedan-type vehicle, the grounding location of the glass antenna device is provided on the upper portion of the C-pillar portion of the vehicle body.

In vehicles outfitted with a side airbag for the rear seats, the side airbag is installed on the same upper portion of the C-pillar portion. That is to say, when the length of the cable from the antenna module to the grounding location is subject to Hmitation as in Comparative Example 1, a place for installing the side airbag cannot be secured.

As explained above, in a glass antenna device with the grounding structure of the present invention, the reception sensitivity is hardly affected by the length of the cable from the antenna module to the grounding position.

Moreover, the reception sensitivity of a glass antenna device with the grounding structure of the present invention is also hardly affected by the bending path of the cable from the antenna module to the grounding location.

Furthermore, a glass antenna device with the grounding structure of the present invention is useful when applied to a vehicle which is equipped with a side airbag for the rear seats. That is, because it is not subject to limitations with respect to the length or position of the cable from the antenna module to the grounding location, it places less of a hmitation on the spot for installing a side airbag.

Claims

1. A grounding structure of a vehicle glass antenna, the vehicle glass antenna comprising: a vehicle window glass; at least one antenna element provided on the vehicle window glass, the at least one antenna element receiving at least very high frequency band waves; an antenna module provided on the vehicle window glass, the antenna module being in electrical contact with the antenna element; and a coaxial cable including an outer conductor, the coaxial cable being connected to the antenna module, the grounding structure comprising a connection that electrically connects the outer conductor to a vehicle body, thus making the vehicle body function as a part of the vehicle glass antenna.

2. The grounding structure according to claim 1, wherein the at least one antenna element does not include a grounding pattern.

3. The grounding structure according to claim 1, wherein the outer conductor is grounded to a pillar of the vehicle body.

4. The grounding structure according to claim 1, wherein the vehicle window glass is a rear window glass.

5. The grounding structure according to claim 1, wherein the antenna module is connected to a receiver by a path that comprises the coaxial cable.

6. The grounding structure according to claim 5, the vehicle glass antenna further comprising a pair of connectors, wherein the path further comprises the pair of connectors.

7. The grounding structure according to claim 6, wherein the coaxial cable comprises a first portion and a second portion, wherein a path from the antenna module to the connectors comprises the first portion and a path from the connectors to the receiver comprises the second portion.

8. The grounding structure according to claim 6, wherein the outer conductor is electrically connected to the vehicle body via at least one of the connectors.

9. The grounding structure according to claim 6, wherein the at least one of the connectors includes a metal fitting that electrically connects to the outer conductor, and the outer conductor is grounded to the vehicle body via the metal fitting.

10. The grounding structure according to claim 1, wherein the antenna element further receives medium frequency band waves.

11. The grounding structure according to claim 1, wherein the length of the coaxial cable between the antenna module and a position where the outer conductor is grounded to a vehicle body is not more than 400 mm.

12. A vehicle glass antenna device comprising: a vehicle window glass; at least one antenna element provided on the vehicle window glass, the antenna element receiving at least very high frequency band waves; an antenna module provided on the vehicle window glass, the antenna module being in electrical contact with the antenna element; and a coaxial cable including an outer conductor, the coaxial cable being connected to the antenna module, wherein the outer cable is grounded to a vehicle body.

13. A vehicle glass antenna device comprising: a vehicle rear window glass; at least one antenna element provided on the vehicle window glass, the antenna element receiving at least very high frequency band waves; an antenna module provided on the vehicle window glass, the antenna module being in electrical contact with the antenna element; and a coaxial cable, one end of the coaxial cable being connected to the antenna module, wherein the other end of the coaxial cable is connected to a connector and the length of the coaxial cable is not more than 400 mm.