EP2273608A1

EP2273608A1 - Vehicle glass antenna

Info

Publication number: EP2273608A1
Application number: EP10167723A
Authority: EP
Inventors: Hidetoshi Oka; Hikaru Mizukami
Original assignee: Nippon Sheet Glass Co Ltd
Current assignee: Nippon Sheet Glass Co Ltd
Priority date: 2009-07-08
Filing date: 2010-06-29
Publication date: 2011-01-12
Also published as: CN101950853A; JP5285521B2; JP2011019081A

Abstract

A vehicle glass antenna (20) is mounted on a window glass (13) provided on a vehicle body (11) so as to close an opening in the vehicle body (11). The glass antenna (20) includes an antenna element (21, 22, 20b) having at least a double-bend pattern.

Description

The present invention relates to an antenna provided on a window glass of a vehicle, and in particular relates to a glass antenna for a vehicle which is suitable for receiving signals in the digital television band.
Digital television (DTV) antennas mounted in vehicles primarily assume the configuration of a film mounted on the window glass of a vehicle due to considerations of installation space and outward appearance. Automobile high-frequency glass antennas assuming the form of a film installed on the window glass of a vehicle are known in the art; e.g. as disclosed in Japanese Patent Application Laid-Open Publication No. 2008-22538 ( JP-A 2008-22538 ).
FIG. 15 hereof shows the glass antenna disclosed in JP-A 2008-22538 .
An antenna conductor 100 shown in FIG. 15 is formed on a windshield 110. The antenna conductor 100 comprises a first antenna element 101, a second antenna element 102, a first connecting conductor 103, and a loop-forming element 104. A U-shaped conductor pattern is formed from the first antenna element 101, the second antenna element 102, and the first connecting conductor 103.
The first antenna element 101 and a feeder 105 are connected via a second connecting conductor 106, and a loop is formed by the first antenna element 101, the first connecting conductor 103, and the loop-forming element 104.
Thus, since the antenna conductor 100 has a loop, the antenna conductor can have a plurality of resonant frequencies, and it is possible to achieve a level of high antenna gain and a high FB ratio (front-to-back power ratio) without compromising the aesthetic appearance, even in a wideband broadcasting frequency.
However, when the distance between the second antenna element 102 and a body edge composed of metal is 1/4 of the wavelength or less, the radiation impedance decreases, the radiation efficiency of the antenna is reduced, and restrictions are imposed on the bandwidth and the directional characteristics.
Therefore, according to the technology disclosed in Japanese Laid-Open Patent Application No. 2008-22538 , the second antenna element 102 serving as the main antenna must be separated from the body edge of the vehicle roof by a distance of approximately 100 mm.
When the distance from the body edge is approximately 100 mm, there is a risk of the second antenna element 102 entering the field of vision of the driver, necessitating an improvement.
In view whereof, there has been a demand for a technique whereby the second antenna element 102 can be brought closer to the body edge.
It is an object of the present invention to provide a glass antenna for a vehicle in which high antenna performance can be achieved even if an antenna element is disposed in close proximity to a body edge.
According to a first aspect of the present invention, there is provided a glass antenna for a vehicle mounted on a window glass provided to a vehicle body so as to close an opening in the vehicle body, which glass antenna comprises an antenna element having at least a double-bend pattern.
In the present invention, providing the glass antenna for a vehicle with at least a double-bend pattern extending along the edge of the opening of the vehicle body thus makes it possible to improve reception sensitivity while minimizing the loss of impedance caused by placing the antenna in close proximity to the body edge of the vehicle.
Preferably, the antenna element is comprised of a linear first antenna element which extends from a feeder into the opening and along an edge of the opening, a second antenna element which bends approximately 180° from a distal end of the first antenna element and extends facing the first antenna element, and a third antenna element which folds approximately 180 degrees from a distal end of the second antenna element and extends facing the second antenna element. Thus, a double-bend pattern can be formed by the first, second, and third antenna elements, making it possible to improve reception sensitivity while minimizing the loss of impedance caused by placing the antenna element in close proximity to the body edge of the vehicle.
Desirably, the third antenna element is disposed at a distance from the edge of the opening that is equal to or less than one-sixteenth of one wavelength of a predetermined frequency. Thus, since the third antenna element positioned at the lowest part of the glass antenna is disposed at a short distance equal to or less than one-sixteenth of one wavelength of a predetermined frequency from the edge of the opening, the third antenna element does not interfere with the field of vision during driving.
In a preferred form, the length of the third antenna element is set to a value obtained by multiplying a shortening rate by one-half of one wavelength of the predetermined frequency. The antenna pattern is accordingly of simple design.
Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is top plan view illustrating a vehicle employing a vehicle glass antenna according to an embodiment of the present invention;
FIG. 2 (a) and (b) are schematic views illustrating the essential structure and component dimensions of the vehicle glass antenna according to the embodiment;
FIG. 3 (a) - (d) are diagrammatical views showing representative patterns of the vehicle glass antenna subjected to measurements;
FIG. 4 is a graph showing a correlation between bend dimensions and sensitivity of the vehicle glass antenna;
FIG. 5 (a) - (c) are graphs showing changes in VSWR for the bend dimensions 0, 25, and 50 mm;
FIG. 6 (a) - (c) are graphs showing changes in VSWR for the bend dimensions 75, 80, and 90 mm;
FIG. 7 (a) - (c) are graphs showing changes in VSWR for the bend dimensions 95, 100, and 105 mm;
FIG. 8 (a) - (c) are graphs showing changes in VSWR for the bend dimensions 110, 115, and 120 mm;
FIG. 9 (a) - (c) are graphs showing changes in VSWR for the bend dimensions 125, 130, and 140 mm;
FIG. 10 is a graph showing a change in VSWR for the bend dimension 150 mm;
FIG. 11 is a view a pattern showing a distance between the main antenna element and the body edge subjected to measurement;
FIG. 12 is a graph showing a correlation between the change in sensitivity and the distance between the main antenna element and the body edge;
FIG. 13 is a view of a pattern showing the element length of the main antenna element subjected to measurement;
FIG. 14 is a graph showing a correlation between the element length of the main antenna element and sensitivity; and
FIG. 15 is a diagram showing a the configuration of a conventional vehicle glass antenna.

A glass antenna for a vehicle according to the present invention can be mounted on a window glass of a vehicle. Specifically, a vehicle 10 is provided with window glass composed of a windshield 13 fitted between left and right front pillars 12L, 12R (L is a symbol indicating left, R is a symbol indicating right; hereinafter likewise) of a vehicle body 11, a rear window 15 fitted between rear pillars 14L, 14R, front door windows 17L, 17R raisably and lowerably mounted to front doors 16L, 16R, and rear door windows 19L, 19R raisably and lowerably mounted to rear doors 18L, 18R, as shown in FIG. 1.
A glass antenna 20 for a vehicle can be mounted on any of the window glasses described above, but in the present embodiment, the antenna is provided in substantially the top center part of the windshield 13. The glass antenna 20 for a vehicle is primarily a DTV antenna designed in order to receive radio waves of terrestrial digital broadcasting using a terrestrial UHF (ultra-high frequency) band for an in-car television.
The details of the glass antenna 20 for a vehicle are described in FIG. 2.
The glass antenna 20 for a vehicle is mounted to the window glass 13 provided to the vehicle body 11 so as to close an opening in the vehicle body 11, as shown in FIG. 2(a). The glass antenna 20 for a vehicle includes an impedance adjustment element 20a as an antenna body, a main antenna element 20b, and a feeder 20c, which are mounted in close proximity to an edge of the opening of the vehicle body 11 (hereinafter referred to as a body edge 24). The numeral 25 indicates a glass edge of the window glass 13 installed in the interior of the vehicle body 11.
The impedance adjustment element 20a is described in further detail.
The impedance adjustment element 20a is composed of a linear conductor, and includes a first antenna element 21 and a second antenna element 22, as shown in FIG. 2(b).
The first antenna element 21 is a linear conductor formed extending along the body edge 24 from the feeder 20c. The second antenna element 22 is a linear conductor formed bent back 180° from a distal end position of the first antenna element 21 and extending facing the first antenna element 21. The two elements herein extend substantially parallel facing each other.
The main antenna element 20b shown as a third antenna element is a linear conductor formed bent back at approximately 180° from the distal end of the second antenna element 22 constituting the impedance adjustment element 20a and extending facing the second antenna element 22. The elements extend substantially parallel facing each other.
The following is a description of an optimal example of the dimensions of the linear conductors described above.
Optimally, the bend dimension a of the first and second antenna elements 21, 22 constituting the impedance adjustment element 20a is 100 mm, the element length d of the main antenna element 20b is 150 mm, and the conductor gaps c between the first and second antenna pattern and between the impedance adjustment element 20a (the second antenna element 22) and the main antenna element 20b are both 5 mm. The reason the conductor gaps c are 5 mm because there is a tradeoff in terms of design between improving the performance or the size of the antenna (reducing the installation space), with size being given priority from an optimal range of 3 mm to 20 mm for the conductor gaps.
The distance b between the main antenna element 20b and the body edge 24 of the vehicle body 11 is optimally 30 mm or less. The basis for this is described hereinafter. The gap e between the feeding point 20c and the bending point of the impedance adjustment element 20a is optimally 5 mm. The dimensions of the feed point (terminal 20b) used herein are preferably 20 mm × 12 mm, and the antenna wire width is 0.2 mm to 1.0 mm.
Designing the impedance adjustment element 20a with a double-bend pattern is equivalent to electrically adding impedance. Additionally the distance b to the body edge 24 of the vehicle body 11, which had been 100 mm in conventional practice, is reduced to 30 mm.
As a result, it is possible to suppress the loss of impedance resulting from the glass antenna 20 for a vehicle being placed in close proximity to the body edge 24 so as not to impede the field of vision of the driver, the result of which is that reception sensitivity can be improved.
The following is a description of the reasons pointing to the optimal values for the bend dimension a of the impedance adjustment element 20a, the distance b between the main antenna element 20b and the body edge 24, and the element length d of the main antenna element 20b, respectively.

(Bend dimension a of impedance adjustment element 20a)

The inventors measured antenna sensitivity while varying the bend dimension a of the impedance adjustment element 20a shown in FIG. 2(b) from 0 to 150 mm and varying the frequency from 400 MHz to 800 MHz.
For ease of reference, a representative example of the impedance adjustment element 20a is shown in FIGS. 3(d) though (d). Specifically, when the bend dimension a is varied while the antenna length d is kept constant, the shape of the impedance adjustment element 20a changes as shown in (a) through (d), for example.
FIG. 4 shows the result of varying the bend dimension a of the impedance adjustment element 20a from 0 to 150 mm and plotting the corresponding sensitivities on a graph. The horizontal axis represents the frequency [MHz] centered on the DTV band (473 MHz to 713 MHz), and the vertical axis represents relative sensitivity [dBd].
The sensitivity curves when the bend dimension a has been varied among 0 (no bend), 25 mm, 50 mm, 75 mm, 80 mm, 90 mm, 95 mm, 100 mm, 105 mm, 110 mm, 115 mm, 120 mm, 125 mm, and 150 mm are indicated by a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, and a14, respectively.
The curve indicated by the bold line a8 shows the sensitivity obtained in the DTV band, which is particularly stable in comparison with the thin lines a1 to a7 and a9 to a14.
The bold line a8 is the sensitivity when the bend dimension a is 100 mm, and it is possible to ascertain that the sensitivity is superior than with the cases of other the bend dimensions a = 25 mm and 75 mm shown in FIG. 3, and a = 150 mm, for example.
However, it is known that the ratio between the maximum voltage and the minimum voltage of a standing wave occurring at the feeding point 20c is indicated by the VSWR (voltage standing wave ratio), representing the degree of matching between characteristic impedance and load impedance of the feeder line. In the event of a perfect match, the VSWR is 1.
When the bend dimension is changed, so does the VSWR. In view of this, the inventors investigated the correlation between the bend dimension and the VSWR. The results are shown in FIGS. 5 through 10.
FIGS. 5 through 10 show the VSWR at points of 473 MHz, 575 MHz, 586 MHz, and 713 MHz, wherein the frequency (in MHz) is plotted on the horizontal axis, the VSWR is plotted on the vertical axis, with 400 MHz being the measurement starting point, and 800 MHz being the measurement ending point.
As a result, it can be seen that the bend dimensions of 90 mm and 95 mm shown in FIGS. 6(c) and 7(a) result in an overall lower VSWR in the DTV band. At 90 mm in FIG. 6(c), 473 MHz is in the middle of a steep varying curve, and 95 mm is therefore more stable.
As described above, it could be ascertained from the VSWR that 95 mm is the preferred value for the bend dimension.
Next, an attempt will be made to describe hypothetically how 95 mm obtained by measurement is a desirable value.
The 473 MHz frequency shown in FIG. 4 is used for the frequency in the hypothetical description.
The 473 MHz frequency has the worst impedance within the DTV band (473 to 713 MHz). Therefore, if a description is provided for 473 MHz, which is the lowest, other frequencies that are more favorable will not need to be described.
The bend dimension can be found by multiplying one wavelength of the predetermined frequency (design frequency)] by (1/4) by the shortening factor. The value of one wavelength of the predetermined frequency (design frequency) is determined by dividing 300 by 473. The bend dimension a, which is obtained by multiplying (300/470) by (1/4) by 0.6, is thus 95 mm. When the bend is 95 mm, there is a sharp drop in sensitivity in the vicinity of 473 MHz in the digital TV band, as shown by a6 in FIG. 4; therefore, in the present embodiment, 100 mm is the preferred value of the optimal bend dimension.
As described above, in the glass antenna 20 for a vehicle according to the embodiment of the present invention, providing the impedance adjustment element 20a with at least a double-bend pattern extending along the edge of an opening in the vehicle body 11 makes it possible to improve reception sensitivity while minimizing the loss of impedance caused by placing the impedance adjustment element 20a in close proximity to the body edge 24. The optimal value for the bend dimension in such circumstances is obtained by multiplying the shortening factor by one-fourth of one wavelength of a predetermined frequency.

(Distance b between main antenna element 20b and body edge 24)

FIG. 11 shows a sensitivity measurement pattern according to the distance between the main antenna element 20b and the body edge 24, and FIG. 12 shows a graph representing the variation in sensitivity.
Specifically, when the distance b' shown in FIG. 11 is changed to 20 mm, 30 mm, 40 mm, 60 mm, 80 mm, 100 mm, and 30 mm (bent at 100 mm), the manner in which the sensitivity changes is shown in FIG. 12 respectively as b1, b2, b3, b4, b5, b6, and b7.
As is evident from FIG. 12, it follows that as the main antenna element 20b is placed in closer proximity to the body edge 24 (100 mm → 20 mm), the sensitivity drops at any frequency and performance worsens. Within the DTV band, it is clear that in the curve indicated by the bold line b7, at which the bend dimension a of the impedance adjustment element 20a is 100 mm and the distance b' between the main antenna element 20b and the body edge 24 is 30 mm (bent at 100 mm), a higher sensitivity is maintained throughout the entire DTV band in comparison with the other graphs b1 to b6.
The impedance adjustment element 20a is provided with a bend of a dimension which matches the frequency at which sensitivity is inadequate due to impedance mismatching, but since matching is the purpose of the bend, the impedance adjustment element 20a must be placed in close proximity to the body edge 24 so as not to form an antenna receiving element. In the present embodiment, the distance is preferably equal to or less than one-sixteenth of one frequency wavelength. For example, when the design frequency is 470 MHz, then b' = 300/470/16 = 40 mm.
In the glass antenna 20 for a vehicle according to the embodiment of the present invention described above, the main antenna element 20b, which is the third antenna element positioned at the lowest point of the bending pattern, is subjected to impedance adjustment by the impedance adjustment element 20a. The main antenna element 20b therefore can be disposed in close proximity to the body edge 24 without impeding the field of vision during driving.

(Element length d of main antenna element 20b)

FIG. 13 shows the sensitivity measurement pattern and its dimensions resulting from varying the element length of the main antenna element 20b, and FIG. 14 shows a graph representing the relationship between element length and sensitivity.
Specifically, the manner in which the sensitivity changes when the length (element length) d' of the main antenna element 20b shown in FIG. 13 is varied in 5 mm increments from 130 mm to 170 mm is indicated in FIG. 14 as d1, d2, d3, d4, d5, d6, d7, d8, and d9, respectively.
As is evident from FIG. 14, it follows that when the element length of the main antenna element 20b is 150 mm, a higher sensitivity is obtained, on average, throughout the entire DTV band.
With the glass antenna 20 for a vehicle according to the embodiment of the present invention described above, the antenna pattern is more readily designed because the element length of the main antenna element 20b is set to a value obtained by multiplying the shortening factor by one-half of one wavelength of a predetermined frequency.
Whereas it shall be apparent that the present invention can be applied to the DTV band within Japan, the present invention can also be applied to digital television bands in other countries, and is suitable as means for optimizing resonant impedance and improving reception sensitivity in cases in which the glass antenna for a vehicle is installed in close proximity to the body edge in order to achieve a favorable field of vision during driving.

Claims

A vehicle glass antenna mounted on a window glass provided to a vehicle body so as to close an opening in the vehicle body, comprising:
an antenna element (21, 22, 20b) having at least a double-bend pattern.
The glass antenna of claim 1, wherein the antenna element comprises:
a linear first antenna element (21) which extends from a feeder (20c) into the opening and along an edge of the opening;

a second antenna element (22) which bends approximately 180° from a distal end of the first antenna element (21) and extends in opposed relation to the first antenna element; and

a third antenna element (20b) which bends approximately 180° from a distal end of the second antenna element and extends in opposed relation to the second antenna element.
The glass antenna of claim 2, wherein the third antenna element (20b) is disposed at a distance from the edge of the opening that is equal to or less than one-sixteenth of one wavelength of a predetermined frequency.
The glass antenna of claim 2 or 3, wherein the length (d) of the third antenna element (20b) is set to a value obtained by multiplying a shortening factor by one-half of one wavelength of the predetermined frequency.