A vehicle window antenna

Abstract: A vehicle window antenna mounted on a window glass (1) set in a window frame (2) of a vehicle has a plurality of antenna elements (4, 5, 6) disposed over a portion of the glass (1) which is spaced from said window frame and arranged to receive radio signals over a relatively broad frequency band. The elements (4, 5, 6) extend generally parallel to the frame (2) and they all have different effective lengths (L4, L5, L6). The antenna elements (4, 5, 6) are arranged concentrically, being substantially in the form of a loop. The space (d, f, h, j) between the antenna elements (4, 5, 6) and the window frame (2) ranges from 20 to 120 mm. Two of the elements are such that each element (4, 5) comprises a first strip conductor determining the effective length of the antenna element and an auxilliary strip element disposed between the ends of said first strip conductor whereby the antenna element (4, 5) takes the form of an almost complete loop.

Description: This invention relates to a window glass for a vehicle, the window glass having an antenna comprising a plurality of antenna elements disposed over a portion of the window glass, each antenna element having an effective length which is different from the effective length or lengths of the other antenna element or elements, and having one end free and the other end connected to a common antenna terminal. Japanese utility model application laid open to public as Kokai Sho-JP-U-61 197714 discloses a vehicle window antenna comprising a single antenna element in the form of a strip conductor extending generally alongside a frame of a rear quarter window of an automobile. The length of the antenna strip conductor as well as the distance thereof from the window frame are chosen such that the maximum antenna gain is attained at the center of the desired frequency band, e.g., very high frequency (VHF) band. With this arrangement, the antenna always provides a relatively high Q factor causing the antenna gain to fall rapidly as the frequency goes further away from the center point of the maximum gain. Therefore, it is not suitable for a broad-band radio receiver, since such a receiver would require an expensive tuner circuit to compensate for the antenna characteristics. DE-A-3 521 732 describes a UHF or VHF antenna for a television where the antenna is formed on the windscreen of a vehicle and includes a pair of antenna elements extending adjacent one or two sides of a window, the elements being connected at one end to a common terminal and being free at the other end, and having different effective lengths. US-A-4 072 953 describes a multiband antenna for a windscreen, the antenna having a fish-pole type element extending along a vertical mid-line of the windscreen, and a frame-type element extending adjacent each side of the windscreen. Each of the two elements has one end connected to a common terminal and the other end free. The effective lengths of the two elements are different. The frame-type element borders the windscreen rim and includes a succession of series-connected horizontally directed S-shaped loops each having a length corresponding to an uneven multiple of a quarter wave length corresponding to the central frequency of the metric wave band (FM and VHF). GB-A-1 559 871 describes a multiband antenna for a windscreen where the antenna has a T-shape with the vertical member at the vertical mid-line of the windscreen and the arms adjacent the top of the windscreen. The arms may be extended to include branches running parallel to the sides of the windscreen and partly alongside the bottom of the windscreen. The antenna may be formed of two elements extending from a common terminal at the bottom of the vertical of the T-shape and disposed symmetrically about the vertical mid-line of the windscreen. In this form, the two elements may be doubled back to form parallel tracks adjacent the periphery of the windscreen. US-A-3 845 489 describes a windscreen antenna for FM and AM reception comprising a T-shaped element and a bipolar element completely surrounding the T-shaped element, with a common terminal connected to both elements at the foot of the T-shape. In modifications of this, US-A-3 845 489 describes symmetrical bipolar elements extending adjacent the edges of the windscreen and having free ends spaced from the ends of the horizontal member of the T-shaped element. US-A-4 331 961 describes an antenna for use in an opening in a car body where three complete loop elements are disposed in a concentric pattern and extend adjacent the rim of the opening. A coaxial loop is attached to the rim of the opening and surrounds the three loop elements. A coaxial antenna lead has its inner conductor coupled to the inner conductor of the coaxial loop, and the loop elements are connected to each other and to the inner conductor of the coaxial loop. It is therefore an object of the invention to provide a vehicle window antenna which overcomes these disadvantages. According to the present invention a window glass defined hereinbefore at the beginning is characterised in that the antenna elements are disposed in a concentric pattern with each element extending adjacent each side of the glass and being so disposed as to be spaced by 20 to 120mm from the sides of a window frame when the glass is installed in the frame. With this arrangement, the antenna can respond well to radio signals covering a relatively broad band of frequencies because of the provision of the plurality of antenna elements with a variety of lengths. In addition, the antenna impedance can be easily optimized by selecting and establishing a suitable special relationship between the window frame and the plurality of the antenna elements, and by appropriate choice of the number of such antenna elements whereby a substantial improvement is made in the antenna gain in those frequencies outside the central portion of the reception band. The window frame may have three sides or more. The antenna of the present invention may be mounted in any suitable window of an automobile, for example, a side window on either side of the automobile. Each antenna element may comprise one or more printed strip conductors. In a preferred embodiment, at least one of the antenna elements comprises a first strip conductor determining the effective length of the antenna element and an auxiliary or ornamental strip element disposed from one end to the other end of the first strip conductor whereby an almost complete loop is formed by the first and auxiliary strip elements. Brief Description of the Drawings In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a front view of a vehicle window glass embodying the invention; Figure 2 is a front view of a vehicle window glass, showing a single loop antenna element mounted on a surface of the glass; Figure 3 is a graph showing an antenna gain against frequency for three different lengths of a single antenna element shown in Figure 2; Figure 4 is-a graph showing a voltage standing wave ratio against frequency for several different spaces formed between the single antenna element and the window frame shown in Figure 2; Figure 5 is a graph showing a relative antenna gain against frequency for several different spaces formed between the single antenna element and the window frame shown in Figure 2. Figure 6 is a graph showing a voltage standing wave ratio against frequency for different numbers of the antenna elements; and Figure 7 is a graph showing a relative antenna gain against frequency for different numbers of the antenna elements. Detailed Description of a Preferred Embodiment Referring first to Figure 1, there is shown a panel of glass 1 installed in a vehicle window, such as rear or rear-quarter window formed on either side of an automobile, e.g., jeep and the like. The window is defined by a frame 2 having four sides 2a, 2b, 2c and 2d arranged to generally form a parallelogram window opening. Correspondingly, the glass 1, which is fitted in the window frame 2, has four sides 1a, 1b, 1c, and 1d. The left-hand of Figure 1 is directed toward the front of the automobile while the top of Figure 1 is directed toward the top of the vehicle. Peripheral portion 3 of the window glass 1 is masked by a coloured material such as black ceramic. A feed pad 10 is masked and mounted on a surface of the glass 1 at the left-hand, upper peripheral region thereof. An antenna feeder (not shown) typically in the form of a coaxial cable is connected to the pad 10 to supply VHF signals collected on the pad 10 to receiver circuitry. In the window, the feed pad 10 gathers VHF signals from a pattern of antenna elements designated by 4, 5 and 6, mounted on the glass surface and constructed in accordance with the invention. To make connection between the antenna pattern and the pad, a lead 10a extends horizontally from the pad 10 to one end of each antenna element 4, 5 and 6. The illustrated antenna elements 4, 5 and 6 as well as the lead 10a are all formed by a printed strip conductor having an effective wire gauge of 0.8 mm. As seen from Figure 1, the antenna pattern is advantageously disposed over a portion or loop-track of the glass 1 which generally extends parallel to and relatively close to each side 2a, 2b, 2c, 2d of the window frame 2. More specifically, the antenna pattern comprises a plurality of, here three, looplike antenna elements 4, 5 and 6 arranged in a concentric manner and each having a figure smaller than but similar to the outline of the glass 1. Each looplike antenna element 4, 5 and 6 comprises four sides and apparently has a similar length of loop. However, the operative or effective lengths of the antenna elements are different from one another. More specifically, the outmost antenna element 4 has a first strip conductor having a length designated l4 and a second strip conductor extending from the one end to the other end of the first strip conductor. There is formed a small space or gap 11 designated k between the first and second strip conductors. It is the first strip conductor that primarily determines the operative length of the antenna element 4. The second strip conductor is ornamental or auxiliary. Similarly, the middle antenna element 5 comprises a first or effective strip conductor having a length designated l5 and a second or ornamental strip conductor extending between the ends of the first strip conductor of the antenna element 5, with a small gap 12 of k formed between the conductors. The innermost antenna element 6, however, consists of a single continous strip conductor having a length designated l6. Other dimensional antenna parameters are also shown in Figure 1. Among them the space or distance formed between the antenna pattern and the sides of the metallic window frame 2 (which may be regarded as a ground) is important because such a space contributes to the antenna characteristics. In Figure 1, d indicates the space between the left frame side 2a and the left side of the outermost antenna element 4, f the space between the bottom frame side 2b and the lower side of the antenna element, h the space between the right sides of frame and antenna element, and j the space between the upper sides thereof. Left, bottom, right and top margins formed between the antenna element 4 and the corresponding edges of the glass 1 are indicated by c , e , g and i , respectively. There is formed a small gap k between the antenna elements 4, 5 and 6. The corners of each antenna element are rounded. The radiuses of curvature at these corners are as follows: For the outermost antenna element 4, both bottom left and right-hand corners are R15 mm, top right-hand R20 mm and top left-hand R13 mm. The middle antenna element 5 has bottom corners of R10 mm, top right-hand corner of R15 mm and top left-hand corner of R8 mm. The innermost antenna element 6 has bottom corners of R5 mm, top right-hand corner of R10 mm and top left-hand corner of R3 mm. Readers will note that there are many dimensional parameters in the antenna arrangement of the invention that are suitably chosen for the design of various antennas and once determined, cooperate with one another to provide the overall antenna performance as desired. Among these parameters, the effective length of each antenna element and the spatial relationship between the antenna elements and the window frame are most critical. The procedure of designing an antenna normally starts when the reception frequency band is specified. It is assumed, here, that such a band covers VHF or FM broadcasing frequencies from 85 to 110 MHz. Since there are plural antenna elements (three in the case of Fig. 1), it will be convenient to first use a single loop antenna element and find the best arrangent thereof for the preselected frequency band of 85 to 110 MHz. Figure 2 shows such a single loop-like antenna element 15 which may correspond to one of the three antenna elements in Figure 1, say, the innermost element 6. Now let the space designated K in Figure 2 formed between the single antenna element 15 and the window frame 2 be given a particular value, tentatively, say, K = 70 mm. Then, we will change the length of the element 15. Figure 3 is a graph of antenna gain versus frequency for three different lengths of the antenna element 15. Curve A was obtained with L = 1,555 mm. Curve B was plotted with L = 1,350 mm and curve C with L = 1,750 mm. Obviously, the best curve is A measured for L = 1,555 mm which is therefore the optimal antenna length when using a single looplike element for FM band of 85 to 110 MHz. The curve A reaches the maximum gain at or around 96 MHz which approximately corresponds to L = 1,555 mm, because we can assume a half-wavelength antenna with λ (wavelength) ≒ 1,555 x 2. We will now change the distance K of the single antenna element 15 from the surrounding metallic frame 2 at a ground potential because a change in such spatial relationship is believed to make a change in the antenna impedance of the element 15, a change in the voltage standing wave ratio (VSWR) and a change in antenna gain, each of which is a function of frequency. Figure 4 is a graph showing VSWR versus frequency for three different values of distance K. VSWR was measured at the junction between a feeder and a radio receiver. Curve A was obtained with K = 70 mm. Curve B was plotted with K = 45 mm and curve C with K = 35 mm. Figure 5 is a graph showing a relative antenna gain against frequency for four different values of the distance K. All curves indicate normalized antenna gains relative to the reference gain obtained with K = 70 mm and shown in Figure 5 by zero dB horizontal line. Curve A was plotted with K = 30 mm, curve B with K = 45 mm, curve C with K = 35 mm and curve D with K = 20 mm. The optimal value of K is found to be K = 30 mm because the antenna gain curve A for that value of K is best stabilized and increased relative to the reference level over the entire band of frequencies concerned. So far we have found L = 1,555 mm and K = 30 mm as the optimal dimensional parameters of a single loop antenna element 15 or 4 for receiving FM broadcasting band of 85 to 110 MHz. According to the teachings of the invention, we will then decide to use a plurality of loop-like antenna elements as illustrated in Figure 1. In consideration of the best dimensions of a single loop element, its antenna characteristics as well as the targeted range of frequencies, we can find the optimal dimensional parameters of the antenna elements added. From the antenna characteristics of Figures 3 to 5, we will see, for example, that the antenna gain in higher frequencies (right-hand side of each graph) is substantially lower relative to its peak. This problem will be overcome primarily by the addition of antenna element(s) having different length(s) and bearing an appropriate spatial relationship with the metallic or conductive window frame 2. The test of the antenna arrangement of Figure 1, which was actually mounted in an automobile, has revealed optimal dimensional parameters thereof as follows. The length ℓ4 of the outermost antenna element 4 is 1,520 mm. The length ℓ5 of the middle antenna element 5 is 1,590 mm and the length ℓ6 of the innermost antenna element 6 is 1,615 mm. Other parameters are a = 66 mm, b = 39 mm, c = 108 mm, d = 39 mm, e = 64 mm, f = 38 mm, g = 80 mm, h = 67 mm, i = 75 mm, j = 40 mm and k = 5 mm. Referring to Figure 6, there is shown a graph of VSWR versus frequency for different numbers of loop-like antenna elements. Curve A was plotted when using three loop antenna elements exemplified in Figure 1. Curve B was obtained with a single loop antenna element such as shown in Figure 2. Two loop antenna elements resulted in curve C and four loop antenna elements provided curve D. As is clearly shown in Figure 6, the curve B with a single antenna element has higher VSWR to the right-hand of the graph, or higher frequency side-band. VSWR is a measure of mismatching of impedance between the antenna and the feeder. Such mismatched antenna impedance was substantially improved by the use of a plurality of loop antenna elements as depicted from curves A, C and D for three, two and four elements, respectively. The antenna gain, which is a measure of the power available for the receiver system, was similarly improved as seen from the graph of Figure 7 showing a relative antenna gain versus frequency for different numbers of loop-like antenna elements. Curve A was measured with the triple-element window antenna shown in Figure 1. Curve B was obtained with a dual-element antenna, and Curve C with a guard-element antenna. Each curve is normalized by the reference antenna gain obtained for a single loop-like antenna element shown in Figure 2, and indicated here in Figure 7 by the zero decibel horizontal line. Having employed a plurality of loop-like antenna elements (three for the curve A, two for the B and four for the C in Figure 7) with suitable dimensions, a considerable improvement was observed on the antenna gain over the reception band and particularly in higher frequencies. Such improvement on the antenna gain may be comparable to the improvement on the voltage standing wave ratio as depicted in Figure 6 in which three elements were used for the curve A, one for the B, two for the C and four for the D. In Figure 7 the increased antenna gain observed in the higher band is shown by a value relative to the reference antenna gain measured for the single loop-like element. The antenna gain of the latter is illustrated in Figure 3 by the curve A, according to which a decreased antenna gain is observed in the higher band. Therefore, we can say that the use of plural loop-like antenna elements has stabilized the actual antenna gain or power available in the entire reception band. This concludes the description of a preferred embodiment of the invention. However, many modifications and variations will be obvious to those of ordinary skill in the art. For example, an ornamental or auxiliary strip conductor of the antenna element as discussed in connection with Figure 1 may be omitted if desired. Each antenna element 4, 5, 6 as well as the lead 10a could be made from a transparent material. The reception band of 85 to 110 MHz discussed in the embodiment is merely an example, and will have a different range according to the receiver system involved. The values of the space d , f , h , j formed between the outmost antenna element 4 and the window frame 2 have been found to range from 20 to 120 mm. While a four-sided window is shown in the embodiment, windows of different shapes such as a triangle, an oval and a circle could be used. Also, a curved window glass such as windshield glass could be employed.

Claims: A window glass for a vehicle, the window glass having an antenna comprising a plurality of antenna elements (4,5,6) disposed over a portion of the window glass (1), each antenna element (4,5,6) having an effective length which is different from the effective length or lengths (ℓ₄, ℓ₅, ℓ₆) of the other antenna element or elements, and having one end free and the other end connected to a common antenna terminal (10), characterised in that the antenna elements (4,5,6) are disposed in a concentric pattern with each element extending adjacent each side of the glass (1) and being so disposed as to be spaced by 20 to 120mm from the sides (2a,2b,2c,2d) of a window frame (2) when the glass (1) is installed in the frame (2). A window glass according to claim 1, characterised in that at least one (4) of the antenna elements (4,5,6) comprises a first strip conductor (l₄) determining the effective length of the antenna element (4) and an auxiliary strip conductor disposed between but spaced from the ends of the first strip conductor (l₄). A window glass according to claim 1 or 2, characterised in that the antenna elements (4,5,6) comprise a plurality of printed strip conductors. A window glass according to any preceding claim, characterised in that the glass (1) has four sides (1a,1b,1c,1d). A window glass according to any preceding claim, characterised in that the glass (1) is shaped to fit in a side window frame (2) of a vehicle.

Inventors: IIJIMA HIROSHI, MURAKAMI HAJIME, MURAKAMI HARUNORI, SAKUKRAI KAORU, IIJIMA, HIROSHI, MURAKAMI, HAJIME, MURAKAMI, HARUNORI, SAKUKRAI, KAORU
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 1989
Main classification: H01Q1
IPC codes: H01Q1