ANTENNA MOUNTED ON VEHICLE WINDOW

Abstract: An antenna, particularly a mobile telephone antenna mounted onto the window of a motor vehicle. The antenna comprises a H-shaped ground plane (1) turned to the horizontal plane and a vertical radiator (2) essentially perpendicular to it and mounted at a distance from it. On either side of the vertical radiator there is at least one auxiliary element (3) parallel to the vertical radiator, the said auxiliary elements being used to adjust the antenna's impedance to the desired value.

Description: Antenna mounted on vehicle window The invention relates to an antenna, particularly to a mobile telephone antenna mounted onto the window of a motor vehicle. The antenna consists of a ground plane and a vertical radiator mounted essentially perpendicular to it at a distance from the ground plane. Antennas mounted onto motor vehicle windows - usually onto the windscreen - and intended for the purpose of radio reception in the AM and FM frequency ranges are well known as such. The solution presented in FI patent 58996 is one example of an antenna mounted onto or within the window of a motor vehicle for the purpose of radio reception in the AM and FM ranges. The use of the window heating element of a motor vehicle as an antenna to facilitate radio reception is also known and several solutions regarding it have been presented. These radio reception antennas are not, of course, appro¬ priate for use as mobile telephone antennas. A mobile te¬ lephone antenna must be able to both transmit and receive signals. Additionally, it must be noted that the impedance of car radios is generally 150 ohms whereas that generally implemented in mobile telephones is 50 ohms. It would appear that the most common antennas intended for radio reception continue to be vertical antennas of various types and lengths that are fastened onto the body of the car. The most commonly used mobile telephone antenna in motor vehicles would appear to be the vertical radiator mounted in the middle of the roof of the vehicle. A prior art antenna of this type is fastened into a hole bored into the roof of the vehicle in such a way that the central conduc- tor of the coaxial cable linking the antenna to the mobile telephone is joined to the radiator and the cable sheath is joined to the roof of the car, whereby the roof forms the ground plane for the antenna. One important parameter of a mobile telephone antenna intended to be used in a motor vehicle is its radiation pattern. Radiation should be concentrated in the horizontal direction as much as possible because the base station is generally at the horizon. Moreover, the radiation pattern in the horizontal plane should be a wide one and symmetri¬ cal in relation to the vertical plane in order for the antenna to function properly regardless of the angle formed between the vehicle's direction of travel vector and the directional vector between the vehicle and the base sta¬ tion. The above mentioned prior art vertical radiators mounted in the middle of the roof of motor vehicles result in good directional radiation patterns in the horizontal plane except that the maximum radiation is directed upwards from the horizontal plane at an angle of about 45°. The present invention does not, as such, aim to achieve a better radiation pattern than that achieved using prior art radiators mounted in the middle of the roof of vehicles. On the other hand, the radiation properties of the vehicle window-mounted antenna in accordance with the invention correspond at least to the radiation properties of the above mentioned prior art radiators. This has also been proven in practical tests conducted with an antenna in accordance with the invention. However, the greatest advan¬ tage offered by the antenna in accordance with the inven¬ tion is that there is no need to bore holes in external vehicle body parts when fastening the antenna to a vehicle. The antenna in accordance with the invention is fastened with glue to a side window, for instance, of a car. In the event that the window is of the opening type, the antenna cable passes inside the door and from there on to the mobile telephone; in the event of a fixed window, the cable is placed inside the upholstery. The best location for the antenna would, of course, be in the middle of the vehicle's windscreen or the rear window, but the resultant restric¬ tions on visibility and other reasons are against it. One major problem associated with prior art mobile telepho- ne antennas fastened onto the external body parts of motor vehicles is the fall in the resale price of the vehicle resulting from the required bored holes. Solutions to this problem have been presented; one such solution is to use suction cups, magnets, etc., in attaching the antenna to the external body parts of motor vehicles. These are, however, rather clumsy solutions and the antenna cable must first pass along the roof of the motor vehicle and from there, by way of the door seal, into the cabin of the vehicle and then to the mobile telephone. Another prior art solution consists of fastening the an¬ tenna onto the outside of the motor vehicle's window and a counterpart onto the inside of the window, in which sol¬ ution the signal is transmitted through the glass. This solution does, however, lead to considerable losses in the signal transmission through the glass. In addition, US patent 4,746,925 describes ~ t mobile te¬ lephone antenna for mounting onto the wind.... of a motor vehicle, the said antenna being composed of two dipole antenna elements. One end of either of the two dipole antenna elements is connected to the antenna cable's cen¬ tral conductor by means of its central electricity conduc¬ tor and the other dipole antenna element's end is connected to the antenna cable's sheath by means of a first electric¬ ity conductor installed adjacent to the central electricity conductor the said first electricity conductor serving as an electrical protector. Additionally, there is, on the other side of the central electricity conductor, a second electricity conductor connected to the antenna cable's sheath and acting as an electrical protector. Additionally, transformer elements protrude from the first and second electrical protectors. An antenna of this type possesses a typical dipole directional pattern; that is to say, a sphere, which means that considerable amounts of energy are directed upwards and downwards in addition to the horizon- tal plane. The effective radiation area of this type of an antenna remains relatively small in relation to the overall space required by it. The dipole elements form the radiati¬ on space while the rest of the antenna is unnecessary space in this respect. The part of the antenna between the dipole elements and the antenna cable forms a balun by means of which the antenna is tuned to a single frequency. Moreover, the structure in question is relatively complicated. The above mentioned problems associated with the prior art are overcome by the solution in accordance with the inven¬ tion. The antenna in accordance with the invention con¬ sists, advantageously, of a ground plane of a material that conducts electricity well (e.g. copper tape) and of a vertical radiator essentially perpendicular to it, the said radiator having on either side of it at a certain distance advantageously at least one auxiliary element parallel to the vertical radiator, the said elements being made of material that conducts electricity well (e.g. copper tape). These auxiliary elements make it possible for the antenna's impedance to be adjusted precisely to the desired value. Additionally, only one model of the antenna in accordance with the invention can be constructed and the user selects the desired frequency of the antenna by cutting along marked lines. Thus, the manufacturing of the antenna re- quires only one tool and one production line. An antenna of this type can be tuned within a frequency range of approxi¬ mately 300 MHz - 3 GHz. For the antenna to be adapted to the antenna cable, the impedance of the antenna and the wave impedance of the antenna cable must be equal at the antenna's resonance fre¬ quency at which the reactive part of the antenna's impe- dance is essentially zero. The most common mobile telephone antenna cable wave impedance is 50 ohms. Unless the impe¬ dance of the antenna and that of the antenna cable are equal, a point of discontinuity will form between them and cause wave reflection leading to decisive weakening of the communication. In the following, the invention is described in more detail referring to one advantageous non-limiting embodiment shown in the appended drawing, in which fig. 1 shows the antenna in accordance with the inven¬ tion, fig. 2 shows the horizontal directional diagram of the antenna in accordance with the invention, fig. 3 shows the corresponding vertical directional diagram and fig. 4 shows the antenna's impedance as a function of the number of auxiliary elements. The motor vehicle window mountable antenna in accordance with the invention shown in figure 1 consists of a ground plane 1 taking the form of a letter H essentially turned to the horizontal plane and of a vertical radiator 2 installed essentially perpendicular to it. On either side of the vertical radiator 2 at a distance from it there are the auxiliary elements 3 installed parallel to the vertical radiator, two on either side of the radiator. These auxili¬ ary elements 3 are used to adjust the antenna's impedance to match that of the antenna cable. These auxiliary el- ements 3 have hardly any influence on the antenna's ra¬ diation pattern. The central conductor of the coaxial cable is connected to the base of the radiator 2 and its sheath is connected to the middle of the ground plane 1. The radiator 2 is installed midway along the ground plane's horizontal direction. The following is an example of the dimensioning, for an NMT-450 mobile telephone and proven by measurements to be advantageous, of the parts of a motor vehicle window moun- table antenna in accordance with the invention. The antenna in accordance with the invention has been executed in such a way that the radiator 2 and the auxili- ary elements 3 are formed of copper tape about 4 mm in width and the ground plane 1 is formed of copper tape about 5 mm in width having the form of a letter H and having been turned essentially to the horizontal plane. The parallel points 4, 5 of the H turned to the horizontal plane are also about 5 mm apart from one another. The auxiliary elements 3 are at a distance of about 2 mm from the ra¬ diator 2 and 2 mm from one another. The auxiliary elements 2 are not in electricity conducting contact with one anot¬ her and neither are they in electricity conducting contact with the radiator 2. The point 4 of the ground plane 1 nearest to the base of the radiator 2 is at a distance of about 10 mm from the base of the radiator 2. Distance here means the distance between the elements' opposite edges; that is to say, the free space left between them. While the width of the antenna elements and the distance between them do not constitute very critical parameters, the calcula¬ tions presented below, computed with measurements as the basis, are established on the said dimensions. Neither is the thickness of the copper tape a very critical factor. As an indication of the order of the dimensions involved, the ratio of the tape width to its thickness is approximately 40:1. The frequency of 455,5 MHz in the NMT-450 mobile telephone network results in a wave length in the air of approximate¬ ly 300/455,5 = 0,659 m. Taking into account the velocity factor of the vehicle's window glass, which depends on the glass and is approximately 0,717 in a car window glass, the electrical length of the radiator 2, reduced by the veloci¬ ty factor, is 659 x 0,717 = 472 mm. Because measurements have shown a 3/4 wave length radiator to be advantageous and also because its length makes it suitable for mounting onto motor vehicle windows, the length of the radiator 2 is obtained as 3/4 x 472 = 350 mm. The appropriate length of the auxiliary elements 3 is 255 mm which is slightly more than 1/2 the wave length (coefficient 1/1,86). Since the auxiliary elements 3 are not in conductive contact with the other parts of the antenna, their capacity coupling is smaller and the wave length in them is thus greater than the length of the radiator 2 computed in the corresponding manner. The auxiliary elements 3 are installed at the same height with respect to their bases and so that the free space on the side of the ground plane 1 of the radiator 2 is approximately 40 mm. The said free space is also not a very critical requirement, but it is necessary for the auxiliary elements 3 to extend below the first capacity maximum of the radiator 2. The length of point 4 of the H- shaped ground plane 1 turned to the horizontal plane and, being nearest to the radiator 2, is advantageously approxi¬ mately 180 mm and that of the longer point 5 is approxima¬ tely 260 mm. Here the ground plane 1 funct ons, together with the coa¬ xial cable, mainly as a grounding capacitance and the influence of the window glass velocity factor on the groun¬ ding element is fairly negligible. The horizontally orien¬ ted H-structure of the grounding plane 1 and the above men- tioned dimensioning serve to efficiently eliminate distur¬ bances resulting from variation in the length of the an¬ tenna cable. These arrangements make it possible to achieve an antenna impedance of 50 ohms at a resonance frequency of 455,5 MHz; that is to say, the antenna is adjusted to the most common¬ ly employed antenna cable whose wave impedance is 50 ohms. By employing the above mentioned principle, the antenna in accordance with the invention can also be designed for an NMT-900 mobile telephone and any other frequency within the range 300 MHz - 3 GHz. In this type of a solution, the coupling of the auxiliary elements 3 to the radiator 2 is inductive and capacitive by nature. These auxiliary elements 3 do not influence the electrical length of the radiator 2. In the antenna in accordance with the invention, the ra¬ diator 2 and the auxiliary elements 3 are located between an imposition film and a protective paper. Once the protec¬ tive paper is stripped away, the imposition film is used in pressing the radiator 2 and the auxiliary elements 3 onto the desired place on the window so that the adhesive on them fastens them onto the glass. Once this has occurred, the imposition film can be removed. Thus, the manufac¬ turing of the antenna is quite simple and readily lends itself to automation. Mass-production of the antenna can be confined to just one size so that the protective paper bears the cutting outlines for antennas appropriate for NMT-450 and NMT-900 antennas, for instance. The antenna in accordance with the invention is also advan¬ tageous from the point of view of manufacturing technique as it possesses good recurrence; that is to say, each manufactured antenna is sufficiently identical. This is due to the structure of the antenna; all elements in it can be defined quite precisely and they are easy to execute. Since the metal parts of motor vehicles cause some distur¬ bance in the NMT-450 mobile telephone frequency, the user may wish to install two antennas - one onto either side window - and thereby achieve an enhanced radiation pattern and audibility. The audibility achieved by this solution is at least as good as that achieved by the best prior art radiator mounted in the middle of the roof of a motor vehicle. In the case of the NMT-900 mobile telephone fre¬ quency, there is no need for this because the higher fre- quency employed in the NMT-900 mobile telephones means that the antenna is not adversely influenced by the body of the motor vehicle in the same way that it is with NMT-450 mobile telephones. Consequently, one antenna mounted on one side window is sufficient. The standing wave ratio of this kind of an antenna is 1:1. This means that the precise length of the antenna cable, for instance, is of no significance. This being so, it is easy to install several antennas side by side when necess- ary. To facilitate access to satellites, two antennas may be used with one installed on a side window of the motor vehicle and the other on either the windscreen or the rear window. This results in horizontally polarized circular polarization. Etching, lamination or some other means may also be used to embed the antenna into the window of the motor vehicle. Instead of using tape, the antenna can also be manufactured using thin wire installed onto the surface of the glass or embedded into the glass. However, if this is done, the antenna becomes considerably more difficult to manage, especially in the case of the surface mounting alternative. The cylindrical conductor on the surface of the glass leads to imprecisions of varying extent and these in turn mean, for example, reduced recurrence at the manufacturing stage. Figure 2 shows the horizontal directional diagram of the antenna shown in figure 1. The antenna was located on the right rear side window of the motor vehicle while the measurements were made. Figure 3 shows the corresponding vertical directional diagram. As becomes evident from figures 2 and 3, the radiation patterns of the antenna in accordance with the invention are quite advantageous and easily fulfil the requirements imposed on mobile telephone antennas as defined at the beginning of this description. Figure 4 shows the influence of the auxiliary elements 3 on the impedance of the antenna. Without the auxiliary el¬ ements 3, the antenna's impedance Z is approximately 75 ohms. The impedance of the antenna as shown in figure 1, with four auxiliary elements 3, two on either side of the radiator 2, is exactly 50 ohms at the resonance frequency and the antenna impedance's reactive part is also zero.

Claims: Claims: 1. An antenna, particularly a mobile telephone antenna mounted onto the window of a motor vehicle, comprising a ground plane (1) and a vertical radiator (2) installed essentially perpendicular to it at a certain distance, characterized in that on either side of the vertical radiator (2) and at a distance from it there is at least one auxiliary element (3) parallel to the vertical radiator (2). 2. An antenna as claimed in claim 1, characterized in that the ground plane (1) is formed of a H pattern turned essentially to the horizontal plane. 3. An antenna as claimed in claim 1 or 2, characterized in that the auxiliary elements (3) are shorter than the ra¬ diator (2). 4. An antenna in accordance with any of claims 1 - 3, cha¬ racterized in that there are advantageously two auxiliary elements (3) on either side of the radiator (2). 5. An antenna in accordance with any of claims 1 - 4, cha¬ racterized in that the vertical radiator (2) is a 3/4 wave length radiator. 6. An antenna in accordance with any of claims 1 - 5, cha¬ racterized in that the ground plane (1), the radiator (2) and the auxiliary elements (3) are advantageously made of a material that conducts electricity well, e.g. copper tape. 7. An antenna in accordance with claim 6, characterized in that the ground plane (1), the radiator (2) and the auxiliary elements (3) are located between an imposition film and a protective paper. 8. An antenna in accordance with claim 7, characterized in that the ground plane (1), the radiator (2) and the auxiliary elements (3) bear adhesive on the side facing the protective paper, the said adhesive being employed to fasten the antenna onto the window.

Inventors: LUOMA ARI-MATTI, LUOMA, ARI-MATTI
Applicants: WINDOW ANTENNA OY
Application year: 1991
Main classification: H01Q1
IPC codes: H01Q1