EP0594809B1 - Radio antenna arrangement located next to vehicle window panes - Google Patents

Abstract

An antenna arrangement for motor vehicles has an outer radio antenna (1) and a window pane (2) mounted in the metallic car body (3). The outer radio antenna (1) is arranged next to the window pane opening, on the car body (3) or on the window pane (2) itself. An optically transparent two-dimensional structure (4) made of a conductive material is mounted on the window pane, next to the outer radio antenna (1). This structure (4) is substantially opaque to electromagnetic radiation propagating through the window pane opening, as far as radio waves in the radio broadcasting frequency domain are concerned, and is connected with low impedance to the metallic car body (3) in the radio broadcasting frequency domain. Its dimensions are selected so that the inside of the vehicle is sufficiently shielded from the electromagnetic radiation generated by the outer antenna.

Description

Radio antennas, e.g. for C-Netz or D-Netz mobile phones, are often mounted as rod-shaped or other shaped antennas near the rear roof edge due to the good antenna function or glued to the rear window as an adhesive antenna. In any case, the antennas protrude from the outside of the vehicle and are therefore, in contrast to e.g. to disc antennas, referred to as external antennas.

The antennas are typically loaded up to 25 W in the case of transmission. Because of the simple mounting option, antennas that are glued onto the rear window and that capacitively couple the antenna signal through the window are particularly advantageous. Such an antenna arrangement, which is glued to the rear window of the vehicle, is e.g. known from DE 39 31 807 A1, EP 0 279 117 A1 and US RE 33743 (E).

The electromagnetic radiation, i.e. the electrical and magnetic fields, reach into the vehicle interior through adjacent window openings. At the usual mounting points on the rear roof edge or with adhesive antennas on the rear window, this is primarily the rear window, but to a lesser extent other neighboring window openings, e.g. that of the rear side windows, for the coupling of the fields into the vehicle interior.

Today's rear windows generally have window heaters which either consist of printed and usually horizontally attached conductors or of a large number of horizontal individual wires which are inserted between the two windows of a laminated glass pane. Conductor structures of this type have a certain shielding effect with regard to the coupling of electromagnetic fields into the interior and reduce the field strengths in comparison to rear windows which have no heating field. However, the effect is low if no special measures are taken with regard to the design of the heating arrangement. This also results from the fact that vertical polarization is used in radio systems and that the radio antennas are usually arranged in the center of the vehicle's longitudinal axis. The resulting field configuration is then such that horizontally arranged heating conductors are not suitable for returning the currents to the antenna base. The shielding effect of normal heating fields is correspondingly low.

In special cases, heating conductors with busbars at the upper and lower edge of the pane are used orthogonally to the maximum pane dimension. Panes with flat vapor-deposited metallic layers are also technically feasible today. With regard to the field configuration, such heated disks are more favorable given for a shielding effect. In spite of this, there is still no adequate shielding effect, since the earthing conditions of the disc heaters designed according to the prior art are undefined for the frequencies of the radio systems.

As measurements show, the field strengths that occur inside the vehicle are usually large. With the transmission powers typically used in mobile communications of up to 25 W, antennas that are mounted on the outer skin of the roof near the rear roof edge already produce electrical and magnetic field strengths that reach the limit values according to DIN 0848 in the head area of the rear passengers or even exceed.

The situation is even more problematic with antennas glued onto the pane. Since very high field strengths occur especially in the vicinity of the antenna base, the limit values according to DIN 0848 are exceeded in larger areas of the passenger compartment, whereby the special design of the antenna has a considerable influence on the field distribution.

In all of these cases, a risk to passengers cannot be ruled out with certainty.

The object of the invention is therefore to provide an antenna arrangement with which the fields in the passenger compartment are significantly reduced, the performance of the external antenna with respect to radio operation being fully retained.

This object is achieved by the characterizing features of claim 1.

The advantages of the invention are, in particular, that roof antennas and adhesive antennas can be used with antenna arrangements according to the invention without any problems, since the field strengths occurring in the passenger compartment can be kept safely below the limit values of DIN draft 0848 at the maximum transmission powers that are common today. The antennas according to the invention thus avoid the disadvantages of the prior art which endanger the safety of passengers.

A particularly great advantage is that the extent of the reduction can be adapted to the respective requirements, such as the maximum transmission power or antenna type used, by appropriate choice of the size, the arrangement and the configuration of the structure 4, so that the technical outlay in each case is not made greater than necessary must become.

Another significant advantage is that in vehicles that are equipped with antenna arrangements according to the invention, the vehicle window, which has the structure 4, can also be heated and can also have structures that as Antenna structures are suitable for radio reception, so that there are no restrictions with regard to the previous function of vehicle windows.

Fig.1:

• a) Antennenanordnung nach dem Stand der Technik mit einer Außenantenne 1, die auf dem Dach in der Nähe der hinteren Scheibenkante montiert ist und deren Felder in den Fahrgastraum hineingreifen (Schnitt).
• b) dito mit einer Klebeantenne.

Fig.2:

Erfindungsgemäße Antennenanordnung mit einer Außenantenne 1, die auf dem Dach in der Nähe der hinteren Scheibenkante montiert ist und deren Felder nicht in den Fahrgastraum hineingreifen (Schnitt).

Fig.3:

Erfindungsgemäße Antennenanordnung nach Fig.2, Draufsicht.

Fig.4:

Antennenanordnung nach der Erfindung, im Schnitt dargestellt, mit einer Klebeantenne und mit einer Struktur 4, die sich über die gesamte Scheibenöffnung erstreckt.

Fig.5:

Antennenanordnung nach der Erfindung nach Fig. 4, aber in der Draufsicht. Die Struktur 4 ist als Gitternetz ausgebildet und so ausgeführt, daß in der unmittelbaren Umgebung des Antennenfüßpunktes 10 eine besonders hohe Wirksamkeit im Hinblick auf die Reduktion der Felder im Fahrzeuginneren gegeben ist.

Fig.6:

Antennenanordnung nach der Erfindung, im Schnitt dargestellt, mit einer Klebeantenne und mit einer Struktur 4, die nur einen Teil der Scheibenöffnung bedeckt. Im übrigen Bereich erstreckt sich zum großen Teil das Heizfeld 20. Die Struktur 4 ist über eine Drahtverbindung 11, deren Induktivität durch die Kapazität 12 kompensiert ist, mit der Karosserie 3 verbunden.

Fig.7:

Antennenanordnung nach der Erfindung nach Fig.6, aber in der Draufsicht.

Fig.8:

Antennenanordnung nach der Erfindung mit einer niederohmigen Verbindung zwischen Struktur 4 und Karosserie 3, die mittels einer Leitungstransformation ausgebildet ist.

Fig.9:

Antennenanordnung nach der Erfindung, im Schnitt dargestellt, mit einer Klebeantenne mit Dipolcharakter, die koaxial von unten gespeist wird und mit einer Struktur 4, die nur längs einer Scheibenkante leitend mit der Karosserie verbunden ist. Im unteren Bereich der Scheibe ist das Heizfeld 20 angeordnet.

Fig. 10:

Antennenanordnung nach der Erfindung nach Fig.9, aber in der Draufsicht. Die Struktur 4 ist vorwiegend aus radial angeordneten Leitern aufgebaut.

Fig.11:

Antennenanordnung nach der Erfindung, bei der gleichzeitig Rundfunksignale empfangen werden.

Embodiments of the invention are shown in drawings and are described below:

Fig.1:

• a) Antenna arrangement according to the prior art with an external antenna 1, which is mounted on the roof near the rear window edge and whose fields reach into the passenger compartment (section).
• b) ditto with an adhesive antenna.

Fig. 2:

Antenna arrangement according to the invention with an external antenna 1, which is mounted on the roof near the rear window edge and whose fields do not reach into the passenger compartment (section).

Fig. 3:

Antenna arrangement according to the invention according to Fig. 2, top view.

Fig. 4:

Antenna arrangement according to the invention, shown in section, with an adhesive antenna and with a structure 4 which extends over the entire window opening.

Fig. 5:

Antenna arrangement according to the invention of Fig. 4, but in plan view. The structure 4 is designed as a grid and is designed such that in the immediate vicinity of the antenna base 10 there is a particularly high level of effectiveness with regard to the reduction of the fields inside the vehicle.

Fig. 6:

Antenna arrangement according to the invention, shown in section, with an adhesive antenna and with a structure 4, which covers only part of the window opening. The heating field 20 extends for the most part in the remaining area. The structure 4 is connected to the body 3 via a wire connection 11, the inductance of which is compensated by the capacitance 12.

Fig. 7:

Antenna arrangement according to the invention according to Figure 6, but in plan view.

Fig. 8:

Antenna arrangement according to the invention with a low-resistance connection between structure 4 and body 3, which is formed by means of a line transformation.

Fig. 9:

Antenna arrangement according to the invention, shown in section, with an adhesive antenna with a dipole character, which is fed coaxially from below and with a structure 4, which is only conductively connected to the body along a pane edge. The heating field 20 is arranged in the lower region of the pane.

Fig. 10:

Antenna arrangement according to the invention according to Figure 9, but in plan view. The structure 4 is predominantly composed of radially arranged conductors.

Fig. 11:

Antenna arrangement according to the invention, in which broadcast signals are received at the same time.

1a and 1b show antenna arrangements according to the prior art. In the example in FIG. 1 a, the external antenna 1 is an antenna which is mounted on the rear roof edge of the vehicle and is mounted in the vicinity of the window opening closed by the window 2. In the example in FIG. 1b, the external antenna 1 is an antenna, the antenna base 10 of which is fastened on the outside of the vehicle window 2; this is usually done by gluing, which is why one speaks of adhesive antennas.

The signal connection between antenna 1 and radio 23 takes place, as usual, via a coaxial line 18. In the example in FIG. 1 a, the outer conductor of the coaxial line 18 is, as usual, electrically conductively connected to the body 3 near the antenna base. In the example in FIG. 1b, the outer conductor of the coaxial line 15 is electrically conductively connected to the body in the vicinity of the antenna base 10.

In the case of transmission, starting from the external antenna 1, electrical field lines 17 result, which close against the metallic body (FIGS. 1 a and 1b). Dielectric displacement currents are linked to the electrical field lines. These currents close as surface currents on the metallic body 3 to the antenna base. Furthermore, magnetic field lines are present, which are not shown in FIGS. 1 a and 1b, and which are perpendicular to the electrical field lines 17, that is, they emerge from the plane of the drawing.

As shown in Fig. La and Fig. 1b, some of the electric field lines 17 and thus also some of the magnetic field lines pass through openings in the body, e.g. through the window openings into the passenger compartment and close against the metallic body 3 in the interior. Persons who are in the area of the fields are thereby exposed to electromagnetic energy.

Particularly high field strengths naturally occur in the immediate vicinity of the external antenna 1. In the example in FIG. 1a, particularly high field concentrations also occur in the part of the pane opening which is adjacent to the external antenna and which is identical to the pane 2, that is to say in the upper region of the pane. In the case of a rear window and an external antenna mounted on the rear roof edge, the rear passengers are particularly at risk in the head area. The risk naturally increases with the amount of transmission power.

In the case of an adhesive antenna, as shown in FIG. 1b, even more field lines reach into the interior, because in the immediate vicinity of the antenna base 10 in the case of antenna arrangements according to the prior art there are no sufficiently large metallic counter surfaces at the potential of the body. Therefore, significantly higher field strengths can be found inside the vehicle. In some cases, in the case of adhesive antennas, the electrical connection of the outer conductor of the coaxial cable 18 to the metallic body 3 in the vicinity of the antenna base 10 is even dispensed with because of the simpler assembly. As a result, jacket waves can then propagate on the coaxial cable 18, which can cause high field concentrations in the interior even at a greater distance from the antenna base. Even antennas with additional conductors arranged as "radials" on the pane do not result in any improvement with regard to the undesired field strengths in the vehicle interior.

In Fig. 1b, part of the rear window, as is common today, is covered with a heating field 20. The heated area is arranged in the lower area of the pane 2 and is smaller than the entire pane opening, so that the adhesive antenna 1 can be mounted above the heating field in free space. The heating conductors of the heating field are not electrically connected to the body 3 for frequencies of the radio service, since the wires 24 supplying the direct current are only connected to the body ground or to the positive connection of the battery at a greater distance. The laying and the length of the wires 24 supplying the direct current (see FIG. 7 for the case of an antenna arrangement according to the invention) are designed according to the prior art under vehicle-specific aspects and not with regard to the electrical effect for the frequencies of the radio service.

The shielding effect of a normal heating field on the coupling of electromagnetic waves, which were emitted by the external antenna 1, into the interior is therefore low in practice. As shown in FIG. 1b, the fields reach into the interior through the heating field 20 essentially without weakening.

Antenna arrangements according to the invention are described below with which the coupling of electromagnetic waves into the interior is significantly reduced. FIG. 2 shows such an antenna arrangement in section, FIG. 3 in a top view for the example of an external antenna 1 mounted on the rear roof edge.

In contrast to FIG. 1 a, in the example of FIGS. 2 and 3, the pane 2 is now provided with a two-dimensional, that is to say flat structure 4 made of conductive material with a low surface resistance in the frequency range of the radio service. In order to achieve the advantages according to the invention, a surface resistance must be effective for the frequency range of the radio service through the structure 4, which surface resistance is significantly lower than the field wave resistance of the free space. Then the structure 4 is suitable for shielding act and can effectively prevent electrical and magnetic fields generated by the external antenna 1 from penetrating into the vehicle interior.

With the current technological possibilities, structure 4 can e.g. be vapor-deposited in the form of a coating. These metallic layers are vapor-deposited very thinly, which maintains the transparency for light and at the same time achieves the required low-impedance for the radio frequencies.

However, structures made of individual conductors, as are known from printed heating fields, which are applied using the screen printing method, are equally suitable for antenna arrangements according to the invention. In order to achieve the desired effect in terms of reducing the coupling of electromagnetic fields into the interior, the teachings of the invention must then be followed with regard to the geometries of the structures used, which will be explained further below. The combined use of a vapor-deposited layer with a printed conductor structure can also be advantageous.

Because of the small penetration depth of high-frequency radio waves due to the skin effect in metallic conductors, a very thin layer is sufficient, which in turn is suitable for direct currents, e.g. would be too high-resistance for the purposes of a window heating. The required low impedance for direct currents can then be set by printed conductors.

In order to achieve the advantages according to the invention, at least one edge of the structure 4 must have a low-resistance connection to the metallic body 3. In the case of Fig. 2 or

3, this is the case on all four edges of structure 4, which results in the advantage of particularly high effectiveness in the sense of the invention. In the example of FIGS. 2 and 3, the pane 2 is inserted into the body 3 in the manner customary today and mechanically connected to the body by means of the pane adhesive 13, which is applied as a bead of adhesive parallel to the outer pane edge.

The electrically low-resistance connection for the frequencies of the radio service can in many cases be particularly simple and therefore advantageous, as assumed in the example of FIGS. 2 and 3, via the flat opposite edges of the body and the edges of the structure 4 via the adhesive bead.

The required low resistance of the connection can ideally be achieved, for example, by means of a silver-containing and therefore highly conductive adhesive 13. This causes the structure 4 to be at the potential of the body. The low-resistance connection to the body 3 can in principle take place in different ways in the antennas according to the invention. The use of a conductive adhesive indicated above is ideal from an electrical point of view, but the high cost of such adhesives is disadvantageous.

For design reasons, there is always an overlap zone of typically 1 to 2 cm width along the edge of the window between the vehicle window 2 and the body 3 in modern vehicles with windows glued in. Because of the comparatively small distance of typically 3 to 4 mm, due to the adhesive bead, between the structure 4 and the opposite surface of the body 3, this results in a not inconsiderable capacitive coupling, which represents a sufficiently low-resistance capacitive connection due to the large-area arrangement, if the window adhesive arranged in this area has a dielectric constant significantly greater than 1. This is the case with the window adhesives currently used with ε _r values of typically greater than 5.

Since the radio services, for which the antenna arrangements according to the invention are preferably used, are generally arranged at frequencies in the UHF range or higher, the capacity thus formed is sufficient to provide a sufficiently low-resistance connection between the structure 4 and the body 3.

Disc adhesives currently used sometimes have low losses at high frequencies, while others are highly lossy due to a high proportion of soot, so that in addition to the capacitive connection there is also a high ohmic conductivity. Even when such window adhesives are used, a low-resistance connection between the body and structure 4 is therefore reliably provided for the frequencies of the radio services.

However, the low-impedance connection between structure 4 and body 3 required for antenna arrangements according to the invention can also be made in a punctiform manner. Fig. 6 shows this possibility in section and Fig. 7 in plan view. In this example, it is assumed that the washer is inserted into the body with a rubber seal, as was customary in the past. A sufficiently low-resistance connection to the body 3 is therefore not always given in all cases because of the edges of the structure 4 and body 3 that are only opposite on the end face. If the connection is not low-resistance, which can be determined by measuring the impedance between structure 4 and body 3, the low-resistance connection can be ensured exclusively or by means of one or more wire bridges from structure 4 to body 3.

Due to the self-inductance of approx. 10 nH / cm, even with a short wire bridge, the low impedance of a wire bridge is not sufficient for the frequency ranges in which the radio systems under consideration are operated, i.e. essentially above about 400 MHz, since a wire bridge with a length of approx. 10 cm, which is difficult to go below, already results in an inductive impedance of approx. 280 ohms in the frequency range of the C network. The low-impedance for the frequencies of the radio service can then be restored by compensating for the intrinsic inductance of the wire by means of a capacitance 12 connected in series. In the example given, a capacitance of approximately 1.2 pF is required.

In this way, there is an effective low-resistance connection between the connection points 25 on the structure 4 and 33 on the body.

The position of this connection point 25 on the structure 4 is preferably chosen where the greatest return currents flow to the antenna base point 10, because this results in the greatest advantageous effect electrically. In the example of FIG. 6, this is the axis of symmetry of the pane at the upper edge, that is to say in the immediate vicinity of the antenna base point 10. The connection point 33 is preferably selected as close as possible to the connection point 25 on the body. The electrical character of this connection is that of a series resonance.

The disadvantage of this version of the low-resistance connection using a wire bridge is the installation and contacting costs. Electrically equivalent and therefore functionally equivalent, the low-resistance connection between the body 3 and the structure 4 which is effective with regard to the connection point 25 can also be achieved by means of a line transformation. This advantageously eliminates the assembly costs for the wire bridge.

Such an embodiment according to the invention is shown in FIG. 8. In this example, the line character results between the upper edge of the structure 4a and the opposing metallic body 3, with the structure 4a running empty on the left and right side. The respective idling is transformed according to the length 36 or 37 into an impedance which takes effect between the structure 4a and the body 3 in the vertical line of symmetry 38 and which, with a suitably selected dimension 36 or 37, results in an AC short circuit with series resonance character. The structure 4a and thus also the dimensions 36 and 37 for the region of the structure 4a lying on the left and for the right of the axis of symmetry 38 are advantageously of the same mirror image. The total horizontal dimension of the structure 4 thus results in 27 as the sum of the dimensions 36 and 37. The dimensions 36 and 37 are typically carried out for the fulfillment of this task in such a way that a lambda-quarter transformation or a transformation with a corresponding similar one Characteristic (by an odd integer multiple of lambda quarter) results. The exact required dimensions 36 and 37 are preferably determined by measuring the impedance between structure 4 and the body on the axis of symmetry 38, since the fields of the line via which the line transformation results are also partially present in the glass of the pane 2 , resulting in an approximately shorter effective wavelength deviating from the free space wavelength.

The result of an antenna arrangement according to the invention is then such that the electrical field lines, coming from the external antenna 1, end on the structure 4 and no longer penetrate into the interior, or only weaken it to a great extent. The same applies to the magnetic fields. In this way, the interior of the vehicle is largely free from the fields of the radio antenna.

The greatest effectiveness, with the advantage of a particularly large reduction in the fields in the interior of the vehicle, naturally arises in an antenna arrangement according to the invention if the structure 4 extends over the entire surface of the window opening. In practice, however, a sufficient, because only slightly reduced, effectiveness can be achieved by the structure 4 extending over the particularly important areas of the pane.

These particularly important areas for achieving advantages according to the invention result on the one hand from the proximity to the foot point of the external antenna 1, in the example of FIG. 2 in the upper middle area of the vehicle window 2. It is known that the highest field concentration occurs there. Correspondingly, in the case of an external antenna 1 which is designed as an adhesive antenna (FIGS. 4 and 5), the area around the base 10 of the antenna is particularly important.

On the other hand, it must be taken into account how close body parts of passengers can approach the respective areas of the pane 2. In any case, the upper area of the panes is therefore very important, since the heads of the rear passengers can come very close to pane 2 there. In contrast to this, the lower area of the pane 2 is considerably further away from the body of the rear passengers. This is particularly true when the disc 2 is arranged relatively flat.

Therefore, the effectiveness of the structure 4 is usually particularly great in the upper region of the pane and there again in the middle, even if the external antenna 1 is attached there in the middle on the pane 2 or in the vicinity on the body 3. For the same reasons, it is then possible to dispense with the structure 4 down to the lower region of the pane 2, without disadvantages relevant in practice, or it can at least be carried out in these regions with less technical effort.

FIG. 4 shows an antenna arrangement according to the invention for an adhesive antenna 1 in a sectional view, in which these aspects are taken into account. Fig. 5 shows the same arrangement in the top view. 10 again designates the antenna base, that is also the area on the pane to which the adhesive antenna 1 is glued. In the case of antenna arrangements according to the invention, this mounting point 10 is preferably inside the area covered by the structure 4, since particularly high field concentrations occur again in the area of the base point 10 of the antenna 1.

The surface of the adhesive antenna 1 facing the pane, by means of which the mechanical connection to the pane is also established, is typically of metallic design in such antennas in such a way that a capacitively sufficiently low-resistance connection passes through the pane is given to a metallic counter surface 14 on the inside of the pane. The inner conductor of a coaxial line 18, which establishes the signal connection to the radio, is connected to this counter surface 14. The outer conductor of the coaxial line 18 is connected to the structure 4 near the counter surface 14 in antennas according to the invention. Because of the shielding effect of the structure 4, there are no jacket waves on the coaxial line 18.

In the example in FIGS. 4 and 5, structure 4 again covers the entire pane surface, with the exception of the area in which the signal is coupled in from the coaxial line 15 to the adhesive antenna 1 through the pane. However, the density of the printed conductors 7 is designed differently in different areas of the structure 4, namely with a high density in the upper area of the pane 2 and there again in the middle, in the lower area and there again at the edges, the conductor density is significantly lower .

For the geometries of the counter surface 14 and the cutout in the structure 4 for this counter surface, concentrically arranged circles (FIG. 5) or rectangles or squares are appropriate. The necessary size of the counter surface 14 and the surface at the base of the adhesive antenna 1 is known from commercially available antenna types and is typically 2 to 4 cm ² when, for example, the frequencies of the C or D network radio telephone are considered.

Of course, the signal supply to the adhesive antenna 1 for antenna arrangements according to the invention can also take place from the outside while maintaining the advantages, that is to say not only capacitively through the pane, as assumed in FIGS. 4 and 5. Because of the problematic cable routing, this technology is hardly used in practice for adhesive antennas.

The two-dimensional, optically transparent structure 4, which is largely impervious to radio waves in the frequency range of the radio service, can be implemented in various ways. In addition to the already mentioned vapor deposition of the pane surface with a thin metallic layer, the realization by wire-shaped conductors, which are applied in the screen printing process, is of particular interest in practice.

For the formation of the low surface resistance required for antennas according to the invention in the frequency range of the radio service, the special configuration of the fields must be taken into account, the penetration of which into the vehicle interior is to be prevented. Flat structures with a thickness that is greater than the depth of penetration at the operating frequency of the radio service are very well suited for forming the structure 4. A close-meshed wire mesh, as shown in Fig. 5, is equivalent in effect. Differences to a surface vapor-deposited structure arise only at a very short distance from the structure 4, that is to say in the range of a few centimeters. Usually, a narrow mesh is understood to mean a mesh size that is not significantly larger than 1/10 of the mean operating wavelength of the radio service. This also applies in connection with antenna arrangements according to the invention. This results in a mesh size of 7 cm for the frequency range of the C-Netz mobile phone and approx. 3 cm for the D-Netz. However, larger mesh sizes are not ineffective, but only have a lower effectiveness, especially in the vicinity of the grille. Mesh widths of substantially more than 1/4 of the mean operating wavelength of the radio service are only suitable to a very limited extent for antenna arrangements according to the invention.

For the reasons mentioned above, it is particularly important to choose the effectiveness in the vicinity of the antenna base point 10 to be large and thus to choose the mesh size sufficiently narrow. At a greater distance from the antenna base point 10, the mesh size can then increase or the number of conductors can decrease, e.g. in the manner shown in FIG. 5, without thereby overall impairing the advantages of the invention.

Another advantageous embodiment of a structure 4 is shown in FIG. 10, in which the return currents to the antenna base point are effectively detected by conductors running in a star shape towards the antenna base point 10. This structure 4a, which is not very extensive in terms of area, considerably reduces the fields inside. FIG. 8 shows a structure similar in area to that of FIG. 10, which extends essentially on the pane 2 in the vicinity of the antenna base point 10.

The effectiveness of a structure for achieving the object according to the invention decreases if the area covered by the structure 4 is made smaller. However, since the field strengths are greatest near the base point and rapidly decrease with increasing distance from it, comparatively small-area structures 4 are also able to significantly reduce the field strengths in the interior of the vehicle. The required areal extension of structure 4 is therefore also dependent on the maximum transmission power used, since with small transmission powers only a slight reduction in the fields is required.

Antenna arrangements according to the invention relate to radio devices with at least medium output power. HF output powers are understood to be in the range above approximately 5 W, with which the field strengths in the passenger compartment exceed the limit values according to DIN 0848 at least in the interior without design according to the invention.

With an RF output power of, for example, 5 W, the area of the pane 2 covered by the structure 4 can naturally be less than, for example, the maximum power of about 25 W used in the C network in the vehicle. As a lower limit for the sensible use of an inventive one Antenna arrangement for a power of approximately 5 W in combination with an antenna shape which produces pronounced fields in the vicinity of the base point 10 can be stated with respect to the dimensions of the structure 4 on the basis of measurements that the dimensions 36 and 37 and 35 (Fig. 8) must not be significantly less than about 1/4 of the mean operating wavelength in the frequency range of the radio service. At higher frequencies, at which the operating wavelength becomes very small, it is necessary not to make the dimensions smaller than about 10 cm in the case of the dimensions 27 and 35. 27 is the sum of the dimensions 36 and 37.

In practice, there are usually other structures on the window 2, at least if it is the rear window of a vehicle, e.g. Heating fields 20 or structures 34 for radio reception antennas.

7, 8 and 10 show examples of antenna arrangements according to the invention in combination with heating fields 20.

In the example in FIG. 7, it is assumed that the heating field 20 does not have the characteristic features of a structure 4 with regard to the design and the electrical wiring in the frequency range of the radio service, e.g. is not connected to the body 3 with low resistance. Therefore, the electromagnetic fields that the radio antenna emits are not or only slightly weakened by the heating field 20. Since, for the reasons mentioned above, in the lower area of the pane in which the heating field 20 is arranged, however, the fields of the external antenna 1 are smaller than in the upper area of the pane and, moreover, the body parts hardly approach the lower part of the pane in practice 7, it is often sufficient to arrange the structure 4 only in the upper region of the pane.

In the example of an antenna arrangement according to the invention, as shown in FIG. 8, it was assumed that shielding is also required in the lower region of the vehicle window 2. For this purpose, the low-resistance connection between the structure of the heating field 20 and the body 3 is e.g. reached again via the busbars 30 of the heating field 20 directly opposite the body panel and via the adhesive bead. In this way, the heating field 20 simultaneously becomes the structure 4, subarea 4b, and thus fulfills the tasks according to the invention with regard to a reduction of the electromagnetic fields in the vehicle interior.

In order to avoid a direct current connection between the busbars 30 and the body by the adhesive in the example in FIG. 8, an adhesive with good dielectric but low ohmic conductivity is advantageous in this case.

In the example of FIG. 10, the heating field 20 is also connected to the body in the area of the busbars with a low impedance and is therefore also a component of the structure 4. If, as in many cases, the heating field in the area of the window opening consists only of horizontal conductors, it is shielding effect is present, but possibly not yet sufficient. The shielding effect of the heating field 20 acting as structure 4 can, however, be improved by additional measures in terms of effectiveness in the sense of the invention. For this purpose, in the example of FIG. 10, three additional conductors 31 arranged almost vertically are provided in the middle, which can conduct currents in the direction of the antenna base 10 and which are arranged on equipotential lines with respect to the heating conductors, so that no heating current flows in the transverse direction. The effect is further improved by the interdigital structure 32, via which the structure 4b is capacitively coupled to the structure 4a.

Different types of antennas are used as external antennas. Widely used are antennas that have a low-impedance base impedance with respect to the base, such as lambda / 4, 5/8 lambda or 3/4 lambda radiators, which have a funnel impedance in the vicinity of the characteristic impedance of 50 ohms of conventional coaxial cables and their advantage in easy adaptation to the power cable. However, the use of these antenna shapes goes hand in hand with large reverse currents on the base area. Depending on the electrical properties of the base area, that is to say on its surface impedance, there may therefore be losses which undesirably reduce the efficiency of the external antenna 1.

This is not a technical problem if the external antenna e.g. is arranged in the center of the roof of a vehicle because the body has a very low and low-loss surface impedance.

In the case of antennas according to the invention, on the other hand, the external antenna 1 is arranged in the vicinity of a window opening formed by a vehicle window 2. If a structure 4 of conductive material with a low surface resistance in the frequency range of the radio service is then applied completely or partially to the vehicle window 2, at least some of the currents flow back to the antenna base via parts of the structure 4. For the solution of the task according to the invention, namely To lower the fields in the passenger compartment significantly, it is sufficient if the surface resistance of the structure is low-resistance compared to the field-wave resistance of the free space, i.e. 377 ohms. A value that is at least 5 times lower in terms of amount can be considered sufficient to significantly reduce the fields in the interior, e.g. to reach 6 dB.

With regard to the coupling of losses into the antenna circuit with the consequence of a reduced efficiency, however, further requirements may have to be made on the surface impedance, which depend on the particular antenna type. In the case of the antennas listed above with comparatively large base point currents, the aim should therefore be to make the surface impedance of the structure as conductive as possible, such as the metallic body. This requires, for example, a surface coating with a correspondingly high level specific conductivity or a correspondingly high thickness, which may result in an inadmissible decrease in the optical transparency.

If this structure 4 is also to take on the function of a window heating at the same time, further requirements with regard to the ohmic conductivity for direct current must be met. The selection of a suitable type of coating therefore plays an important role in antenna arrangements according to the invention, especially in the case of external antennas 1 with large base point currents. However, since coatings with very different electrical properties are available today, there is no restriction on the applicability of antenna arrangements according to the invention.

In the case of printed conductor structures, the surface resistance can be changed via the spacing between the conductors. In addition, e.g. by means of a galvanically applied very thin layer, the surface resistance for the frequencies of the radio service can be set within wide limits independently of the resistance for direct current (heating field), since the depth of penetration is extremely small at high frequencies, while the entire cross section is full of current in direct current.

With other antenna types, a different situation arises with regard to the requirements for the surface impedance of the structure 4. Such antenna shapes are characterized by a low current at the antenna base point, with the result that even low currents on an adjacent base area or structure 4. As a particular advantage, this does not result in any requirement for low resistance compared to 377 ohms that goes beyond the object of the invention.

Such antenna shapes are e.g. Lambda / 2 dipoles fed below, which, however, can only be adapted to the characteristic impedance of coaxial cables. The use of antennas according to DE 40 07 824 A1 (FIG. 9) which are coaxially fed through the base point and are particularly suitable for adhesive antennas is particularly advantageous for antenna arrangements according to the invention.

Because of the typical field configuration with field lines that essentially close between the dipole halves, there are only very small return currents on the structure 4. This therefore does not have to take on the function of a counterweight, so that no significant losses occur in the structure 4 if the structure fulfills the object of the invention of preventing the coupling of electrical energy from the radio antenna in the passenger compartment. The low currents on structure 4 result from comparatively few field lines that strike structure 4. Therefore, the load on the passenger compartment with electrical energy is in principle already comparatively low with this type of antenna. The required degree of reduction of the fields in the passenger compartment is consequently also less than for antennas with large feed currents at the base. The share of The area of the pane 2, which must be provided with the structure 4 in order to be able to rule out a hazard to the occupants, is therefore also smaller than in the case of external antennas with large base currents.

In modern vehicles, disc antennas are increasingly used for radio reception. These antennas partly use the heating field, partly also separate structures 34 as antenna elements. Antenna arrangements according to the invention can advantageously be combined with the known antenna structures 34 for radio reception. An example of this is shown in FIG. 11. In this example, structure 4 is formed from several areas. The area 4a which extends in the immediate vicinity of the antenna base 10 is e.g. low-resistance connected to the body 3 at the upper edge of the structure 4a via the adhesive bead. Due to the low-resistance connection in the area of the busbars, the heating field 20 is a component of the structure 4, namely the area 4b. The structures 34 serve in a known manner as antenna elements for radio reception, e.g. particularly advantageous in the embodiment of an active antenna. The antenna structures 34 become part of the structure 4 in that they are connected to the body with low impedance via wire bridges compensated for the frequencies of the radio service, which thereby have the character of series resonance circuits with a replacement inductor 28 and a series-connected capacitance 12. Because of the very small value of the capacitance 12, lower frequencies, e.g. of the LMK and VHF radio broadcasting area then only a negligible capacitive load on the structures 34, as a result of which the performance of the radio receiving antenna is not inadmissibly impaired.

Claims (20)

1. Antenna arrangement for motor vehicles, comprising an external radio antenna (1) for the reflection of electromagnetic signals, and a vehicle window (2) in metallic automotive bodywork (3), wherein the external radio antenna (1) is mounted, on the bodywork or on the vehicle window, adjacent to the window aperture of the vehicle formed by the vehicle window (2),
   characterised in that
   on the vehicle window (2), at least in parts thereof and adjacent to the external radio antenna (1), there is mounted a flat structure (4) of conductive material with low surface resistance in the frequency range of the radio communication service, said structure being optically substantially transparent but substantially impermeable to radio waves within the frequency range of the radio communication service in terms of the electromagnetic radiation coming through the window aperture, and wherein this flat structure (4) is connected in low-resistance manner to the metallic bodywork (3)at least at one side of the margin of the flat structure (4) for the frequency range of the radio communication service, and wherein the dimensions of the flat structure (4) are chosen so that the electromagnetic radiation produced by the external antenna (1) is sufficiently screened from the interior of the vehicle.
2. Antenna arrangement according to claim 1,
   characterised in that
   the horizontal dimension (27) of the structure (4) is at least half the mean operating wavelength of the radio communication service, but not less than 10 cm, and the vertical dimension (35) of the structure (4) is at least a quarter of the mean operating wavelength of the radio communication service, but likewise not less than 10 cm.
3. Antenna arrangement according to one of claims 1 and 2,
   characterised in that
   the external antenna (1) is mounted on the vehicle window (2) as an adhesively-mounted antenna, and the mounting point (10) of the adhesively-mounted antenna lies within the region covered by the structure (4).
4. Antenna arrangement according to one of claims 1 to 3,
   characterised in that
   the structure (4) is formed as a flat coating (5).
5. Antenna arrangement according to one of claims 1 to 3,
   characterised in that
   the structure (4) is formed as an arrangement (6) of wire-form conductors (7) which are set parallel to one another or are imprinted as grid structures or are set between glass plates.
6. Antenna arrangement according to claim 5,
   characterised in that
   the spacings between the wire-form conductors (7) in the region of the foot (10) of the antenna are less than 1/10 of the mean free-space wavelength of the radio communication service.
7. Antenna arrangement according to one of claims 1 to 6,
   characterised in that
   the low-resistance connection for the frequency range of the radio communication service between the structure (4) and the metallic bodywork (3) is effected by means of the window adhesive.
8. Antenna arrangement according to one of claims 1 to 6,
   characterised in that
   the low-resistance connection for the frequency range of the radio communication service between the structure (4) and the metallic bodywork (3) has electrically the character of a series resonant circuit with a resonance frequency which corresponds approximately to the centre frequency of the radio communication service.
9. Antenna arrangement according to claim 8,
   characterised in that
   the character of the series resonant circuit is produced by an inductively functioning wire connection (11) and a capacitor (12) connected in series therewith having a Suitably chosen value, and this connection is effective electrically between a terminal (13) on the window and a terminal (14) on the bodywork, and the terminal (13) of the wire connection is arranged on the structure (4) adjacent to the mounting point of the external antenna (1) at the edge of the structure (4) overlapping the bodywork (3) and the terminal (14) is arranged on the bodywork (3) at a small distance from the terminal (13) on the opposite side.
10. Antenna arrangement according to claim 8,
    characterised in that
    the character of the series resonant circuit is produced by means of a line transformation between the upper margin of the structure (4) and the opposing edge of the bodywork (3) such that the low-resistance connection with the metallic bodywork (3) results between an imaginary terminal (13) at the edge of the structure (4) adjacent to the mounting point (10) of the adhesively-mounted antenna (1) and the bodywork (3) which lies opposite at a small distance therefrom, and in that the horizontal dimensions of the structure are chosen correspondingly.
11. Antenna arrangement according to one of claims 1 to 10,
    characterised in that
    the structure (4) is provided over the whole surface of the window, with the exception of the region of signal coupling to the antenna, and consequently extends at all four margins up to the edge of the window aperture.
12. Antenna arrangement according to one of claims 1 to 10,
    characterised in that
    the structure (4) is provided only in partial regions (9) of the window surface, and the structure (4) extends at three margins up to the edge of the window aperture.
13. Antenna arrangement according to one of claims 1 to 10,
    characterised in that
    the structure (4) is provided only in partial regions of the window surface, and the structure (4) extends only at one margin up to the edge of the window aperture.
14. Antenna arrangement according to one of claims 1 to 13,
    characterised in that
    the external antenna (1) is a type of antenna with high foot currents and therefore high return currents to the foot (10) of the antenna.
15. Antenna arrangement according to one of claims 1 to 13,
    characterised in that
    the external antenna (1) is a type of antenna with low return currents to the foot (10) of the antenna.
16. Antenna arrangement according to claim 15,
    characterised in that
    the external antenna (1) is an adhesively-mounted antenna and the signal coupling for this adhesively-mounted antenna (1) is effected capacitively through the window.
17. Antenna arrangement according to one of claims 1 to 16 with the exception of 13,
    characterised in that
    the structure (4) consists of two regions (4a) and (4b) and the region (4b) is used simultaneously as a heating field (20) for heating this part of the window (20).
18. Antenna arrangement according to one of claims 1 to 17,
    characterised in that
    other vehicle windows in the region of the external antenna are likewise provided with a structure (4).
19. Antenna arrangement according to one of claims 1 to 17,
    characterised in that
    antenna structures (34) for the receiving of radio broadcast signals are also provided on the vehicle window.
20. Antenna arrangement according to claim 19,
    characterised in that
    the antenna structures (34) are constituents of the structure (4).

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