EP0124055B2 - Active windscreen antenna for all kinds of polarization - Google Patents

Description

The invention relates to an active windshield antenna for motor vehicles for VHF and LMK radio reception according to the preamble of claim 1.

With active windshield antennas, it is known that it is possible to receive all wave ranges (LMK and FM) as with the standard rod antenna on the motor vehicle. The advantage here is that the integration in the vehicle body meets vehicle-specific requirements such as mechanical robustness, long service life, easy installation, avoidance of unnecessary air turbulence much better than the standard rod antenna.

Active windshield antennas with the properties described above are known for example from DE-GM 78 08 489.

It turns out, however, that a motor vehicle antenna which is designed according to this prior art, in the case of vertically or circularly polarized waves, types of polarization such as those e.g. are frequently used in the USA for VHF broadcasting, only provides extremely unsatisfactory reception powers, whereas the reception powers in the FM range with horizontal polarization and in the LMK range are at least equivalent to those of the standard antenna. Another disadvantage is the position of the antenna amplifier that is predetermined for the antenna to function properly. A change in the antenna configuration shown in Figure 1 of DE-GM 78 08 489 by changing the connection point leads to poorer reception properties, particularly in the FM range.

The invention has for its object to realize an active windshield antenna according to the preamble of claim 1, the reception power of which has good reception properties both horizontally and vertically and circularly polarized VHF reception field and in the vertically polarized LMK range and the requirement is met should also be that the mounting point for the amplifier near the metal frame can be freely chosen with regard to vehicle-specific aspects.

An antenna arrangement has also become known (FR-A-2 513 021) which, although it is a two-band antenna, is not an active antenna of the type described. This known antenna arrangement is always composed of two sections, namely a section for frequency modulation (FM) and a section for amplitude modulation (AM), these two sections being connected to one another in series. The sections are separated by an inductor, the inductor acting as an open circuit for the FM range and a short circuit for the AM range. The known antenna arrangement is therefore not able to achieve the described task, namely to achieve the desired reception results, without this separation by the inductance. It is also essential that an antenna amplifier, as required by an active antenna, is not present in this known antenna arrangement. This also contradicts a solution to the task described.

The object is achieved in an active windshield antenna according to the preamble of claim 1 by the characterizing features of claim 1.

Such an antenna has the particular advantage of delivering very good reception results with horizontal as well as with vertical and circular polarization in the FM range. In contrast to the previously known antennas according to DE-GM 78 08 489 and DE-GM 75 276 21, which are inferior to a vertical comparison antenna in the transition from horizontal polarization to vertical polarization, the antenna according to the invention delivers good results in all types of polarization. Compared to the antenna arrangement according to FR-A-2 513 021, there is a significant difference in the omission of the subdivision of the antenna configuration into a conductor section for the FM range and one for the AM range, with the advantage thereby achieved of the uniformity of the antenna conductors for FM and AM reception.

In many cases, in particular in the case of an antenna integrated into the vehicle body, such as in the case of the windshield antenna, the location for the antenna connection point and the amplifier is predetermined by vehicle-specific constraints. The constraints often result from the assembly sequence in the manufacture of the motor vehicle and from the requirement that the antenna amplifier be exchangeable and retrofitted. With an antenna amplifier and a conductor configuration according to the invention, the reception properties that can be achieved are largely independent of the specified installation location. A comparison of the conductor or wire length with a view to adapting to the current vehicle body or windshield opening is largely unproblematic with the given conductor configuration.

In claims 2 to 9 further and advantageous embodiments of the invention are characterized.

• Fig. 1 eine aktive Windschutzscheibenantenne für alle Polarisationsarten nach der Erfindung;
• Fig. 2 eine Antenne nach der Erfindung mit Verstärkermontage rechts unten;
• Fig. 3 eine Antenne nach der Erfindung mit Verstärkermontage rechts oben;
• Fig. 4 eine Antenne nach der Erfindung mit Verstärkermontage rechts unten und minimaler Leiter- bzw. Drahtlänge.

The invention is explained in the following description in exemplary embodiments with reference to the drawing. Show it:

• 1 shows an active windshield antenna for all types of polarization according to the invention;
• Figure 2 shows an antenna according to the invention with amplifier assembly bottom right.
• 3 shows an antenna according to the invention with an amplifier assembly at the top right;
• Fig. 4 shows an antenna according to the invention with amplifier assembly lower right and minimum conductor or wire length.

Fig. 1 shows the front window of the vehicle with a view from the passenger compartment. The antenna amplifier 5 is mounted in the vicinity of the metal frame 2 and is connected at its ground point to the metal frame of the windshield via the connection 8 (input connection). The antenna amplifier 5 has at its input a branching of the transmission paths for signal components from the VHF frequency range and the LMK frequency range. Such a signal branch is e.g. known from DE-PS 21 15 657 or from DE-PS 21 66 898 or DE-PS 19 19 749. The transmission path for the LMK frequency range contains an amplifier with a high-impedance input impedance, as is e.g. is known from DE-PS 20 21 331 or DE-PS 25 54 828 or DE-PS 25 54 829. It is essential for reasons of sensitivity that the circuit for branching the signals does not load the input impedance of the LMK amplifier in the LMK frequency range with a low resistance, so that the input impedance of the overall amplifier 5 in the LMK frequency range is overall high-resistance. The output line 6 of the antenna amplifier 5 is led to a receiver 7. The first input connection 3 of the antenna amplifier 5 is connected to the one end of the antenna conductor 4a, 4b via the shortest possible connecting line.

The interaction of the conductor configuration given by the antenna conductors 4a and 4b with the LMK amplifier 10 is explained below. The conductor 4b makes the essential contribution to reception. In the interest of good reception in the LMK area, the conductor 4b should not be chosen too short and if possible use the full pane height. Although the reception field strength in the lower part of the conductor 4b is opposite to the field strength in the upper part 4a, the full conductor length is preferable from the point of view of the LMK reception due to the associated increased capacitance.

In many cases it would be sufficient for LMK reception to attach the amplifier to the metal frame 2 on the vertical line of symmetry 9. Such a mounting point is often not possible for vehicle-specific reasons. To bridge the distance between the break point 12 on the line of symmetry 9 and the predetermined mounting location of the amplifier 5, the use of a conventional coaxial line would be obvious. However, such a line has the disadvantage of a large parallel capacitance, which is accompanied by a loss of sensitivity. The design of the conductor 4a according to the invention has the decisive advantage of a comparatively small capacity in conjunction with an additional contribution to reception. This contribution is due to the high concentration of the electric field lines in the vicinity of the metal frame 2, which brings about a noteworthy improvement in reception even when the conductor 4a is at a small distance from the metal frame 2. This makes it possible to choose the distance A of this conductor 4a from the metal frame 2 so small that it does not significantly impair the driver's or passenger's view and the design of the windshield. Distances A significantly less than 10mm between the conductor 4a and the metal frame 2 should, however, be avoided.

The function of an active windshield antenna according to the invention for reception in the FM range is explained in more detail below with reference to FIG. 1. As is known, the pane opening 1 enclosed by the conductive frame 2 can be regarded approximately as a slot radiator which can be optimally excited by a wave with an electrical field strength vector oriented in the direction of the vertical line of symmetry 9. In the VHF band, a resonance-like increase in the field strength along the line of symmetry 9 occurs when the width of the window opening corresponds to approximately half a wavelength, as is usually the case with cars customary today. In the case of horizontally as well as vertically and circularly polarized waves, field components in the direction of the symmetry line 9, which excite the disk resonance, are present because of the pane inclined in the vehicle.

A conductor or wire structure such as the conductor part 4b is therefore strongly coupled to the receiving field. The coupling of the conductor part 4a, on the other hand, can be changed via the distance A, small distances A between the conductor 4a and the frame 2 resulting in low power consumption from the field and large distances A leading to higher power consumption from the field. The distance A can therefore also be used to change the ratio of the contributions to reception by the conductor parts 4a and 4b in the FM range.

Although maximizing the VHF signal power available at the end of the conductor part 4a would in many cases suggest the choice of the maximum possible distance A of approximately 7 cm from the frame 2 from a safety point of view, in practice the distance A will often be chosen to be significantly smaller because this results in results in a significantly more attractive antenna design and the view through the pane is less impaired.

Distances A down to approx. 10 mm from Frames 2 are justifiable because the total power consumption from the field by the conductor 4b is large and, especially in connection with a noise-optimized active VHF signal path 13 in the amplifier 5, the resulting signal-to-noise ratio in comparison to the standard system with the standard antenna is usually already superior.

The available output power of the structure can be optimized by varying the distance 11 between the end of the vertical conductor part 4b and the frame 2, the maximum possible length often being found to be the optimal length for all polarizations. In order to prevent a high-frequency short circuit at the end of the conductor part 4b with the frame 2, the distance 11 should not be less than approx. 2 mm.

For a mounting point of the amplifier specified from a vehicle-specific point of view, the optimal distance 11 between the end of the conductor part 4b and the frame 2 must be found by the fact that for different distances 11 by longer measurement runs with statistical travel routes, the mean antenna output voltage resulting from the input resistance of a measurement receiver structure under investigation compared to the mean output voltage of the reference antenna is determined. After measuring the impedance of the conductor or wire structure, this voltage across a load resistor that is not matched to the power must then be converted to the available average output power, that is to say to the power that would have been achievable with a respective power-matched load.

The optimum distance 11 is when this available signal power is at a maximum, the value of the impedance of the conductor or wire structure itself being almost insignificant, since low-loss transformation circuits can be used in the antenna amplifier 5 within the transmission path for the FM frequency range 13. Such transformation circuits are expediently designed such that noise adaptation for the input transistor is achieved if an active, amplifying three-pole element such as a field effect transistor or a bipolar transistor is used in the transmission path 13, or it is necessary to adapt the power to the characteristic impedance of the connecting cable 6 to the receiver 7, if the transmission path 13 is designed passively.

Passively executing the transmission path 13 for the VHF frequency range is, however, only expedient if the achievable signal-to-noise ratio in combination with the receiver with the pane antenna structure is equivalent to that of the standard rod antenna. As a rule, it is necessary to ensure sufficient limit sensitivity via a noise-adapted amplifier stage.

Fig. 2 shows an advantageous embodiment of the invention, in which the antenna amplifier 5 and the antenna connection are mounted in the vicinity of the lower right corner of the metal frame and the break point 12 is in the vicinity of the upper horizontal part of the metal frame and the conductor 4a parallel to it right and to the horizontal upper part of the metal frame.

In this example, the amplifier is installed in the area of the dashboard, an area in which there is often enough space and which is usually also easily accessible for retrofitting an antenna or for replacing the antenna amplifier. It is also advantageous at this installation location that only a relatively short connecting cable to the receiver is required.

FIG. 4 shows an embodiment of the invention with the same mounting point for the amplifier as in FIG. 2, but with a minimized conductor or wire length on the disk and, compared to the structure according to FIG. 2, a simple geometry of the conductor arrangement with only a single break point , which results in cost advantages in production. However, the risk of interference coupling from the engine compartment to the antenna structure is greater in an embodiment according to FIG. 4 because of the closer proximity of the conductor part 4a to the interference sources such as the ignition. Therefore, such a structure can preferably only be used for vehicles with good interference suppression.

3 shows a likewise advantageous attachment of the amplifier in the upper right corner of the metal frame, an area in which there is often enough space for the antenna amplifier in the right spar or under the vehicle headlining. The connecting cable can often simply be led down under the plastic cover with which the metal bar is clad. The conductor or wire structure itself has the same advantages as the structure according to FIG. 4, namely the advantage of simplicity and that of inexpensive manufacture, but is not at risk with regard to the coupling in of motor faults.

Claims (9)

1. Active windscreen aerial for motor vehicles for the reception of VHF and long/medium-wave radio signals, which comprises the metallic frame (2) of the windscreen (1) of the motor vehicle, and an aerial wire structure mounted on or in the windscreen (1) and which consists of at least two substantially rectilinear conductors connected to each other, of which at least one extends parallel to the metallic frame (2), and which comprises an aerial amplifier (5) having two input terminals (3,8) and an output lead (6), and in which the aerial amplifier (5) is mounted in the region of the metallic frame (2), and the conductor structure connected to the aerial amplifier (5) and the first input terminal (3) of the aerial amplifiers (5) and the second input terminal (8) of the aerial amplifier (5) are connected to the conductive frame (2) which surrounds the windscreen, and the output lead (6) of the aerial amplifier (5) is connected to the receiver, wherein the aerial amplifier (5) has separate transmission paths (14,13) for the long/medium-wave band and the VHF band and the transmission path of the long/medium-wave band (14) comprises a transmission quadripole which has at its input a long/medium-wave amplifier with a high-resistance input impedance in the long/medium-wave band, and the input impedance of the aerial amplifier (5) is high-resistance in 5 the long/medium-wave band, characterised in that the aerial wire structure consists of an aerial wire (4a, 4b) and is connected at one end to the first input terminal (3) of the aerial amplifier and the part (4a) of the aerial wire extending parallel to the metallic frame (2) at a first spacing (A) is bent at a kink point (12) at the intersection with the vertical line of symmetry (9) of the frame (2) and in its extension part (4b) along the line of symmetry (9) extends at least over a quarter of the depth of the windscreen (1) up to a second spacing (11) from the opposing part of the metallic frame (2), wherein the first spacing (A) and the second spacing (11) are dimensioned such that for the long/medium-wave band and the VHF band such power ratios exist at the input terminal (3) of the aerial amplifier that good reception properties are present both for horizontally and also for circularly polarised VHF received fields and in the vertically polarised long/medium-wave band.
2. Active windscreen aerial according to claim 1, characterised in that the end of the conductor part (4b) extending along the line of symmetry (9) terminates at the second spacing (11), for which at one end of the aerial wire (4a) the available signal power in the VHF band with circular polarised reception fields is a maximum.
3. Active windscreen aerial according to claim 1 or 2, characterised in that the conductor part (4b) extending along the line of symmetry (9) extends from the kink point (12) substantially up to the opposing horizontal part of the metallic frame (2) and said second spacing (11) is at least 2 mm.
4. Active windscreen aerial according to one of claims 1 to 3, characterised in that the first spacing (A) is chosen to be in the range greater than 1 cm and less than 7 cm.
5. Active windscreen aerial according to one of claims 1 to 4, characterised in that the aerial amplifier (5) and the one end of the aerial wire are mounted in the region of the one lower corner of the metallic frame (2) and the kink point (12) is located adjacent to the upper horizontal part of the metallic frame (2) (Fig. 2).
6. Active windscreen aerial according to one of claims 1 to 4, characterised in that the aerial amplifier (5) and the one end of the aerial wire are mounted adjacent to an upper corner of the metallic frame (2) and the kink point (12) is located adjacent to the upper horizontal part of the metallic frame (2) (Fig. 3).
7. Active windscreen aerial according to one of claims 1 to. 4, characterised in that the aerial amplifier (5) and the one end of the aerial wire are mounted in the region of a lower corner of the metallic frame (2) and the kink point (12) is located adjacent to the lower horizontal part of the metallic frame (2) (Fig. 4).
8. Active windscreen aerial according to one of claims 1 to 7, characterised in that the transmission path (13) for the VHF band includes at least one amplifying active tripole, wherein the first active tripole is noise-matched to the impedance of the aerial wire at the connection end (4a) with the help of a low-loss network connected in advance of it.
9. Active windscreen aerial according to one of claims 1 to 7, characterised in that the transmission path (13) for the VHF band consists of a low-loss matching network and there is power matching between the one end of the aerial wire (4a) and the output lead (6) of the aerial amplifier (5).

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