The present invention relates to an antenna device for emitting and/or receiving electromagnetic radiation, such as radar radiation in, for example, a car radar appar¬ atus. The antenna device comprises a rotatable reflector, a feeder which is fixed relative to the antenna device and interacts with the reflector, a motor for driving the reflector and a tachometer for monitoring the rotation of the reflector and for commanding the drive motor.

In certain contexts, for example within the car radar field, there is a need for an antenna device having an antenna beam which can sweep a limited range of angle in the order of magnitude of some ten degrees, for example 20°, but which does not need to cope with large angle sectors or a complete revolution.

A conventional method for sweeping a region is to rotate both the feeder and the antenna reflector. In this way, both small and large ranges of angle can be swept. Known realizations of this method have proved however to be both costly and space-consuming and are not suitable for use in a car radar context, for example, in which the space which might be available for use is very limited and the need for a low price is extremely important.

It is also known to realize the sweeping electronically using a fixed antenna. In order to realize a sweeping of the region according to the above, as is required in a car radar context, the solutions remain complicated and expensive and limitation to a smaller sweeping range in which simpler solutions can be utilized has an adverse effect upon the functioning and reliability of the car radar.

In order to realize sweeping within a limited region, it has previously been proposed to utilize a rotatable

reflector in combination with a fixed feeder. The reflec¬ tor can in this case be driven, for example, by means of a stepping motor.

The object of the present invention is to provide an antenna device according to the principle of the preced¬ ing paragraph, having a fixed feeder and a rotatable reflector, which is simple, robust, compact and reliable, whilst at the same time demanding a low price. This is achieved by virtue of an antenna device having a drive motor for driving the reflector and a tachometer for monitoring the rotation of the reflector in the form of two interacting flag motors (voice coil motors) . The antenna device is here characterized in that the drive motor and the tachometer are constituted by so-called flag motors, i.e. motors having flat coils fitted so as to move in a magnetic field created by permanent magnets and having the coil ends directed towards the permanent magnets, the rotors of the motors being mutually motiona- lly coupled to each other by mechanical means.

It is worth pointing out that flag motors per se are known and are used, inter alia, for positioning in hard disks and CD-players. In this case, no use is made of the interaction between two motors and the field of applica¬ tion in question is totally different, having different drive and command requirements.

According to an advantageous embodiment, the rotors of the flag motors are rigidly connected and are arranged so as to rotate about a common rotary axle, which common rotary axle is arranged so as to support the reflector of the antenna device. This embodiment has advantageous characteristics in terms of, inter alia, compactness and motional stability. The rotors of the flag motors can herein advantageously be constituted by a common compo¬ nent which is designed in one piece. Furthermore, a bearing member supporting the reflector of an antenna device can be designed in one piece with the rotor of the

drive motor and/or tachometer.

According to another advantageous embodiment, the drive motor is disposed behind and the tachometer in front of the common rotary axle which supports the reflector of the antenna device. This positioning of the drive motor and tachometer enables effective use to be made of the space available in the antenna device. In order to make even more effective use of the space, according to yet another advantageous embodiment the rotor of the drive motor and the rotor of the tachometer can be fitted each in its own non-coincident, parallel plane. The drive motor is normally dimensioned to be substantially more * powerful than the tachometer and is therefore more space- consuming. The placement of the more space-consuming drive motor behind the rotary axle which supports the reflector of the antenna device, in combination with a more compact tachometer in front of the rotary axle, enables the best possible use to be made of the space in the antenna device, without the sweep of the antenna being thereby disturbed.

In order to limit the rotary motions of the rotors, according to a further embodiment the drive motor and/or the tachometer can further be provided with stop members which limit the rotary motions of the rotors in question. Preferably, the stop members contain motion-damping material, such as rubber, for example.

The invention is to be described in greater detail below with reference to appended drawings, in which:

Figure 1 shows diagrammatically, in cut side view, an example of an antenna device according to the invention, Figure 2 shows diagrammatically, in top view, the antenna device according to Figure 1, Figure 3 shows diagrammatically, in perspective view, an example of a flag motor, and

Figure 4 shows, in perspective view, an example of the shape of the rotor of a flag motor and its rotor winding.

The antenna device is described below with reference to Figures 1 and 2. The antenna device is limited by a casing comprising a bottom part 1 having side walls 2, 3, an arched front part 4 constituting the radome and sub- reflector, and a cover 5. A reflector 6 is fitted rotata- bly around an axle 7 by a bearing member 10 mounted on each side of the reflector in bearings 8, 9 of, for example, the ball-bearing type or some other suitable form of bearing. The reflector 6 is rotated about the- axle 7 by means of a drive motor comprising a fixed motor part 11 and a movable motor part or rotor 12. The rotary motion is detected by a tachometer having a fixed part 13 and a movable part or rotor 14. A fixed feeder 15 in the form of a horn is supported by a supporting element 16 fitted between the drive motor and the bearing 8. Adjoin¬ ing the supporting element, accommodation is also pro¬ vided for the microwave unit 17 of the antenna device. In the bottom part of the casing 1 there is additionally space for the electronics 18 and for cooling fins 19. The electronics can be mounted on a printed circuit card.

During operation, a radar signal in the microwave range is emitted by the feeder 15. The signal is reflected from the sub-reflector 4 back towards the reflector 6. The reflector 6 reflects the signal towards the sub-reflector 4, which this time transmits the signal owing to the fact that the polarisation of the signal has changed during the course of reflection as a result of the signal having been reflected from the sub-reflector. An incoming signal to the radar device follows the same course as the emitted signal, but in the reverse order. By rotating the reflector 6 to and fro about the rotary axle 7 using a drive motor 11, 12, the radar beam is made to sweep within a range of angle which is determined by the rotary motion of the reflector in interaction with the sub-

reflector 4. The tachometer 13, 14 monitors the rotary motions of the reflector and, via a servo (not shown) , commands the drive motor 11, 12 to work within the established sweep frequency.

The drive motor and the tachometer are designed according to the principle for flag motors. Figures 3 and 4 show the diagrammatic structure of a flag motor. The flag motor 20 contains a plane rotor 21 provided with a coil 22 on each side, preferably glued or fixed-cast in the rotor. The rotor 21 is rotatable about an axle 27. On each side of the rotor 21 there are permanent magnets 23 and 24 respectively, which yield a constant field. Arrows 25 and 26 indicate the direction of the magnetic field in different parts of the permanent magnets. Each permanent magnet may possibly be replaced by two separate magnets of opposing magnetisations. When used as a drive motor, the coil of the rotor is fed with drive voltage. By measuring the voltage across the coil of the rotor, which is a function of its speed, the flag motor can be used as a tachometer. When used as a tachometer, there is no need for magnets as strong as when used as a drive motor and the flag motor can then be made considerably smaller.

According to the embodiment described with reference to Figures 1 and 2, the rotors 12, 14 of the tachometer and drive motor are fixedly connected and constructed in a common piece together with the supporting member 10. The rotors can also, of course, be constituted by separated components which are suitably coupled together. The rotors 12, 14 and the supporting member 20 can be con¬ structed in reinforced plastic, for example polyamide plastic with glass-fibre reinforcement. Good stability and adequate dimensional tolerances are obtained. It is also possible to use die-cast metal, but then account would have to be taken of induction currents in the metal. The tachometer 13, 14 is accommodated on the front side of the reflector 6, but, because of its small bulk, does not encroach upon the region within which the radar

signal is transmitted. The drive motor 11, 12, which has a greater bulk, is accommodated on the rear side of the reflector. To ensure that the tachometer encroaches as little as possible upon the space in front of the reflec¬ tor 6, the rotor 14 of the tachometer is connected to the rotor 12 of the drive motor in such a way that the rotor 14 of the tachometer rotates in a plane closer to the bottom part 1 of the antenna device than to the rotor 12 of the drive motor.

In the known flag motor shown in Figure 3, stop members 28, 29 have been inserted in order to limit the motion of the rotor. The stop members are clad with rubber or some other suitable damping material. The primary task of the stop members is to protect the flag motors from damage in the switched-off state. During operation, the rotors of the flag motors reverse before they reach the stop members.