GB2243955A

GB2243955A - Circular polarisation antenna

Info

Publication number: GB2243955A
Application number: GB9104214A
Authority: GB
Inventors: Jean Leflour
Original assignee: Valeo Securite Habitacle SAS
Current assignee: Valeo Comfort and Driving Assistance SAS
Priority date: 1990-03-02
Filing date: 1991-02-28
Publication date: 1991-11-13
Anticipated expiration: 2011-02-28
Also published as: DE4105826A1; GB2243955B; FR2659173B1; FR2659173A1; GB9104214D0

Abstract

A circular polarisation antenna for an automotive remote control driver recognition system comprises a first winding, (L1), a second winding (L2) mounted inside the first winding with the axes of these two windings nearly perpendicular, and an excitation winding (L3) coaxial with the second winding. The first and second windings are closed by tuning capacitors (C1, C2) to give phase quadrature emission. The first winding is wound on a hollow coil carrier (2) which is very short by comparison with its transverse dimensions. The second winding is wound on a ferrite bar (1) having a longitudinal dimension substantially greater than its transverse dimension and being elongated in cross section, the minor dimension of which is no greater than the longitudinal dimension of the coil carrier. The bar (1) is supported by the coil carrier (2) at its two ends. The coil carrier is annular, and the bar (1) extends over the whole length of the internal diameter of the coil carrier. <IMAGE>

Description

1 AN EXTRA FLAT CIRCULAR POLARISATION ANTENNA, IN PARTICULAR FOR A REMOTE

CONTROL SYSTEM INCORPORATED IN AN AUTOMOBILE This invention relates to a circular polarisation antenna.

It is first necessary to mention that, although in this application attention is directed mainly to an anteni-i;- intended for the emission of radio frequency waves, such an antenna may also be used for reception or, account of the principle of reciprocity, and such an application is accordingly included within the field of the present invention.

The antenna cif the present invention is, in particular, adapted for use in interactive remote control fc;i incorporation in an automotive vehicle.

In such remote control systems, which must permit recognition cif the driver, an on-board m(--)dL-ile, iriccirpc:lre-te---., in the automobile, transmits an interrogation sLgnal to &!-,. electronic identification device, which is generally in the form of a badge or a flat card carried by the driver on his person or in one of his pockets. If certain conditions artsatisfied, the card responds to the interrogation signal on receipt of the latter, by transmitting back towards the onboard module a coded identification signal which is unique to his card and therefore to his key. If this is recognised by the module on board the vehicle, the latter then carries out a certain number of actions, for example unlocking of the doors, automatically adjusting the position of the driving mirrors, the steering wheel, and so on.

It can thus be seen that there is a requirement to transmit a radio signal between an antenna incorporated in the vehicle and connected to the on-board module, and the card -1 .L carried by the driver. Since identification is carried out "on the hoof", that is to say without the conductor having to take the card out of his pocket and introduce it into a reader or similar device, it is not possible to know beforehand in what orientation the card lies at the instant of interrogation. Now, this card is flat, and may therefore lie in certain positions in which the electromaEnetic field received by the windin5z w,,,i-ii-h is incorporated into the card is weak or even non- existent, for example when the plane of the card lies perpien'--L--,-,-!;- =,-to the plane of the transmitting winding of the vehicle.

One known solution to this difficulty consists in a wave with circular instead of a. si-ple- w.znJt- which is polarised in a straight line as is the case the wave is emitted by an insulated windingi, in such a Twe,; as tc-i set up a rotating, field such that, c-,f orientation of the flat winding. in the card, there- w-51.1 always be a component cif the magnetic field is able tr produce a non-zero em',.

In order to produce such a rotating field, it is usual tremploy two windings with axes that are perpendicular to each other, supplied separately with signals of the same

Jth frequency and the same amplitude, but out of phase Pi. each other by 90 degrees. Such an antenna produces a rotating field which has very good circularity of its polarisation: from the radio-electric point of view, this enables the problem of interrogation of the driver's identification card on the hoof to be most satisfactorilly resolved.

However, from the mechanical point of view, this soluticn involves a relatively cumbersome antenna system, in, 3 the two identical windings are arranged in a cruciform pattern. This makes in awkward to instal the antenna in an automobile or to mount it in the cabin of the latter. This installation problem is made worse by the fact that, in order not to perturb the electromagnetic transmission, it is necessary to avoid having any metal components or walls situated close to the antenna.

One of the objects of the present invention is to overcome this drawback, and to propose a circular polarisation antenna for a remote control system incorporated in an automobile which, while satisfactorily providing the radioelectric functions described above, will also not be bulky, so that it can readily be mounted in the cabin of a vehicle, or in some part of the superstructure of the latter.

In particular, it will be seen that a very flat antenna can be provided in accordance with the invention, which may be easily incorporated in, for example, a rear view mirror of the automobile.

This last possibility has numerous advantages. For example, the antenna is not visible, it is situated outside the cabin and is therefore not influenced by metal components of the bodywork, and finally it is mounted within an element which includes no, or very few, metal components which would otherwise alter the transmission (since rear view mirrors are mainly made of plastics materials and glass, which are materials transparent to radio-electric waves).

To this end, the invention provides a circular polarisation antenna for a remote control system incorporated in an automotive vehicle, wherein the antenna comprises a first winding, a second winding mounted within the first winding 4 and having an axis orientated substantially but not exactly at right angles to the axis of the first winding, together with a third, excitation, winding which is connected to a signal source and which is mounted coaxially with the second winding, the first and second windings being closed by respective tuning capacitors whereby to transmit in phase quadrature, the first winding being wound on a hollow coil carrier having. a longitudinal dimension substantia-liv smaller than its transverse dimension, the sec.--InO wind-in_being wound on a magnetic core in the form of a bar the long-tudinal-I dimension of which is substantially greate-r L - than the transverse dimensions there-of. the said bar h&,zi-E an elongated cross section the minor dimension of which Js equ;31 to cir less than the longitudinal dimens-ion cf ti'le coil carrier, the said bar being Eupporte-d toy its tw- longitudinal ends in the coil carrier, and the coil arrier being annular in shape with the said bar extending, over the full length of the inner diameter of the said anni.11ar r-oil carrier.

Among features of the antenna of the invention which are preferred, though without limitation, are the (1) The cross section of the magnetic core is substantially rectangular. with its larger side disposed substantially parallel to the plane of the first winding.

(2) The coil carrier defines two parallel end faces, with the turns of the first winding being disposed between them, the second winding being disposed within -P i net the said coil carrier wholly within the space de. between the said end faces.

(23) The said bar is force-fitted into the internal 1 opening of the annular coil carrier.

(4) Retaining means for the two ends of the said bar are provided at the internal surface of the coil carrier.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a front view of an antenna in accordance with the invention; and Figure 2 is a top plan view, in cross section taken on the line II-II in Figure 1, of the same antenna.

The antenna shown in the drawings is shown in the form of a flat circular antenna, but this shape is not to be teken as limiting: it could equally well be made in a rectangular, square, polygonal or other shape while retaining the same basic structure.

The antenna essentially comprises a core 1 of ferromagnetic material (ferrite) in the form of a flat bar which is arranged along a diameter of a circular ring 2 which constitutes a coil carrier, with the longitudinal axis of the bar and that of the coil carrier being substantially perpendicular to each other. A first winding Ll is wound on to the ring 2, with the two terminals of the winding L1 being connected through a capacitor Cl, so as to constitute a first resonant circuit L1 Cl. A second winding L2 is wound on to the ferrite core 1, with the two terminals of the winding L2 being connected through a capacitor C2, s::i as to constitute a second resonant L2 C2, which is tuned to the same frequency as the first resonant circuit Ll Cl.

Finally, a third winding L3 is also wound on to the core 1. - L3 is an excitation winding, the terminals of The winding which are connected to a radio frequency source S which produces the signal to be transmitted by the antenna.

The windings L2 and L3, being wound on to the same ferrite core 1. are coupled, thus enabling a radic. frequency wave to be emitted by the winding L2 in a firs-1 direction polarisation.

By contrast, as regards the windings Li and L3, it is necessary that their respective axes should not be exact-1perpendicular to each other, for otherwise their mutual coupling would be zero. This is why, in order to enatilecoupling to take place between these windings, and consequently to permit the emission of a radio freque-n--y wave by the winding Ll in a second direction of polarisation, the axis of the ferrite core 1 is inclined very slightly with respect to the plane of the ring.;an--i thus the plane of the winding Ll), as can be seen in Figu -;j defined between these two axes, which will 2 The angle C -L be referred to as the coupling angle, will be so ad-justed as to obtain the required coupling index between the windings Ll and L3.

In practice, it has been found that it is possible to obtain entirely satisfactorily results with a coupling angle of a few degrees (typically 1 to 2 degrees), which gives a double advantage, namely that:

(a) since the Coupling angle is small, the circularity cif the polarisation remains very good, since the windings. L! and L2 have axes which remain almost perpendicular to each other, and r e 7 (b) the antenna retains its very flat shape, with the core 1 being preferably entirely contained within the envelope defined by the end faces 3 and 3' of the ring 2. The antenna thus has no projecting part.

Preferably, and again in order to preserve the very flat form of the antenna, the core 1 is chosen to be not a circular or square bar, but a bar of severely flattenei rectangular cross section, as can be seen by comparing Figures 1 and 2: or it may be of a comparable shape, fer example a circular shape which is truncated in cross section in two parallel planes equidistant from the tentra2 axis.

The calculation of the inductances of the windings Li and L2 will be conveniently carried out with a view to tot'n windings producing fields which are of substantially the same amplitude (about 10 or 20%) and which will be as precisely as possible in quadrature. The inductance of the winding L3 will be mainly determined in such a way as to optimise the adaptation of impedence as between the generator S and the antenna itself (typically with a value of 50 W.

Thus, in one example, a ferrite core is used having a cross section of S x 14 mm and a length of 66 mm, placed within a ring 8 mm thick, having an outer diameter of 80 mm, an internal diameter of 70 mm and a winding diameter of 72 mm. The winding Ll is of 40 turns, giving an inductance of 25 Q. The winding L2 consists of 100 turns, giving an inductance of S20 Q. Finally, the winding L3 has five and a half turns, giving an inductance of 2 PH. The resonant circuits Ll C1 and L2 C2 are tuned to 120 kHz by means of capacitors CI and C2 having capacitances of 6.8 nF and 3.2 nF respectively.

8 The antenna thus constructed, with circular polarisation, has a thickness of only 8 mm for an overall diameter of---n C mm. These dimensions are perfectly compatible with those of existing, conventional antennas of the reflector type, in which it would be possible readily to mount such an antenna, though such mounting of the latter is not limiting.

As regards the assembly of the components the various methods may be adopted. The bar 1 may first be force-fitted into the ring 2, this being accompanied by a slight resilient deformation of the latter. To this the ring 2 may be made in an appropriate plastics m,-:tei-iEl.

In a variant on this method, there may be provided, on inner surface of the ring 2', retaining means such as reinforcements which are adapted to the shape of the b71r and diametrically opposed to each other. Altprnatixlel-,,,, parallel ribs may be formed, between which the respe-tit.. Ifends of the bar 1 may be located.

In addition, in order to facilitate positioning of the 2 are preferat;-L,,, 1 at the angle G, the bar 1 and the ring so dimensioned that, when the bar 1 is in the correct angular position, its outwardly offset side faces are then just flush with the respective side faces of the ring 2.

The present invention is of course in no way limited to the embodiment described above and shown in the drawings, but embraces any variant or modification within the spirit of the invention that would be seen to be practicable by the person in the art. In particular, although as stated above, an antenna in accordance with the present inventic.n may be mounted in a driving -L - mirror, any other location Jof course possible. Such locations include mountinE. within - L.

9 1 the head lining, in door frames, in the rear bumper, or within the bodywork.

Claims

1. A circular polarisation antenna for a remote eciritrt--31 system incorporated in an automotive vehicle, wherein the antenna comprises a first winding, a second winding mounted within the first winding and having an axis orientated substantially but not exactly at right angles to the axis of the first winding. together with a third, exc2Ltat.i--,Y-., winding which is connected to a signal source and which is mounted coaxially with the second winding, the first andl second windings being closed by respective tuninj capacitors whereby to transmit in phase quadrature, the first winding being wound on a hollow coil carrier having a longitudinal dimension substantially smaller than its transverse dimension, the second winding, being wound on a magnetic core in the form of a bar, the longitudinal dimension of which is substantially greater than the transverse dimensions thereof, the said bar having En elongated cross section the minor dimension of which is equal to or less than the longitudinal dimension of the coil carrier, the said bar being supported by its tx,.,.j longitudinal ends in the coil carrier, and the c,-il carrier being annular in shape with the said bar extending over the full length of the inner diameter of the said annular coil carrier.

2. An antenna according to Claim 1, wherein the cross section of the magnetic core is substantially rectangular, with its larger side disposed substantially parallel to the plane of the first winding.

An antenna according to any one of the preceding Claims, wherein the coil carrier defines two parallel enc, faces, with the turns of the first winding being disposed between them, the second winding being disposed within the I 11 said coil carrier wholly within the space defined between the said end faces.

4. An antenna according to any one of the preceding Claims., wherein the said bar is force-fitted into the internal aperture of the annular coil carrier.

5. An antenna according to any one of Claims 1 to 3, further including retaining means for the two ends of the said bar, at the internal surface of the coil carrier.

6. A circular polarisation antenna for a remote control system incorporated in an automotive vehicle, si-ibstaritiall---7 as herein described with reference to the accompanying drawings.

Published 1991 at The Patent Office. Concept House. Cardiff Road, Newport, Gwent NP9 1 RH. FurLher copies may he obtained from Sales Branch. Unit 6, Nine Mile Point, Cwmfelinfach, Cross Keys, Newport, NP1 7HZ. Printed by Multiplex techniques lid, St Mary Cray, Kent.