CARD COMPRISING AN ANTENNA FIELD OF THE INVENTION The present invention relates to a card comprising an antenna in the form of a flat coil. The antenna allows the card to communicate with a card reader in a contactless fashion. Cards of this type can be used, for example, as bank cards, telephone cards, identity cards, access cards, etc.

BACKGROUND OF THE INVENTION A so-called contactless integrated circuit card typically comprises a card body having embedded therein an antenna connected to an integrated circuit. The antenna is typically in the form of a flat coil. The flat coil is formed by a conductive track on a support sheet.

The conductive track extends in a spiral-like fashion close to the periphery of the card. That is, the turns of the flat coil have dimensions that substantially correspond to the dimensions of the card. United States Patent number 5,337,063 describes a contactless integrated circuit card. Figure 6 of this patent publication illustrates a typical configuration of such a card.

The antenna, which is in the form of a flat coil connected to the integrated circuit, has a certain resonant frequency. The card concerned will be able to communicate with a reader that emits an electromagnetic field whose frequency is sufficiently close this resonant frequency.

SUMMARY OF THE INVENTION An object of the invention is to allow satisfactory performance at moderate cost.

To that end, a card comprising an antenna in the form of a flat coil, which comprises an outer turn having dimensions that are substantially equal to those of the

card, is characterized in that the flat coil comprises an inner turn having dimensions that are substantially smaller than those of the card.

The invention takes the following aspects into consideration. Many applications require that a card can communicate with a reader when the card is within a certain distance from the reader. Whether communication can take place or not basically depends on two factors: the strength of the electromagnetic field that the reader emits and the sensitivity of the card to that electromagnetic field. The more sensitive the card is, the less strong the electromagnetic field needs to be. In principle it holds that less strong the electromagnetic field that a reader needs to emit, the cheaper the reader is. Furthermore, an electromagnetic field of moderate strength is also desirable for reasons of health and non- interference with other electronic equipment. Thus, it is desirable that a card is relatively sensitive to the electromagnetic field that a reader emits.

The sensitivity of a card depends, amongst other things, on the layout of the antenna, which is in the form of a flat coil. The larger the turns of the flat coil are, the more sensitive the card is. Thus, in principle, it is advantageous that the turns of the flat coil are close to the periphery of the card. In other words, it is advantageous that the turns have dimensions that are substantially equal to those of the card.

The sensitivity of a card further depends on the resonant frequency of its antenna. The closer the resonant frequency is to the frequency of the electromagnetic field that the reader emits, the more sensitive the card will be. The frequency of the electromagnetic field that the reader emits will be referred to as reader frequency hereinafter.

The resonant frequency depends, amongst other things, on the number of turns. In practice, the number of turns will be an integer, 1,2, or 3, etc. The resonant

frequency changes in relatively large steps by changing the number of turns. However, there are other parameters that allow the antenna to have a resonance frequency that is sufficiently close to the reader frequency.

The resonant frequency varies as a function of the width of the conductor track that forms the flat coil.

The conductor track will be visible if it is relatively wide, for example, if it is wider than 500 micrometer.

This may be undesirable for esthetic reasons. On the other hand, the smaller the width of the conductor track is, the greater the electrical resistance of the antenna will be. The greater the electrical resistance of the antenna, the less sensitive the card will be. It is therefore desirable that the conductor is just sufficiently narrow so that it is barely visible, but not much narrower. A width comprised between 100 and 500 micrometers constitutes a suitable value.

In principle, it is possible to bring the resonant frequency sufficiently close to the reader frequency by connecting a capacitor in parallel to the flat coil. The capacitor should have a value that is comprised within a relatively narrow range. Consequently, the capacitor needs to have a relatively precise value or the capacitor needs to be adjustable. In either case, the capacitor will be relatively expensive.

A card in accordance with the invention, has an antenna in the form of a flat coil comprising an outer turn having dimensions that are substantially equal to those of the card, and comprising an inner turn having dimensions that are substantially smaller than those of the card.

The inner turn allows the resonant frequency to be sufficiently close to the reader frequency without connecting a capacitor in parallel to the flat coil. The inner turn further allows the resonant frequency to be sufficiently close to the reader frequency using a conductor track that is sufficiently narrow so that it is

barely visible or even not visible at all, but not too narrow so that the antenna has a sufficiently low electrical resistance. The fact that the inner turn has dimensions that are substantially smaller than those of the card, is, as such, not an optimal design choice with regard to sensitivity. However, any loss in sensitivity, which is due to the relatively small inner turn, is generally more than compensated by the gain in sensitivity achieved by the aforementioned aspects.

Overall, the invention allows a satisfactory sensitivity at relatively modest cost.

These and other characteristics and advantages of the invention will appear on reading the following description of a particular and non-limiting embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of a card in accordance with the invention during manufacture ; and Figure 2 is a view analogous to Figure 1 showing the card an adjustment procedure that is optional.

DETAILED DESCRIPTION The card described below is intended in conventional manner to incorporate an integrated circuit and then embedded in a further body so as to enable the integrated circuit to be powered and so as to enable it to exchange data with a reader that emits a magnetic field.

With reference to the figures, the card of the invention given overall reference 1 comprises a support sheet 2 having a face 3 on which there extends a conductor track given overall reference 4. The conductor track can be made by silkscreen printing a conductive ink on the support sheet, by being cut out and then fixed to the support sheet, or by etching a copper sheet previously deposited on the support sheet, or by any other appropriate method of manufacture. The conductor

track has a width that is preferably comprised between 100 and 500 micrometers. A width of 250 micrometers, or close thereto, is a particularly suitable value.

The conductor track 4 forms a set 5 of outer turns 6,7 together with an inner turn 8. The conductor track 4 has enlarged ends 9 forming terminals for connection to the integrated circuit.

The outer turns 6,7 are of dimensions that are substantially identical (same areas and perimeters).

The inner turn 8 has dimensions that are smaller than those of the set of turns 5. The dimensions of the inner turn 8 are determined so that the inductance of the antenna is such that the resonant frequency of the assembly comprising the integrated circuit and the antenna corresponds to a predetermined value that ensures communication can take place over a satisfactory range between the card and a reader.

With reference more particularly to Figure 1, the antenna 1 has segments 10,11 which extend on the face 3 of the support sheet 2 and which are connected electrically in parallel with a segment 12 of the inner turn 8.

With reference more particularly to Figure 2, the inductance of the antenna 1 may optionally be adjusted by disconnecting two of the parallel-connected segments, in this case the segments 10 and 12, so that only one of the parallel segments remains connected to the remainder of the antenna, in this case the segment 11. Under such circumstances, only the segment 11 is included in the inner turn 8.

The segments 10 and 12 are disconnected by being cut by punching the antenna 1 (the punched zones are referenced 13 in Figure 2). This disconnection could also be performed merely by removing a portion of the conductor track.

It is thus possible to adjust the inductance of the antenna as a function of the electrical characteristics

of the integrated circuit that is to be connected thereto firstly by modifying the dimensions of the inner turn 8 during manufacture of the antenna 1 and secondly by selecting that one of the parallel segments 10,11, and 12 which will make it possible to obtain an inner turn 8 of dimensions that are exactly matched while disconnecting the other segments.

The embodiments described hereinbefore illustrate the following basic characteristics. A card (1) comprises an antenna in the form of a flat coil. The flat coil comprises an outer turn (6) having dimensions that are substantially equal to those of the card. The flat coil further comprises an inner turn (8) having dimensions that are substantially smaller than those of the card.

Naturally, the invention is not limited to the embodiment described and variants can be applied thereto without going beyond the ambit of the invention as defined by the claims.

In particular, the antenna need have only one outer turn.

Furthermore, the antenna need not have parallel segments or the segments can be connected electrically in parallel to a segment of the innermost turn of the set of outer turns.

Although the segments are shown as being geometrically parallel to one another, they could be at angles to one another.