Abstract:
In a general aspect, a mobile communications apparatus can include a housing and inductive coupling communication circuitry configured to communicate in peer-to-peer mode with a like apparatus. The inductive coupling communication circuitry can include a communication controller and an inductive antenna. The inductive antenna can have at least one loop arranged in series between a first connection point and a second connection point and be proximate to a back wall of the housing. The inductive antenna can be, when the mobile communications apparatus is placed back-to-back in longitudinal and latitudinal alignment with a like mobile communications device, asymmetric with a like inductive antenna of the like mobile communications device, where a ratio of a sum of respective surface areas of mutually opposite surfaces of loops of a same rank of the inductive antenna divided by a sum of respective surface areas of the like inductive antenna is 0.8 or less.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/792,317, filed Mar. 11, 2013, which is a continuation of, and claims priority to International Application No. PCT/FR2011/000562, filed Oct. 19, 2011. U.S. patent application Ser. No. 13/792,317 and International Patent Application No. PCT/FR2011/000562 both claim priority benefit to French Patent Application No. 10 04096, filed Oct. 19, 2010. The disclosures of U.S. patent application Ser. No. 13/792,317, International Patent Application No. PCT/FR2011/000562 and French Patent Application No. 10 04096 are all are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the present invention relate to an apparatus including inductive coupling communication circuitry configured to communicate in peer-to-peer mode with an identical apparatus, the communication circuitry including a communication controller and a planar antenna coil tuned to a working frequency, linked to the controller by way of two connection points and including N loops arranged in series between the two connection points. 
         [0003]    Embodiments of the present invention also relate to a method of limiting a frequency detuning between first and second identical planar antenna coils turned to a working frequency when the antenna coils are arranged opposite each other at a distance less than 10 millimeters. 
         [0004]    Embodiments of the present invention also relate to a method for establishing a communication by inductive coupling between two identical apparatuses, each including inductive coupling communication circuitry configured to communicate in peer-to-peer mode. 
         [0005]    Embodiments of the present invention relate in a general manner to inductive coupling communication techniques also known as “NFC” (Near Field Communications). 
         [0006]    A communication by inductive coupling generally uses a passive NFC device and an active NFC device, each equipped with an antenna coil tuned to a working frequency, for example 13.56 MHz. The active device emits a magnetic field oscillating at the working frequency, and sends data to the passive device by modulating the magnetic field. The passive device receives a voltage induced by the magnetic field, which it demodulates to extract the data contained therein, and sends data to the active device by charge modulation. 
         [0007]    Apparatuses equipped with NFC devices, in particular NFC mobile telephones, generally have three operation modes: a reader mode, a card emulation mode, and a peer-to-peer mode. Embodiments of the present invention relate particularly to communications in peer-to-peer mode between two identical apparatuses. 
         [0008]    During a communication in peer-to-peer mode, two apparatuses are arranged opposite each other, one being in active mode and emitting the magnetic field, the other being in passive mode and receiving the magnetic field in the form of an induced voltage in its antenna coil. 
         [0009]    The conception of the antenna coil should generally take into consideration standards and constraints applicable to contactless cards, of those applicable to contactless card readers, as well as constraints specific to the peer-to-peer operating mode. 
         [0010]    The standard ISO/IEC 7810 specifies a rectangular format “ID-1” for contactless cards used mainly in the transportation and payment sectors. 
         [0011]    For the reading of contactless cards, the format of the reader and of its antenna is not standardized. However, the magnetic field produced by the reader&#39;s antenna coil must respect certain limits and its performances are verified during the certification of the reader. For applications in the transportation sector, the certification tests are based on the standard ISO/IEC 10373-6, whereas the standard “EMV contactless protocol 2.0” is generally applied in the payment sector. 
         [0012]    The antenna coil of an NFC apparatus is generally planar and extends proximate to the internal face of a wall of the housing of the apparatus, for example its back wall. A communication in peer-to-peer mode thus requires placing the walls, proximate to the antenna coils, of the two devices mutually opposite each other. 
         [0013]    In such a configuration, the antenna coils of the two apparatuses may be closely juxtaposed, their respective loops being mutually opposite. As the back walls of the housings are generally thin, they may be very close to one another, generally at a distance less than 10 millimeters. 
         [0014]    Embodiments of the present invention include the observation that when a small distance separates two identical antenna coils placed opposite each other, a strong inductive coupling results, which detunes the frequency of each antenna coil and causes its Q factor to drop. This has the consequence of greatly reducing the amplitude of both the signal transmitted by the emitting antenna coil and the voltage received by the receiving antenna coil. Unexpectedly, these losses may be such that they prevent a communication between the two apparatuses, despite the fact that they are very close to one another. 
         [0015]    As an example, the transmission losses were evaluated between two identical antenna coils opposite each other, each antenna including two imbricated planar loops extending along a rectangular outline of 31 millimeters (mm) in length and of 51 mm in width, the planes of the antenna coils being separated by a distance of 5 mm then by 1 mm. With a separation distance of 5 mm, the maximum transmission is obtained at a frequency of around 14 MHz instead of at 13.56 MHz, which reveals a slight detuning caused by the coupling, and the transmission losses are on the order of −6 dB, signifying that only 50% of the magnetic field is transmitted. 
         [0016]    With a separation distance of 1 mm, the antenna coils are completely detuned with a low Q factor and the maximum transmission occurring at around 23 MHz. Thus, bringing the two antenna coils closer together leads in this case to a large shift of their tuning frequency, here from 13.56 MHz to 23 Mhz. Consequently, since the antenna coils are no longer tuned to the working frequency of 13.56 MHz, the transmission losses at 13.56 MHz greatly increase, and are on the order of −10 dB: only 32% of the magnetic field is transmitted. 
         [0017]    It may therefore be desired to improve the quality of a communication by inductive coupling between two identical planar antenna coils turned to a working frequency, in particular when the antenna coils are arranged opposite each other at a distance less than 10 millimeters. 
         [0018]    The patent application US 2004/0256468 discloses antenna coils integrated within an NFC telephone destined to read contactless cards and deprived of the peer-to-peer communication mode. The antenna coils shown in the  FIGS. 6, 7A, and 8  of this document present a symmetric structure relative to a longitudinal Z-axis of the telephone ( FIG. 11 ), and an asymmetrical structure relative to an X-axis perpendicular to the Z-axis. The antenna coils shown in the  FIGS. 9 and 10  of this document include four loops and present an asymmetrical form relative to the longitudinal Z-axis, such that a contactless card can be read even when it is arranged at a point distant from the center of the antenna coil, for example above a keypad  71  of the telephone ( FIG. 11 ). 
       SUMMARY OF THE INVENTION 
       [0019]    Embodiments of the invention relate to an apparatus including inductive coupling communication circuitry configured to communicate in peer-to-peer mode with an identical apparatus, the communication circuitry including a communication controller and a planar antenna coil tuned to a working frequency, linked to the controller by way of two connection points and including N loops arranged in series between the two connection points. The antenna coil presents, relative to a longitudinal median or a transversal median axis of the antenna coil, an asymmetry corresponding to a coverage rate less than or equal to 0.6 if it includes 4 or more loops, less than or equal to 0.7 if it includes 3 loops, or less than or equal to 0.8 if it includes 1 or 2 loops. The coverage rate is equal to the sum of mutually opposite surfaces of loops of the same rank divided by the sum of surfaces of loops of the antenna coil, when the antenna coil is arranged in maximum mutual opposition with an identical antenna coil. 
         [0020]    According to one embodiment, the number N of loops of the antenna coil arranged in series between the two connection points is at most equal to 3. 
         [0021]    According to one embodiment, the antenna coil presents a coverage rate less than or equal to 0.7, irrespective of the number of loops it has. 
         [0022]    According to one embodiment, the antenna coil includes at least one loop having a zero coverage rate. 
         [0023]    According to one embodiment, the antenna coil includes at least one composite loop including two juxtaposed loops not having any common surface. 
         [0024]    According to one embodiment, the antenna coil includes at least one supplementary tuning loop arranged between one of the two connection points and at least a third connection point, a supplementary tuning loop not intervening in the determination of the coverage rate of the antenna coil. 
         [0025]    According to one embodiment, the antenna coil is arranged proximate one wall of the apparatus presenting a longitudinal median axis, and the antenna coil presents a longitudinal median axis that is the same as the longitudinal median axis of the wall. 
         [0026]    According to one embodiment, the antenna coil is arranged proximate one wall of the apparatus, in a plane situated at less than 5 millimeters from an external face of the wall. 
         [0027]    Embodiments of the invention also relate to a method of limiting frequency detuning between first and second identical planar antenna coils turned to a working frequency, when the antenna coils are arranged opposite each other at a distance less than 10 millimeters, each antenna coil including N loops arranged in series between two connection points of the antenna coil. The method includes conferring to each antenna coil, relative to a longitudinal median or a transversal median axis of the antenna coil, an asymmetry corresponding to a coverage rate less than or equal to 0.6 if it includes 4 or more loops, less than or equal to 0.7 if it includes 3 loops, or less than or equal to 0.8 if it includes 1 or 2 loops, wherein the coverage rate is equal to the sum of mutually opposite surfaces of loops of the same rank divided by the sum of surfaces of loops of the antenna coil, when the antenna coils are in maximum mutual opposition. 
         [0028]    According to one embodiment, the method includes a step of choosing a number N of loops arranged in series between the two connection points that is at most equal to 3. 
         [0029]    According to one embodiment, the method includes a step of conferring to each antenna coil a coverage rate less than or equal to 0.7, irrespective of the number of loops it has. 
         [0030]    According to one embodiment, the method includes a step of providing in each antenna coil at least one loop having a zero coverage rate. 
         [0031]    According to one embodiment, the method includes a step of providing in each antenna coil at least one composite loop including two juxtaposed loops not having any common surface. 
         [0032]    According to one embodiment, the method includes a step of providing in each antenna coil at least one supplementary tuning loop arranged between one of the two connection points and at least a third connection point, and not taking into consideration the supplementary tuning loop in the determination of the coverage rate of the antenna coil. 
         [0033]    Embodiments of the invention also relate to a method for establishing an inductive coupling communication between two identical apparatuses each including inductive coupling communication circuitry configured to communicate in peer-to-peer mode, the communication circuitry including a communication controller and a planar antenna coil tuned to a working frequency, linked to the controller by way of two connection points and including N loops arranged in series between the two connection points, including a step of placing walls of the two apparatuses mutually opposite such that the antenna coils are arranged opposite each other at a distance less than 10 millimeters. The method further includes conferring to each antenna coil, relative to a longitudinal median or a transversal median axis of the antenna coil, an asymmetry corresponding to a coverage rate less than or equal to 0.6 if it includes 4 or more loops, less than or equal to 0.7 if it includes 3 loops, or less than or equal to 0.8 if it includes 1 or 2 loops, the coverage rate being equal to the sum of mutually opposite surfaces of loops of the same rank divided by the sum of surfaces of loops of the antenna coil, when the antenna coils are in maximum mutual opposition. 
         [0034]    According to one embodiment, the method includes a step of choosing a number N of loops arranged in series between the two connection points that is at most equal to 3. 
         [0035]    According to one embodiment, the method includes a step of conferring to each antenna coil a coverage rate less than or equal to 0.7, irrespective of the number of loops it has. 
         [0036]    According to one embodiment, the method includes a step of providing in each antenna coil at least one loop having a zero coverage rate. 
         [0037]    According to one embodiment, the method includes a step of providing in each antenna coil at least one composite loop including two juxtaposed loops not having any common surface. 
         [0038]    According to one embodiment, the method includes a step of providing in each antenna coil at least one supplementary tuning loop arranged between one of the two connection points and at least one third connection point, and not taking into consideration the supplementary tuning loop in the determination of the coverage rate of the antenna coil. 
         [0039]    According to one embodiment, the method includes conferring to the antenna coil a form presenting a longitudinal median axis, and arranging the antenna coil proximate a wall of the apparatus presenting a longitudinal median axis situated at mid-distance between the lateral edges of the wall, such that the longitudinal median axis of the antenna coil is the same as the longitudinal median axis of the wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0041]      FIG. 1  schematically shows a portable apparatus including NFC communication circuitry, 
           [0042]      FIG. 2  schematically shows a cross-sectional view of the apparatus of  FIG. 1  placed back to back with an identical apparatus, 
           [0043]      FIG. 3  schematically shows a top view of an embodiment of an antenna coil according to the invention, 
           [0044]      FIG. 4  shows two antenna coils identical to that of  FIG. 3 , arranged in maximum mutual opposition, 
           [0045]      FIG. 5  schematically shows a top view of another embodiment of an antenna coil according to the invention, 
           [0046]      FIG. 6  shows two antenna coils identical to that of  FIG. 5 , arranged in maximum mutual opposition, 
           [0047]      FIGS. 7 to 10  schematically show top views of other embodiments of antenna coils according to the invention, 
           [0048]      FIG. 11  schematically shows a top view of a variation of the antenna coil of  FIG. 3 , and 
           [0049]      FIG. 12  is an electrical diagram of an antenna circuit including the antenna coil of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]      FIG. 1  shows a portable apparatus DV, such as a mobile telephone. The apparatus DV includes, arranged in a housing  1 , an application device D 1  and a communication device D 2 . The devices D 1 , D 2  are linked by a communication interface  5 , for example a data bus. 
         [0051]    The application device D 1  includes for example a radio communication circuit GSMCT and a processor PROC. The circuit GSMCT is linked to a UHF antenna  16  and is configured to ensure the establishment of a data link with a telecommunication network. The processor PROC is for example a baseband processor. 
         [0052]    The device D 2  is an NFC communication device including an NFC controller, designated NFCCT, equipped with an antenna circuit ACT. The antenna circuit ACT includes an antenna coil AC and components CX (such as capacitors) allowing the antenna coil AC to be tuned to a working frequency, for example 13.56 MHz. The controller NFCCT is configured to ensure the establishment of communications by inductive coupling between the apparatus DV and an external NFC component (not shown), by way of the antenna coil AC. The external component may be a contactless card when the device D 2  is operating in reader mode, a contactless card reader when the device D 2  is operating in card emulation mode, or an apparatus DV′ identical to the apparatus DV when the device DV is operating in peer-to-peer mode. The processor PROC of the application device D 1  may be configured as a host processor of the controller NFCCT, to manage applications in reader mode or in peer-to-peer mode. 
         [0053]      FIG. 2  schematically shows a cross-sectional view of the apparatus DV. The antenna coil AC is also shown in cross-section and includes a planar conductor  10  arranged on an isolating support  11  and proximate to a wall  2  of the housing  1 , for example its back wall. 
         [0054]      FIG. 2  also shows another apparatus DV′ identical to the apparatus DV. The apparatus DV′ includes an antenna coil AC′ identical to the antenna coil AC and including a conductor  10 ′ on an isolating support  11 ′ and arranged proximate to a wall  2 ′ of the housing  1 ′ of the device DV′. 
         [0055]    The configuration shown is that of a peer-to-peer communication between the two devices DV, DV′. The wall  2 ′ of the apparatus DV′ is placed against the wall  2  of the apparatus DV, the antenna coils AC, AC′ being mutually opposite and very close to each other, generally at a distance less than 10 millimeters. The antenna coils are generally stuck to the wall  2  by way of an adhesive strip. A double-sided adhesive has a thickness on the order of 30 micrometers, greatly less than a millimeter. Moreover, a wall of a mobile telephone may be locally thinned and have a thickness less than a millimeter. Thus, the minimum distance between the two antenna coils may be on the order of a millimeter. 
         [0056]    Embodiments of the invention relate to structures of the antenna coil AC decreasing the frequency detuning of each AC, AC′ during a peer-to-peer communication mode corresponding to the configuration of  FIG. 2 . It is desired to limit this frequency detuning to a range of plus or minus 20% when the antenna coils AC, AC′ are arranged in maximum mutual opposition, which in practice corresponds to the worst case scenario in terms of frequency detuning. 
         [0057]    The present invention includes the following observations: 
         [0058]    the frequency detuning increases as the number of loops of the antenna coils AC, AC′ increases, 
         [0059]    the provision of an asymmetry in the structure of the antenna coils AC, AC′ decreases the frequency detuning, 
         [0060]    the asymmetry of antenna coils AC, AC′ should be greater as the number of loops increases, in order to compensate for the impact of the number of loops on the frequency detuning. 
         [0061]    In order to quantify the necessary asymmetry and to determine the conception rules applicable to antenna coils having a different number of loops, the present invention provides a parameter termed “coverage rate”. This coverage rate is determined when the antenna coil is arranged with maximum mutual opposition with itself (that is, with another identical antenna coil), and is equal to the sum ΣFSWi of the mutually opposite surfaces FSWi of loops SWi of the same rank of the antenna coil, divided by the sum ΣSWi of the surfaces SWi of loops Wi of the antenna coil. 
         [0062]    The arrangement termed “maximum mutual opposition” is an arrangement in which the sum ΣFSWi of mutually opposite surfaces FSWi of loops SWi of the same rank of the antenna coil is at a maximum. This arrangement is generally obtained when the longitudinal median axis and the transversal median axis of two identical antenna coils are aligned (i.e. superimposed). 
         [0063]    The present invention also includes the discovery that in order to limit the frequency detuning of the antenna coil AC to an acceptable range of values, for example on the order of plus or minus 20%, when it is arranged in maximum mutual opposition with an identical antenna coil AC′, the antenna coil AC should have, relative to its longitudinal median or transversal median axis, an asymmetry corresponding to a coverage rate less than or equal to 0.6 if it includes 4 or more loops, less than or equal to 0.7 if it includes 3 loops, or less than or equal to 0.8 if it includes 1 or 2 loops. 
         [0064]    It will clearly appear by the following that the axis along which an antenna coil AC presents an asymmetry must be the axis along which it is placed mutually opposite the antenna coil AC′ during a peer-to-peer application. This axis is generally the longitudinal median axis of the antenna coil if it is parallel to or the same as the longitudinal median axis of the housing  1 , because the apparatuses DV, DV′ are generally arranged in the same direction when they are placed in peer-to-peer mode. If the users are requested to orient the apparatuses in inverted directions during a peer-to-peer communication (that is, top to bottom), the asymmetry will thus be provided relative to an axis of the antenna coil that is transversal to the longitudinal median axis of the wall  2  of the housing. In order to simplify the terminology, in the following, the term “longitudinal median axis” of an antenna coil designates the axis of the antenna coil that is parallel to or the same as the longitudinal median axis of the wall of the housing including the antenna coil, including the hypothesis where the geometric longitudinal median axis of the antenna coil is transversal to this axis (the case of an antenna coil that does not extend over the entire length of the wall and presents a rectangular form transversal thereto). 
         [0065]    Embodiments AC 1 , AC 2 , AC 3 , AC 4 , AC 5 , AC 6  of the antenna coil AC will now be described. 
         [0066]    The antenna coil AC 1  shown in  FIG. 3  includes a first loop W 1  of an essentially rectangular form (the figure shows sharp angles that could be rounded in practice) and a second loop W 2  also of rectangular form, extending inside the loop W 1 . The loops W 1 , W 2  are formed by the conductor  10  on the isolating support  11 . The extremities of the conductor  10  are connected to connection terminals or contacts C 1 , C 2  linked to the controller NFCCT (Cf.  FIG. 1 ). The conductor  10  presents here a crossing or overlapping zone  12  where it passes above itself by way of an isolating layer (not shown). In other embodiments, the overlapping zone could be provided by another conductor, buried in the isolating support  11 . 
         [0067]    In order to maximize the surface of the antenna coil AC and to increase the communication distance of the device DV in reader mode or in card emulation mode, the loop W 1  here occupies the majority of the surface of the wall  2 . The loop W 1  thus presents a longitudinal median axis AW 1  that is the same as a longitudinal median axis A 2  of the wall  2  (the axis A 2  extending at mid-distance between the edges of the wall  2 ). The axis AW 1  also forms the global longitudinal median axis A 1  of the antenna coil AC 1 , that is to say an axis situated at mid-distance between the extreme lateral edges of the antenna coil. In  FIG. 3 , “L 1 ” designates the width of the loop W 1 . The axes AW 1 , A 1  are thus at a distance equal to L 1 /2 of the two edges of the loop W 1 . 
         [0068]    The loop W 2  has a width L 2  less than L 1  and presents, at mid-distance from its edges (L 2 /2), a longitudinal median axis AW 2  that is shifted relative to the longitudinal median axis A 1 , AW 1 . More precisely, the left edge of the loop W 2  extends here at a distance L 3  of the longitudinal median axis A 1 , AW 1  representing 5% of its width L 2 . 
         [0069]    When the antenna coil AC 1  is arranged in maximum mutual opposition with an identical antenna coil AC 1 ′, as shown in  FIG. 4 , the mutually opposite surface FSW 2  of the loop W 2  of the antenna coil AC 1  and of a loop W 2 ′ of the same rank of the antenna coil AC 1 ′ is equal to 1/10 of the surface SW 2  of the loop W 2 . In this arrangement, the mutually opposite surface FSW 1  of the loop W 1  of the antenna coil AC 1  and of a loop W 1 ′ of the same rank of the antenna coil AC 1 ′ is equal to the surface SW 1  of the loop W 1  because the loop W 1  is perfectly symmetrical. 
         [0070]    The coverage rate may thus be calculated by considering that the surface SW 1  of the loop W 1  is equal to 1 (the reference surface being set to one) and by considering, by way of example, that the surface SW 2  of the loop W 2  is equal to half that of the loop W 1 . It follows that: 
         [0071]    the surface SW 1  of the loop W 1 =1 
         [0072]    the surface SW 2  of the loop W 2 =0.5 
         [0073]    the surface FSW 1  of the loop W 1  opposite the loop W 1 ′=1 
         [0074]    the surface FSW 2  of the loop W 2  opposite the loop W 2 ′= 
         [0075]    0.1*SW 2 =0.1*0.5=0.05 
         [0076]    ΣFSWi=1+0.05=1.05 
         [0077]    ΣSWi=1.5 
         [0078]    The coverage rate is thus equal to: 
         [0079]    ΣFSWi/ΣSWi=1.05/1.5=0.7 
         [0080]    In one embodiment, the distance L 3  is equal to 0. In this case, the area covered by the loop W 2  does not include a portion to the left of the longitudinal median axis A 1  of the antenna coil and the loop presents an individual coverage rate equal to zero (complete asymmetry). The antenna coil AC 1  thus presents a coverage rate equal to 1/1.5, thus 0.66. In another embodiment, the loop W 2  presents an individual coverage rate equal to 0 and a surface equal to 0.45 times the surface of the loop W 1 . In this case, the antenna coil AC 1  presents a coverage rate equal to 1/1.45, thus 0.69. 
         [0081]      FIG. 5  shows an antenna coil AC 2  including a single loop W 3 , of asymmetric form. The loop W 3  presents a longitudinal median axis AW 3  that forms the global longitudinal median axis A 1  of the coil, and is the same as the longitudinal median axis A 2  of the wall  2 . The loop W 3  presents straight portions S 1  of the conductor  10  that are parallel to the axis AW 3  and straight portions S 2 , S 3 , S 4  of the conductor  10  that follow directions X 2 , X 3 , X 4  having angles a 2 , a 3 , a 4  included between 10° and 80° relative to the longitudinal median axis AW 3 . 
         [0082]    As shown in  FIG. 6 , the form of the loop is here determined so that it presents a coverage rate less than 0.7: 
         [0083]    the surface SW 3  of the loop W 3 =1 
         [0084]    (the surface being set to one), 
         [0085]    the surface FSW 3  of the loop W 3  opposite a loop W 3 ′ of an antenna coil AC 2 ′ identical to AC 2 =0.7, 
         [0086]    ΣFSWi=FSW 3 =0.7 
         [0087]    ΣSWi=SW 3 =1 
         [0088]    ΣFSWi/ΣSWi=0.7/1=0.7 
         [0089]      FIG. 7  shows an antenna coil AC 3  including two loops W 4 , W 5 , the loop W 5  extending inside the loop W 4 . The conductor  10  presents a crossing point  12  situated at the lower part of the loops, opposite the contacts C 1 , C 2 . The loops W 4 , W 5  have essentially the same form as the loop W 3  previously described, and each presents an individual coverage rate less than 0.7. It follows that the coverage rate of the antenna coil AC 3  considered in its entirety is also less than 0.7. 
         [0090]      FIG. 8  shows an antenna coil AC 4  including a large loop W 6  of essentially rectangular form, inside of which a composite loop W 7  including two loops W 7   a , W 7   b  juxtaposed and separate (i.e. not having a common surface) that are also of essentially rectangular form. The antenna coil AC 4  presents a longitudinal median axis A 1  corresponding to the longitudinal median axis AW 6  of the loop W 6  and coinciding here with the longitudinal median axis of the wall  2  of the apparatus. The conductor  10  forming the antenna coil presents two crossing zones  12 ,  12 ′ near the loops W 7   a , W 7   b.    
         [0091]    The loops W 7   a , W 7   b  are juxtaposed such that the resulting composite loop W 7  presents an individual coverage rate equal to 0. More particularly, the loop W 7   a  presents a surface less than or equal to a forth of the surface of the loop W 6  and is arranged in the upper right quadrant of the loop W 6 . The loop W 7   b  presents a surface less than or equal to a forth of the surface of the loop W 6  and is arranged in the lower left quadrant of the loop W 6 . 
         [0092]    Thus, when the antenna coil AC 4  is arranged in maximum mutual opposition with an identical antenna coil AC 4 ′, the loop W 7   a  is not opposite corresponding loops W 7   a ′, W 7   b ′ of the antenna coil AC 4 ′ and the loop W 7   b  is not opposite corresponding loops W 7   a ′, W 7   b ′ of the antenna coil AC 4 ′, the composite loop W 7  thus presenting a coverage rate equal to 0. If the surface of each loop W 7   a , W 7   b  is equal to ¼ of the surface of the loop W 6 , the coverage rate of the antenna coil AC 4  is equal to (1+0+0)/(1+0.25+0.25), thus 1/1.5=0.66. 
         [0093]      FIG. 9  shows an antenna coil AC 5  including an essentially rectangular large loop W 8  and two birectangular loops, the loop W 9  extending inside the loop W 8  and the loop W 10  inside the loop W 9 . The loop W 8  presents a longitudinal median axis AW 8  forming the longitudinal median axis A 1  of the antenna coil AC 5 , which is here the same as the longitudinal median axis A 2  of the wall  2  of the housing. The loops W 9 , W 10  each follow a path corresponding to the perimeter of a form including two rectangles R 1 , R 2  arranged next to each other and on opposite sides of the longitudinal median axis A 1 . The rectangle R 2  presents the same width as the rectangle R 1  and a length equal to a quarter of the length of the rectangle R 1 . Because of this, the individual coverage rate of each loop W 9 , W 10  is equal to twice the surface of R 2  divided by the surface of R 1  plus the surface of R 2 , that is 2*1*0.25/(1+0.25), considering that the surface of the rectangle R 1  is equal to 1, thus a coverage rate equal to 0.4. 
         [0094]    By disregarding the surface of the conductor  10  forming the loops and the distance between the loops W 8  to W 10 , and by considering that the loop W 8  presents a surface of 1, that the rectangle R 1  presents a surface of 0.5, and that the rectangle R 2  presents a surface of 0.25, the coverage rate of the antenna coil AC 5  is calculated in the following manner: 
         [0095]    the surface SW 8  of the loop W 8 =1 
         [0096]    (the surface being set to one), 
         [0097]    the surface SW 9  of the loop W 9 =0.5+0.25*0.5=0.625 
         [0098]    the surface SW 10  of the loop W 10 =0.5+0.25*0.5=0.625 
         [0099]    the surface FSW 8  of the loop W 8  opposite a loop W 8 ′ of an antenna coil AC 5 ′ identical to AC 5 =1 
         [0100]    the surface FSW 9  of the loop W 9  opposite a loop W 9 ′ of the antenna coil AC 5 ′=2*0.5*0.25=0.25 
         [0101]    the surface FSW 10  of the loop W 10  opposite a loop W 10  of the antenna coil AC 5 ′=2*0.5*0.25=0.25 
         [0102]    ΣFSWi=1+0.25+0.25=1.5 
         [0103]    ΣSWi=1+0.625+0.625=2.25 
         [0104]    Thus: 
         [0105]    ΣFSWi/ΣSWi=1.5/2.25=0.66 
         [0106]      FIG. 10  shows an antenna coil AC 6  including two birectangular loops W 11 , W 12 , each following a path corresponding to the perimeter of a form including two rectangles R 1 , R 3  arranged next to each other. The loops W 11 , W 12  present a longitudinal median axis AW 11 , AW 12  that forms the longitudinal median axis A 1  of the antenna coil AC 6  and is here the same as the longitudinal median axis A 2  of the wall  2  of the housing. The rectangles R 1  and R 3  have the same width and are arranged on opposite sides of the axis A 1 . It may be supposed for example that the rectangle R 3  has a length equal to half the length of the rectangle R 1  (the diagram of  FIG. 10  merely being approximate). In this case, by considering that the surface of the rectangle R 1  is equal to 1 and the surface of the rectangle R 3  is equal to 0.5, the individual coverage rate of each loop W 11 , W 12  is equal to twice the surface of R 3  divided by the sum of the surface of R 3  and the surface of R 1 , that is (2*0.5)/1.5, thus 0.66. The loops having the same form, and by disregarding the surface of the conductor  10  and the inter-loop distance, the coverage rate of the antenna coil AC 6  is essentially equal to the individual coverage rate of each of the loops, thus 0.66. 
         [0107]    Embodiments of the present invention also apply to “multi tap” antenna coils, of the type disclosed in the international application WO 2010/066799. This type of antenna coil includes at least one supplementary tuning loop arranged between one of the two connection terminals C 1 , C 2  of the antenna coil to the controller NFCCT, and at least a third connection terminal C 3 . In this case, the supplementary tuning loop does not intervene in the determination of the coverage rate of the antenna coil but is taken into consideration for the determination of the arrangement in maximum mutual opposition, from which the coverage rate is calculated. 
         [0108]    An example of a multi-tap antenna coil AC 7  is shown in  FIG. 11 . In this example, the antenna coil AC 7  is of the same form as that of the antenna coil AC 1  of  FIG. 3  and includes a large loop W 13  of the same form as that of the loop W 1  of  FIG. 3 , and a small loop W 14  of the same form as that of the loop W 2  of  FIG. 3 . 
         [0109]    The antenna coil AC 7  differs from the antenna coil AC 1  in that the contact C 2  is arranged at the end of the loop W 14 , and in that a contact C 3  is arranged at the end of the loop W 13 . Thus, the loop W 14  extends between the contacts C 1  and C 2 , and the loop W 13  extends between the contacts C 2  and C 3 . 
         [0110]    An electrical diagram shown in  FIG. 12  shows that the loop W 14  is linked to the controller NFCCT by way of the contacts C 1 , C 2 , whereas the loop W 13  is used as a supplementary tuning loop. Thus, tuning components CX may be arranged between the contacts C 1  and C 2  and/or between the contacts C 2  and C 3 , and/or between the contacts C 1  and C 3 . 
         [0111]    In such an embodiment, the coverage rate of the antenna coil is determined relative to the loop W 14  only, with respect to an arrangement of the antenna coil in maximum mutual opposition with a similar antenna coil AC 7 ′ (Cf. for example  FIG. 4 ). The arrangement in maximum mutual opposition is however determined by taking into consideration all the loops W 13 , W 14  of the antenna coil. In conformance with the definition previously provided, the arrangement in maximum mutual opposition is thus an arrangement wherein the sum ΣFSWi of mutually opposite surfaces FSWi of loops SWi of the same rank of the antenna coil is at a maximum. Once the maximum mutual opposition has been determined, the loop W 13  is not taken into consideration for the calculation of the coverage rate of the antenna coil AC 7 , which is thus equal here to the individual coverage rate of the loop W 14 . The individual coverage rate of the loop W 14  is equal to 0.1 if the left edge of the loop W 14  extends to the left of the longitudinal median axis A 1  of the antenna coil, at a distance L 3  that is equal to 5% of the width of the loop W 14 . 
         [0112]    It will clearly appear to the skilled person that the present invention is susceptible to various alternative embodiments and applications. Embodiments of the invention apply, in a general manner, to any application in which two tuned antenna coils must be arranged mutually opposite and at a small distance from each other, when this distance is susceptible of detuning the antenna coils and of reducing the transmission of the electrical signal between the two antenna coils. 
         [0113]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.