Abstract:
The present invention relates to an antenna system comprising on a substrate, at least a first and a second printed radiating elements, each supplied by a feed line, with, between the two radiating elements, at least one transmission line comprising a first extremity and a second extremity. The first and the second extremities of the transmission line are respectively coupled to the first and the second radiating elements according to a coupling function with a ratio 1:b, b&gt;1 and a phase φ, linked to the physical difference between the radiating elements, the length of the transmission line bringing a phase difference θ such that θ compensates for φ. 
     The invention applies to antennas compatible with WIFI.

Description:
[0001]    The present invention relates to a compact antenna system, more particularly an antenna system for a wireless communication device, such as multi-standard digital platforms. 
       BACKGROUND OF THE INVENTION 
       [0002]    The digital platforms on the current market offer multi-services through wireless links. Therefore, they must be capable of supporting various standards, especially the standards implemented for wireless high bit rate communications such as the IEEE802.11a, b, g standards, and now the 802.11n standard for the WIFI function. This type of wireless communication also takes place inside closed premises where, in particular, very penalizing electromagnetic wave propagation conditions are observed. To improve the system loss and the bit rate between two wireless devices, a technique known as MIMO (for ‘Multiple Output Multiple Input’) is used. This technique requires at least two antennas, a good de-correlation as well as a good isolation between the antennas. 
         [0003]    To respond to the problem of the isolation between two antennas, the solution typically used is to spatially distance the antennas from each other in order to ensure a sufficient isolation. However, this solution does not allow a compact system to be obtained. 
         [0004]    Another solution allowing the isolation between two antennas to be improved has been presented in the article by A. DIALLO, C. LUXEY, Ph. LE THUC, R. STARAJ, G. KOSSIAVAS, entitled “Enhanced two-antenna structures for universal mobile telecommunications system diversity terminal”. IET Microwaves, Antennas and Propagation, vol. 2, no 1, p. 93-101, February 2008. This solution proposes to connect two PIFA type antennas, i.e. F-inverted antennas by means of a conductive line. This suspended conductive line is directly connected to the antenna at the antenna short circuit point and can compensate for the electromagnetic coupling existing between the two antennas. This line brings a fraction of the signal from an antenna to the other, which isolates them more or less according to the length of the line. 
         [0005]    It has also been proposed to add quarter wave notches between two antennas to increase the isolation between antennas. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to a specific solution applying to slot type antennas, such as ¼ wave or ½ wave slots, annular slots, tapered slots (TSA, Vivaldi) and also to patch type antennas or other printed antennas. 
         [0007]    Therefore, the present invention relates to an antenna system comprising on a substrate, at least a first and a second printed radiating elements, each supplied by a feed line, with, between the two radiating elements, at least one transmission line comprising a first extremity and a second extremity, characterized in that the first and the second extremities of the transmission line are respectively coupled to the first and the second radiating elements according to a coupling function with a ratio 1:b, b&gt;1 and a phase φ, linked among other things to the physical difference between the radiating elements, the length of the transmission line bringing a phase difference θ such that θ compensates for φ. 
         [0008]    According to a preferential embodiment, the radiating elements are slot type antennas and the transmission line is a slot line. The radiating elements can also be patches and, in this case, the transmission line is a microstrip or a coplanar line. 
         [0009]    The coupling function is achieved by positioning a portion of the radiating element parallel to the corresponding end of the transmission line, the distance d between the parts in parallel as well as the length of the parts in parallel determining the parameters of the coupling function. 
         [0010]    Moreover, the total length of the transmission line allows the component of the complex signal coming from the other antenna to be minimized, which allows a good isolation between the two slot type radiating elements to be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Other characteristics and advantages of the present invention will emerge upon reading the description of a preferential embodiment of the present invention, this description being made with reference to the drawings attached in the appendix, in which: 
           [0012]      FIG. 1  is a diagrammatic representation of a MIMO system with two antennas explaining the principle of the present invention. 
           [0013]      FIG. 2  is a diagrammatic top representation of two slot type radiating elements to which the present invention applies. 
           [0014]      FIG. 3  shows curves giving, according to the frequency, the impedance matching of each of the antennas and the isolation between the two radiating elements. 
           [0015]      FIG. 4  is a diagrammatic top plan view of an antenna system in accordance with the present invention. 
           [0016]      FIG. 5  shows the impedance matching and isolation curves of the system of  FIG. 4  according to the frequency. 
           [0017]      FIG. 6  diagrammatically shows various embodiments of the present invention in which the distance D has been varied between the parallel parts of the transmission line and of the radiating elements. 
           [0018]      FIGS. 7   a  and  7   b  respectively show in a) the impedance matching curves according to the frequency and to the value of D and b) the isolation curves between the two radiating elements according to the distance D. 
           [0019]      FIG. 8  is a diagrammatic representation of various embodiments of the invention according to the electrical length θ of the transmission line. 
           [0020]      FIGS. 9   a  and  9   b  respectively show the impedance matching and isolation curves of the various embodiments of  FIG. 8 . 
           [0021]      FIG. 10  is a diagrammatic top plan view of an antenna system in accordance with another embodiment of the present invention. 
           [0022]      FIGS. 11   a  and  b  show the impedance matching and isolation curves according to the frequency respectively of an antenna system without a transmission line  FIG. 11   a  and as shown on  FIG. 10 ,  FIG. 11   b.    
           [0023]      FIG. 12  is a diagrammatic top plane view of an antenna system in accordance with still another embodiment of the present invention. 
           [0024]      FIGS. 13   a  and  b  show the impedance matching and isolation curves according to the frequency respectively of an antenna system without a transmission line  FIG. 13   a  and as shown on  FIG. 12 ,  FIG. 13   b.    
           [0025]      FIG. 14  is a diagrammatic top plane view of an embodiment variant of the present invention. 
           [0026]      FIG. 15  is a diagrammatic top plane view of another embodiment variant of the present invention. 
           [0027]      FIGS. 16   a  and  b  and  FIGS. 17   a  and  b  respectively show the impedance matching curves (curves a) and the isolation curves (curves b) of the embodiment of  FIG. 15  with no transmission line and with the transmission lines as shown in  FIG. 15 . 
           [0028]    To simplify the description, the same elements have the same references as the figures. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    The principle implemented in the present invention will first be explained with reference to  FIG. 1  which shows two antennas A 1  and A 2  using the MIMO technology. 
         [0030]    To benefit the most from the contribution of the MIMO technology, each antenna must transmit a signal in a propagation channel specific to it, i.e. at the antenna system level, the antennas must be decoupled and, firstly, isolated.  FIG. 1  diagrammatically shows a system with two antennas used for reception. In this case, each antenna receives a differentiated signal P, i.e. P 1  on antenna A 1  and P 2  on antenna A 2 . 
         [0031]    Since the two receiving antennas are close, they couple together according to a ratio 1:a with a&gt;1 and a phase φ related to the distance between the two antennas. As a result, antenna A 1  receives a signal P 1 +aP 2 e iφ , likewise antenna A 2  receives a signal P 2 +aP 1  e iφ . 
         [0032]    In accordance with the present invention, an element providing a coupling function is added in the actual structure of each antenna with a coupling ratio 1:b with b&gt;1. These two coupling elements are connected by a transmission line having an electrical length with a phase difference of θ. So, the adjustment of the value of θ with respect to θ allows the component of the complex signal from the other antenna to be minimized. 
         [0033]    According to an embodiment of the present invention and as shown on  FIG. 2 , the two antennas are achieved with two slot type radiating elements  1 ,  2 . Preferably, slots  1  and  2  have been etched on a metallized substrate  3 . The radiating slots, which can be quarter wave or half wave slots, have a length such that λg/4 or λg/2, λg being the guided wavelength at the operating frequency of the antenna system. To limit their size, slots  1  and  2  are folded at 90°, with their short circuited extremities facing each other. However, other structures can be envisaged without leaving the scope of the present invention, in particular linear slots. 
         [0034]    As known and as shown in  FIG. 2 , the slot type radiating elements  1  and  2  are supplied by electromagnetic coupling by a feed line respectively  4 , 5  made using microstrip technology on the substrate side opposite to the metallized side. Each microstrip line extends to an excitation port, respectively  6 ,  7 , by a line section  8 ,  9  forming an impedance transformer. In this case, the line/slot coupling can be achieved as described in the published patent application WO2006/018567 in the name of Thomson Licensing. 
         [0035]    A system such as shown in  FIG. 2  has been simulated by using the IE3D commercial software (from Zeland) based on the moments method. 
         [0036]    The electromagnetic simulations were performed by using an FR4 type substrate with the following characteristics: 
         [0037]    Permittivity=4.4. 
         [0038]    Loss tangent=0.023. 
         [0039]    Substrate thickness=1.4 mm. 
         [0040]    Metallization thickness=17.5 μm. 
         [0041]    In this case, two radiating elements  1 ,  2  consisting of quarter wave slots with a slot width of 0.3 mm were produced, the two radiating elements being distant by a length of 29.5 mm. 
         [0042]    The simulation results are given by the curves of  FIG. 3  which show the impedance matching parameters S 11  and S 22  according to the frequency of the two radiating elements and isolation S 21  according to the frequency between the two radiating elements. The curves of  FIG. 3  show an isolation of only 11.5 dB for operating frequencies of 2.4 GHz. 
         [0043]    In accordance with the present invention and as shown in  FIG. 4 , a transmission line  10  constituted by a slot line is placed between the two radiating elements  1 ,  2  to form, as explained with reference in  FIG. 1 , a coupling element with the radiating elements. 
         [0044]    More precisely, and as shown in  FIG. 4 , the two radiating elements  1 ,  2  comprise a slot portion  1   a ,  2   a  which corresponds to the part folded to 90° to limit the system size. Each extremity  10   a  of the transmission line  10  is positioned parallel to the slot portions  1   a ,  2   a  of the radiating elements  1  and  2  of the antenna system. The length L of the part  10   a  and the distance d between the element  10   a  of the transmission line and the portions respectively  1   a  and  2   a  of the radiating elements are chosen to make a coupling with each of the radiating elements as explained with reference to  FIG. 1 . 
         [0045]    Moreover, to allow its integration between the two radiating elements  1  and  2 , the transmission line  10  is curved, as shown in  FIG. 4 . The length L′ of the transmission line  10  between the two coupling elements is chosen to optimize the isolation between the two radiating elements  1  and  2  by compensating for the phase shift φ as will be explained in a more detailed manner hereafter. 
         [0046]    The structure shown in  FIG. 4  is an example of optimized configuration for the transmission slot line and for the two radiating elements in order to minimize the total size of the antenna system. This structure has been simulated like the structure of  FIG. 2 . The simulation results are shown in  FIG. 5 . 
         [0047]    It is noted that the 50 Ohm impedance matching on the two ports  6  and  7  is greater than −14 dB in the frequency band corresponding to the 802.11b, g standard, namely the 2.4 GHz band. The isolation between the two accesses is greater than 27 dB in the frequency band considered whereas, as mentioned with reference in  FIG. 2 , without the slotted transmission line, the isolation was only 11.5 dB for the same size. 
         [0048]    The influence of various parameters, such as the distance d between the ends  10   a  of the transmission line and the portions  2   a  and  1   a  of the slot type radiating elements and the length of the transmission line with respect to the desired result will be shown hereafter with reference to  FIGS. 6 to 9 . 
         [0049]      FIG. 6  allows the impact of the coupling of slot type radiating elements to the slot type transmission line to be shown by the adjustment of the distance d between the two extremities  10   a  and the portions of slots respectively  2   a,    1   a,  as shown in  FIGS. 6   a, b, c, d . In this case, the length L of the slot portion at the coupling level is fixed and is equal to 52 mm whereas D varies in steps of 0.6 mm with d=1 mm, the optimum distance. 
         [0050]      FIG. 6   a  corresponds to a distance D 1  equal to the distance d+1.2 mm.  FIG. 6   b  corresponds to D 2 =d+0.6 mm.  FIG. 6   c  corresponds to D 3 =d, optimum distance and  FIG. 6   d  corresponds to D 4 =d−0.6 mm. 
         [0051]    On  FIGS. 7   a  and  7   b , for each of the four configurations D 1 , D 2 , D 3 , D 4  above, the 50 Ohm impedance S 11  matching curve for a slot type radiating element in the 2.4 GHz band and the S 12  insulation curve between the two slot type radiating elements in the same band have been represented. 
         [0052]    These curves show that, for an impedance matching level better than −17 dB, the adjustment of the distance D allows to obtain an optimum isolation better than 17.5 dB. 
         [0053]    On  FIG. 8 , various lengths and positions for the slot type transmission line integration between the radiating elements have been shown, to show the influence of the physical length and therefore of the slot line phase coupled to the two radiating elements. The phase of the slot line between the two couplers varies from 90°+θ (L 1  configuration) to −90°+θ (L 5  configuration) in steps of 45° (L 2 , L 3 , L 4  configurations), where the value of θ is 225° at the 2.45 GHz frequency, i.e. a length of 52 mm. For the five L 1 , L 2 , L 3 , L 4 , L 5  configurations shown in  FIG. 8 , the distance between the extremities of the transmission slot line and the portions of the radiating slots is identical and equal to d=1 mm. 
         [0054]    For each of these five configurations,  FIGS. 9   a  and  9   b  show respectively the 50 Ohm impedance matching curve with access of a radiating element in the 2.4 GHz band and isolation curve between the two radiating elements in the same frequency band. These curves show that, for an impedance matching level better than −12 dB, the adjustment of the length of the slot type transmission line allows an optimum isolation better than 18 dB to be obtained. 
         [0055]    Another embodiment of the present invention will now be described with reference to  FIGS. 10 and 11 . In this case, each radiating element  20 , 21  consists of a tapered slot such as for example a Vivaldi type antenna. In a standard manner, the tapered slot is supplied by is electromagnetic coupling by a microstrip  22 , 23 . In accordance with the present invention, a transmission line  24  constituted by a slot line is provided between the two tapered slots such that the extremities  24   a  of the slot line are parallel to the tapered edge  20   a  and  21   a  of the tapered slots. In this case, the coupling function takes place after the line/slot transition, i.e. on a part of the radiating element profile. 
         [0056]    FIGS.  11 - a  and  11 - b  show respectively the parameters S of the configuration without transmission line and the configuration of  FIG. 10 . These curves show an impedance matching level better than −10 dB in the 2.4 GHz frequency band for the two configurations. So, according to the principle implemented in this configuration, the isolation between antennas, initially greater than 6 dB (FIG.  11 - a ), is improved to reach in this example a level greater than 19 dB. 
         [0057]    Yet another embodiment of the present invention will now be described with reference to  FIGS. 12 and 13 . In this case, the radiating elements are constituted by patches  30  and  31 . 
         [0000]      FIG. 12   a  shows two patches  30  and  31  of side 30 mm on a substrate FR4 with the same characteristics as above. The two patches are spaced by 4 mm from edge to edge.  FIG. 13   a  shows the parameters S of such a structure, where the two patch antennas are matched to −10 dB around 2.45 GHz. The isolation around this frequency is −9.5 dB. 
         [0058]      FIG. 12   b  shows two patches  30  and  31  in the same configuration as above. In this case, the coupling functions are placed on one of the sides  30   a  and  31   a  of the patch in order to have an electromagnetic coupling. The transmission line  32  between the two couplers C is a microstrip line, the length of which allows the isolation to be adjusted.  FIG. 13   b  shows the parameters S of such a structure, where the two antennas are matched to −10 dB around 2.45 GHz. The isolation around this frequency is 19 dB, i.e. an improvement of almost 10 dB. 
         [0059]    Other embodiments of the present invention will now be described with reference to  FIGS. 14 to 17 . 
         [0060]    On  FIG. 14 , an antenna system such as shown in  FIG. 4  is used. However, in this embodiment, a second slot type transmission line  11  is integrated in the same manner as the first transmission slot line  10  in an area such that that it is possible to make two couplers  11   a ,  10   a,    1   a  and  11   a ,  10   a,    2   a  and link them together by means of two transmission lines  10  and  11 . The length of the transmission line and the distance between each transmission line and the radiating elements are adjusted in order to reject either a frequency close to the antenna operating frequency, or a more distant frequency to reject a frequency which is undesirable for the operation of the antenna system. In the case where the transmission line is a slot line, this can be done between the line/slot transition and the short-circuit plane of the slot type radiating element  1 ,  2  or on the other side of the line/slot transition. 
         [0061]    In  FIG. 15 , another embodiment with 3 radiating elements A 10 , A 20 , A 30  has been shown; the element in the middle A 20  must be isolated from the other two elements. 
         [0062]    Hence, in comparison with  FIG. 4 , a third quarter wave slot A 30  is added as shown in  FIG. 15 . Two coupling functions (C 1 ′ and C 1 ″) are arranged on the radiating element A 20  and a coupling function (C 2  and C 3 ) on each of the other two radiating elements A 10  and A 30 . A first slot line L′ 1  links coupling functions C 1 ′ to C 2  respectively of the radiating element A 10  and the radiating element A 20 . A second slot line L′ 2  links coupling functions C 1 ′ to C 3  respectively of the radiating element A 10  and of the radiating element A 30 . The second slot line L′ 2  is integrated in the same manner as the first slot line L′ 1  in an area such that it is possible to place two couplers and link them together by means of a transmission line. 
         [0063]      FIGS. 16   a  and  16   b  show the parameters S of the configuration of  FIG. 15  but without a transmission line whereas  FIGS. 17   a  and  17   b  show the same parameters but for the configuration of  FIG. 15 . As shown in  FIGS. 17   a  and  17   b , the 50 Ohm impedance matching in the 2.4 GHz frequency band is better than 13 dB. Hence, according to the principle implemented in this configuration, the isolation between antennas, initially greater than 9 dB (FIG.  16 - a ) is improved to reach in this example a level greater than 18 dB.