Abstract:
The invention relates to a dual-band planar antenna formed by at least one slot of closed shape fabricated on a printed substrate having a perimeter equal to kλ f , and two supply lines supplying power to the slot via two accesses separated by (2m+1)λ f /4, where λ f  is the guided wavelength in the slot and k and m integers greater than 0, the slot comprising means modifying the operating frequency, one of the supply lines being situated on the said means. The invention is especially applicable to antennas used in domestic wireless networks (IEEE802-11a or Hyperlan 2 standards.

Description:
[0001]     The present invention relates to a planar antenna and more especially to a dual-band planar antenna of the slot type designed for wireless networks operating in distinct frequency bands.  
       BACKGROUND OF THE INVENTION  
       [0002]     In regard to the deployment of wireless mobile networks in the domestic environment, the design of the antennas is confronted with a particular problem that results from the manner in which the various frequencies are allocated to these networks. Thus, in the case of domestic wireless networks using the IEEE802.11a or Hyperlan2 standard, two distinct frequency blocks, operating in the 5 GHz band, have been allocated to the various service providers as can be seen in the table below.  
                       TABLEAU A                       Technology   Application   Frequency bands (GHz)                   Europe BRAN/   Domestic networks   (5.15-5.35)       HYPERLAN2        (5.47-5.725)       US-IEEE 802.11a   Domestic networks   (5.15-5.35)               (5.725-5.825)                  
 
         [0003]     For this reason, in order to cover the two frequency bands, whether it be for a single standard or for two standards simultaneously, various solutions have been proposed.  
         [0004]     The most obvious solution consists in using a broadband antenna that covers, at the same time, the two frequency bands defined above. However, this type of antenna covering a broad band of frequencies generally has a complex structure and is expensive. The use of a broadband antenna also has other drawbacks such as the degradation in the performance of the receiver owing to the width of the noise band and to the scrambler capable of operating over the whole band covered by the antenna, this band also comprising the band not allocated to the specific applications in the range 5.35 GHz to 5.47 GHz.  
         [0005]     The use of a broadband antenna implies more severe filtering constraints for the transmitter in order to conform to power transmission profiling masks, namely the maximum powers allowed for transmissions both within the allocated band and outside of this band. This leads to additional losses and a higher cost for the equipment.  
         [0006]     Furthermore, in wireless networks, at any given time an antenna covers a channel having a bandwidth of around 20 MHz situated in one or the other of the two bands. An alternative solution allowing the drawbacks associated with broadband antennas to be avoided would be to use an antenna whose band of frequencies can be adjusted.  
         [0007]     Thus, planar antennas formed, as shown in  FIG. 1 , by an annular slot  1  are known and which operate at a given frequency f determined by the perimeter of the slot, this slot being supplied by a supply line. More precisely, on a substrate formed by a normal printed circuit metallized on both faces, the annular slot  1 , which can be of circular shape or of any other closed shape, is fabricated by etching of the side forming the ground plane of the antenna. The supply line  2  is provided for supplying power to the slot  1 , notably by electromagnetic coupling. This is, for example, formed by a line using microstrip technology, positioned on the opposite side of the substrate from the slot  1  and, in the embodiment shown, oriented radially with respect to the circle forming the slot.  
         [0008]     The microstrip line—annular slot transition of the antenna is arranged in a known manner such that the slot  1  is located in a short-circuit plane of the line, in other words in a region where the currents are highest. Thus, the supply line after the line-slot transition has a length of around λm/4, where λm is the guided wavelength under the microstrip line. This length can be an odd multiple of λ m /4 if the line is terminated by an open circuit, or an even multiple of λ m /4 if the line is terminated by a short circuit. Moreover, the diameter p of the slot operating in its fundamental mode is chosen in a known fashion such that p=λ f , where λ f  is the guided wavelength in the slot.  
         [0009]     Under these conditions, the distribution of the fields in the slot is as shown in  FIG. 2  with two regions of maximum field (CO) and two regions of minimum field (CC). For this reason, it is possible to place a second supply line on the slot at a short-circuit region CC without however degrading the matching at the access on the first supply line while still achieving a good isolation between the two accesses.  
         [0010]     Accordingly, the present invention uses this type of structure to obtain a dual-band antenna.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Consequently, the subject of the present invention is a dual-band planar antenna formed by at least one slot of closed shape fabricated on a printed substrate having a perimeter equal to kλ f , the said slot being supplied by two supply lines, the two lines supplying power to the slot via two accesses separated by (2m+1)λ f /4, where λ f  is the guided wavelength in the slot and k and m integers greater than 0, characterized in that the slot comprises means modifying the operating frequency, one of the supply lines being situated on the said means.  
         [0012]     According to a first embodiment, the means modifying the operating frequency are constituted by protrusions cut out from the slot. The protrusions can be placed on the inner rim of the slot or on the outer rim of the slot. They are square or rectangular in shape. The dimensions of the protrusion as a function of the two operating frequencies are given by the equation:  
         2   ×         f   2     -     f   1           f   2     +     f   1           =     A   ×         W   c     ⁢     L   c           ∏             ⁢           ⁢     R     may   .     2               
 
 where f 1  and f 2  are the central operating frequencies on each of the supply lines, W C  the width of the protrusion, L C  the length of the protrusion, R moy  the mean radius of the slot and A a multiplier coefficient. 
 
         [0013]     According to another embodiment of the present invention, the means modifying the operating frequency are formed by a symmetric gradual variation of one of the rims of the slot near the open-circuit regions or near the short-circuit regions. In this case, one of the rims can be circular and the other elliptical.  
         [0014]     According to another feature of the present invention, the supply lines are coupled with the slot according to a line-slot coupling of the Knorr type.  
         [0015]     According to yet another feature of the present invention, the supply lines are magnetically coupled with the slot according to a tangential line-slot transition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Other features and advantages of the present invention will be described below with reference to the appended drawings in which:  
         [0017]      FIG. 1 , previously described, is a schematic plan view of an antenna of the annular slot type supplied by a microstrip line, according to a line-slot transition of the Knorr type.  
         [0018]      FIG. 2  is a schematic view showing the field distribution inside the annular slot.  
         [0019]      FIG. 3  is a schematic top plan view of a first embodiment of a dual-band planar antenna according to the present invention.  
         [0020]      FIG. 4  shows the matching and isolation curves of the antenna shown in  FIG. 3 .  
         [0021]      FIGS. 5   a  and  5   b  show radiation patterns of the slot antenna according to the present invention when the supply is through the access  1  and through the access  2 , respectively.  
         [0022]      FIG. 6  is a schematic top plan view of a second embodiment of a dual-band planar antenna according to the present invention.  
         [0023]      FIG. 7  shows the matching and isolation curves of the antenna shown in  FIG. 6 .  
         [0024]      FIG. 8  shows the matching curves S 11  and S 22  as a function of frequency when the mean radius of the annular slot antenna is varied.  
         [0025]      FIG. 9  shows the matching curves S 11  and S 22  as a function of the frequency of an annular slot antenna when the dimensions of the protrusion are varied.  
         [0026]      FIG. 10  is a curve showing the difference in frequency as a function of the relative size of the protrusion.  
         [0027]      FIGS. 11   a ,  11   b ,  FIGS. 12   a ,  12   b ,  FIGS. 13   a ,  13   b ,  FIGS. 14   a ,  14   b,    FIGS. 15   a ,  15   b ,  FIGS. 16   a ,  16   b,  are respective schematic plan views and curves showing the matching and isolation as a function of the frequency of various embodiments of dual-band antennas according to the present invention.  
         [0028]      FIG. 17  and  FIG. 18  show antennas according to the present invention in which the closed shape of the slot is not circular, and  
         [0029]      FIG. 19  is a schematic view of another embodiment of the present invention in which the supply lines are tangential to the slot.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     Various embodiments of the present invention will now be described, with reference to FIGS.  3  to  19 . In these figures, in order to simplify the description, the same elements may be given the same reference numbers.  
         [0031]     FIGS.  3  to  5  relate to a first embodiment of the present invention. In this case, as shown in  FIG. 3 , the dual-band planar antenna is essentially formed by a circular annular slot  10 , fabricated in a known manner on a printed substrate. According to the present invention, protrusions  11   a ,  11   b  are introduced into the slot. In this embodiment, the protrusions  11   a ,  11   b consist of square cutouts provided on the internal perimeter of the slot  10 . The two protrusions  11   a ,  11   b  are diametrically opposed in the case of an annular slot  10  that is dimensioned so as to operate in its fundamental mode, as explained above.  
         [0032]     Furthermore, in order to be able to operate over two distinct frequency bands, the antenna according to the present invention comprises a first supply line  12   a  which crosses the annular slot  10  at equal distances from the two protrusions  11   a ,  11   b , as shown in  FIG. 3 . The coupling between the line  12   a , formed in the conventional manner using microstrip technology, is a coupling of the Knorr type in the embodiment shown. In addition, the annular slot can also be supplied by a second supply line  12   b . This second supply line  12   b  is coupled to the slot according to a Knorr-type coupling at the protrusion  11   a.    
         [0033]     For a better understanding of the present invention, a simulation of a dual-band antenna such as that shown in  FIG. 3  is produced. In this case, the following dimensions have been used:  
         [0034]     R int =6.6 mm, R ext =7 mm, R moy =6.8 mm, W s =0.4 mm, W m =0.3 mm, L m =L m ′=8.5 mm, L 50Ω =4.6 mm and W 50Ω =1.85 mm.  
         [0035]     The simulation was carried out using a commercially available electromagnetic software package (IE3D, from the company Zeland). In addition, the square protrusions are 1.29 mm on each side. The results of the simulation are presented in  FIGS. 4 and 5 .  
         [0036]      FIG. 4  shows the matching curves S 11  and S 22  when the access is through  1  for the curve  1  or when the access is through  2  for the curve  2 , respectively. Thus, it can be seen from the curves that the operation through the access  1  is lower in frequency than for a standard annular slot, namely 5.35 GHz instead of 5.625 GHz, whereas the operation through the access  2 , shown by the curve  2 , is similar to that of a standard annular slot antenna, namely 5.68 GHz instead of 5.625 GHz. In this case, a dual-band structure with closely-spaced operating bands is therefore obtained. According to the curves, it can therefore be seen that the matching bands are of about the same width, whichever access is considered, and that the isolation between the accesses is greater than −21 dB on the two matching bands, the isolation being given by the curve  3 .  
         [0037]     Furthermore, as shown in  FIGS. 5   a  and  5   b,  the radiation pattern of the dual-band planar antenna in  FIG. 3  is similar to that of a circular slot antenna,  FIG. 5   a  showing the radiation pattern when the slot is supplied through the access  1  at 5.4 GHz, whereas  FIG. 5   b  shows the radiation pattern when the slot is supplied through the access  2  at 5.6 GHz.  
         [0038]     With reference to  FIGS. 6 and 7 , a second embodiment of the present invention will now be described. In this case, the dual-band planar antenna is formed by an annular slot  20  having a circular inner rim  20   a  and an elliptical outer rim  20   b . The perturbations are therefore obtained by the resulting gradual widening of the slot.  
         [0039]     As shown in  FIG. 6 , this slot  20  is supplied by a first supply line  21 , fabricated using microstrip technology and supplying the slot  20 , according to the Knorr method, at a region of minimum field which is located between the two protrusions. This line  21  corresponds to the access  1 . In addition, the annular slot  20  is also supplied by a second supply line  22 . This supply line  22  crosses the slot  20  at the protrusions formed by the widest sections of the slot, the supply being effected by electromagnetic coupling according to the Knorr method.  
         [0040]     This structure has also been simulated using the IE3D package, with a mean radius R moy =6.8 mm. In addition, the protrusions are effected by taking a slot width of 0.4 mm at the access  1 , namely at the intersection with the supply line  21 , and a width of 0.8 mm at the access  2 , namely at the intersection with the supply line  22 . Between these two points, the width of the slot varies progressively from 0.4 mm to 0.8 mm. The results of the simulation are given by the curves in  FIG. 7 . As for the first embodiment, the operating band is different for the access  1 , giving the curve  1 , and for the access  2 , giving the curve  2 . Thus, the operating frequency is 5.39 GHz when the access  1  is supplied and 5.905 GHz when the access  2  is supplied. This second embodiment therefore allows the operating frequency through the access  1  and the operating frequency through the access  2  to be modified.  
         [0041]     With reference to  FIGS. 8, 9  and  10 , certain modifications will now be described which can be effected, notably on the embodiments in  FIGS. 3 and 6 , in order to obtain an operation in the desired frequency bands.  
         [0042]     Thus, as shown in  FIG. 8 , it can be seen that a modification in the mean radius of the initial annular slot allows the operating frequency of the two sub-bands to be modified. If the mean radius R moy  is increased, the operating frequency of the two sub-bands is reduced, as is illustrated by the curves in  FIG. 8  in which the curves in bold show the matching as a function of frequency for a mean radius R=6.8 mm, whereas the thin curves show the matching as a function of frequency for a mean radius of 7 mm.  
         [0043]     Moreover, the dimensions of the perturbation created in the slot can be reduced to obtain operating modes that are less separated in frequency, as is illustrated in  FIG. 9 . In this figure, the curves in bold represent, in the second embodiment, a widening of the slot to 0.8 mm, whereas the thin curves represent a widening of the slot to 0.6 mm.  
         [0044]     Based on the above observations, a design rule has been found for determining the dimensions of the protrusion in the case of the embodiment in  FIG. 3 . This design rule allows the size of the protrusion to be determined as a function of the difference between the two chosen operating frequencies, yielding the equation:  
         2   *         f   2     -     f   1           f   2     +     f   1           =     A   *         W   c     *     L   c           π             ⁢     *           ⁢     R   may   2                 
 
         [0045]     where f 1  and f 2  are the central operating frequencies on the access  1  and on the access  2 , respectively, W c  the width of the protrusion, L c  the length of the protrusion, R moy  the mean radius of the slot and A a multiplier coefficient.  
         [0046]     The simulations yielded the curve in  FIG. 10  which shows the frequency difference as a function of the relative size of the protrusion.  
         [0047]     Various possible variants for the dual-band planar antenna according to the invention will now be described with reference to  FIGS. 11A, 11B  to  16 A,  16 B.  
         [0048]     The figures with reference A are schematic drawings of the antenna, whereas the figures with reference B give the matching and isolation curves, namely curve  1  for the access  1 , curve  2  for the access  2  and curve  3  for isolation.  
         [0049]     In  FIG. 11A , a dual-band planar antenna according to the present invention is shown schematically, comprising a circular annular antenna  30  having two protrusions  31  provided on the outside, on the outer rim of the annular antenna  30 . In this case, the protrusions  31  are square in shape. As described with reference to  FIG. 3 , this annular slot is supplied by a first supply line  32  crossing the slot at equal distances from the two protrusions  31  and by a second supply line  33  crossing the slot at one of the protrusions  31 . The simulation results for this dual-band antenna are given in  FIG. 11B , in the case of a square protrusion on the outer rim with the dimension W c =1.29 mm.  
         [0050]      FIG. 12A  shows a dual-band planar antenna formed by a circular annular slot  40  having two rectangular protrusions  41  on the inner rim of the slot  40 . As in  FIG. 11A , this annular slot is supplied by two supply lines  42 ,  43  where, as in  FIG. 11A , one is placed equidistant from the two protrusions and the other at one of the protrusions. The simulation results for this dual-band antenna are given in  FIG. 12B .  
         [0051]      FIG. 13A  shows an annular slot  50  in the shape of a clover leaf operating in its first harmonic mode. For this reason, the slot has a perimeter p equal to 2λ f . In this case, the protrusions are obtained by a widening of the slot, as indicated by  50 A and  50 B. As in the case of the embodiment in  FIG. 6 , this slot  50  is supplied by two supply lines  51  and  52 , one of the supply lines  52  crossing the slot at its largest part, whereas the other supply line  51  crosses the slot  50  at its narrowest part. The simulation results for a dual-band antenna of this type are given in  FIG. 13B .  
         [0052]     The embodiments in  FIGS. 14A  to  16 A show a dual-band antenna formed from two concentric annular slots. The use of multiple slots allows the band to be broadened. In this case, the protrusions can be positioned on the first and the second slots for the same access or different accesses or simply on one or the other of the two slots.  
         [0053]     Accordingly, the dual-band antenna shown in  FIG. 14A  comprises two concentric annular slots  60 ,  62 . In this embodiment, the outer annular slot  60  has two rectangular protrusions  61  on its outer rim, whereas the inner circular slot  62  has two rectangular protrusions  63  on its inner rim. In this embodiment, the protrusions  61  are perpendicular to the protrusions  63 . As in the embodiment in  FIG. 3 , the annular slots are supplied by a first common supply line  64  that cuts across the two slots in the direction of the protrusions  61  and by a second common supply line  65  that cuts across the two slots in the direction of the protrusions  63 .  
         [0054]     The results of the simulation for the antenna in  FIG. 14A  are given in  FIG. 14B .  
         [0055]      FIG. 15A  shows an embodiment in which the two slots are formed by concentric circular annular slots  70  and  72 . In this case, the protrusions  71  and  73  are placed in the same plane, with the protrusions  71  positioned on the outer rim of the outer slot  70  and the protrusions  73  positioned on the inner rim of the inner slot  72 . In this case, the first supply line  74  is symmetrically positioned between the protrusions  71 ,  73 , whereas the second supply line  75  cuts across the two annular slots at the protrusions  71  and  73 .  
         [0056]     The simulation results for a slot such as is shown in  FIG. 15A  are given in  FIG. 15B .  
         [0057]     According to another embodiment shown in  FIG. 16A , the multiple slots are formed by two concentric circular annular slots  80 ,  81 . In this case, only one of the slots, namely the annular slot  81 , has rectangular protrusions on its inner rim  82 . These two slots are respectively supplied by a first supply line  83  cutting across the slots at equal distances from the two protrusions  82  and by a second supply line  84 , cutting across the slots at the protrusions  82 .  
         [0058]     The simulation results for such a dual-band antenna are given in  FIG. 16B .  
         [0059]      FIGS. 17 and 18  show other embodiments of the present invention. In this case, the slot antenna has a shape other than circular, namely a square slot in the case of  FIG. 17 . This square slot, with reference  90 , has inner protrusions  91  on two sides and is supplied, as in the case of the embodiment in  FIG. 3 , by two supply lines, namely one supply line  93  cutting across the slot  90  at one of the protrusions  91  and one supply line  92  cutting across the slot at equal distances from the two protrusions  91 .  
         [0060]      FIG. 18  shows a slot in the shape of a lozenge  100 . In this case, the outer rim of the slot is a lozenge  100 A, whereas the inner rim  100 B has a polygonal shape having a straight section at two of the corners, so as to obtain a protrusion formed by a widening of the slot. As in the case of the embodiment in  FIG. 7 , the slot is supplied by two supply lines  101  and  102 , one of the lines  102  cutting across the slot at its widened corner, whereas the other line  101  cuts across the slot at a corner equidistant from the two widened corners.  
         [0061]      FIG. 19  shows an embodiment of a dual-band antenna formed by an annular slot  110 , having two protrusions  111  on its inner rim. In this case, the annular slot is supplied through two accesses  1 ,  2 , by two supply lines  112  and  113  which create a magnetic coupling tangentially to the slot  110 , one of the supply lines being tangent to the slot at one of the protrusions  111 , whereas the other line  112  is tangent to the slot at a point equidistant from the protrusions  111 .  
         [0062]     It will be clear to those skilled in the art that the embodiments heretofore described are only presented by way of examples and can be modified in numerous ways without straying from the scope of the appended claims.