Patent Publication Number: US-6903690-B2

Title: Internal antenna of small volume

Description:
The present invention relates to an internal antenna of small volume. 
   More precisely, the present invention relates to an. antenna which can be disposed axially inside the housing of an electronic appliance of very small thickness, the antenna including its own ground plane, or co-operating, for example, with printed circuits having metallization suitable for acting as the equivalent of a ground plane. 
   The manufacturers of mobile telephones tend to offer appliances of smaller and smaller size and also of smaller and smaller thickness. 
   In order to reduce size, so-called “internal” antennas are used, i.e. antennas which are located entirely inside the housing of the mobile telephone. 
   As internal antennas, it is possible to consider using so-called “PIFA” antennas, which are essentially constituted by a radiating element, and which must necessarily operate together with a ground plane. In order to ensure that the antenna operates well, the ground plane must be disposed at a distance of about 7 millimeters (mm) from the radiating element for the GSM frequency bands of 900 GHz to 1800 GHz. The total thickness of the antenna can be too thick for it to be usable in mobile radiotelephones of very small size. Faced with this difficulty, proposals have been made to use external antennas of very small thickness. The problem which is encountered with such antennas offset from the ground plane is that their performance is degraded if the ground plane is small. In addition, the specific absorption rate (SAR) of the electromagnetic field is high. 
   An object of the present invention is to provide an internal antenna of very small volume, the ground plane naturally being preferably that of the appliance in which the antenna is mounted. 
   According to the invention, this object is achieved by a small volume antenna comprising:
         a conductive ground plane;   a first conductive surface placed in an antenna plane substantially parallel to the ground plane and partially surrounding a portion of the antenna plane, and presenting first and second ends;   a second conductive surface forming a main radiating assembly disposed essentially in said portion of the antenna plane, said two conductive surfaces not being connected together by any conductive electrical connection;   an antenna conductor connected to said second conductive surface;   first electrical connection means for connecting a first end of the first conductive surface to a first zone of the ground plane; and   second electrical connection means for connecting said first surface at least in the vicinity of the second end of the first conductive surface to a second zone of the ground plane that is distinct from the first zone;   the assembly constituted by said first conductive surface, the portion of the ground plane electrically interconnecting the first and second zones, and the two connection means presenting an opening.       

   It will be understood that in this antenna, a first portion of the radiating element is constituted both by a first conductive surface placed in an antenna plane parallel to the ground plane and by the ground plane itself. A main radiating element constituted by a second conductive surface is disposed in the space defined by said first portion of the radiating element. This configuration can operate in highly satisfactory manner with a distance of 2 mm to 3 mm being provided between the ground plane and the antenna plane in which the first conductive surface and the major part of the second conductive surface are disposed. 
   The term “not being connected together by any conductive electrical connection” means that the only connection that might possibly exist between the two conductive surfaces consists in capacitance, self-induction, or a combination of these components. 
   The term “for connecting said first surface at least in the vicinity of the second end of the second surface to the ground plane” means that the electrical connection has one end connected to the first surface either directly at its second end or else close to its second end so that only a small portion of said first surface extends beyond the point of connection. 
   It will thus be understood that by means of the antenna of the invention, the conductive surfaces forming the radiating element can be disposed at a very small distance from the ground plane, which is naturally preferably the ground plane of the appliance in which the antenna is mounted. 
   In a first embodiment, the first and second conductive surfaces are made on a face of an insulating support or a dielectric substrate that is substantially parallel to the ground plane. 
   In a second embodiment, the first and second conductive surfaces are cut-out pieces of metal sheet which are connected to the ground plane and are mounted thereon. These portions may be mechanically connected by an adhesive tape of the high temperature Kapton type. 
   Preferably, in the second embodiment, said first and second electrical connection means are extensions of the piece of sheet forming the first conductive surface, said extensions being bent through a right angle and having ends bonded to the ground plane. 
   According to another characteristic of the invention, the opening is formed in said first conductive surface. In another variant embodiment, the opening is made in the ground plane on the electrical path interconnecting said two connection zones. 
   Also preferably, the antenna includes impedance-matching means between said first and second conductive surfaces. 
   Also preferably, the antenna has second impedance-matching means which are mounted on the assembly constituted by the first conductive surface and the portion of the ground plane interconnecting the connection zones. 
   Also preferably, the second impedance-matching means are constituted by an open-ended slot made in the ground plane. 

   
     Other characteristics and advantages of the invention appear better on reading the following description of various embodiments of the invention given as non-limiting examples. The description refers to the accompanying figures, in which: 
       FIG. 1  is a simplified view of a first embodiment of the antenna showing the principle on which the antenna is made; 
       FIG. 1A  shows a variant of the antenna shown in  FIG. 1 ; 
       FIG. 2  is a more detailed perspective view of an embodiment of the antenna when the conductive surfaces are made on a dielectric substrate; 
       FIG. 3  is a side view of the  FIG. 2  antenna; 
       FIGS. 4  to  6  show various embodiments of the second impedance-matching means on the ground plane; 
       FIG. 7  is a perspective view of an embodiment of the antenna in which the two conductive surfaces are made using cut-out portions of sheet metal; 
       FIG. 7A  is a plan view of the ground plane of the  FIG. 7  antenna; 
       FIG. 8  is a graph plotting variations in standing wave ratio (SWR) as a function of frequency for an antenna in accordance with the invention; and 
       FIG. 9  is a Smith chart for an antenna in accordance with the invention. 
   

   With reference initially to  FIG. 1 , a simplified embodiment of the antenna is described to set out the principles on which it is made. This figure shows a ground plane  10  which is preferably the ground plane of the appliance in which the antenna is mounted, particularly when the appliance is a mobile telephone. The dimensions of the ground plane may be 105 mm×35 mm. In this embodiment, the antenna includes an insulating support  12  or a dielectric substrate of thickness 0.8 mm and of dimensions 31 mm×13 mm which is held parallel to the ground plane  10  by means not shown. The distance e between the substrate or insulating support  12 , and the ground plane  10  lies in the range 2 mm to 3 mm. It can thus be seen that the complete antenna presents a volume that is very small. On the insulating support  12 , e.g. substantially rectangular in shape, there is made a first conductive surface  14 , e.g. in the form of first metallization. This first metallization  14  may follow three of the edges of the insulating support  12 , while leaving one edge  16  thereof free. More generally, the first metallization  14  surrounds a portion  12   a  of the insulating support in part. The first metallization has two ends given respective references  14   a  and  14   b  which are extended by two bent conductive tabs  18  and  20 , the free ends  18   a  and  20   a  of these tabs being bonded by a conductive material to the ground plane  10 . In this embodiment, the first conductive surface  14  has an opening  22  which is thus made on the insulating support  12 . With reference to the connection zones  24  and  26  of the conductive tabs  18  and  20 , the first conductive surface  14 , with the exception of its opening  22 , forms a closed electric circuit which is looped by the portion  30  of the ground plane that is represented in simplified manner by dashed lines in FIG.  1 . 
   The antenna has a second conductive surface  32  which, in this embodiment, is formed entirely on the top face of the insulating support  12  which is preferably a dielectric substrate made of FR4 type epoxy-impregnated fiberglass. This conductive surface  32  is made on the portion  12   a  of the insulating support which is partially surrounded by the first conductive surface. In the embodiment shown in  FIG. 1 , the second conductive surface  32  is constituted by two conductive elements  34  and  36  interconnected by a connection zone  38 . This second conductive surface  32  shown in  FIG. 1  corresponds to the case where the antenna is to have frequency passbands that are sufficient for the intended operation. The connection zone  38  is connected by a bent conductive tab  40  to a connection zone  42  of an antenna conductor  44  in such a manner that this conductive tab connects the axial conductor of the antenna cable  44  to the connection zone  38  (see FIG.  3 ). 
   Taking the above-described conductive surfaces as a whole, it can be considered that there is a first conductive assembly constituted by the first metallization  14 , by the connection tabs  18  and  20 , and by the electrical path  30  interconnecting the two connection zones. This first conductive assembly is provided with an opening  22 . In the space surrounded by the first conductive assembly as described above there is disposed the second conductive surface  32  which constitutes the main part of the radiating element of the antenna, the first conductive surface also constituting a radiating element. 
   Naturally the shielding  44   b  of the antenna cable  44  is connected to the ground plane  10 : in the zone referenced L 1  (FIG.  3 ). 
   The antenna preferably also has first impedance-matching means represented symbolically by reference  46  between the two conductive surfaces  14  and  32 . These first impedance-matching means are preferably obtained by ensuring that the distance e′ between the first conductive surface and the second conductive surface over a given length has a value that is suitable for obtaining the desired impedance. 
   The embodiment of  FIG. 1A  differs from that of  FIG. 1  only with respect to the following point: 
   The first conductive surface  14  is extended by a short conductive portion  15  which extends away from the connection point  14 ′ a  between the tab  18  and said first surface. 
   With reference now to  FIGS. 2 and 3 , there follows a description in greater detail of how an antenna of the type shown in  FIG. 1  is embodied. In this figure, there can be seen the insulating support  12 , the ground plane  10 , the first conductive surface  14  (it should be observed that it does not include the opening  22 ), and the connection tabs  18  and  20  for the first conductive element. There can also be seen the second conductive surface  32  with its two portions  34  and  36  and its connection tab  40  to the central conductor of the antenna cable  44 . 
   In the particular embodiment shown in  FIGS. 2 and 3 , it can be seen that it is possible for the second conductive surface  32  to be made not only on the top first face  12   b  of the insulating support  12 , but also by a portion  32 ′ made on the edge face of the insulating support and on its bottom face  12   d.  This disposition serves to increase the area of the second conductive surface without increasing the space occupied by the antenna. 
   This figure also shows an open-ended slot  50  in the ground plane  10  going from the non-metallized zone  52  surrounding the connection point of the antenna to the edge of the ground plane. Functionally, this slot  50  performs exactly the same role as the opening  22 . In this figure, there can also be seen a second slot  54  made in the ground plane and constituting second impedance-matching means. This slot  54  is connected to the open slot  50 . It is thus itself functionally open.  FIG. 3  shows more clearly the connection with the antenna coaxial cable  44 . In particular, there can be seen the electrical connection between the shielding  44   b  of the cable and the ground plane  10 , and the connection between the central conductor  44   a  and the connection tab  40 . The shielding of the cable  44  is connected to the ground plane  10  in the zone referenced L 1 . 
   In this embodiment, the distance  e  between the conductive surface made on the dielectric substrate  12  and the ground plane lies in the range 2.5 mm to 3 mm, the thickness of the insulating support being about 0.8 mm, and the dimensions of the insulating support substrate  12  possibly being 13 mm by 31 mm. It can thus be seen that the antenna of the invention is effectively of small thickness and also presents a volume that is very small (less than or equal to 1 cubic centimeter (cm 3 )). By means of its disposition, this antenna includes its own ground plane which, as mentioned above, is preferably the ground plane of the appliance in which the antenna is mounted. 
     FIGS. 4  to  6  show various examples of shapes  54 ′ for the slot made in the ground plane to constitute the second impedance-matching means and which are connected to the open-ended slot  50 . In  FIG. 5 , the slot  54 ′ is open-ended at both ends. 
   In these figures, L 1  represents the point of connection with the shielding of the antenna conductor  44 . 
     FIG. 7  shows a variant embodiment in which the conductive surfaces constituting the radiating assembly are made as cut-out pieces of conductive sheet metal. In this figure, there can be seen the first conductive surface referenced  14 ′ which presents a first end  14   a  connected to the ground plane by a bent tab  60  and a second end  14   b  connected to the ground plane by a second bent tab  62 . The second conductive surface is constituted by a piece of sheet metal given overall reference  32 ′ and disposed in the same plane as the sheet  14 ′. The sheet  32 ′ is cut in such a manner that the overall radiating element is tuned to the wavelengths in which the antenna is to operate. The connection zone of the antenna is connected by a conductive tab  64  to the antenna cable  44  (not shown). In order to ensure that the two pieces of sheet metal  14 ′ and  32 ′ constituting the two conductive surfaces have sufficient mechanical strength, these pieces of sheet metal are provided with mechanical support tabs such as  64 ,  66 ,  68 , and  70 . Naturally, these tabs  66 ,  68 ,  70  must not constitute electrical connections with the ground plane  10 . They are therefore bonded to the support of the ground plane in zones that do not have any metallization as can be seen more clearly in FIG.  7 A. In the example shown in  FIG. 7 , the first conductive surface constituted by the sheet  14 ′ does not have an opening as shown in FIG.  1 . This opening is again constituted by an open-ended slot  72  made in the ground plane. 
     FIG. 7A  shows the ground plane  10  in plan view to show in particular the connection zone of the tab  64  connected to the central conductor of the antenna coaxial cable, a slot  72  which is open-ended and which acts functionally as the opening  22  formed in the first conductive surface of the  FIG. 1  antenna, and a second slot  74  which does not have an open end in the periphery of the ground plane and which advantageously constitutes the second impedance-matching means. 
     FIG. 8  is a curve plotting SWR for an antenna of the invention as a function of frequency (F). This antenna corresponds more particularly to the embodiment of  FIG. 2  with the ground plane shown in FIG.  4 . 
   Mark  1  corresponds to 880 MHz (megahertz), mark  2  to 960 MHz, mark  4  to 1710 MHz, and mark  5  to 1880 MHz. It can be seen that very wide passbands are obtained in the frequency ranges used in telephony. 
     FIG. 9  is a Smith chart for the same antenna with impedance plotted in polar coordinates as a function of frequency. 
   The chart shows that in the operating frequency ranges of the antenna, impedance is close or very close to 50 ohms, and the two loops B 1  and B 2  demonstrate that there are two well-marked frequency bands.