Abstract:
A small sized structure for a plurality of resonance frequency bands in a PIFA antenna system for at least two mobile frequency bands distant from each other including a ground connection and a HF power supply connection. The PIFA antenna system includes at least two antenna branches which are disposed essentially side-by-side and in parallel to each other in the form of a strip and are connected to each other at a base thereof in order to serially connect the antenna branches which extend at a predetermined distance from each other, thereby forming a slit and are provide with straight segments for producing a capacitance coupling between the branches. The ground connection is preferably arranged at the free end of one of the antenna branches, the HF power supply connection is mounted on the external edge of the branch of the PIFA antenna structure provided with the ground connection. The width of the antenna branches, the length thereof and the slit therebetween are calculated in such a way that the PIFA antenna structure is provided with two resonance frequency bands arranged at a desired distance to each other.

Description:
FIELD OF TECHNOLOGY  
       [0001]     The present disclosure relates to a PIFA antenna arrangement for at least two mobile radio frequency bands which are separated from one another, having a ground connection and an RF supply connection.  
       BACKGROUND  
       [0002]     One such PIFA antenna arrangement is known, for example, from EP 0 997 974 A1, in which two planar antenna branches are provided, for each of which a common ground connection and a common RF supply connection are provided. The two antenna branches are connected in parallel with one another, and are intended for one resonant frequency in each case. The antenna branches have considerably extended antenna surfaces in each case, so that the PIFA antenna structure requires a large amount of space overall.  
       SUMMARY  
       [0003]     Against this background, the present disclosure is based on providing a PIFA antenna structure for a plurality of resonant frequency bands, and which is designed to be space-saving.  
         [0004]     A PIFA antenna arrangement is disclosed for at least two mobile radio frequency bands, which are separated from one another, having a ground connection and an RF supply connection, in which the PIFA antenna arrangement has at least two antenna branches, which run parallel to one another (i.e., essentially alongside one another) and are previously in the form of strips and are connected to one another at a foot point in order to connect the antenna branches in series. The antenna branches run at a predetermined distance from one another in order to form a gap, the antenna branches have straight sections in order to produce capacitive coupling between the antenna branches the ground connection is preferably arranged at a free end of one of the antenna branches where the RF supply connection is arranged at the outer edge of the antenna branch of the PIFA antenna structure, at which the ground connection is provided. The widths of the antenna branches, the lengths of the antenna branches and the gap between the antenna branches are preferably of such a size that the PIFA antenna structure has two resonant frequency bands with a desired separation from one another.  
         [0005]     A structure such as this for a PIFA antenna arrangement allows a transmission characteristic and a reception characteristic to be produced for two different mobile radio frequency bands. The major parameters for setting the desired resonant frequency bands are the widths, the lengths and the width of a gap between the antenna branches. In detail, the ratio between the areas of the two antenna branches and the ratio between the widths of the two antenna branches corresponds, to a rough approximation, to the ratio between the two resonant frequency bands. The ratio between the positions of the two resonant frequency bands in a frequency spectrum can likewise be influenced by varying the width of the gap. Accordingly, a PIFA antenna structure may be optimized for specific application requirements by modification of the two ratios that have been mentioned, and the width of the gap, in which case it is possible to set not only the position of the two resonant frequency bands, but also their bandwidth.  
         [0006]     The width of one antenna branch is preferably less than 1/15 of the wavelength of the higher-frequency frequency band. This has the advantage that the antenna branch is narrow, so that the overall volume of the antenna becomes smaller. Furthermore, there is stronger coupling between the antenna branches. In addition, the ratio between the first and the second resonant frequency can be changed more easily. It is particularly preferable for the width of one antenna branch to be less than 1/20 of the wavelength of the higher-frequency frequency band.  
         [0007]     The magnitude of the distance between the ground connection and the RF supply connection should preferably be matched to one of the resonant frequencies, specifically the higher resonant frequency, in order to define the higher-frequency frequency band. In most cases, there is a fixed ratio between the distance between the ground point and the RF supply connection and the (mid-) wavelength of the higher-frequency resonant frequency band.  
         [0008]     In general, the two antenna branches run over essentially the same length to the foot point. However, it is also possible for one of the two antenna branches to have a length which differs from the length of the other antenna branch, for example by being greater or less than it. In this case, care should be taken to ensure that the inductive and capacitive coupling between the two antenna branches is in the desired orders of magnitude, which are important for the respective bandwidth of the resonant frequency bands.  
         [0009]     It is likewise possible to provide for the predetermined distance between the two antenna branches not to be constant but to have a predetermined profile in the area in which the antenna branches run alongside one another.  
         [0010]     It is also possible for the antenna branches which run alongside one another to have common bends, thus increasing the inductive coupling between the two antenna branches. A measure such as this can be adopted when the PIFA antenna structure has to be accommodated in a particularly space-saving manner, for example in the housing of a mobile telephone.  
         [0011]     Under an exemplary embodiment, PIFA antenna arrangement has two antenna branches which run substantially parallel to one another, at least in places, and disclosed are in the form of strips. The branches are connected to one another at a second foot point in order to connect the two further antenna branches in series with one another, the further antenna branches running in a predetermined distance from one another over one section in order to form a gap, the further antenna branches have straight sections in order to produce capacitive coupling between the antenna branches, and the ground connection is arranged between the antenna branches and the further antenna branches. A further supply connection is arranged at the outer edge of the antenna branches of the PIFA antenna structure, at which the ground connection is provided, and the widths of the further antenna branches, the lengths of the further antenna branches and the gap between the further antenna branches are of such a size that the PIFA antenna structure has two further resonant frequency bands with the desired separation from one another.  
         [0012]     The exemplary embodiment represents a combination of two substantially identical PIFA antenna arrangements the aforementioned structure. The extended PIFA antenna arrangement is thus able to receive and transmit in four different resonant frequency bands. To this extent, the embodiment provides a so-called “quad-band antenna structure”, which is of particular interest at the moment for the development of antenna structures which can be used for standard international mobile radio frequency ranges (GSM850, EGSM900, PCN1800 and PCS1900).  
         [0013]     It is preferable for the RF supply connection and the further RF supply connection to be arranged on opposite sides of the ground connection, and to be joined together to form a common RF supply line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The various objects, advantages and novel features of the present disclosure will be more readily apprehended from the following Detailed Description when read in conjunction with the enclosed drawings, in which:  
         [0015]      FIG. 1  illustrates a plan view of a PIFA antenna arrangement with two antenna branches, according to a first exemplary embodiment of the invention;  
         [0016]      FIG. 2  illustrates an equivalent circuit of the PIFA arrangement shown in  FIG. 1 ;  
         [0017]      FIG. 3  illustrates a schematic representation of a frequency spectrum of the PIFA arrangement shown in  FIG. 1 ;  
         [0018]      FIG. 4  illustrates a plan view of a PIFA antenna arrangement according to a second exemplary embodiment;  
         [0019]      FIG. 5  illustrates a plan view of a PIFA antenna arrangement according to a third exemplary embodiment;  
         [0020]      FIG. 6  illustrates a plan view of a PIFA antenna arrangement according to a fourth exemplary embodiment;  
         [0021]      FIG. 7  illustrates a plan view of a PIFA antenna arrangement according to a fifth exemplary embodiment;  
         [0022]      FIG. 8  is a graph illustrating a simulation result for the frequency response of the PIFA antenna arrangement shown in  FIG. 1 , optimized for the EGSM900 and Bluetooth frequency bands;  
         [0023]      FIG. 9  is a graph illustrating a simulation result for the frequency spectrum of the PIFA antenna arrangement shown in  FIG. 1 , optimized for the EGSM900 and PCN1800 frequency bands;  
         [0024]      FIG. 10  illustrates a perspective view of a PIFA antenna arrangement according to a sixth exemplary embodiment; and  
         [0025]      FIG. 11  is a graph illustrating the frequency response of the PIFA antenna arrangement shown in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION  
       [0026]      FIG. 1  illustrates a folded PIFA arrangement (F-PIFA) which is generally L-shaped for compactness reasons. The PIFA antenna arrangement has two antenna branches Z 1 , Z 2 , with the first antenna branch Z 1  having a first width W 1 , and the second antenna branch Z 2  having a second width W 2 . The two antenna branches Z 1 , Z 2  are connected in series and are connected to one another at a foot point F. In addition, they run substantially parallel to one another, and alongside one another. The PIFA antenna arrangement shown in  FIG. 1  is also characterized by the external dimensions of the antenna branch Z 1 , specifically a first length B 1  between a free end and bend point K in the L shape, and a second length B 2  between the bend point K and the foot point F.  
         [0027]     A gap SP with a width T 1 , which remains essentially constant over the lengths of the antenna branches Z 1 , Z 2 , is defined between the two antenna branches Z 1 , Z 2 .  
         [0028]     A ground connection G is provided at a free end FE of the first antenna branch Z 1 , to be precise at the outer edge of the first antenna branch Z 1 , facing away from the gap SP. An RF supply connection S for RF signals is provided on the first antenna branch Z 1 , at a distance from the ground point G. The distance between the ground point G and the RF supply connection S is optimized for one of two resonant frequencies of the PIFA antenna structure. The PIFA antenna arrangement illustrated in  FIG. 1  is arranged at a distance H 1  from a circuit board (not illustrated), on which contact is also made with the ground connection G and the RF supply connection S.  
         [0029]     The following parameters are of particular importance for the ratio between the frequency of the first resonant frequency band and of a second resonant frequency band of the PIFA antenna structure: the ratio of the areas of the first antenna branch Z 1  and of the second antenna branch Z 2 , the width T 1  of the gap SP and the distance between the ground point G and the RF supply connection S. Primarily, the three parameters mentioned above must be matched for optimization of the PIFA antenna arrangement for a desired frequency spectrum with two resonant frequency bands, and this can be carried out by a person skilled in the art, by simple experiments.  
         [0030]      FIG. 2  shows an equivalent circuit of the PIFA antenna arrangement shown in  FIG. 1 . The first antenna branch Z 1  is represented in  FIG. 2  by a first inductance L 1 , a first capacitance C 1  and a first non-reactive resistance R 1 , while the second antenna branch Z 2  is reproduced by a second inductance L 2 , a second capacitance C 2  and a second non-reactive resistance R 2 . Coupling between the first antenna branch Z 1  and the second antenna branch Z 2  is represented by a third capacitance C 3  and a third inductance L 3 . In this case, the magnitude of the third capacitance C 3  depends primarily on straight sections of the two antenna branches Z 1 , Z 2 , running alongside one another, or else on the width T 1  of the gap SP. In contrast, the inductive coupling between the two antenna branches Z 1 , Z 2 , which is represented by the third inductance L 3 , is governed by curved sections of the two antenna branches Z 1 , Z 2 , alongside one another. In the present exemplary embodiment, a first curved section occurs in the area of the bend point, while a second curved section is provided by the foot point. The inductive coupling between the two antenna branches Z 1 , Z 2  is particularly strongly pronounced in these two areas.  
         [0031]     Furthermore,  FIG. 2  shows the ground connection G and the RF supply connection S. A signal between these two connections is coupled to the two antenna branches Z 1 , Z 2  by means of a transformer.  
         [0032]      FIG. 3  shows a typical frequency spectrum for the PIFA antenna arrangement, as has been explained with reference to  FIG. 1 . The frequency spectrum has two resonant frequency bands, which are annotated f 1  and f 2  in FIG.  3 . The value of f 1  is governed essentially by the distance between the ground connection G and the RF supply connection S. The precise position of the resonant frequency band for the frequency f 2  depends on the ratio between the areas/widths W 1 , W 2  of the two antenna branches Z 1 , Z 2  and the width T 1  of the gap SP. For given lengths B 1 , B 2 , the area ratio between the two antenna branches Z 1 , Z 2  can thus be modified by variation of the width ratio W 1 /W 2 , in order to achieve a desired position for the second resonant frequency band for the frequency f 2 .  
         [0033]     FIGS.  4  to  7  show three modified embodiments of the PIFA antenna arrangement shown in  FIG. 1 . In the embodiment which is illustrated in  FIG. 4 , the antenna branch Z 2  has a reversal point at approximately the same level as the ground connection G. Two sections of the antenna branch Z 2  are located essentially parallel to one another from this reversal point.  
         [0034]     The difference between the PIFA antenna structure shown in  FIG. 1  and that shown in  FIG. 5  is that the antenna branches Z 1 , Z 2  are three-dimensional.  
         [0035]     Beyond the RF supply connection S, the antenna branch Z 1  has a cross section which is essentially right-angled. This also applies to the antenna branch Z 2 .  
         [0036]     The embodiment shown in  FIG. 6  of a PIFA antenna arrangement is distinguished by the two antenna branches Z 1 , Z 2  not being in the form of elongated elements, but by their width or general structure varying, starting from the foot point F. In particular, the width W 1  of the first antenna branch Z 1  as well as the width W 2  of the second antenna branch Z 2  vary, in each case from the foot point F to the opposite end of the relevant antenna branch Z 1 , Z 2 .  
         [0037]     The further embodiment of a PIFA antenna arrangement as illustrated in  FIG. 7  is a generalized example wherein the external shape of the PIFA antenna arrangement is comparatively irregular. As can be seen from  FIG. 7 , it is sufficient for the functionality of the PIFA antenna structure for the two antenna branches Z 1 , Z 2  to run approximately alongside one another and parallel to one another. The respective overall lengths of the antenna branches Z 1 , Z 2  may also differ from one another. In comparison to the PIFA antenna arrangement shown in  FIG. 1 , the PIFA antenna arrangement shown in  FIG. 7  has two curved areas for the two antenna branches Z 1 , Z 2 , thus increasing the inductive coupling between the two antenna branches Z 1 , Z 2  in comparison to the PIFA antenna arrangement shown in  FIG. 1 . The PIFA antenna arrangement shown in  FIG. 7  also has the foot point F, at which the first antenna branch Z 1 , which originates from the ground connection G, is connected to the second antenna branch Z 2 , in the form of a series circuit.  
         [0038]     Two frequency spectra (reflection spectra) of PIFA antenna arrangements will be explained with reference to  FIGS. 8 and 9  which correspond to the embodiment disclosed in  FIG. 1 . The graphs in each case show the magnitude |S 11 | as a function of the frequency in MHz.  
         [0039]     The parameters of the PIFA antenna arrangement shown in  FIG. 1  were chosen to be as follows in order to obtain the frequency spectrum shown in  FIG. 8 : 
    W 1 =W 2 =T 1 =2 mm, B 1 =36 mm, B 2 =14 mm, H 1 =6 mm.    
 
         [0041]     This means that the PIFA antenna structure has a volume of 1.58 cm3, which means a very compact structure.  
         [0042]     The choice of the parameters in the manner mentioned above results in the frequency spectrum shown in  FIG. 8 , which has pronounced resonant frequency bands both in the EGSM900 frequency range and in the Bluetooth frequency range. To this extent, the PIFA antenna structure is matched for transmission and reception of signals from the two standard mobile radio frequency ranges.  
         [0043]     The frequency spectrum shown in  FIG. 9  is likewise based on a PIFA antenna arrangement of the type shown in  FIG. 1 . The relevant parameter magnitudes are as follows: 
    W 1 =4, W 2 =T 1 =2 mm, B 1 =36 mm, B 2 =18 mm, H 1 =7 mm.    
 
         [0045]     This results in an antenna volume of 2.94 cm3, which is somewhat greater than that in the previous example. A PIFA antenna structure such as this has resonant frequency bands for the EGSM900 and PCN1800 standard mobile radio frequency ranges, as can be seen directly from  FIG. 9 .  
         [0046]     For illustrative purposes,  FIGS. 8 and 9  show the positions of the relevant standard mobile radio frequency ranges separately in the form of a dashed-dotted line or dashed line.  
         [0047]     Another exemplary embodiment of a PIFA antenna arrangement with an essentially rectangular outer edge is shown in  FIG. 10 . The PIFA antenna arrangement is designed to transmit and receive in a total of four different standard mobile radio frequency ranges. With regard to the designation of components and parameters for the PIFA antenna arrangement illustrated in  FIG. 10 , the same reference symbols are used for components and parameters with the same effect as in  FIG. 1 .  
         [0048]     Fundamentally, the PIFA antenna arrangement shown in  FIG. 10  corresponds to a combination of two PIFA antenna arrangements as shown in  FIG. 1 , with the ground connection G defining a junction point between the two PIFA antenna arrangements.  
         [0049]     The PIFA antenna arrangement illustrated in  FIG. 10  has two pairs of antenna branches, specifically a first pair Z 1 , Z 2  and a second pair Z 3 , Z 4 . In this case, the antenna branches Z 3 , Z 1  are connected to the ground connection G, with their “free ends” coinciding.  
         [0050]     The third exemplary embodiment of the PIFA antenna structure has two foot points F 1 , F 2 , which are defined as follows: the two antenna branches Z 1 , Z 2  together describe a general U-shape, whose free ends govern the positions of the foot points F 1 , F 2 . In this case, the width W 1  of the antenna branches Z 1 , Z 3  is the same. In alternative exemplary embodiments, these widths may also differ from one another.  
         [0051]     The antenna branches Z 2 , Z 4  are located in the interior of the general U-shape which is described by the antenna branches Z 1 , Z 3 . The antenna branch Z 2  runs from the foot point F 1  parallel to and alongside the antenna branch Z 1 , extends by a specific distance beyond the ground connection G, and is bent back in the final section, so that the antenna branch Z 2  is partially folded.  
         [0052]     The antenna branch Z 4  originates from the foot point F 2 , but first of all runs essentially at right angles to a straight section of the antenna branch Z 3  that is adjacent to the foot point F 2 . As soon as the antenna branch Z 4  has reached a predetermined distance from the opposite antenna branch Z 2 , it is folded back and runs alongside its initial straight section. As soon as the antenna branch Z 4  has reached a predetermined distance, specifically the width T of a gap SP 1  between the antenna branch Z 3  and the antenna branch Z 4 , it runs alongside and parallel to the antenna branch Z 3 .  
         [0053]     The antenna branches Z 2 , Z 4  have the same width W 2 . In alternative embodiments, these widths of the antenna branches Z 2 , Z 4  may also differ from one another. A PIFA antenna structure element formed by the antenna branches Z 1 , Z 2  has a gap SP 2  whose width corresponds to the width T. The gap widths between the two PIFA antenna structure elements may, of course, also be different. The widths of the respective gaps SP 1  and SP 2  are governed by sections of mutually associated antenna branches running alongside one another in parallel, such as Z 3  and Z 4 , as well as Z 1  and Z 2 .  
         [0054]     The PIFA antenna structure shown in  FIG. 10  has a common (not illustrated) RF excitation circuit, which is formed on a circuit board (not illustrated). The PIFA antenna structure is at a distance H 1  from the circuit board and has two RF supply connections S 1 , S 2 , of which the supply connection S 1  is associated with the antenna branch pair Z 1 , Z 2 , and the RF supply connection S 2  is associated with the antenna branch pair Z 3 , Z 4 . The two RF supply connections S 1 , S 2  are joined together to form a common RF supply connection S, so that the same excitation signals are available for the PIFA antenna structure at the locations defined by the RF supply connections S 1 , S 2 .  
         [0055]     With regard to capacitive and inductive coupling, the antenna branches Z 1 , Z 2 , Z 3  and Z 4  behave in a similar way to the antenna branches Z 1 , Z 2  shown in  FIG. 1 .  
         [0056]      FIG. 11  shows a frequency spectrum of the PIFA antenna structure as shown in  FIG. 10 , with predetermined values for the major parameters. These values are chosen as follows: 
        W 1 =3 mm, W 2 =2 mm, T=1 mm.          
         [0058]     The overall width of the PIFA antenna structure is 36 mm, and the overall length of the PIFA antenna structure is 24 mm. This results in an antenna volume of 6.0 cm3. The distance H 1  between the circuit board and the PIFA antenna structure is 7 mm. The spatial position of the four antenna branches (Z 1 , Z 2 , Z 3  and Z 4 ) is in each case evident from  FIG. 10 , which was discussed above.  
         [0059]     As is evident from the frequency spectrum shown in  FIG. 11 , the PIFA antenna arrangement has resonant frequency bands for the GSM850, EGSM900, PCN1800 and PCS1900 standard mobile radio frequency ranges, thus providing a so-called “quad-band” antenna. The frequency spectrum shown in  FIG. 11  is also a simulated spectrum.  
         [0060]     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.