Abstract:
A dual band antenna device has a first conducting layer acting as resonator plane for the antenna device, a dielectric body on which said first conducting layer is provided and a second conducting layer, that is in substantial parallel with the first conducting layer, and acting as ground plane. The first conducting layer comprises two branches, and both branches will contribute to the matching of the antenna device in both hands.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a dual band antenna for a handset. Such an antenna includes a metallic plate or layer acting as ground plane for the antenna, a resonator plate or layer acting as radiating element(s), and a feeding point supplying the signal to the antenna.  
           [0002]    The applicant launched recently a new GSM dual band phone named Nokia 3210™. This phone has a dielectric antenna body covered by a metallic pattern forming two radiating elements—one for each band. The dielectric antenna body is inside the phone snapped onto a metallic shield acting as resonator plane. The antenna used in Nokia 3210™ is a PIFA (Planar Inverted F-Antennas) antenna and is described in GB 9828533.1, GB 9828364.1, and GB 9828535.6—all filed in December 1998.  
           [0003]    WO 95/24746 describes a single band internal antenna having a dielectric body coated with a metallic layer on two substantially parallel surfaces.  
           [0004]    U.S. Pat. No. 5,764,190 describes a capacity loaded PIFA according to which an extra plate is interposed in between the ground plane and the radiating element.  
           [0005]    U.S. Pat. No. 5,764,190 describes how to provide a longitudinal slit in the resonator layer in order to obtain two radiating elements. A capacitive feeding concept is used.  
           [0006]    A letter by Z. D. Lui and P. S. Hall, “Dual-Frequency Planar Inverted-F Antenna”, is published in IEEE Transactions on Antennas and Propagation, October 1997, Volume 45, Number 10. This letter describes a number of solutions—one of these having a rectangular patch for the 900 MHz band. This patch is provided with an L-shaped slot separating one quarter of the 900 MHz band for acting as resonating element in 1800 MHz band. GSM works in the 900 MHz band (uplink: 890-915 MHz (mobile to base-station), and downlink: 935-960 MHz (base-station to mobile)) and in the 1800 MHz band (uplink: 1710-1785 MHz (mobile to base-station), and downlink: 1805-1880 MHz (base-station to mobile)).  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is to provide a dual band antenna having a reduced overall size.  
           [0008]    This object is achieved by a dual band antenna device having a first conducting layer acting as resonator plane for the antenna device, a second conducting layer, that is substantial parallell with the first conducting layer, and acting as ground plane, and a dielectric body on which said first conducting layer is provided. The first conducting layer comprises two branches, and both branches will contribute to the matching of the antenna device in both hands. Hereby the full patch area may be used either for radiaing an electromagnetic field or for mating the antenna.  
           [0009]    Preferably the first one of said two branches is quarter-wave resonant in a first one of said two bands, and half-wave resonant in a second one of said two bands, while the second one of said two branches provides a resonant matching in said first one of said two bands, and will appear as a quarter-wave resonant stub in said second one of said two bands. When the antenna device is used in a GSM dual band phone the two bands will have center frequencies in approximately 920 MHz and in approximately 1800 MHz, respectively.  
           [0010]    By placing the strips of the feeding means in parallel close together the Q-value of the antenna will be reduced and hence the bandwidth of the antenna will be increased. Also this arrangement provides better flexibility for the patch layout since the feed occupies less area on the patch.  
           [0011]    According to the referred embodiment the antenna elements constituted by the branches have been folded in order to reduce the RF coupling between the two branches. This can be done by locating the open ends away from each other, as well as aligning the currents of the two at 90 degrees angle. Hereby the capacitive coupling between the open ends of the stubs (electrical field) will be reduced. Furthermore the inductive coupling between the branches where the currents are strong (close to the feed and at 1800 MHz at the middle of the 900 MHz as well) will be reduced. Locating the feed close to the edge of the PCB will also increase bandwidth.  
           [0012]    Besides minimizing the coupling voltage/voltage and current/current of the two branches, the layout distributes the currents in a large area of the patch, which is desirable. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    For a better understanding of the present invention and to understand how the same may be brought into effect reference will now be made, by way of example only, to accompanying drawings, in which:  
         [0014]    [0014]FIGS. 1 and 2 illustrates in perspective a preferred embodiment of a hand portable phone according to the invention seen from the front and rear side, respectively.  
         [0015]    [0015]FIG. 3 schematically shows the essential parts of a telephone for communication with a cellular or cordless network.  
         [0016]    [0016]FIG. 4 shows in perspective view the antenna body mounted onto a metallic inner cover of the phone shown in FIGS. 1 and 2.  
         [0017]    [0017]FIGS. 5 and 6 illustrates in perspective details of the antenna body according to the invention seen from the front and rear side, respectively.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0018]    [0018]FIGS. 1 and 2 shows a preferred embodiment of a phone according to the invention, and it will be seen that the phone, which is generally designated by  1 , comprises a user interface having a keypad  2 , a display  3 , an on/off button  4 , a speaker  5 , and a microphone  6  (only openings are shown). The phone  1  according to the preferred embodiment is adapted for communication via a cellular network, but could have been designed for a cordless network as well.  
         [0019]    According to the preferred embodiment the keypad  2  has a first group  7  of keys as alphanumeric keys, two soft keys  8 , two call handling keys  9 , and a cursor navigation key  10 . The present functionality of the soft keys  8  is shown in separate fields in the display  3  just above the keys  8 , and the call handling keys  9  are used for establishing a call or a conference call, terminating a call or rejecting an incoming call.  
         [0020]    [0020]FIG. 3 schematically shows the most important parts of a preferred embodiment of the phone, said parts being essential to the understanding of the invention. The preferred embodiment of the phone of the invention is adapted for use in connection with a GSM 900 MHz and a GSM 1800 MHz network. The processor  18  controls the communication with the network via the transmitter/receiver circuit  19  and an internal antenna  20  that will be discussed in details below.  
         [0021]    The microphone  6  transforms the user&#39;s speech into analog signals, the analog signals formed thereby are A/D converted in an A/D converter (not shown) before the speech is encoded in an audio part  14 . The encoded speech signal is transferred to the processor  18 , which i.a. supports the GSM terminal software. The processor  18  also forms the interface to the peripheral units of the apparatus, including a RAM memory  17 a and a Flash ROM memory  17   b,  a SIM card  16 , the display  3  and the keypad  2  (as well as data, power supply, etc.). The audio part  14  speech-decodes the signal, which is transferred from the processor  18  to the earpiece  5  via a D/A converter (not shown).  
         [0022]    According to the preferred embodiment of the invention the antenna is based upon the PIFA principle. In order to achieve optimum performance at two frequency bands, the GSM 900 MHz band and GSM 1800 MHz band, according to the preferred embodiment shown in FIGS. 4, 5 and  6 , the patch  24  consists of two branches  25 ,  26  connected in parallel to the feed of the antenna. One branch  26  is quarter-wave resonant at approximately 920 MHz (center of GSM 900 MHz band), the other branch  25  provides a resonant matching at approximately 1800 MHz (center of 1800 MHz band). At 1800 MHz, the 900 MHz branch  26  will basically be half-wave resonant, whereas the 1800 MHz branch  25  will appear as a quarter-wave resonant stub. However, both branches  25 ,  26  will in both bands contribute to the matching of the antenna  20 .  
         [0023]    In FIG. 4 the rear cover of the phone shown in FIGS. 1 and 2 has been removed in order to expose the internal parts of the phone. It is seen how the antenna  20  is fixed to a Printed Circuit Board  22  of the phone by means of a screw  21 . The antenna  20  is coated with metallic patches  24  constituting the radiating antenna elements, while metallic shielding cans  23  provides the ground plane of the PIFA antenna.  
         [0024]    In order to reduce the size of the antenna without sacrificing bandwidth, the patches have been folded in a specific manner. Bandwidth will benefit from reducing the RF coupling between the two branches  25 ,  26 . What is desired is to reduce the capacitive coupling between the open ends  27 ,  28  of the stubs (electrical field) and reduce the inductive coupling between the branches where the currents are strong (close to the feed  29  and at 1800 MHz at the middle of the 900 MHz as well). This can be done by locating the open ends away from each other, as well as aligning the currents of the two at 90 degrees angle. Locating the feed  29  close to the edge of the PCB will also increase bandwidth.  
         [0025]    Besides minimizing the coupling voltage/voltage and current/current of the two branches  25 ,  26 , the layout distributes the currents in a large area of the patch, which is desirable.  
         [0026]    The two branches  25 ,  26  will influence each other regarding tuning of the center frequencies. The obvious way of tuning the antenna is to increase/decrease the length of the branches, but this will not provide optimum tuning since they both affect the 900 MHz as well as the 1800 MHz frequencies. In order to simultaneously matches both bands, capacitive coupling between the two branches as well as between the first part and the end  28  of the 900 MHz branch  26  has been used. Also, the inductance along the length of the patches has been carefully tuned for achieving best bandwidth as well as centering both bands of operation. The feeding of the patch consists of two strips  29 ,  30 —one of these strips  29  is connected to the RF feed provided on the PCB  22  via a not shown standard spring connector, and the other strip  30  is connected to ground of the PCB  22 , and a screw  21  is used for ensuring a sufficient mechanical pressure. The strips  29 ,  30  have been located close together in order to reduce the Q-value of the antenna  20  and hence increase the bandwidth of the antenna. Also this arrangement provides better flexibility for the patch layout since the feed occupies less area on the patch.  
         [0027]    From FIG. 6 it is seen how the antenna  20  is provided with guide pins  30  to prevent the antenna  20  against a displacement relative to the PCB  22 . It has been verified that the antenna as claimed fulfills the requirements for type approval for a GSM 900/1800 MHz phone. This means that the antenna provides a sufficient gain in both frequency bands. The overall width W of the antenna is 36 mm, the length L of the antenna is  19  mm and the height H is 9 mm.