Abstract:
An antenna device for a handportable phone having a first conducting layer acting as resonator plane, a second conducting layer arranged substantially in parallel with the first conducting layer and acting as ground plane for the antenna device, and feeding means connected to said first and the second conducting layer for feeding an RF signal to the antenna device. The first conducting layer is provided as a radiating pattern having a common frequency part coherent with two further parts where said common frequency part in combination with said two further parts defines a first radiating element and a second radiating element, respectively. Capacitive couplings between the first and the second conducting areas adjacent to the crossing areas between said common frequency part and the respective one of said two further parts.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a dual band antenna for a handset. Such a 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. When the ground plane and the resonator plane are electrically coupled the feeding point will be placed in a position where the antenna is matched to the RF output of the handset. Such antennas are known as Planar Inverted F-Antennas (PIFA). 
     Until a few year ago all phones for cellular communication were equipped with an extendable antenna element, as known from e.g. the phone sold under the tradename Nokia 2110™. Later on this extendable antenna element were substituted by an external helix antenna, as known from e.g. the phone sold under the tradename Nokia 6110™. Recently the applicant has launched a phone sold under the tradename Nokia 8810™ and this phone includes an internal antenna based on the PIFA concept. The antenna is a so-called single band antenna and the present version it is adapted for GSM in the 900 MHz band (uplink 890-915 MHz and downlink 935-960 MHz). The antenna element will have an electric length corresponding to a quarter wavelength and placing a dielectric between the ground a resonator plane the over physical dimensions is reduced. The overall dimensions of the PIFA are reduced to 32×20×4 mm. 
     WO 95/24746 describes an internal antenna having a dielectric body coated with a metallic layer on two substantially parallel surfaces. This antenna is a single band antenna for the GSM 900 MHz band only. Basically a plastic body is molded and with metal. Afterwards a pattern is created in the metallic layer by removing parts of the coated surfaces by milling. This concept has been used in the phone marketed by Hagenuk under the tradename Global Handy™. 
     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. This requires that a two-shot molding process be used in addition to several coating processes. 
     A letter by C. R. Rowell and R. D. Murch, “A Compact PIFA suitable for dual frequency 900/1800 MHz operation”, is published in IEEE Transactions on Antennas and Propagation, April 1998, Volume 46, Number 4. This letter is written by the inventors mentioned in U.S. Pat. No. 5,764,190, and describes further improvement of the three layered antenna concept. The improvement includes providing of a longitudinal slit in the resonator layer in order to obtain two radiating elements. The RF signal is fed to the radiating elements via the intermediate plate. 
     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 a L-shaped slot separating one quarter of the 900 MHz band for acting as resonating element in 1800 MHz band. The two resonating elements are interconnected in the bottom of the slot the common feeding point is provided in this interconnection. Furthermore the two resonating elements are shortened in this interconnection by means of a number of shorting pins. Hereby the coupling between the two radiating elements is reduced. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a dual band antenna for a handset having a form that can be integrated into the per se known type of handset cabinets. 
     This purpose is obtained by an antenna device having a first conducting layer acting as resonator plane, a second conducting layer arranged substantially in parallel with the first conducting layer and acting as ground plane for the antenna device, and feeding means connected to said first and the second conducting layer for feeding an RF signal to the antenna device. The first conducting layer is provided as a radiating pattern having a common frequency part coherent with two further parts where said common frequency part in combination with said two further parts defines a first radiating element and a second radiating element, respectively. Capacitive couplings between the first and the second conducting areas adjacent to the crossing areas between said common frequency part and the respective one of said two further parts. Hereby the common frequency part carries currents from both frequency bands and thereby constitutes a part of the two radiating elements of the antenna. 
     Preferably the antenna device according to the invention has a dielectric body on which the first conducting layer is coated onto. By manufacturing the dielectric body in an injection molding process including two shots the manual handling of the antenna device is minimized during manufacture. The material used in one of the two injection molding shots is a resin repelling metal in a subsequent coating process, while the material used in the other shot is a resin to which metal in the subsequent coating process adhere. 
     The dielectric body may advantageously be provided with coupling means for establishing a releasable interconnection with a separate metal body acting as the second conducting layer of the device. 
     The feeding means may advantageously include a bore through the dielectric body as a connection via connecting the first conducting layer to a connection pad on the rear side of the dielectric body. 
     Preferably the capacitive coupling between said first and said second conducting layers is partly obtained by reducing the distance between said first conducting layer and said second conducting layer. 
     The dielectric body is provided with releasable interconnection with a separate metal body acting as the second conducting layer of the device. The metal body is provided with a stepwise raised part to establish the capacitive coupling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 schematically illustrates a preferred embodiment of a hand portable phone according to the invention. 
     FIG. 2 schematically shows the essential parts of a telephone for communication with a cellular or cordless network. 
     FIG. 3 shows in details the antenna feeding concept in cross-section. 
     FIG. 4 shows in perspective the antenna body and the metal shield of a phone prior to assembly. 
     FIG. 5 shows in plan view of the antenna body and the metal shield when assembled. 
     FIG. 6 shows in perspective view of the antenna body seen from below. 
     FIG. 7 shows in perspective view of the antenna body seen from above. 
     FIG. 8 shows a first alternative embodiment based on the antenna body shown in FIGS. 6 and 7. 
     FIG. 9 shows a second alternative embodiment based on the antenna body shown in FIGS. 6 and 7. 
     FIG. 10 shows a third alternative embodiment based on the antenna body shown in FIGS. 6 and 7. 
     FIG. 11 illustrates the two shot mold process for manufacturing the antenna according to the invention. 
     FIGS. 12 and 13 illustrates the radiation pattern for the 900 MHz band and 1800 MHz band, respectively. 
     FIGS.  14 ( a ) and ( b ) illustrates the preferred steps for performing the injection molding of the antenna according to the antenna. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 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. 
     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 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. 
     FIG. 2 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 the GSM 900 MHz and GSM 1800 MHz network, but, of course, the invention may also be applied in connection with other phone networks. The processor  18  controls the communication with the network via the transmitter/receiver circuit  19  and an antenna  20  that will be discussed in details below. 
     The microphone  6  transforms the user&#39;s speech into the analog signals formed thereby are A/D converted in an AND 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). 
     The Antenna Structure 
     As seen from FIGS. 3,  4  and  5  the preferred embodiment of the antenna device according to the invention comprises two basic parts—a shield  24  acting as ground plane for the antenna and a dielectric body  40  coated with a metallic pattern  41  where the latter acts as resonator plane. Printed Circuit Board (PCB)  21  carries a plurality of non-shown electric components, and the shield  24  is used as a per se known EMC shielding can for these components. The shield  24  is along its periphery connected to the ground of the PCB  21  and the shield  24  is therefore well suited as ground plane for the antenna. A resilient metallic leg  23  of a connector  22  is soldered onto the PCB  21  and connects a not shown RX/TX path on the PCB  21  to the antenna body part  40  via connection pin  43  having an internal metallized via  42  coherent with the metallic pattern  41 . 
     The antenna element will be positioned in the upper rear part of the phone. 
     As seen from FIG. 4 the shield  24  has a number of flanges  25  for guiding the shield relatively to the cover of the phone. The shield  24  is secured to the PCB  21  by means of screws or the like passing through the holes  30 . The shield  24  is furthermore provided with a punch out  26  for a not shown SIM card connector. The SIM card is placed against the edges of the punch out  26  and secured in this position by not shown locking means. In the top of the shield  24  there is provided a plane area  27  acting as ground plane for the antenna, and this area is also provided with an punch out  29  through which the antenna connector  22  extends. Furthermore the plane area  27  has two resilient spring tongues  28  used for improving the grounding of the antenna along one side (the topside) of the antenna body  40 . The shield  24  is formed with a step  28  reducing the distance between the ground plane and the resonator plane in this area. 
     In FIGS. 6 and 7 the antenna body  40  is shown, and from top view (FIG. 7) it is seen that the feeding point  42  of the antenna is positioned on the tip of a tongue  45  of the metallic pattern. The feeding point  42  is provided as a plated via coming from the rear side of the antenna body  40  and transferring the RF signal between the PCB  21  and the resonator elements of the antenna. A metal island  44  surrounds the via on the top of the connection pin  43 . The shape of the tip of the tongue  45  corresponds to the form of the metal island  44  in order to ease the production. 
     The tongue  45  on the metallic pattern  41  is defined in between a main slit  46  and a minor slit  47  branching from the mid one third of the main slit  46 . The length of the main slit  46 , and more in particular the circumference, has a substantial influence on the resonance frequency of the two radiating elements  48  and  49  of the antenna. The tongue  45  is coherent with the two radiating elements  48  and  49  via a common frequency part  39 . The two radiating elements  48  and  49  are according to the preferred embodiment dedicated for the GSM 900 MHz band and the 1800 MHz band, respectively. 
     The Radiating Element in the 900 MHz Band 
     The surface currents on the radiating element  48  in the 900 MHz band starts from the feeding point  42  and continues along a broad passage (common frequency part  39 ) having a rounding  52  which allows the surface current (illustrated by arrows A) to whirl easily around the bottom  53  of the slit  46 . Hereby the distribution of the surface currents becomes more evenly distributed compared to an embodiment having this passage provided with straight parallel edges. It has been observed that this rounding  52  increases the gain of the radiating element  48  in the direction normal to the radiation element. 
     At the end of the slit  46  there is provided a capacitive coupling  58  (FIG. 6) with the ground plane. This coupling  58  reduces the GSM 900 MHz resonance frequency. When the distance between the termination of the metallic layer and the ground plane is decreased, the capacitive coupling  58  is increased and thereby the GSM 900 MHz resonance frequency is lowered. 
     The circumference and thereby the length of the GSM 900 MHz resonator element  48  is determining for the GSM 900 MHz resonance frequency. This circumference of the GSM 900 MHz resonator element  48  does not affect the gain of this element. 
     The longer the tip  59  of the resonator element  48  is, the lower will the 900 MHz resonance frequency be. However the tip  59  must not come to close to the point  60  on the 1800 MHz resonator element  49  near the opening of the slit  46  because this will increase the coupling between the two radiating elements and the grounding point adjacent to the opening of the slit  46 . If the coupling to ground from the tip  59  is increased the gain of the 900 MHz resonator element  48  will become decreased. 
     It has been observed that a constant width of the slit  46  and a broad ending (the width is increased towards the end) of the 900 MHz resonator element  48  gives the highest gain figures. 
     The length and more in particular the circumference of the slit  46  has a substantial influence on the 900 MHz resonance frequency—the longer the slit  46  is, the lower will the resonance frequency be. 
     The width of the slit determines both the resonance frequency and the gain. A thinner slit  46  gives a higher 900 MHz resonance frequency (partly due to the fact that the circumference is shorter, partly due to the negative coupling of opposite currents) as well as a lower overall gain (due to the negative coupling of the currents running along the two sides of the slit  46 . 
     Normally the slit  46  will be designed for maximum gain. However it has been observed that a wide slit  46  results in a low resonance frequency and in a slightly lower gain. This might be due to the fact that the minimum width of the resonator element  48  is reduced in order to maintain the overall size of the antenna body. This will affect the ability of the resonator element  48  to guide the surface currents in an effective manner. However the width of the resonator element  48  may then be increased by letting the element  48  have an extension  68  wrapping around the smooth edge of the antenna body. This will lower the resonance frequency of the element  48  due to the increase incircumference, but the gain will be reduced, too. The gain reduction is caused by the fact that the electromagnetic field is kept inside the structure. 
     Terminating the slit  46  in a bend portion  66  as shown in FIG. 8 may increase the resonance frequency. The angle between the main portion  46  and the bend portion  66  will preferably be around 90symbol 176\f “Symbol”\s 12°. 
     Alternatively the slit  46  is continued as a downwardly extending portion  67  into the capacitive coupler  58  as shown in FIG.  9 . This will reduce the overall gain of the 900 MHz band. 
     The Radiating Element in the 1800 MHz Band 
     The surface currents on the radiating element  49  in the 1800 MHz band starts from the feeding point  42  and passes the common frequency part  39  around the end of the second slit  47 . The second slit  47  increases the bandwidth in the GSM 900 MHz band and reduces the bandwidth in the GSM 1800 MHz band. However it has been observed that the improvement of the bandwidth in the lower frequency band is higher than the bandwidth reduction in the higher frequency band. It is believed that this is due to the fact that the surface currents has to run in a quite diffuse way—see the arrows B in FIG.  7 —and thus resulting in paths having different lengths, which causes the resonator element  49  to resonate at different frequencies in a continuos frequency band. 
     The width of this slit  47  has an impact on the bandwidth in the GSM 1800 MHz band. The wider the slit  47  is the lower will the bandwidth of the upper frequency band be. At the same time a wide slit will reduce the gain of the GSM 900 MHz band resonating element  48 . Therefore the slit will be provided with a minimum width in the range 0.8 mm and with a length in the range 4.2 mm. This minimum width ensures a minimum coupling between the two resonator elements  48  and  49  and is mainly determined by the manufacturing process where a shot molding process is used according to the preferred embodiment. The length of the slit  47  determines the bandwidth of the 900 MHz band and the gain of the 1800 MHz band. The longer the slit  47  is the higher will the bandwidth in the 900 MHz band be, and the lower will the gain in the 1800 MHz band be. 
     A cut  61  decouples the two frequency bands by forcing the 900 MHz current not to run on a capacitive 1800 MHz coupler  54 . Reducing the width of the metal pattern between the end of the slit  47  and the cut  61  will have the same effect as increasing the width of the slit  47 . 
     The 1800 MHz band resonating element  49  is terminated in a shorting surface  56  which is biased toward the shield  24  acting as ground plane for the antenna. A metalized pin  51  lowers the resonance frequency of the 1800 MHz band and is moreover used as a gripping arm for attaching the antenna to the shield  24 /PCB 21. The reason for the resonance frequency lowering is that the surface currents (the arrow C in FIGS. 6 and 7) can pass around the pin  51  before coming to ground on the rear side of the shield  24  and thus run a longer electrical distance. 
     Another pin  57  similar to the pin  51  is provided for fixing the antenna to the shield  24 . However the pin  57  is not metalized and does only serve a mechanical purpose. In both sides of the antenna body there is provided protrusions  55  for establishing snap connection to the shield  24  having similar recesses  65 . 
     With reference to FIG. 6 it is seen that the capacitive coupler  54  is provided as a metallic pattern part on a wall extending towards the shield  24 . This coupler  54  reduces the 1800 MHz band resonance frequency—the closer to the ground plane the pattern is terminated the higher coupling there will be and this causes a lower resonance frequency. 
     The Antenna Body. 
     Basically the antenna body  40  as shown in perspective view in FIGS. 6 and 7, is provided as plastic body in a two shot molding process. According to the preferred embodiment of the invention the plastic materials used for the two shot&#39;s needs to have basically specified characteristics—primarily with regard to electrical properties of the antenna body. Advantageously the plastic material or the dielectric material for internal antenna is selected as being a crystalline polymer synthesized from styrene monomer. A surface of such a plastic body may not be coated (plated) with metal while a surface of the same plastic but provided as a compound with an appropriate catalyst may be plated. 
     The metallic material will adhere to the compound plastic only and a pattern useful as the strip lines for the antenna may be created. Idemitsu Petrochemical Co., Ltd. markets a dielectric material useful for the manufacturing of the antenna body  40  under the trade name XAREC®. According to the preferred embodiment two variants Xarec S-131 (GF 30%) and Xarec SP-150 (GF 30%) are used for the first and second shot, respectively. The preferred dielectric material is syndiotactic polystyrene (SPS). Alternative materials having similar properties may be used, e. g. Questra QA 802 or Catalyzed SPS RTP 4699×7900. 
     The required characteristics for the material in order to be used in an antenna is appropriate electrical properties, such as dielectric constant and loss factor, and an ability to keep these properties for a long time. Basically this requires that the water absorption rate is low in order to secure that the dielectric properties of the antenna remain substantially at the same level. Otherwise the absorbed water will affect the dielectric properties of the antenna body. Xarec S-131 (GF 30%) and Xarec SP-150 (GF 30%) have a water absorption/24 h at 0.05% according to the ASTM D 570 test method. 
     Basically the properties of these dielectric materials may be found from the associated data sheets. However the materials has been selected primarily due to their dielectric constant in the range 3.0-3.1 which affects the relation between the resonance wave length and the wavelength in free air. Furthermore the water absorption rate is very important because the presence of water in the dielectric material will heavily affect the dielectric properties thereof. 
     The Preferred Method for Manufacturing the Antenna 
     According to the preferred embodiment of the invention a method for manufacturing the antenna body  40  as described above will comprise steps of injection molding followed by plating steps for establishing the required metallic pattern. 
     FIG. 14 ( a ) illustrates the basic steps in the injection molding proces. As a first step the a cavity is created in between a first an a second mold part,  101  and  102  respectively. This cavity is created by moving a first tool  120  towards a second tool  121  as shown by the arrow A. The first tool  120  has a two identical mold parts (second mold parts  102  and  106 ), and the second tool  121  has three mold parts (a third mold part  103  and two first mold parts  101  and  105  adjecent thereto). The resin is shot (first shot) into the cavity created by the first mold part  101  and the second mold part  102  whereby a first body part  100  is created (the geometrical form of the body is simplyfied in FIG. 14 ( a ) and ( b ) compared with the actual form shown in FIG.  11  and FIG.  12 ). The resin used for this shot repels metal in a later metalization process. The resin is injected through an inlet  104  provided in the first mold part  101 . 
     Then the two tools  120  and  121  are separated as indicated by the arrow B in FIG.  14 ( b ), and the first body part  100  is maintained in the second mold part  102 . The tool  120  is then displaced so the the second mold part  102  becomes aligned with the central third mold part  103 . The tools  102  and  103  will form a cavity having the form of the final antenna body part  40  shown in FIGS. 6 and 7. However the first body part  100  fills a substantial part of this cavity whereby the residual cavity for receiving the second resin corresponds to the body part  110 . The tools with the cavity in which the first body part  100  is placed is preheated whereby the second resin when shot into the cavity integrates with the first resin body to form a coherent antenna body. This coherent body is given the reference number  112 . The resin used for the second shot allow metal metal to adhere in a later metalization process. The resin is injected through an inlet  104  provided in the third mold part  103 . 
     In plating the plating process, e.g. an electroless dyp process, a 10-12 μm Cu-plating is added to the surface of an antenna body in a pattern defined by the two mold shots. The Cu-layer is finally protected by a thin Ni-layer having a thickness around 1-2 μm. The Ni-layer protects the current carrying Cu-layer. Finally the part is dipped in a chromate solution in order to passivate the Nickel surface. The metal does only adhere to the resin used in the second shot of the injection mold process. 
     By using this lateral displacement of the tool  120  the first resin is shot into cavity partly defined by one of the second mold parts  102  at the same time as the second resin is shot into cavity partly defined by other of the second mold parts  106 . The the first resin is provided through the outlet  104  in one of the first mold parts  101  and  105 , while the second resin is provided through the outlet  104  in the third mold part  103 . Only the one of the first mold parts  101  and  105  that is aligned with one of the the second mold parts  102  and  106  injects resin during a shoot. 
     Size of the Dielectric Body 
     The antenna body described with reference to FIGS. 3-11 is designed as a dual band antenna for the GSM 900 MHz band and the GSM 1800 MHz band has in the preferred embodiment an overall width around 45 mm, an overall height around 37 mm and overall thickness around 9 mm. The overall length of the GSM 900 path  48  is 50-55 mm. The overall length of the GSM 1800 MHz path  49  is 20-30 mm. 
     Radiation Pattern 
     FIGS. 12 and 13 illustrates the radiation pattern for the 900 MHz band and 1800 MHz band, respectively. The S 11  minimum return loss has been measured to −17 dB for the GSM 900 MHz band and to −34 dB for the GSM 1800 MHz band. The bandwidth at S 11 =−6 dB is 78 MHz (8 MHz excess) for the GSM 900 MHz band and 180 MHz (10 MHz excess) for the GSM 1800 MHz band, respectively. 
     The maximum gain is 1.6 dBi for the GSM 900 MHz band and 5.2 dBi for the GSM 1800 MHz band, respectively. The maximum gain at band edges is 0.8dBi for the GSM 900 MHz band and 3.23 dBi for the GSM 1800 MHz band, respectively. The estimated efficiency at centers is 70% for the GSM 900 MHz band and 60% for the GSM 1800 MHz band, respectively. The center frequencies are 925 MHz and 1795 MHz, respectively. 
     FIG. 12 shows that the power radiated in the GSM 900 MHz band through the rear side of the phone is 1.6 dBi, while the power radiated in the opposite direction is at least 1.6 dB lower. FIG. 13 shows that the power radiated in the GSM 1800 MHz band through the rear side of the phone is 5.2 dBi, while the power radiated in the opposite direction is almost negligible.