Patent Publication Number: US-8537072-B2

Title: Antenna for mobile terminal unit

Description:
TECHNICAL FIELD 
     The present invention relates to an antenna for a mobile terminal unit, such as a mobile telephone, comprising electronic circuits and at least one antenna radiator molded with a non conductive cover of the mobile terminal unit and a connection between the circuits and the at least one antenna radiator. The antenna is intended for operation in one or several frequency bands. The invention also includes a mobile terminal comprising such an antenna. 
     BACKGROUND ART 
     Several types of antennas intended for integration in mobile terminals are available today. The most common type is the so-called PIFA (Planar Inverted F Antenna) antenna. These antennas normally consist of a radiator element made of metal sheet or flex-film applied to a plastic carrier mounted at a certain distance above ground. Other types exist, all having in common that they are integrated inside the covers of the phone and taking up valuable space. 
     In mobile terminals of today there is a requirement for more compact terminals and above all thinner terminals which is of special interest for mobile terminals such as mobile phones. 
     Solutions to integrate the radiating elements of the terminal in the outer cover have therefore been proposed. In this way the number of components is reduced and the thickness of the cover can be used to separate the radiator from the ground as far as possible which is positive for a good antenna performance. This also makes it possible to reduce the thickness of the phone as the cover, including the radiator, becomes part of the antenna. This means that no space is required inside the cover for the radiator as is the case with conventional solutions, thus making the phone thinner. 
     EP 1439603 proposes a solution with an antenna radiator integrated in a cover and fed through a special feeding element. 
     A problem with integrating the antenna radiator in the cover in prior art solutions is that it will become more sensitive to influences from the hand of the user of the mobile terminal. Both the signal strength and frequency of the antenna will be affected when the hand covers the antenna or part of the antenna causing a reduced coverage area for the antenna, and possibly a dropped call, as well as reduced talk time as the terminal has to increase output power to compensate for the decreased antenna performance. 
     Thus there is a requirement for an improved antenna performance where the antenna radiator is integrated in the cover to a mobile phone. 
     DISCLOSURE OF INVENTION 
     An object with the present invention is to provide an antenna that can be integrated in the cover of a mobile terminal unit, henceforth exemplified with a mobile phone, that eliminates the drawbacks with prior art and to accomplish a solution with very compact outer dimensions with easy and cost effective manufacturing. 
     To take advantage of the relatively large area of a cover to a mobile phone several antennas can be integrated in the cover such as GSM, TV and FM antennas or separate antennas for receiving and transmitting thus reducing the number of components even further making the assembly efficient and cost effective. 
     These objects are met by providing an antenna for a mobile terminal unit comprising radio frequency circuits and at least one antenna radiator molded with a non-conductive cover of the mobile terminal unit and a connection between the radio frequency circuits and the at least one antenna radiator where the connection between the radio frequency circuits of the mobile terminal unit and the antenna radiator is made non-galvanic through at least one dielectric interface being integrated with the cover. 
     By using the dielectric interfaces having a relatively high dielectric constant, preferably around 20, the antenna will be less sensitive to affects from a hand holding the mobile phone and covering, or partly covering, the antenna. However higher values for the dielectric constant can be used as well as lower values down to at least 10. The sensitivity will be decreased in relation to the difference in dielectric constant between the hand and the dielectric interface. 
     A further advantage with the present invention is the possibility of improved matching, i.e. to adjust the impedance of the antenna to the impedance of the RF-circuits of the mobile phone. Good matching means that transmission losses between the RF-circuits and the antenna are minimized. The invention allows the matching to be performed by adjusting different parameters of the dielectric interface as will be described in detail below. This eliminates the need to perform matching with discrete components. 
     The antenna radiator included in the cover can be of ground dependent or ground independent type. A ground dependant antenna interacts with a ground plane in the mobile phone, usually one of the layers in a multilayer Printed Circuit Board (PCB) comprising electronic components of the mobile phone. The ground independent type of antenna can work without the interaction of a ground plane. 
     The antenna is preferably integrated in the outer rear cover of a mobile phone. However integration can also be performed in an internal non-conductive cover or in any suitable non conductive part of the mobile phone. 
     The invention also includes a mobile terminal comprising the antenna according to claim  1 . 
     Further advantages are achieved if the invention also is given one or several characteristics according to the dependent claims not mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described in more detail with reference to the enclosed drawings where: 
         FIG. 1  shows schematically a mobile phone comprising an antenna according to the present invention. 
         FIG. 2  is a cross cut of the cover showing radiating element and Dielectric Interface (DI) molded to the cover. 
         FIG. 3  is a cross cut of the cover showing radiating element and Dielectric Interface (DI) snapped to the cover. 
         FIG. 4  is a schematic top view of one embodiment of the invention showing a conventional PIFA-type of multiband antenna structure with Dielectric Interfaces for RF and ground embedded in a rear cover to a mobile phone. 
         FIG. 5  is a schematic top view of an antenna radiator of PIFA-type structure with Dielectric Interfaces for RF and ground. 
         FIG. 6  is a schematic top view of an antenna radiator with three half wave resonant, ground independent elements, fed by a common Dielectric Element. 
         FIG. 7  is a schematic top view of an antenna radiator with a “treestructure” of quarter wave elements with different resonances. 
         FIG. 8  is a schematic top view of an antenna radiator comprising a patch element. 
         FIG. 9  is a schematic top view of an antenna radiator with a double resonant coil structure. 
     
    
    
     EMBODIMENT(S) OF THE INVENTION 
     The invention will in the following be described in detail with reference to the drawings. 
       FIG. 1  shows a mobile phone  101  comprising a control unit  107  configured to control communication with a mobile communication system  103 . A keyboard  113 , a display  115  and radio frequency (RF) circuits  109  are connected to the control unit  107  which together with an antenna  111  are arranged to establish a radio-interface  105  for communication with the mobile communication system  103 . 
       FIG. 2  shows the radiator  201  embedded in the rear cover  203 . The cover can be the rear non-conductive cover of a mobile phone, an internal non-conductive cover or an internal non-conductive component such as a plastic carrier for a speaker. In the case of a clam shell type of phone the radiator is preferably embedded in the rear top or bottom cover. 
     The radiator can be embedded in the cover using IMF (In Mould Foil), IMD (In Mold Decoration) or IML (In Mold Label) technology. The radiator can be placed close to the outer surface  205  of the cover or the inner surface  207 . When the radiator is placed at the surface  205 , a protective film may be applied to the exterior surface prior to the molding process, or subsequently. 
     The radiator  201  can be curved in two or three directions and shaped as to follow the shape of the cover  203 . The radiator can be any type of RF-suitable, conductive material as known by the person skilled in the art. Preferably a copper foil with a thickness of 5 μm or thicker is used. A flex film including the radiator pattern can also be used. When using IML technology, the label being the radiator, can be pre shaped to the desired form. 
     A DI (Dielectric Interface)  209 , located in a groove in the cover  203 , is molded to the cover  203  after the in-mold of the radiator  201 . Alternatively it can be included in any of the in-mold processes mentioned above. Any other conventional methods, as e.g. gluing, can be used to attach the DI  209  to the cover  203 . DI has conductive pads  211  for contact with the RF circuits  109 . The contact can be established with any conventional method such as a pogo-pin  213  or a contact pin  215 . These contact elements  213  and  215  are attached to either a pad on a Printed Circuit Board (PCB) holding the RF-circuits  109  or to the conductive pad  211  of the DI. The attachment can be made in any conventional way as e.g. by soldering so as to establish a galvanic contact between the DI and RF-circuits  109 . The DI is manufactured of a dielectric material with a relatively high dielectric constant, preferably around 20, but higher and lower values down to at least 10 can also be used. The DI can be realized as blocks or pucks of dielectric material having a conductive pad  211 . One embodiment of DI is a ceramic resonator such as e.g. a coaxial resonator well known to the skilled person. The top surface  217  of DI is in a preferred embodiment substantially parallel to the radiator  201 . 
     The distance of the gap  219  can be varied in order to obtain a good impedance match between the radiator and RF circuits  109 . The impedance matching can also be tuned by using different ∈-values for DI and by changing the surface area DI is facing towards the radiator  201 . The invention thus provides several parameters for tuning the impedance such as; the ∈-value, gap distance and surface area of DI facing the radiator. This makes it possible to obtain good matching and avoiding the need to use discrete electronic components in the RF-circuit  109 . 
     The radiator  201  in  FIG. 2  is a single radiator with a RF-feed realized through the DI  209 . The radiator in this case can be a monopole antenna not requiring a ground plane for its operation or any other type of ground independent antenna type. 
       FIG. 3  differs from  FIG. 2  only in the way the DI is attached to the cover. In this embodiment the DI is attached by snapping it onto the cover using hatches  301  or some other snapping means well known to the skilled person. 
       FIG. 4  is a top view of a radiator  401  molded with a cover  400 . In this case the radiator has a typical PIFA configuration with two branches seen from the grounding point. This makes it possible to have two resonances, one for each branch, and thus accomplishing a dual band antenna. This antenna type requires the cooperation of a ground plane. In a mobile phone the ground plane of the PCB can be used. Connection to the ground plane is accomplished by a DI  405  designed in the same way as the DI  209  described above. The gap distance for the DI  405  to the radiator  401  is typically around 0.1 mm in order to create a sufficient coupling at the lowest operating frequencies between radiator and ground. The RF-feed is accomplished with DI  403  in the same way as described for  FIG. 2  above. The in-molding of the radiator and attachment of the two DI are realized in the same way as described above. In this way a separate unit comprising cover, antenna, impedance matching through DI and contact pads to the RF-part of the mobile phone is accomplished. 
     In an embodiment with a flex film as the radiating element the DI can be mounted flush to the bottom surface of the flex film substrate and the flex film substrate then becomes the dielectric material between the DI and the radiator which is applied on the top surface of the flex film substrate. 
     By using a DI interfaces having a dielectric constant preferably around 20, but also higher and lower values down to at least 10 can be used, an “insulation” between a hand holding the phone and covering part of, or the whole radiator, embedded in the cover and the RF and ground parts in the phone is realized. In a conventional solution the hand will affect the antenna performance in a negative way. The amplitude of received or transmitted signals will be reduced and the frequency response for the antenna will be shifted, both effects giving decreased antenna performance. These negative effects will be much reduced by introducing the “insulation” through DI and the high dielectric constant. 
     Another embodiment not shown in the figures is that the two DI  403  and  405  can be put on a common substrate like a PCB or the PCB itself constitutes the dielectric material and is divided into as many DI units as required, each DI having a conductive pad for contacting the RF-circuits  109 . 
       FIG. 5  is a top view of an antenna comprising one radiator  501  with PIFA configuration and a separate ground connection part  503 . The DI  504  for the ground feed is located above ground connection part  503 . The RF-feed is handled by DI  505 . In this way there will be a well-defined grounding point  507  of the PIFA with the possibility to locate the ground feed at a convenient position “off” the PIFA location. 
       FIG. 6  is a top view of another embodiment where the antenna consists of three radiator elements,  601 ,  602  and  603 . Each element is half wave resonant with different resonance frequencies as the length is different for each element. The elements are fed by a common DI  605  located approximately in the centre of each element. A halfway resonant antenna is not dependent on the cooperation of a ground plane and a ground feed is therefore not required in this embodiment. A half wave resonant antenna has an electrical length that is a half wavelength of the resonant frequency of the antenna as is well known to the person skilled in the art. 
       FIG. 7  is a top view of a radiator comprising a “treestructure” of quarter wavelength elements  701 ,  702 ,  703  and  704 , having an electrical length corresponding to a quarter of the wavelength of the resonant frequency. This type of radiator is ground dependent and thus requires a ground feed. A ground feed pad  705  is therefore included in the radiator pattern as well as a RF-feed pad  707 . The feeding is provided with DI  706  for ground and DI  708  for RF. The separate feeding pads ensures efficient electromagnetic coupling between RF-feed and ground in the mobile phone and the antenna. The pads can also be located at positions convenient for matching suitable feeding points in the mobile phone. 
     Another embodiment of the radiator is shown in  FIG. 8 . It is a top view of a patch radiator  801  fed a certain point through the DI  803 . The patch radiator has to cooperate with a ground plane, which in this case can be the ground plane of the PCB of the mobile phone, but does not need a separate ground feed. The patch can be designed to be single or multi resonant as is well known in the art. The feeding location is chosen as to achieve a good impedance match. 
       FIG. 9  is yet another embodiment of a dual resonant coil type of radiator element seen from the top. The radiator has a coil  901  with ground feed pad  905  and shortcircuiting part or short  909 . The short is effective for higher frequencies and thus makes the coil length shorter for these frequencies. For lower frequencies the short is not working and the electrical length for lower frequencies will correspond to the total coil length. The ground feed is made through DI  903  and the RF-feed through DI  907 . This accomplishes an efficient and compact dual resonant antenna. 
     Mobile phones of today often require several antennas for different functions of the phone as e.g TV, FM-radio, GPS, Bluetooth. Sometimes it is also required to have separate receiving and transmitting antennas. As the invention makes it possible to use a large surface for antennas, e.g the complete back cover of the mobile phone it will be possible to integrate several antennas in the cover as for instance a GSM and TV-antenna. 
     The embodiments described above are only possible examples of how to realize the invention and should not be limiting. Over and above the embodiments described above it is of course possible within the scope of the invention to include any suitable radiating element, either ground dependent or ground independent. It is also a possibility to include separate parasitic elements that are fed from a main radiator only.