Abstract:
An antenna device for a portable radio communication device operable in at least a first and a second frequency band, includes first and second electrically conductive planar radiating elements. The first radiating element has a feeding portion connectable to a feed device of the portable radio communication device. The second radiating element includes a grounding portion connectable to ground. A controllable switch is arranged between the first and second radiating elements for selectively interconnecting and disconnecting the radiating elements. The state of the switch is controlled by means of a control voltage input. A first filter is arranged between the feeding portion and the control voltage input, to block radio frequency signals. By providing a high pass filter between the first and second radiating elements above a ground plane, quad-band operation is provided with high efficiency in a physically small antenna device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/597,192 filed Oct. 28, 2008, which issued Jun. 22, 2010 as U.S. Pat. No. 7,741,998 which, in turn, is a National Stage of International Application No. PCT/SE2005/000115 filed Feb. 1, 2005 (published Aug. 11, 2005 as WO2005/074070) claiming priority to Swedish Application No. 0400203 filed Feb. 2, 2004. The entire disclosure of the above applications are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to antenna devices and more particularly to a controllable internal multi-band antenna device for use in portable radio communication devices, such as in mobile phones. The invention also relates to a portable radio communication device comprising such an antenna device. 
     BACKGROUND 
     Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages connected with using internal antennas, of which can be mentioned that they are small and light, making them suitable for applications wherein size and weight are of importance, such as in mobile phones. A type of internal antenna that is often used in portable radio communication devices is the so-called Planar Inverted F Antenna (PIFA). 
     However, the application of internal antennas in a mobile phone puts some constraints on the configuration of the antenna, such as the dimensions of the radiating element or elements, the exact location of feeding and grounding portions etc. These constraints may make it difficult to find a configuration of the antenna that provides a wide operating band. This is particularly important for antennas intended for multi-band operation, wherein the antenna is adapted to operate in two or more spaced apart frequency bands. In a typical dual band phone, the lower frequency band is centered on 900 MHz, the so-called GSM 900 band, whereas the upper frequency band is centered around 1800 or 1900 MHz, the DCS and PCS band, respectively. If the upper frequency band of the antenna device is made wide enough, covering both the 1800 and 1900 MHz bands, a phone operating in three different standard bands is obtained. In the near future, antenna devices operating four or even more different frequency bands are envisaged. 
     The number of frequency bands in passive antennas is limited by the size of the antenna. To be able to further increase the number of frequency bands and/or decrease the antenna size, active frequency control can be used. An example of active frequency control is disclosed in the Patent Abstracts of Japan 10190347, which discloses a patch antenna device capable of coping with plural frequencies. To this end there are provided a basic patch part and an additional patch part which are interconnected by means of PIN diodes arranged to selectively interconnect and disconnect the patch parts. Although this provides for a frequency control, the antenna device still has a large size and is not well adapted for switching between two or more relatively spaced apart frequency bands, such as between the GSM and DCS/PCS bands. Instead, this example of prior art devices is typical in that switching in and out of additional patches has been used for tuning instead of creating additional frequency band at a distance from a first frequency band. 
     The Patents Abstracts of Japan publication number JP2000-236209 discloses a monopole antenna comprising a linear conductor or on a dielectric substrate, see  FIG. 1 . Radiation parts of the antenna are composed of at least two metal pieces connected through diode switch circuits. The radiation elements have feed points connected to one end of a filter circuit, which cuts of a high-frequency signal. A signal V Switch  is used to control the diode switch. The disclosed configuration is limited to monopole or dipole antennas. Also, the object of the antenna according to the above mentioned Japanese document is not to provide an antenna with a small size. 
     A problem in prior art antenna devices is thus to provide a multi-band antenna of the PIFA type with a small size and volume and broad frequency bands which retains good performance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an antenna device of the kind initially mentioned wherein the frequency characteristics provides for four comparatively wide frequency bands while the overall size of the antenna device is small. 
     Another object is to provide an antenna device having better multi-band performance than prior art devices. 
     The invention is based on the realization that several frequency bands can be provided in a physically very small antenna by arranging the antenna so that first portions of two radiating elements are interconnected for radio frequency signals and second portions of the radiating elements are selectively interconnectable by means of a switch controlled by means of a DC voltage. This DC voltage is applied to a control input wherein a filter arrangement that is provided between the RF feeding portion and the DC control input blocks RF signals. 
     According to a first aspect of the present invention there is provided an antenna device as defined in claim  1 . 
     According to a second aspect of the present invention there is provided portable radio communication device as defined in claim  10 . 
     Further preferred embodiments are defined in the dependent claims. 
     The invention provides an antenna device and a portable radio communication device wherein the problems in prior art devices are avoided or at least mitigated. Thus, there is provided a multi-band antenna device having an antenna volume as small as about 3 cm 3  which means a size of the antenna that is reduced as compared to standard multi-band patch antennas but still with maintained RF performance. Also, the bandwidths of the antenna device according to the invention can be improved as compared to corresponding prior art devices but without any increase in physical size, which is believed to be a result of the use of the dual band antenna structure. 
     The switch is preferably a PIN diode, having good properties when operating as an electrically controlled RF switch. 
    
    
     
       SUMMARY OF THE INVENTION 
       The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a description of a prior art monopole antenna; 
         FIG. 2  shows a schematic diagram of a PIFA antenna device according to the invention; 
         FIGS. 2   a  and  2   b  shown the PIFA antenna of  FIG. 2  in a first and a second operating mode, respectively; 
         FIG. 2   c  is a frequency diagram of the operating modes of the antenna shown in  FIG. 2   
         FIG. 3  is an overview of a printed circuit board arranged to be fitted in a portable communication device and having an antenna device according to the invention; 
         FIG. 4  shows an embodiment of the antenna device wherein capacitive coupling between radiating elements is provided by means of a conductive sheet; 
         FIG. 5  shows yet another embodiment of the antenna device wherein capacitive coupling between radiating elements is provided by means of a meandering interface between the radiating elements; 
         FIG. 6  shows yet another alternative radiating element configuration; 
         FIG. 7  shows an alternative embodiment of an antenna device according to the invention wherein three radiating elements are provided; 
         FIGS. 7   a - d  show different operating modes of the antenna device shown in  FIG. 7 ; 
         FIG. 8  is a perspective view of an exemplary foldable phone in which may be used the PIFA antenna shown in  FIG. 2  according to an exemplary embodiment; and 
         FIG. 9  is a side view of an example embodiment in which radiating elements (shown in  FIG. 4 ) are on one side of an example multi-layer flex film and the conductive sheet (also shown in  FIG. 4 ) are on the other side of the multi-layer flex film. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a detailed description of preferred embodiments of an antenna device according to the invention will be given. In the description, for purposes of explanation and not limitation, specific details are set forth, such as particular hardware, applications, techniques etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be utilized in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, apparatuses, and circuits are omitted so as not to obscure the description of the present invention with unnecessary details. 
       FIG. 1  has been described in the background section and will not be dealt with further. 
     In  FIG. 2 , there is shown an antenna device, generally designated  1 . The antenna device comprises a first generally planar rectangular radiating element  10  made of an electrically conductive material, such as a sheet metal or a flex film, as is conventional. A source RF of radio frequency signals, such as electronic circuits of a portable radio communication device, is connected to a feeding portion  12  of the first radiating element. 
     The antenna device also comprises a second generally planar rectangular radiating element  20 . A switch element  30  is provided between the two radiating elements  10 ,  20 . This switch element is preferably a PIN diode, i.e., a silicon junction diode having a lightly doped intrinsic layer serving as a dielectric barrier between p and n layers. Ideally, a PIN diode switch is characterized as an open circuit with infinite isolation in open mode and as an short circuit without resistive losses in closed mode, making it suitable as an electronic switch. In reality the PIN diode switch is not ideal. In open mode the PIN diode switch has capacitive characteristic (0.1-0.4 pF) which results in finite isolation (15-25 dB @ 1 GHz) and in closed mode the switch has resistive characteristic (0.5-3 ohm) which results in resistive losses (0.05-0.2 dB). 
     The first and second radiating elements  10 ,  20  are also capacitively interconnected by means of a high pass filter, shown as a capacitor  32  in the figures. The high pass filter allows RF signals to pass and this means that the two radiating elements from an RF point of view is one single element, as will be described further with reference to  FIGS. 2   a - c.    
     The first and second radiating elements  10 ,  20  are arranged at a predetermined distance above a ground plane, such as a printed circuit board described below under reference to  FIG. 3 . 
     A DC control input, designated V Switch  in the figures, for controlling the operation of the PIN diode is connected to the first radiating element  10  via a filter block  16  to not affect the RF characteristics of the antenna device. This means that the filter characteristics of the filter block  16  is designed so as to block RF signals. In the preferred embodiment, the filter block  16  comprises a low pass filter. 
     Finally, the second radiating element is connected directly to ground at a grounding portion  22 . This grounding portion functions for both RF signals emanating from the RF input and DC signals emanating from the control input. 
     The antenna is preferably designed to 50 Ohms. 
     The switching of the antenna device functions as follows. The RF source and other electronic circuits of the communication device operate at a given voltage level, such as 1.5 Volts. The criterion is that the voltage level is high enough to create the necessary voltage drop across the PIN diode, i.e. about 1 Volt. This means that the control voltage V Switch  is switched between the two voltages “high” and “low”, such as 1.5 and 0 Volts, respectively. When V Switch  is high, there is a voltage drop across the PIN diode  30  and a corresponding current there through of about 5-15 mA. This voltage drop makes the diode conductive, effectively electrically interconnecting the two radiating elements  10 ,  20  at the diode  30 . 
     With the control voltage V Switch  “low”, there is an insufficient voltage drop across the PIN diode  30  to make it conductive, i.e., it is “open”. The second radiating element is then effectively connected to the first radiating element only through the capacitor  32 . 
     The size and configuration of the two radiating elements are chosen so as to obtain the desired resonance frequencies, such as the 850 and 1800 MHz bands with the switch open and the 900 and 1900 MHz bands with the switch closed. 
     Now turning to  FIG. 2   a , it is shown therein how the two radiating elements  10 , from an RF point of view operate as one single radiating element having a general C-shape. This is because the capacitor  32 , operating as a high pass filter, functions as an “RF bridge” between the two radiating elements. Switch  30  in the form of a PIN diode is open, i.e., non-conductive in  FIG. 2   a  because the control voltage V Switch  is low, i.e. zero Volts. No DC current flows through the diode. The C-shape of the combined radiating elements in combination with the position of the feeding portion  12  makes the arrangement resonate at two frequencies, effectively making it suitable for dual band operation. 
     In  FIG. 2   b , switch  30  is closed, i.e., the diode is conductive. This effect is achieved when a high control voltage V Switch  is applied to the control input, see  FIG. 2 . This voltage creates a DC current that flows through the LP filter  16 , across the first radiating element  10 , through the diode  30 , across the second radiating element  20  and to ground via the grounding portion  22 . With the switch  30  closed, i.e., with the diode conductive, the RF bridge between the two radiating elements is broadened. This is clearly seen in  FIG. 2   b  when compared to  FIG. 2   a.    
     This change of geometry of the effective radiating elements adjusts the resonance frequencies of antenna device. This is seen in  FIG. 2   c , wherein the dashed curves correspond to the operating mode shown in  FIG. 2   a  and the solid curves correspond to the operating mode shown in  FIG. 2   b . The means that an antenna device which can operate in four different frequency bands is obtained, such as the above mentioned 850/900/1800/1900 MHz bands. 
     The adjustment of the resonance frequencies shown in  FIG. 2   c  can be used to an advantage in so-called fold phones. In this kind of communication devices, the resonance frequency of an internal antenna element tends to move downwards in frequency when the position of the phone is changed from folded to unfolded mode. With the inventive antenna device, when the phone is unfolded, the movement of the resonance frequencies can be counteracted by closing the switch  30 . Thus, with the phone folded, the control voltage V switch2  is low and with the phone unfolded, the control voltage is high. The antenna device then operates as a dual band antenna with essentially constant resonance frequency irrespective of the operating mode of the communication device (folded/unfolded). 
     The adjustment of the resonance frequencies shown in  FIG. 2   c  can also be used to an advantage in dual band bar phones. In the frequency bands used for mobile communication, the transmit (TX) and receive (RX) frequencies are separated by approximately 45-90 MHz. By using frequency adjustment, near optimum efficiency can be obtained by adjusting the frequencies to the TX and RX frequencies instead of the broader frequency band incorporating the TX and RX frequencies. 
     In  FIG. 3  the two radiating elements  10 ,  20  are shown arranged generally parallel to and spaced apart from a printed circuit board (PCB)  70  adapted for mounting in a portable communication device  80 , such as a mobile phone. The PCB functions as a ground plane for the antenna device. The general outlines of the communication device is shown in dashed lines in  FIG. 3 . Typical dimensions for the antenna device  1  is a height of approximately 4 millimeter and a total volume of about 3 cm 3 . 
     It will be appreciated that all components except for the two radiating elements  10 ,  20 , the switch element  30 , and the capacitor  32  can be provided on the PCB, thus facilitating easy assembly of the antenna device. This is further facilitated by the fact that there is no separate feeding of the switch element. 
     A conventional production method of antenna devices is to provide an electrically conductive layer forming the radiating portions of the antenna on a carrier made of a non-conductive material, such as a polymer or other plastic material. The carrier is thus made of a heat-sensitive material and a small heating area is desired to keep the temperature as low as possible when soldering components to the antenna device. 
     In  FIG. 4 , there is shown how the capacitive bridge can be provided by means of a conductive sheet  34  provided under part of the two radiating elements  10 ,  20  at the RF bridge location. If a multi-layer flex film is used to provide the radiating elements, the radiating elements  10 ,  20  can be provided on one side of the flex film and the conductive sheet  34  on the other. In this way, discrete components are avoided to provide the capacitive coupling between the radiating elements. 
     In  FIG. 5 , there is shown how the capacitive bridge can be provided by means of a meandering interface between the two radiating elements  10 ,  20 . Also in this way, discrete components are avoided to provide the capacitive coupling between the radiating elements. 
     In  FIG. 6  there is shown an alternative configuration of the radiating elements. In all aspects, this antenna device operates as the one described above with reference to  FIGS. 2 and 2   a - c.    
     In an alternative embodiment shown in  FIG. 7 , generally designated  100 , an additional third radiating element  140  is provided together with a second control input, designated V switch2  connected to the third radiating element via a low pass filter  142 . The third radiating element is connected to the second radiating element  120  by means of a second switch  144  in the form of a PIN diode. 
     Also, in the embodiment shown in  FIG. 7 , the first radiating element  110  is connected to ground at a grounding portion  114  via a high pass filter  118  blocking DC signals. Finally, the second radiating element  120  is connected to ground at a grounding portion  122  via a low pass filter  124  blocking RF signals. Thus, in this embodiment, there are separate grounding portions for RF signals and DC (i.e., control) signals. 
     The antenna device of  FIG. 7  operates as follows. The first control voltage V switch  functions as in the first embodiment shown in  FIG. 2 . Thus, high voltage creates a current flowing through the first switch  130  and to ground through the low pass filter  124 . With the second control voltage V switch2  low, the second switch  144  is non-conductive. This means that the third radiating element  140  is effectively disconnected from the second radiating element, see  FIGS. 7   a  and  7   b.    
     With the position of the feeding portion  112  and the first switch  130  open as in  FIG. 7   a , the first and second radiating elements  110 ,  120  interconnected by means of the capacitor  132  resonates at a first frequency. With the first switch closed as in  FIG. 7   b , the combination of the first and second radiating elements resonates at a second frequency. 
     With the second switch  144  closed as in  FIGS. 7   c ,  7   d , i.e., with the second control voltage high, the combination of the first, second, and third radiating elements  110 ,  120 ,  140  resonates at a third or fourth frequency, depending on whether the first switch  130  is open or closed. Thus, quad band operation is provided with this configuration. 
     Preferred embodiments of an antenna device according to the invention have been described. However, it will be appreciated that these can be varied within the scope of the appended claims. Thus, a PIN diode has been described as the switch element. It will be appreciated that other kinds of switch elements can be used as well. 
     The radiating elements in  FIGS. 2 ,  3 , and  7  have been described as being essentially planar and generally rectangular. It will be appreciated that the radiating elements can take any suitable shape, such as being bent to conform with the casing of the portable radio communication device in which the antenna device is mounted. 
     One switch has been shown to interconnect two radiating elements. It will be appreciated that more than one switch, such as several parallel PIN diodes can be used without deviating from the inventive idea.