Abstract:
This invention relates to an actively tunable patch antenna comprising a ground plane, a planar radiator, a feed point, a grounding line and first and second antenna branches separated from each other by a groove, the patch antenna further comprising one or more additional grounding points between the planar radiator and the ground plane. The invention further relates to a mobile terminal utilizing the tunable patch antenna of the invention.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to actively tunable patch antennas. The invention relates more specifically to actively tunable patch antennas used internally in mobile terminals.  
       BACKGROUND OF THE INVENTION  
       [0002]     Demand for smaller mobile terminals is growing all the time. At the same time there is a need for the mobile terminal to be able to operate on several continents and on several frequency bands (for example GSM 900, GSM 1800 and GSM 1900). And in the future, when new mobile terminals with new technologies are implemented, the mobile terminal should be able to operate on the frequency bands of these technologies also (for example WCDMA 2000 or US GSM 850).  
         [0003]     The diminishing size of the mobile terminal and the requirement to be able to operate on several frequency bands sets requirements for the design of the mobile terminal. Especially small antenna structures with a wide bandwidth on several frequency bands is difficult to implement.  
         [0004]     One implementation of a small antenna structure used in mobile terminals is a planar inverted F antenna (PIFA). Bandwidth of this type of antennas as such is narrow, but with the modifications described in the publication EP 1 202 386 it is possible to construct an antenna with wide bandwidth. In the antenna of the aforesaid EP publication grooves are added to the antenna element in order to lower the Q-value of the antenna and increase the bandwidth.  
         [0005]     However, this type of antenna described in the EP 1 202 386 can only operate properly on one or two frequency bands. Especially two lower frequency bands (for example GSM 850 and GSM 900) can not easily be implemented at the same time. This would require the antenna to be high enough to work properly on both bands and have sufficient bandwidth and radiation efficiency.  
         [0006]     In the publication EP 0 993 070 is described one type of planar inverted F-antenna, comprising one antenna branch which has an electrical length of ¼ wavelength. The antenna described in the publication can be tuned via additional grounding points. Its operation is however limited to only one frequency band.  
         [0007]     There have been some attempts to actively tune the antenna to different frequency bands, but these configurations have been difficult to implement, especially on the lower frequencies. Also the bandwidth at the actively tuned frequency has been narrow, and the radiation efficiency low.  
       SUMMARY OF THE INVENTION  
       [0008]     The object of this invention is to provide a small antenna structure to be used for example in mobile terminals. The antenna of the invention can be actively tuned to operate on different frequency bands. The antenna structure according the invention is low in height allowing the manufacture of lower and smaller antennas. At the same time the antenna is equally or even more efficient than the earlier antennas. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  presents a slot antenna element according to an embodiment of the invention.  
         [0010]      FIG. 2  present a dual slot antenna element according to an embodiment of the invention.  
         [0011]      FIG. 3  presents the S 11  of the slot antenna according to an embodiment of the invention without and with extra grounding.  
         [0012]      FIG. 4  presents the efficiency of the slot antenna according to an embodiment of the invention without extra grounding and with extra grounding.  
         [0013]      FIG. 5  present the S 11  of the dual slot antenna according to an embodiment of the invention without extra grounding and with extra grounding in the lower band branch.  
         [0014]      FIG. 6  presents the efficiency of the dual slot antenna according to an embodiment of the invention without extra grounding and with extra grounding in the lower band branch.  
         [0015]      FIG. 7  presents the S 11  of the dual slot antenna according to an embodiment of the invention without extra grounding and with extra grounding in the upper band branch.  
         [0016]      FIG. 8  presents the efficiency of the dual slot antenna according to an embodiment of the invention without extra grounding and with extra grounding in the upper band branch.  
         [0017]      FIG. 9  presents a block diagram of a part of a mobile terminal according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     The radiating part of the antenna element according to an embodiment of the invention consists of one or two branches of a planar radiator starting at a feed point and ending to a grounding point forming a loop similarly as in the prior art antenna elements. The electrical length of the branches of the radiator is half wavelength. If the planar radiator consist of two branches, both branches can have their own grounding points, or they can have common grounding point if the ends of the branches are located close each other. There are also extra grounding points which are added to antenna element, these extra grounding points being located at a location between the feed point and the normal grounding point.  
         [0019]     Normally the planar radiator consist of only one branch if the antenna element is to be used on only one frequency band, for example on the frequencies near 900 MHz or on the frequencies near 1800 MHz. If the antenna element is to be used on both lower and higher frequency bands the planar radiator usually consist of two branches. It is not necessary for both branches to have extra grounding points if there is no need to tune the antenna element on one both bands.  
         [0020]     The characteristics of the antenna element can be altered by changing the places of the feed point, grounding points and the extra grounding points. Two examples of the antenna element according to an embodiment of the invention are described hereinafter.  
         [0021]      FIG. 1  presents the structure of the slot antenna element according to an embodiment of the invention. The slot antenna element  100  comprises a ground plane  101  and a planar radiator  102 . The material between the ground plane  101  and the radiator  102  is electrically non conductive. The antenna  100  also comprises a feed point  103 , a grounding line  104  for a ground point and a groove  105 . Said groove is a portion that is not electrically conducting and can be implemented as described in the European patent publication EP 1 202 386 which is incorporated herein by reference. The principles of dimensioning of the groove and the antenna structure is also described in said publication.  
         [0022]     The slot antenna element of the  FIG. 1  also comprises an extra grounding line  106  at the edge of the radiator. This extra grounding line  106  provides an extra grounding point which increases the resonance frequency of the antenna. This extra grounding point may also be implemented as a point formed via (i.e., a point formed lead-through between different layers of the circuit board) in the area of the radiator. This extra grounding  106  can be implemented as a switch, which is open when no extra grounding is in use, and connected when it is desired for the antenna to operate on higher frequencies.  
         [0023]     The effect of the extra grounding can be seen in the  FIGS. 3 and 4 . In the  FIG. 3  is presented the S 11  (input reflection coefficient) of the antenna with and without the extra grounding ( 302  and  301  respectively). As can be seen from the figure, with the extra grounding the resonance frequency of the antenna increases about 160 MHz. The amount of resonance frequency increase is dependent on how near the feed point  103  the extra grounding  106  is located.  
         [0024]     In the  FIG. 4  is presented the radiation efficiency of the antenna with and without the extra grounding ( 402  and  401  respectively). As can be seen from the figure, the resonance frequency of the antenna has increased about 160 MHz, but the radiation efficiency is approximately the same on the two frequencies in use.  
         [0025]      FIG. 2  presents the structure of a dual slot antenna element according to the another embodiment of the invention. The dual slot antenna element  200  comprises a ground plane  201 , a planar radiator  202 , a feed point  203 , a grounding line  204  for a ground point and a grooves  205   a  and  205   b . Said grooves are portions that are not electrically conducting and can be implemented similarly as the groove of the previous example. One end of the branches of the planar radiator  202  defined by the grooves  205   a  and  205   b  is located at the feed point  203  and the other end at the grounding line  204 .  
         [0026]     The dual slot antenna element of the  FIG. 2  also comprises an extra grounding line  206   a  at the edge of the radiator and extra grounding via  206   b . The extra grounding line  206   a  is located at the lower frequency branch of the antenna and it provides an extra grounding point which increases the resonance frequency of the lower frequency band of the antenna. This extra grounding point may also be implemented as a point formed via in the area of the radiator.  
         [0027]     The extra grounding via  206   b  is located at the higher frequency branch of the antenna and it provides an extra grounding point which increases the resonance frequency of the upper frequency band of the antenna. This extra grounding point may also be implemented as an extra grounding line in the area of the radiator. Both extra groundings  206   a ,  206   b  can be implemented as switches, which are open when no extra grounding is in use, and connected when the antenna is wanted to operate on higher frequencies.  
         [0028]     The effect of the extra grounding can be seen in the  FIGS. 5, 6 ,  7  and  8 . In the  FIG. 5  is presented the S 11  of the antenna with and without the extra grounding ( 502  and  501  respectively) on the lower frequency band. As can be seen from the figure, with the extra grounding the resonance frequency of the antenna increases also about 160 MHz at the lower frequencies, while the resonance frequency of the higher frequencies is unaltered.  
         [0029]     In the  FIG. 6  is presented the radiation efficiency of the antenna with and without the extra grounding ( 602  and  601  respectively) on the lower frequency band. As can be seen from the figure, the resonance frequency of the antenna has increased about 160 MHz, and the radiation efficiency is about 1 dB higher.  
         [0030]     In the  FIG. 7  is presented S 11  and in  FIG. 8  the radiation efficiency of the antenna with and without the extra grounding ( 702 ,  802  and  701 ,  801  respectively) on the higher frequency band. As can be seen, the resonance frequency of the antenna only changes a small amount on the lower frequencies when extra grounding is added to the higher frequency band. At the same time the resonance frequency on the higher frequencies changes the same 160 MHz as on the other examples. The radiation efficiency presented in  FIG. 8  is about the same with or without the grounding.  
         [0031]     As can be seen from the examples above, the adding of extra grounding does not affect the efficiency of the antenna. However the antenna can be operated on two or more frequency bands depending on the antenna structure. Tuning of the antenna can also be altered by varying the dimensioning of the antenna, for example by adding more capacitances or widening the antenna element.  
         [0032]     In  FIG. 9  is presented a block diagram of a part of a mobile terminal utilizing the planar antenna element according to an embodiment of the invention. The mobile terminal comprises a control unit  901  for controlling a transceiver unit  902  and the extra grounding switches  903 . There might be some other functions for the control unit  901  which are not shown or described herein. Extra grounding switches  903  connect the extra grounding points  904  to the antenna element  905  if it is desired to change the operating frequency of the antenna element  905 .  
         [0033]     For example the mobile terminal might be operating on an area where there are both GSM 1800 and WCDA 2000 networks, and the terminal is currently using the WCDM 2000 network. There might come a situation when the signal power received at the base station is inadequate, and the base station orders the mobile terminal to switch to another network in order to maintain the connection. The control unit  901  orders the transceiver unit  902  to change to the new frequency band, in this case the GSM 1800 band, and at the same time it connects or disconnects extra grounding points  904  required for the antenna element  905  to operate properly on this new frequency band.  
         [0034]     For the one skilled in art it is obvious that the description above does not limit the scope of the invention, and that the different alternatives of the invention are defined by the claims. For example there can be more than two additional extra grounding points if the antenna is wanted to operate on more than two additional frequency bands.