Abstract:
There is provided an antenna for tuning a resonant frequency. The antenna includes a first and a second arms connected to the antenna feeding portion at a common end thereof. The second arm has each of the plurality of branches including a switch for selecting a length of an electrical loop formed by the second arm and an end of a ground plane, each of the switches is connected to the ground plane. A first resonant frequency performed by the first arm is higher than a second resonant frequency by the second arm when each of the switches is open, and the first resonant frequency is lower than a third resonant frequency by the second arm when one of the switches is selected to connect the second arm and the ground plane so that the length of the electrical loop is maximum.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-082770, filed on Mar. 30, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an antenna. 
     BACKGROUND 
     New mobile telephone communication standards have been defined. The standards such as Long Term Evolution (LTE) and LTE-Advanced, and a set of standards for the fourth generation of mobile telephones (4G) have been developed by the Third Generation Partnership Project (3GPP), which is a standardization organization, and the International Telecommunication Union (ITU) respectively. It is expected in these standards that frequencies ranging from a few 100 MHz to about 3.5 GHz will be used. Furthermore, when the Worldwide Interoperability for Microwave Access (WiMAX) or wireless local area network (LAN) function is to be included in wireless terminals in the future, antennas to be included in the terminals may transmit and receive electromagnetic waves with a frequency of about 6 GHz. 
     The antennas of mobile telephones are primarily required to be small. Secondly, the antennas are required to have a higher capability to be able to handle multiple frequency bands over a wide frequency range. Various antennas for the purpose of use in multiple frequency bands are proposed. Japanese Laid-open Patent Publication No. 2007-300398 discloses an antenna configured as illustrated in  FIG. 9 . The antenna is a monopole antenna with multiple arms  214  and  216  connecting with a radiating element  212  via branching portions  213  and  215 , where reference numerals  210  and  211  are a circuit board and a feeding point respectively. The individual arms  214  and  216  have different resonant frequencies each other. 
     Further, Japanese Laid-open Patent Publication No. 2000-124728 discloses an antenna configured as illustrated in  FIG. 10 . The antenna has been disclosed as conventional multi-frequency antenna, where the antenna includes a tabular conductor  316 , capacitances  317 , a feeding point  318 , switches  319 , and an excitation source  321  on a virtual ground plane  300 . This antenna is a loop antenna that may select one of multiple impedances by using switches  319 . 
     SUMMARY 
     According to an aspect of the disclosed technique, there is provided an antenna for tuning a resonant frequency and operable with a ground plane connected through an antenna feeding portion. The antenna includes a first arm connected to the antenna feeding portion at an common end thereof, a second arm connected to the antenna feeding portion at the common end thereof and having a plurality of branches, each of the plurality of branches including a switch for selecting a length of an electrical loop formed by the second arm and an end of the ground plane, each of the switches individually connected to each of the plurality of branches and the ground plane, wherein a first resonant frequency performed by the first arm is higher than a second resonant frequency performed by the second arm when each of the switches is open, and the first resonant frequency is lower than a third resonant frequency performed by the second arm when one of the switches is selected to connect the second arm and the ground plane so that the length of the electrical loop is maximum. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a frequency-adjustable antenna according to an embodiment; 
         FIG. 2  is a diagram illustrating an operation of the antenna of the embodiment; 
         FIGS. 3A and 3B  are diagrams illustrating the relationship between the dimensions of an adjustment arm and a resonant frequency; 
         FIGS. 4A and 4B  are diagrams illustrating the relationship between the dimensions of the adjustment arm and the resonant frequency; 
         FIGS. 5A and 5B  are diagrams illustrating the relationship between the dimensions of a band arm and the resonant frequency; 
         FIGS. 6A and 6B  are diagrams illustrating calculation examples of return loss data; 
         FIGS. 7A and 7B  are diagrams illustrating another calculation example of return loss data; 
         FIGS. 8A and 8B  are diagrams illustrating another calculation example of return loss data; 
         FIG. 9  is a diagram of a first related art; and 
         FIG. 10  is a diagram of a second related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The first antenna illustrated in  FIG. 9  may handle multiple frequency bands, however the antenna may not handle a wide frequency range from a few 100 MHz to 6 GHz, which is expected to be employed in the future. 
     The second antenna illustrated in  FIG. 10  may be also disadvantageous in handling a wide frequency range, while the antenna is only adjustable around a specific frequency band of a few 100 MHz. The second antenna is configured to change its resonant frequency by changing the impedance by using the switches. The antenna is not configured to handle a wide frequency range. 
     Hereinafter, an embodiment will be descried in detail.  FIG. 1  is a diagram of a frequency-adjustable antenna according to an embodiment realized by the disclosed technique.  FIG. 2  is a diagram illustrating an operation of the antenna of the embodiment. 
     The antenna  100  may be mounted, together with a ground plane  105  that is earthed, on a printed circuit board  107  in a case  108  of a mobile phone so that the antenna  100  may be applicable to a mobile phone and even a future-generation mobile telephone terminal. The antenna  100  includes two radiating arms, a first arm is a band arm  102  and a second arm is an adjustment arm  101 . The adjustment arm  101  is a loop antenna that forms an electrical loop  201  ( FIG. 2 ) with the ground plane  105 . The band arm  102  is a reverse-L-shaped monopole antenna. 
     The dimensions of the band arm  102  are designed to contribute to extend the band of the antenna. The dimensions of the adjustment arm  101  are designed to control the adjustment operation of the antenna  100 . The band arm  102  is provided to be more distant than the adjustment arm  101  from an end side of the ground plane  105 . 
     A switched capacitor bank  103  including a plurality of parallel branches is connected as a radio-frequency switch bank to the adjustment arm  101 . At each switch branch of the switched capacitor bank  103 , as illustrated in  FIG. 2 , a switched capacitor  110  is formed between the adjustment arm  101  and the ground plane  105 . By selecting which switched capacitor  110  to turn on, as illustrated in  FIG. 2 , the peripheral length of the electrical loop  201  is changed, thereby changing the operating band. Accordingly, the frequency band may be adjusted. 
     As illustrated in  FIG. 1  or  2 , the adjustment arm  101  and the band arm  102  share a arm  112  of which end is connected to an antenna feeding portion  104 . As described above, the antenna  100  of the embodiment has two main sections, the adjustment arm  101 , which has the role of adjustment, and the band arm  102 , which extends the frequency band. Since the antenna  100  may realize a plurality of resonant structures by using the switch branches of the switched capacitor bank  103 , the antenna  100  may provide an adjustable range over a wide range. 
     Referring to  FIG. 2 , the adjustment arm  101  has a length L TA  expressed by the following equation:
 
 L   TA =2 ×L   TAF   +L   TAA   (1)
 
where L TAA  denotes the length of a long side of the adjustment arm  101  that is parallel to the end side of the ground plane  105 , and L TAF  denotes the length of each of two short sides of the adjustment arm  101  that are orthogonal to the end side of the ground plane  105 . The band arm  102  has a L BA  expressed by the following equation:
 
 L   BA   =L   BAF   +L   BAA   (2)
 
where L BAA  denotes the length of a long side of the band arm  102  that is parallel to the end side of the ground plane  105 , and L BAF  denotes the length of a short side of the band arm  102  that is orthogonal to the end side of the ground plane  105 . The relationship between L TA  and L BA  is carefully designed so as to achieve both the adjustment capability and the band extension capability.
 
       FIGS. 3A ,  3 B,  4 A, and  4 B are diagrams illustrating the relationships between the dimensions of the adjustment arm  101  and the resonant frequency. Firstly, as illustrated in  FIGS. 3A and 3B , the case where the switch at the left end of the switched capacitor bank  103  in  FIG. 2  is turned on, and the adjustment arm  101  and the ground plane  105  are connected to each other will be considered. In this case, the adjustment arm  101  having the length L TA  expressed by equation (1) forms a large closed loop  202  in  FIG. 3B  that provides a first resonant frequency f TAL1  and a second resonant frequency f TAL2  as illustrated in  FIG. 3A . The distance between the switch branch portion and the antenna feeding portion  104  is set to be equal to the length L TAA  of the long side of the adjustment arm  101  when the switched capacitor at the left end in  FIG. 2  is turned on. When the switched capacitor is turned off, the adjustment arm  101  forms a large open loop  203  in  FIG. 4B  that provides a first resonant frequency f TAO1  and a second resonant frequency f TAO2  as illustrated in  FIG. 4A . The resonant frequencies have the following relationships:
 
 f   TAL1   &gt;f   TAO1   ,f   TAL2   &gt;f   TAO2 .
 
       FIGS. 5A and 5B  are diagrams illustrating the relationship between the dimensions of the band arm  102  and the resonant frequency. The band arm  102 , which is reverse-L-shaped and which has the length L BA  expressed by equation (2), is designed so that its first resonant frequency f BA1  results in between f TAO1  and f TAL1 . That is, the relationship represented by the following expression is set:
 
 f   TAO1   &lt;f   BA1   &lt;f   TAL1   (3)
 
In this case, the following relationship holds:
 
 L   TA   &gt;L   BA   (4)
 
The reason f BA1  is set to satisfy the expressions (3) and (4) is that these expressions allows; the resonant frequency not to overlap a loop resonant frequency having a cancelling effect in a loop resonance; and the resonant frequency not to become equal to or less than f TAO1  in the lower frequency band which does not contribute to extend the band.
 
       FIG. 6A  is a diagram illustrating a calculation example of return loss data in the case where the length L BA  of the band arm  102 , which is expressed by equation (2), is about 27 mm; the length L TA  of the adjustment arm  101 , which is expressed by equation (1), is about 49 mm; and the first resonant frequency is set so as to satisfy the relationship represented by expression (3).  FIG. 6B  is a diagram illustrating a calculation example of return loss data in the case where the band arm  102  is not provided. In  FIGS. 6A and 6B , the parameter indicates the switched capacitor turned on such that “sw pos00 ON” means “only the leftmost switched capacitor  102  in  FIG. 2  is on and other switched capacitors are off. Even in  FIGS. 7A and 8A , the parameters will have the same meaning of parameters in  FIGS. 6A and 6B . 
     As illustrated in  FIG. 6B , when there is no band arm  102 , it is difficult to set the resonant frequency to a lower band less than or equal to 3 GHz. In contrast, as illustrated in  FIG. 6A , the structure of the embodiment illustrated in  FIG. 1  in which the band arm  102  is provided may set the resonant frequency to a lower band near 2 GHz. Multiple characteristic curves illustrated in  FIG. 6A  represent the case where switching is made among the switch branches of the switched capacitor bank  103 . By switching among the switch branches of the switched capacitor bank  103  in accordance with these characteristic curves, the resonant frequency may be variously changed over a wide frequency range. 
     In the embodiment, the band arm  102  also serves to limit the band extension so that the resonant frequency will not fall outside the designed conditions, as described above.  FIG. 7A  is a diagram illustrating a calculation example of return loss data in the case where the relationship represented by expression (3) is not satisfied, and, as illustrated in  FIG. 7B , the band arm  102  is too long.  FIG. 8A  is a diagram illustrating a calculation example of return loss data in the case where the relationship represented by expression (3) is not satisfied, and, as illustrated in  FIG. 8B , the band arm  102  is too short. These drawings indicate that sufficient broadband characteristics are not achieved in any of these cases. That is, the relationship represented by expression (3) is very important. 
     According to the above-described embodiment, a small, broadband-adjustable antenna that is a combination of a loop antenna and a monopole antenna may be realized. The antenna may be easily formed on a printed circuit board. In this case, the adjustment arm  101 , the band arm  102 , and the switched capacitor bank  103  are provided on the same side of the printed circuit board as the ground plane  105 . The antenna of the present embodiment may be mounted so that no elements included in the antenna are provided on the opposite side of the printed circuit board. Accordingly, the utilization ratio of the printed circuit board may be improved. 
     In the above-described embodiment, the switched capacitor bank  103  is not limited to a switched capacitor, but extends to various other devices that may operate as radio frequency switch banks. 
     The disclosed technique may be used in, for example, antennas of wireless devices whose resonant frequencies are adjusted to a broad band greater than or equal to 4 GHz, and antennas of the next-generation mobile telephones requiring operation in multiple bands ranging from 600 MHz to 6 GHz. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.