Abstract:
In a compound portable terminal which supports two communication standards using different frequencies when an antenna is prepared for each communication standard, radio interference occurs in two antennas, and there is a possibility that bit errors increase. To avoid this possibility, there is a problem that two antennas need to be separated from each other as far as possible. To solve this problem, two elements are disposed in the vicinity of each other, a feeding terminal and a ground terminal are disposed in one of the elements, and a ground terminal is disposed in the other element. The other element has no feeding element. When transmission is performed from the feeding terminal, an induced current is produced in the element having no feeding terminal, and each element is excited by individual resonance frequency, and operate as antennas.

Description:
This application claims priority to prior Japanese Patent Application JP 2003-293440, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a built-in antenna device for use in a small-sized portable radio such as a cellular phone, which is capable of independently adjusting a plurality of frequencies in order to cope with a plurality of frequency bands. 
   In Japan, a total number of subscribers of both a cellular phone service and a personal handy phone system (PHS) service exceeds 82 million (as of the end of June, 2003), and the mobile phones are generally carried (or common) items since different frequency bands are provided between the cellular phone service and the PHS service, a mobile phone used for both services must be designed to handle the different frequency bands. 
   Moreover, there has been a rising demand for speeding up data communication in a field of radio communication as well as that of cable communication. There has also been a demand for enhancement of multi-accessability indicating that many terminals can access one base station, and this is a problem inherent in the radio communication. To meet these demands, second generation systems such as PDC (Personal Digital Communications) and GSM (Global System for mobile Communications) in which a time division multiple access (TDMA) is adopted are shifting to third generation systems such as cdma2000 and W-CDMA in which a code division multiple access (CDMA) is adopted. In the shifting, a different frequency band exclusive for a third generation system is allocated, but a service area thereof has spread at a slow pace because of costs. As a realistic solution of this problem, a compound portable (or mobile) terminal that can be used both in the second and third generations has appeared. 
   Furthermore, in recent years, in consideration of design, portability and the like, portable terminals have increased in which in-built antennas built in housings of them are adopted rather than antennas protruding to the outside. This tendency can also be seen in the above-described compound portable terminal. 
   In the compound portable terminal, in addition to a plurality of built-in radios in accordance with the corresponding frequency bands, a plurality of antennas are also required. In the compound portable terminal designed assuming that the terminal is carried by a user, it is difficult to build all the antennas in the terminal. 
   Additionally, when a plurality of antennas are miniaturized and built in the terminal, interference is frequently caused by resonance among the antennas, and this has been a bottleneck in designing a miniature radio. However, the problem of radio interference has been substantially solved by use of a duplexer. 
   As an example of a prior technique against these problems, Japanese Patent Application Laid-Open No. 2003-087025 has been proposed. That is, a cellular phone has been disclosed in which a first antenna is disposed through the upper surface of a housing, a second antenna is disposed in lower portion of the housing, which is hard to be covered by an operator&#39;s hand, and accordingly two antennas are separated from each other to prevent resonance from being generated. 
   However, in the prior technique, restrictions on arrangement of the antennas are involved, and there has been a problem in designing a circuit of the cellular phone. Especially, the second antenna disposed in the lower portion of the housing is required to reduce influences caused by covering with the hand. Therefore, further restrictions are imposed on a circuit arrangement. 
   SUMMARY OF THE INVENTION 
   To solve the above-described problems, an object of the present invention is to provide an antenna device having a plurality of antenna elements arranged in the vicinity of one another in the same plane. 
   According to the present invention, there is provided an antenna device comprising a driven element including a feeding terminal and a ground terminal. A parasitic element includes a ground terminal and no feeding terminal. The driven element produces a magnetic field by feeding from the feeding terminal, and the parasitic element produces an induced current by the magnetic field. 
   Furthermore, the driven element is disposed in parallel with the parasitic element. 
   Moreover, the driven element is excited by a natural resonance frequency of the driven element during the feeding. 
   Additionally, the parasitic element is excited by a natural resonance frequency of the parasitic element during the production of the induced current. 
   Moreover, the feeding terminal and the ground terminal of the driven element and the ground terminal of the parasitic element are spring terminals. 
   Furthermore, the feeding terminal and the ground terminal of the driven element and the ground terminal of the parasitic element are spring connectors. 
   Additionally, the driven element is integrated with the parasitic element. 
   Moreover, the driven element is integrated with the parasitic element by means of a resin. 
   There is provided a cellular phone comprising an antenna device comprising a driven element including a feeding terminal and a ground terminal. A parasitic element includes a ground terminal and no feeding terminal. The driven element produces a magnetic field by feeding thorough the feeding terminal, and the parasitic element produces an induced current therein by the magnetic field. 
   In the cellular phone, the driven element and the parasitic element are rectangular, and the driven element is disposed in parallel with the parasitic element. 
   In addition, the antenna device is fixed to a rear case of the cellular phone. 
   Moreover, the antenna device is fixed to the rear case by a double-faced adhesive tape. 
   Furthermore, the cellular phone further comprises a substrate. The antenna device is fixed to the substrate. 
   Furthermore, the feeding terminal and the ground terminal of the driven element and the ground terminal of the parasitic element are spring terminals. The antenna device is fixed to the substrate by the spring terminals. 
   Additionally, the feeding terminal and the ground terminal of the driven element and the ground terminal of the parasitic element are spring connectors. The antenna device is fixed to the substrate by the spring connectors. 
   Furthermore, the above-described cellular phone further comprises a cushion. The cushion is disposed between the antenna device and the substrate except the feeding terminal and the ground terminals. 
   Additionally, the antenna device is pressed and fixed onto the rear case of the cellular phone by the cushion. 
   In the present invention, two antenna elements are disposed in an upper portion of a housing of a cellular phone. The upper portion is hard to be influenced by operator&#39;s hands. When feeding is made to one of the antenna elements, an induced power is induced in the other antenna element. The antenna elements are excited by natural resonance frequencies, respectively, and are operated as antennas. Moreover, the above-described constitution is not influenced by the operator&#39;s hands, and, as a result, the constitution is useful in raising an antenna gain. 
   Furthermore, since both the antenna elements disposed in the vicinity of each other, a degree of freedom in a circuit constitution increases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an antenna element for use in an antenna device according to a first embodiment of the present invention; 
       FIG. 2  is a perspective view of the antenna device according to the first embodiment of the present invention; 
       FIG. 3  is a perspective view in a case where a double-faced adhesive tape is attached to the antenna device of  FIG. 2 ; 
       FIG. 4  is a sectional view of a cellular phone using the antenna device of  FIG. 2 ; 
       FIG. 5  is a perspective view in a case where the antenna device of  FIG. 2  is attached to a substrate; 
       FIG. 6  is a perspective view of an antenna device with a cushion according to a second embodiment of the present invention; 
       FIG. 7  is a sectional view of a cellular phone using the antenna device of  FIG. 6 ; and 
       FIG. 8  is a perspective view of an antenna element according to a third embodiment of the present invention 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described hereinafter in detail in accordance with embodiments. 
   First Embodiment 
     FIG. 1  is a perspective view of an antenna element  1  of an antenna device ( 100  of  FIG. 2 ) according to a first embodiment of the present invention.  FIG. 2  is a perspective view showing a constitution of the antenna device  100  constituted of the antenna element  1  of  FIG. 1 .  FIG. 3  is a perspective view of the antenna device  100  of  FIG. 2  to whose rear surface (an upper side in  FIG. 2 ) a double-faced adhesive tape  5  is attached and which is fixable into a cellular phone ( 200  of  FIG. 4 ).  FIG. 4  is a sectional view of the cellular phone  200  after the antenna device  100  is disposed.  FIG. 5  is a perspective view schematically showing a positional relation between a substrate  7  and the antenna device  100  in a case where the antenna device  100  of  FIG. 2  is disposed on the substrate  7 . 
   As illustrated in  FIG. 1 , the antenna element  1  of the present invention includes a parasitic antenna element  2  and a driven antenna element  3 . The parasitic element  2  has a main rectangular plate and a ground spring terminal  21  diagonally extending ahead (downward in  FIGS. 1 to 5 ) from the main rectangular plate. The parasitic element  2  does not have a feeding terminal. On the other hand, the driven element  3  has a main rectangular plate and a ground spring terminal  22  and a feeding spring terminal  23 . The terminals  22  and  23  diagonally extend ahead (downward in  FIGS. 1 to 5 ) from the main rectangular plate of the driven element  3 . The main plates of the parasitic antenna element  2  and the driven element  3  are disposed in the vicinity of each other. Particularly, the main plates of the element  2  and  3  are located in parallel to each other in the name plane. 
   As shown in  FIG. 5 , the parasitic element  2  is connected onto the substrate  7  by the ground spring terminal  21 . The driven element  3  is also connected onto the substrate  7  by the ground spring terminal  22  and the feeding spring terminal  23 . 
   The ground spring terminals  21  and  22  are connected to a ground pattern (not shown) of the substrate  7 , and grounded. The feeding spring terminal  23  is connected to a feeding pattern (not shown) of the substrate  7 , and is connected to a radio circuit (not shown). It is to be noted that in the present embodiment, each terminal is a spring terminal, but does not have to be the spring terminal. This is because any spring is not required, when the device is fixed to a rear case  8  by the double-faced tape  5  as illustrated in  FIG. 4 . 
   The antenna device  100  of the present invention is constituted of the antenna element  1 , a resin  4 , and additionally the double-faced adhesive tape  5  as illustrated in  FIG. 3 . 
   Moreover, the cellular phone  200  is constituted of the antenna device  100 , the substrate  7 , additionally the rear case  8 , and a front case  9  as illustrated in  FIG. 4 . 
   The substrate  7  is provided with a receiving section  10  and a transmitting section  11 . Additionally, the receiving section  10  and the transmitting section  11  are omitted from  FIG. 5 . 
   In  FIG. 2 , the resin  4  is charged into a lower part of the antenna element  1 , under the parasitic element  2  and the driven element  3 , and accordingly the parasitic element  2  is fixed to the driven element  3 . On the other words, the resin  4  integrates the parasitic element  2  with the driven element  3 . The resin  4  keeps a fixed interval between the parasitic element  2  and the driven element  3 . In the present invention, feeding is executed to the driven element  3  via the feeding spring terminal  23 . The feeding causes an electric field around the driven element  3  and the electric field causes an induced current in the parasitic element  2 . When the feeding has a first frequency equal to a natural resonance frequency of the driven element  3 , the driven element  3  is excited by the natural resonance frequency thereof during the feeding. On the other hand, when the feeding has a second frequency equal to a natural resonance frequency of the parasitic element  2 , the parasitic element  2  is excited by the natural resonance frequency thereof during the feeding. IF the first and the second frequencies are different from each other, both the antenna elements function as the different antennas for two frequency bands. 
   Meanwhile, when the interval between the parasitic element  2  and the driven element  3  changes in accordance with a direction and/or a state of the antenna device  100 , a constant induced current cannot be produced. Therefore, the resin  4  fixes the interval. 
   When the antenna device  100  is mounted on the substrate  7  in the constitution of  FIG. 2  as such, a total weight of the antenna device  100  is held by the ground spring terminals  21  and  22  and the feeding spring terminal  23 , and the device lacks a physical stability. As a measure, as shown in  FIG. 3 , when the double-faced adhesive tape  5  is attached to the upper surface of the antenna device  100  to fix the device to the rear case  8 , an excessive stress can be prevented from being applied to the above-described group of terminals  21 - 23 . A component (receiving section  10  in the present embodiment) can be effectively mounted also on a portion of the substrate  7  in front of the antenna device  100  (under the antenna device  100  in  FIG. 4 ). It is to be noted that the present embodiment does not deny that the device is held by the spring terminals  21 ,  22  and  23  as long as a sufficient strength can be held. 
     FIG. 4  is the sectional view of the cellular phone  200  to which the antenna device  100  is fixed by the double-faced adhesive tape  5 . The antenna device  100  is connected to the rear case  8  by the double-faced adhesive tape  5 . It is seen that any excessive stress is not applied to the terminals  21 - 23  and that a space for mounting a component can be secured also under the antenna device  100 . 
     FIG. 5  is the perspective view schematically showing a positional relation on the substrate  7  in a case where the antenna device  100  is disposed on the substrate  7  of the cellular phone  200  of  FIG. 4 . It is seen that the antenna device  100  is disposed in an upper portion of the cellular phone  200 , when the substrate  7  is disposed in the cellular phone  200 . Here, the upper portion is one of end portions in the longitudinal direction. Assuming a usual use method, the receiving section is disposed in the upper portion, and the transmitting section is disposed in an lower portion, but the present invention is not necessarily limited to this constitution. 
   It is to be noted that a problem of radio interference between the parasitic element  2  and the driven element  3  remains unsolved, but the interference can be suppressed using a duplexer (not shown). 
   Furthermore, adjustment of the natural resonance frequency of the parasitic element  2  can be modified by change of the shape and adjustment of a gap between the element and the substrate. Similarly, the natural resonance frequency of the driven element  3  can be modified by change of the shape and adjustment of a gap between the element and the substrate. Thus, the parasitic element  2  and the driven element  3  can be individually modified with respect to their natural resonance frequencies. 
   Second Embodiment 
   A second embodiment of the present invention will be described hereinafter with reference to  FIGS. 6 and 7 . 
     FIG. 6  is a perspective view of an antenna device  101  according to a second embodiment of the present invention.  FIG. 7  is a sectional view of a cellular phone  201  using the antenna device  101 . 
     FIG. 6  is the perspective view of the antenna device  101 . The device is different from the antenna device  100  of the first embodiment in that a cushion  6  is disposed instead of the double-faced adhesive tape  5 . Accordingly, the antenna device  101  is not fixed to the rear cover  8  but to a substrate  7 . The second embodiment is inferior to the first embodiment in a mountable area of the substrate  7 , but is superior to the first embodiment in ease of assembling. This is because the substrate  7  can be incorporated in the rear cover  8  while confirming that the antenna device  101  is fixed to the substrate  7 . To increase the mountable area of the substrate  7 , the cushion  6  may be hollowed (or shaped into a frame). Alternatively, the cushion  6  may be constituted of a group of small cushions, and the substrate  7  and a resin  4  may be fixed to each cushion. 
     FIG. 7  is the sectional view of the cellular phone  201  using the antenna device  101 . The cushion  6  is disposed between the resin  4  (or the elements  2  and  3 ) and the substrate  7 . The antenna device  101  is pressed onto an inner wall surface of the rear cover  8  by the cushion  6 . 
   It is to be noted that the device does not have to be necessarily pressed onto the inner wall surface of the rear cover  8  as long as a certain distance can be secured between the substrate  7  and the pair of the parasitic element  2  and driven element  3  of the antenna device  101  by the cushion  6 . This is because an effect similar to an effect obtained by pressing the device onto the inner wall surface is obtained by spring stresses of the ground spring terminals  21  and  22  and the feeding spring terminal  23 , and a reaction force of the cushion  6 . 
   Third Embodiment 
   A third embodiment of the present invention will be described hereinafter with reference to  FIG. 8 . 
     FIG. 8  is a perspective view showing a constitution of an antenna element  12 . The present embodiment is different from the embodiment of  FIG. 1  in that the element is connected to the substrate  7  via spring connectors. 
   The parasitic element  2  is connected to the substrate  7  by a ground spring connector  31 . It is to be noted that if there is a fear as to the element fixed by one point, one or more non-conductive spring connectors may be used to mechanically connect the element  2  to the substrate  7 . 
   The driven element  3  is fixed to the substrate  7  by a ground spring connector  32  and a feeding spring connector  33 . One or more non-conductive spring connectors may be used to mechanically connect the element  3  to the substrate  7 . 
   When the spring connectors  31 - 33  are used, a design for reducing, to the utmost, or eliminating contact of the element with an inner wall surface of a rear cover  8  becomes possible. 
   While this invention has thus far been described in conjunction with the preferred embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners. For example, the cushion  6  instead of the resin  4  may be used for integration of the parasitic element  2  and driven element  3 .