Patent Publication Number: US-8111200-B2

Title: Planar antenna and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a planar antenna and an electronic device. 
     2. Description of Related Art 
     Portable devices such as a handy terminal having a wireless communication function, PDA (Personal Digital Assistant), etc. have been known. A planar multiband antenna has been proposed as an antenna for wireless communication which is provided in a portable device (see JP-A-2007-13596, for example). Since the multiband antenna has a planar shape, it can easily be stored in the portable device. Moreover, wireless communications in a plurality of resonance frequency bands can be performed. 
     An inversed F-shaped antenna having an inversed F-shaped antenna element has also been known as an antenna for wireless communication. Furthermore, a multiband inversed F-shaped antenna has also been proposed (see JP-A-10-93332, for example). 
     However, the conventional multiband inversed F-shaped antenna has a plurality of rectangular antenna elements, and the band width of each resonance frequency is structurally narrow. 
     Since the conventional multiband inversed F-shaped antenna has a cubic resonance structure, a storage space for the antenna has to be large. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a main object of the present invention to extend the band width of a resonance frequency band in a multiband antenna and also reduce a storage space for the antenna. 
     According to a first aspect of the present invention, there is provided a planar antenna, including: a film formed of a planar insulating material; an antenna portion which is a planar conductor on the film; and a ground portion which is a conductor to be grounded, wherein the antenna portion includes: at least one first short stub; a first antenna element which is connected to the ground portion through the at least one first short stub and whose shape has such an angle that a distance between the first antenna element and the ground portion increases with increasing distance from a feeding point along the ground portion, the feeding point being provided between the first antenna element and the ground portion; a second short stub; and a second antenna element which is connected to the first antenna element through the second short stub. 
     According to a second aspect of the present invention, there is provided a planar antenna, including: a film formed of a planar insulating material; an antenna portion which is a planar conductor on the film; and a ground portion which is a conductor to be grounded, wherein the antenna portion includes: at least one first short stub; a first antenna element which is connected to the ground portion through the at least one first short stub, a feeding point being provided between the first antenna element and the ground portion; a plurality of second short stubs; and a second antenna element which is connected to the first antenna element through the second short stubs. 
     According to the present invention, the band width of each of the plurality of resonance frequency bands can be extended in the multiband antenna, and a storage space for the antenna can be reduced. 
     Furthermore, according to the present invention, the band width of the resonance frequency band corresponding to a second antenna element can be extended in the multiband antenna, and also a storage space for the antenna can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  is a front view showing a handy terminal according to preferred embodiments of the present invention; 
         FIG. 2A  is a perspective view of a back side of the handy terminal; 
         FIG. 2B  is a perspective view of one side of the handy terminal; 
         FIG. 2C  is a perspective view of a top side of the handy terminal; 
         FIG. 3  is a block diagram showing a circuit configuration of the handy terminal; 
         FIG. 4  shows a configuration of a planar antenna according to the embodiments; 
         FIG. 5  shows a connection configuration between the planar antenna and a coaxial cable in the embodiments; 
         FIG. 6  shows a configuration of a basic multiband planar antenna; 
         FIG. 7  shows routes of current flowing through the planar antenna of the embodiments; 
         FIG. 8  shows a relationship between a frequency and an S parameter in the planar antenna of the embodiments, and routes of current under resonance around a second frequency; 
         FIG. 9  shows an antenna element and a ground element around a feeding point P; 
         FIG. 10A  shows current distribution per unit length when a radio wave having a first resonance frequency is radiated in the planar antenna of the embodiments; 
         FIG. 10B  shows current distribution per unit length when a radio wave having a second resonance frequency is radiated in the planar antenna; 
         FIG. 11A  is a perspective view showing a film, an antenna conductor portion and an insulating layer; 
         FIG. 11B  is a cross-sectional view of the film, the antenna portion and the insulating layer; 
         FIG. 12A  shows a configuration of a planar antenna according to a first modification; 
         FIG. 12B  shows a configuration of another planar antenna according to the first modification; 
         FIG. 13  shows a configuration of a planar antenna according to a second modification; 
         FIG. 14  shows a configuration of a planar antenna according to a third modification; and 
         FIG. 15  shows a configuration of a planar antenna having three resonance frequencies. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to preferred embodiments of the present invention and first to third modifications as illustrated in the accompanying drawings. The present invention is not to be considered limited to what is shown in the drawings and the following detailed description. 
     The embodiments of the present invention will be described with reference to  FIGS. 1 to 10 . First, a device configuration of the embodiments will be explained with reference to  FIGS. 1 to 5 . 
       FIG. 1  is a front view showing a front-side configuration of a handy terminal  1  according to an embodiment.  FIG. 2A  is a perspective view showing a perspective configuration of the back side of the handy terminal  1 ,  FIG. 2B  is a perspective view showing a perspective configuration of one side of the handy terminal  1 , and  FIG. 2C  is a perspective view showing a perspective configuration of the top side of the handy terminal  1 . 
     The handy terminal  1  as an electronic device according to the embodiment is a portable terminal having functions such as input of information through a user&#39;s operation, storage of information, bar-code scanning, etc. The handy terminal  1  has ha as a function of performing wireless communication with an external device through an access point according to the wireless LAN (Local Area Network) system, and a cellular phone communication function based on the GSM (Global System for Mobile Communications) system. 
     The electronic device of this embodiment is not limited to the handy terminal  1 , and it may contain electronic devices such as PDA (Personal Digital Assistance), a cellular phone, a portable communication terminal, a portable device having a wireless communication function such as a portable computer, etc. 
     As shown in  FIG. 1 , the handy terminal  1  has a display unit  14 , a trigger key  3 A, various kinds of keys  3 C, etc. on the front surface of a case  2 . The handy terminal  1  has trigger keys  3 B on both the side surfaces of the case  2 . The trigger keys  3 A,  3 B serve to accept light irradiation of a scanning unit  10  described later and a trigger operation input of bar-code scanning. The various kinds of keys  3 C are character input keys of numerals, etc., and function keys for accepting the input of various kinds of functions such as mode switching, etc. 
     As shown in  FIGS. 2A ,  2 B and  2 C, the handy terminal  1  has a planar antenna  30 , a coaxial cable  40  as a feeding cable, a main board  4 , a chassis portion  5  as a second conductor portion, a GSM module  5   a , a battery  6 , a key board  3   a , a scanning unit  19 , etc. 
     The respective parts of the handy terminal  1  are connected to the main board  4 . The planar antenna  30  is an antenna used for the cellular phone communication as described above. Furthermore, the planar antenna  30  is fixed to the chassis portion  5  through screws. The planar antenna  30  will be described in detail later. 
     The chassis portion  5  is a chassis portion of the GSM module  5   a , etc. The chassis portion  5  is formed of metal (conductor) of magnesium alloy, aluminum or the like, and electrically grounded. Therefore, the chassis portion functions as a ground portion of the planar antenna  30 . The chassis portion  5  is regarded as being substantially rectangular, and the length in the short-side direction (lateral direction) and the length in the long-side direction (longitudinal direction) are represented by L 1  and L 2 , respectively. The lengths L 1  and L 2  correspond to the resonance frequencies of radio waves transmitted and received by the planar antenna  30 . The GSM module  5   a  is a module for performing the cellular-phone communication and is connected to the planar antenna  30  through a coaxial cable  40 . 
     The scanning unit  19  is a reading module for applying light such as a laser beam or the like to a bar-code and receiving and binarizing reflection light from the bar-code to read data of the bar-code. The battery  6  supplies power for the power supply of the handy terminal  1 . The key board  3   a  is provided with the trigger key  3 A and the various kinds of keys  3 C thereon, and outputs key input signals of these keys to the main board  4 . 
       FIG. 3  is a block diagram showing the circuit configuration of the handy terminal  1 . 
     As shown in  FIG. 3 , the handy terminal  1  has CPU (Central Processing Unit)  11  as a controller, an input unit  12 , RAM (Random Access Memory)  13 , a display unit  14 , ROM (Read Only Memory)  15 , a wireless communication unit  16  as a communication unit having a planar antenna  30 , a flash memory  17 , a wireless LAN communication unit  18  having an antenna  18   a , a scanning unit  19 , I/F (Inter Face)  20 , etc., and the respective parts are connected to one another through a bus  21 . 
     The CPU  11  controls the respective units of the handy terminal  1 . The CPU  11  reads out a specified program from the ROM  15  which stores a system program and various application programs, loads the specified program into the RAM  13 , and carries out various processing in cooperation with the program loaded into the RAM  13 . 
     In cooperation with the various programs, the CPU  11  accepts an input of operating information through the input unit  12 , reads out various information from the ROM  15 , reads out and writes various information from and into the flash memory  17 . Moreover, the CPU  11  controls the wireless communication unit  16  so that the handy terminal  1  can communicate with an external device through a base station using the planar antenna  30 . The CPU  11  controls the wireless LAN communication unit  18  so that the handy terminal  1  can communicate with an external device through an access point using the antenna  18   a . The CPU  11  controls the scanning unit  19  to read data of a bar code. The CPU  11  communicates with an external device through the I/F  20  using a communication cable. 
     The input unit  12  includes the trigger keys  3 A,  3 B and various keys  3 C, and outputs a key input signal of each key pressed by an operator to the CPU  11 . The input unit  12  may be designed to integrate with the display unit  14  so that a touch panel can be formed. 
     The RAM  13  is a volatile memory for temporarily storing information. The RAM  13  has a working area for temporarily storing various programs executed by the CPU  11  and various data associated with these programs. 
     The display unit  14  has a display such as liquid crystal display (LCD) and electro luminescent display (ELD). The display unit  14  executes a display processing in accordance with a signal from the CPU  11 . 
     The ROM  15  is a read only storage unit in which various programs and data are stored. 
     The wireless communication unit  16  is connected to the planar antenna  30 , and transmits/receives information to/from a base station through the GMS type communication by using the planar antenna  30 . In this embodiment, multiband wireless communication whose resonance frequency bands are set to about 800 [MHz] band (frequency f 2  band) and about 1900 [MHz] band (frequency f 1  band) is performed as the GSM type communication. The planar antenna  30  is also matched with these resonance frequency bands. However, the present invention is not limited to this embodiment, and the planar antenna  30  and the wireless communication unit  16  may be adapted to other resonance frequency bands or designed to perform wireless communications based on other wireless communication systems. 
     The flash memory  17  is a storage unit which can read out and write information such as various kinds of data. 
     The wireless LAN communication unit  18  is connected to the antenna  18   a . The wireless LAN communication unit  18  sends and receives information to and from an external device using the antenna  18   a  through an access point via wireless LAN communication. 
     The scanning unit  19  has a light emitting unit for laser beam or the like, a light receiving unit, a gain circuit, a binarizing circuit, etc. In the scanning unit  19 , light emitted from the light emitting unit is applied to a bar-code, and reflection light from the bar-code is received and converted to an electrical signal by the light receiving unit. The electrical signal is amplified in the gain circuit, and converted to data of a white and black bar-code image in the binarizing circuit. As described above, the scanning unit  19  reads a bar-code image, and outputs the data of the bar-code image concerned to CPU  11 . 
     The I/F  20  sends and receives information to and from an external device through a communication cable. The I/F  20  is a cable communication unit using universal serial bus (USB), for example. 
     Next, a configuration of the planar antenna  30  will be explained with reference to  FIG. 4 . 
       FIG. 4  shows a configuration of the planar antenna  30 . 
     The planar antenna  30  includes a film  30 A, an antenna conductor portion  30 B and an insulating layer  30 C. The film  30 A is a film of FPC (Flexible Print Circuit), and is formed of an insulator such as polyimide. The antenna conductor  30 B is formed of a single planar conductor such as copper foil, and is print-wired on the film  30 A. The insulating layer  30 C is formed of an insulator as a film of FPC, for example, and is formed on the film  30 A and the antenna conductor  30 B. The insulating layer  30 C has hole portions  30 C 1  and  30 C 2  for soldering. 
     The antenna conductor  30 B comprises an antenna portion  310  and a ground portion  320  as a first conductor portion. Power is supplied to the antenna portion  310  and the ground portion  320  is grounded. The antenna portion  310  has an antenna element  311  as a first antenna element, short stubs  312  and  313  as first short stubs, an antenna element  314  as a second antenna element, and short stubs  315  and  316  as second short stubs. The ground portion  320  has a ground element  321  and screw holes  322 . 
     The antenna element  311  is an antenna element for resonance at a higher resonance frequency f 1  of the two resonance frequencies f 1  and f 2 . The antenna element  311  has an almost triangular shape so that a vertex of the triangle is located in the neighborhood of the ground element  321 . 
     The coaxial cable  40  is connected between the vertex portion of the antenna element  311  (corresponding to the position of the hole portion  30 C 1 ) and an opposed portion to the vertex portion of the ground element  321  (corresponding to the position of the hole portion  30 C 2 ) by soldering. This connection portion is referred to as a feeding point P. 
     The antenna element  311  is connected to the short stub  312  at one end in the longitudinal direction thereof, and also connected to the short stub  313  so as to be spaced from the connection position of the short stab  312  at a predetermined distance. Furthermore, the short stub  312  is connected to the ground element  321 . The short stub  313  is connected to the ground element  321  so as to be spaced from the connection position between the short stub  312  and the ground element  321  at a predetermined distance. 
     The antenna element  314  is an antenna element for resonance at a lower resonance frequency f 2  of the two resonance frequencies f 1  and f 2 . The antenna element  314  is strip-shaped, and has a bending portion at some midpoint thereof. The bending portion is matched with the mount space of the planar antenna  30 , and the present invention is not limited to this shape. The antenna element  314  is connected to the short stub  315  at one end in the longitudinal direction thereof, and also connected to the short stub  316  so as to be spaced from the connection point of the short stub  315  at a predetermined distance. Furthermore, the short stub  315  is connected to a predetermined position of an intermediate portion in the longitudinal direction of the antenna element  311 . The longitudinal directions of the antenna element  311 , the antenna element  314  and the ground element  321  are set to one another. 
     The ground element  321  is trapezoidal, however, the present invention is not limited to this shape. The ground element  321  is fixedly connected and electrically conducted to the chassis portion  5  by fixing screws through the screw holes  322 . Therefore, the ground portion  320  and the chassis portion  5  integrally function as the ground. 
     Next, the connection between the planar antenna  30  and the coaxial cable  40  at the feeding point P will be described with reference to  FIG. 5 . 
       FIG. 5  is a diagram showing the connection configuration between the planar antenna  30  and the coaxial cable  40 . The film  30 A and the insulating layer  30 C are omitted from the illustration of  FIG. 5 . 
     The coaxial cable  40  has a core wire  41  formed of copper wire or the like, an insulator  42  formed of polyethylene or the like, an external conductor  43  formed of a meshed copper wire or the like and a protection coating member  44  as an insulator which are successively arranged concentrically. The core wire  41  at one end of the coaxial cable  40  is passes through the hole portion  30 C 1  and connected to the antenna element  311  by soldering, and the external conductor  43  is passes through the hole portion  30 C 2  and connected to the ground element  321  by soldering. 
     The other end of the coaxial cable  40  is connected to the GSM module  5   a . The core wire  41  at the other end of the coaxial cable  40  is connected to a power feeding terminal of the GSM module  5   a , and the external conductor  43  is connected to the ground of the GSM module  5   a . High frequency power is supplied from the GSM module  5   a  through the coaxial cable  40  to the feeding point P. 
     Next, the details of the planar antenna  30  will be described with reference to  FIGS. 6 to 10 . In order to simplify the description and the illustration, the antenna conductor portion  30 B of the planar antenna  30  will be described, and the description and illustration of the film  30 A and the insulating layer  30 C are omitted. 
     First, the operation principle of a basic multiband planar antenna  50  will be described. 
       FIG. 6  is a diagram showing the configuration of the basic multiband planar antenna  50 . The planar antenna  50  is an antenna resonating at frequencies f 1  and f 2 . 
     As shown in  FIG. 6 , the planar antenna  50  has an antenna portion  510  and a ground portion  520 . The antenna portion  510  has a antenna element  511 , short stubs  512  and  513 , an antenna element  514  and a short stub  515  which are rectangular in shape. 
     The antenna element  511  is disposed so that the longitudinal direction thereof is parallel to the ground portion  520 , and connected to the ground portion  520  through the short stubs  512  and  513 . The antenna element  514  is disposed so that the longitudinal direction thereof is parallel to the longitudinal direction of the antenna element  511 , and connected to the antenna element  511  through the short stub  515 . A feeding point P is provided between one end of the short stub  513  and the ground portion  520 . 
     The ground portion  520 , the short stub  513 , the antenna element  511  and the short stub  512  constitutes a minute loop portion, and loop current flows into the loop portion, whereby the impedance matching is established and the depth of the resonance is adjusted. 
     The length L 3  of a route passing through the short stub  512  and the antenna element  511  as a current-flowing route is set to ¼ of the wavelength λ 1  of the resonance frequency f 1 . Likewise, the length L 4  of a route passing through the short stub  512 , the antenna element  511 , the short stub  515  and the antenna element  514  as a current-flowing route is set to ¼ of the wavelength λ 2  of the resonance frequency f 2 . Therefore, the planar antenna  50  functions as a multiband antenna resonating when the radio waves of the two resonance frequency f 1 , f 2  bands are transmitted/received, thereby obtaining a high gain. 
     As described above, the planar antenna  50  is the multiband antenna of the two resonance frequency f 1 , f 2  bands. However, there is a little mode resonating in each frequency band, so that the band width thereof is relatively narrow. 
     Next, the planar antenna  30  according to this embodiment will be described. First, as in the case of the planar antenna  50 , the planar antenna  30  resonates at the two resonance frequencies f 1 , f 2  (f 1 &gt;f 2 ). The length of a route passing through the short stubs  312 ,  313  and the antenna element  311  as a current-flowing route is set to ¼ of the wavelength λ 1  of the resonance frequency f 1 . Furthermore, the length of a route passing through the short stubs  312 ,  313 , the antenna element  311 , the short stubs  315 ,  316  and the antenna element  314  as a current-flowing route is set to ¼ of the wavelength λ 2  of the resonance frequency f 2 . 
     Next, the configuration of broadening the resonance frequency band of the planar antenna  30  will be described. 
       FIG. 7  is a diagram showing a route for current flowing through the planar antenna  30 . 
     As shown in  FIG. 7 , the planar antenna  30  has an antenna portion  310  and a ground portion  330 . The ground portion  330  is a ground portion when the ground portion  320  and the chassis portion  5  are regarded as being integral with each other. 
     By designing the antenna element  311  in a substantially triangular shape, a plurality of length-different routes (a plurality of modes) through which current flows can be formed between the feeding point P in the neighborhood of the vertex of the triangle of the antenna element  311  and each end in the longitudinal direction of the antenna element  311  so as to be displaced inside, and thus the antenna element  311  resonates at the different modes, thereby securing the plurality of routes having different lengths through which current flows. The length of the route varies in accordance with the magnitude of the frequency. Therefore, the resonance frequency band is extended by the routes of the plurality of lengths. 
     Next, another configuration of broadening the resonance frequency band of the planar antenna  30  will be described. 
       FIG. 8  is a diagram showing a relationship between frequency and an S parameter in the planar antenna  30 , and routes of current under resonance around a second frequency f 2 . The S parameter is called as a scattering matrix (S matrix) or a scattering parameter. The S parameter represents a passage/reflection power characteristic of a circuit network. 
     As shown in  FIG. 8 , the S parameter with respect to frequency is low around frequencies f 1  and f 2 . A high gain is obtained in the band widths containing the frequencies f 1  and f 2  as the resonance frequencies. As shown in  FIG. 8 , the S-parameter low portions around the frequencies f 1  and f 2  are broad, and thus the band width of the resonance frequency thereof is extended. 
     As compared with the planar antenna  50 , in the planar antenna  30 , the antenna element  314  resonating at the frequency f 2  is short-circuited to the antenna element  311  resonating at the frequency f 1  through the short tub  315  and the short tub  316 . As shown in  FIG. 8 , in the band of the frequency f 2 , when the resonating frequency is lower than the frequency f 2 , current flows through the route passing through the short stub  312 , the antenna element  311 , the short stub  315  and the antenna element  314 . 
     When the resonating frequency is in the neighborhood of the center of the band of the frequency f 2 , current flows in the route passing through the short stub  312 , the antenna element  311 , the short stub  315  and the antenna element  314  and in the route passing through the short stub  312 , the antenna element  311 , the short stub  316  and the antenna element  314 . When the resonance frequency is higher than the frequency f 2 , the current flows through the short stub  312 , the antenna element  311 , the short stub  316 , and the antenna element  314 . 
     Therefore, the place to which current is concentrated varies due to the provision of the short stub  316 , and thus the same advantage obtained when the plurality of antenna elements different in length exist is obtained, and the band width of the resonance frequency band of the lower frequency f 2  out of the two resonance frequencies f 1  and f 2  can be extended. 
     Next, the impedance matching in the planar antenna  30  will be described. 
       FIG. 9  is a diagram showing the antenna element  311  and the ground element  321  in the neighborhood of the feeding point P. 
     As shown in  FIG. 9 , the antenna element  311  has a substantially triangular shape having angles θ 1  and θ 2  with respect to the ground element  321  with the feeding point P at the center. When the angles θ 1  and θ 2  increase, the impedance viewed from the feeding point P increases. Furthermore, when the angles θ 1  and θ 2  decrease, the impedance viewed from the feeding point P is lowered. The matching of the impedance viewed from the feeding point P can be established by adjusting the angles θ 1  and θ 2 . 
     Next, the shape and size of the ground portion will be described. 
       FIG. 10A  is a diagram showing current distribution per unit length in the planar antenna  30  when a radio wave having the resonance frequency f 1  is emitted. 
       FIG. 10B  is a diagram showing current distribution per unit length in the planar antenna  30  when a radio wave having the resonance frequency f 2  is emitted. 
     In  FIGS. 10A and 10B , it is assumed that the current per unit length increases as the drawing color is shifted from black to white. 
     As shown in  FIG. 2A  and  FIGS. 10A and 10B , in the planar antenna  30 , the length of the side S 1  in the short-side direction (horizontal direction) of the ground portion  330  is represented by L 1 , and the length of the side S 2  in the longitudinal direction (vertical direction) is represented by L 2 . Here, the horizontal direction and the vertical direction are defined with respect to the ground surface when the handy terminal  1  is used. The length L 1  is set to ¼ of the wavelength λ 1  of the higher resonance frequency f 1 , and the length L 2  is set to ¼ of the wavelength λ 2  of the lower resonance frequency f 2 . 
     As shown in  FIG. 10A , in the planar antenna  30 , an area in the vicinity of the side S 1  resonates and current concentrates there when the radio wave of the resonance frequency f 1  is emitted, so that the emitted radio wave is a horizontally-polarized wave. 
     As shown in  FIG. 10B , in the planar antenna  30 , an area in the vicinity of the side S 2  resonates and current concentrates there when the radio wave of the resonance frequency f 2  is emitted, so that the emitted radio wave is a vertically-polarized wave. As described above, the polarization direction of the wave transmitted/received to/from the planar antenna  30  can be varied. 
     The base station receives the radio wave of the vertically-polarized wave. Electrical waves having low frequencies are little reflected, and thus it is preferable to set the radio wave of the lower frequency f 2  to the vertically polarized wave in conformity with the base station. Accordingly, the length of the side S 2  in the vertical direction of the ground portion  330  is set to correspond to the resonance frequency f 2 . 
     Next, the process of manufacturing the planar antenna  30  will be described. 
       FIG. 11A  is a diagram showing the perspective configuration of the film  30 A, the antenna conductor  30 B and the insulating layer  30 C, and  FIG. 11B  is a diagram showing the cross-sectional configuration of the film  30 A, the antenna conductor  30 B and the insulating layer  30 C. 
     As shown in  FIG. 11A , the antenna conductor  30 B is first formed on the film  30 A. The formation of the antenna conductor  30 B is implemented by a method such as etching on the film  30 A, adhesion of adhesive agent, double-sided adhesive tape or the like. 
     The insulating layer  30 C is formed on the film  30 A on which the antenna conductor  30 B has been formed. The formation of the insulating layer  30 C is implemented by adhesion of film of FPC or the like. However, the present invention is not limited to this manner, and a method of painting coating material for insulation or the like may be used. The coating material for insulation comprises a resist material such as insulating ultraviolet effect resin or the like. The insulating layer  30 C is formed so that the hole portions  30 C 1  and  30 C 2  are set at the soldering position of the feed point P. 
     As shown in  FIG. 5 , the coaxial cable  40  is soldered to the hole portions  30 C 1  and  30 C 2  of the insulating layer  30 C shown in  FIGS. 11A and 11B . A soldering manufacturer may solder the hole portions  30 C 1  and  30 C 2  which are formed in advance, and an unskilled person can easily solder and prevent the soldering position from being displaced. Furthermore, the unnecessary film  30 A and insulating layer  30 C are deleted. 
     As described above, according to the embodiments, in the planar antenna  30 , the antenna element  311  is connected to the ground portion  320  through the short stubs  312  and  313 , and the feeding point P is provided between the antenna element  311  and the ground portion  320 . The antenna element  311  has a triangular shape with such an angle that a distance between the antenna element  311  and the ground portion  320  increases with increasing distance from the feeding point P along the ground portion  320 . The antenna element  311  has such a length that the antenna element  311  resonates at the high frequency f 1 . The antenna element  314  has such a length that the antenna element  314  resonates at the low resonance frequency f 2 . With this structure, the antenna element  311  has a plurality of antenna current routes different in length. Therefore, the band width of each of the resonance frequency f 1  band and the resonance frequency f 2  band can be extended. Moreover, because the planar antenna  30  (the antenna portion  310  and the ground portion  320 ) is planar, the storage space can be reduced. Furthermore, the handy terminal  1  having the planar antenna  30  can perform the wireless communication in which the respective band widths of the resonance frequency f 1  band and the resonance frequency f 2  bands are wide. 
     The antenna element  314  is connected to the antenna element  311  through the two short stubs  315 ,  316 . Therefore, there exist a plurality of routes in which current flows through at least one of the short stubs  315  and  316  at a frequency in the neighborhood of the frequency f 2 , and thus the band width of the resonance frequency f 2  band can be extended. 
     In the ground portion  330 , the length of the side S 1  is set to ¼ of the wavelength λ 1  of the radio wave of the resonance frequency f 1 , and the length of the side S 2  is set to ¼ of the wavelength λ 2  of the radio wave of the resonance frequency f 2 . Therefore, the polarization direction of the radio wave emitted from the planar antenna  30  can be varied, and the gain can be increased. Particularly, the side S 2  which is vertical when the handy terminal  1  is used is set to correspond to the lower resonance frequency f 2 , so that the polarization direction of the radio wave of the resonance frequency f 2  having a small gain can be set to the same vertically polarized wave direction as the base station, and thus the gain can be increased. 
     Furthermore, the ground portion  330  comprises the ground portion  320  formed on the film  30 A and the chassis portion  5 . Accordingly, the chassis portion  5  can be used as the ground, and the planar antenna  30  (the antenna portion  310 , the ground portion  320 ) and the storage space thereof can be more greatly reduced. Furthermore, the material of the ground portion  320  is reduced, and thus the cost can be reduced. 
     The planar antenna  30  has the insulating layer  30 C. Therefore, even when the packaging density of the planar antenna  30  in the handy terminal  1  is increased, the antenna conductor portion  30 B can be prevented from being short-circuited to the other parts, the cable, etc. 
     The insulating layer  30 C has the soldering hole portions  30 C 1  and  30 C 2  for the coaxial cable  40  at the feeding point P. Therefore, the soldering position can be fixedly provided at the manufacturing stage, and variation of the antenna performance due to production tolerance can be eliminated. 
     (First Modification) 
     A first modification of the above embodiment will be described with reference to  FIGS. 12A and 12B . The device configuration of this modification resides in that the planar antenna  30  in the handy terminal  1  is replaced by planar antennas  60   a  and  60   b . The configuration of the planar antennas  60   a  and  60   b  will be mainly described, and the description of the other configuration is omitted. 
       FIG. 12A  is a diagram showing the configuration of the planar antenna  60   a . As shown in  FIG. 12A , the planar antenna  60   a  comprises an antenna portion  610   a  and a ground portion  630  as an antenna conductor portion. The antenna portion  610   a  has an antenna element  611  as a first antenna element, a short stub  612  as a first short stub, an antenna element  614  as a second antenna element and short stubs  615  and  616  as second short stubs. However, the ground portion  630  contains the ground portion of the antenna conductor portion and the chassis portion  5 . The antenna element  611 , the antenna element  614 , the short subs  615  and  616  and the ground portion  630  are the same as the antenna element  311 , the antenna element  314 , the short stubs  315 ,  316  and the ground portion  330  of the planar antenna  30  in this order. 
     In the planar antenna  60   a , a feeding point P is provided between the antenna element  611  and the ground portion  630 . Furthermore, the planar antenna  60   a  has a film and an insulating layer (not shown) as in the case of the film  30 A and the insulating layer  30 C of the planar antenna  30 . 
     The planar antenna  60   a  is provided with a single short stub  612  instead of the two short stubs  312  and  313 , and one end of the antenna element  611  is short-circuited to the ground portion  630  through the short stub  612 . The short stub  612  has such a shape and width that the empty area between the short stubs  312  and  313  is filled. 
     According to the planar antenna  60   a  of this modification, the same advantage as the planar antenna  30  can be obtained. This is because the current flowing in the short stub  612  concentrates on both the right and left end portions thereof and thus the short stub  612  has the same function as the short stubs  312  and  313 . Furthermore, three or more short stubs may be provided to connect the antenna element  611  and the ground portion  630 . 
       FIG. 12B  is a diagram showing the configuration of a planar antenna  60   b . As shown in  FIG. 12B , the planar antenna  60   b  comprises an antenna portion  610   b  and a ground portion  630  as an antenna conductor. The antenna portion  610   b  has an antenna element  611 , a short stub  612 , an antenna element  614  and a short stub  617  as a second short stub. The planar antenna  60   b  has a film and an insulating layer (not shown) as in the case of the film  30 A and the insulating layer  30 C of the planar antenna  30 . 
     The planar antenna  60   b  is provided with a single short stub  617  instead of the two short stubs  615  and  616 , and one end of the antenna element  614  is short-circuited to the antenna element  611  through the short stub  617 . The short stub  617  has such a shape and width that the empty area between the short stubs  615  and  616  is filled. 
     According to the planar antenna  60   b  of this modification, the same advantage as the planar antenna  30  is obtained. This is because current flowing in the short stub  617  concentrates on both the right and left end portions and thus the short stub  617  has the same function as the short stubs  615  and  616 . Three or more short stubs may be provided to connect the antenna element  611  and the antenna element  614 . 
     (Second Modification) 
     A second modification of the above embodiment will be described with reference to  FIG. 13 . The device configuration of this modification resides in that the planar antenna  30  in the handy terminal  1  is replaced by a planar antenna  70 . The configuration of the planar antenna  70  will be mainly described, and the description of the other configuration is omitted. 
       FIG. 13  is a diagram showing the configuration of the planar antenna  70 . As shown in  FIG. 13 , the planar antenna  70  has an antenna portion  710  and a ground portion  730  as an antenna conductor portion. The antenna portion  710  has an antenna element  711  as a first antenna element, a short stub  712  as a first short stub, an antenna element  714  as a second antenna element, and short stubs  715  and  716  as second short stubs. The ground portion  730  contains the ground portion of the antenna conductor portion and the chassis portion  5 . The antenna element  714 , the short stubs  715  and  716  and the ground portion  730  are the same as the antenna element  314 , the short stubs  315  and  316  and the ground portion  330  of the planar antenna  30  in this order. The planar antenna  70  has a film and an insulating layer (not shown) as in the case of the film  30 A and the insulating layer  30 C of the planar antenna  30 . 
     The antenna element  711  has a right triangular shape, and a feeding point P is provided between a vertex portion at the lower side of  FIG. 13  and the ground portion  730 . That is, the antenna element  711  has a shape with such an angle that a distance between the antenna element  711  and the ground portion  730  increases with increasing distance from the feeding point P along the upper side of the ground portion  730 . 
     The short stub  712  is located below the short stub  715 . The antenna element  711  is short-circuited to the ground portion  730  through the short stub  712 . The antenna element  711 , the ground portion  730  and the short stub  712  constitutes a loop. 
     According to the planar antenna  70  of this modification, the same advantage as the planar antenna  30  can be obtained. Furthermore, the position of the feeding point P provided between the antenna element and the ground portion may be changed to another position in the loop constructed by the short stub. The antenna element corresponding to the resonance frequency f 1  may have a shape with such an angle that a distance between the antenna element and the ground portion increases with increasing distance from the feeding point P along the upper side of the ground portion. 
     (Third Modification) 
     A third modification of the above embodiment will be described. The device configuration of this modification resides in that the planar antenna  30  in the handy terminal  1  is replaced by a planar antenna  80 . The configuration of the planar antenna  80  will be mainly described, and the description of the other configuration is omitted. 
       FIG. 14  is a diagram showing the configuration of the planar antenna  80 . As shown in  FIG. 14 , the planar antenna  80  comprises, as an antenna conductor portion, an antenna portion  810  and a ground portion  820  as a first conductor. The antenna portion  810  has an antenna element  811  as a first antenna element, short stubs  812  and  813  as first short tubs, an antenna element  814  as a second antenna element and short stubs  815  and  816  as second short stubs. The antenna element  811 , the short stubs  812  and  813 , the antenna element  814 , the short stubs  815  and  816  and the ground portion are the same as the antenna element  311 , the short stubs  312  and  313 , the antenna element  314  and the short stubs  315  and  316  in this order. 
     In the planar antenna  30 , the ground portion  320  which is the conductor sandwiched between the film  30 A and the insulating layer  30 C is provided integrally with the chassis portion  5  and considered as the ground portion  330 . The planar antenna  80  has a ground portion  820  which is a conductor sandwiched between the film and the insulating layer. In the antenna portion  810 , a feeding point P is provided between the antenna element  811  and the ground portion  820 . The ground portion  820  is not directly electrically connected to the chassis portion  5 . 
     As in the case of the ground portion  330 , the ground portion  820  is rectangular, and the length in the short-side direction thereof is represented by L 1  while the length in the longitudinal direction thereof is represented by L 2 . The length L 1  is set to the length of ¼ of the wavelength λ 1  of the higher resonance frequency f 1 , and the length L 2  is set to the length of ¼ of the wavelength λ 2  of the lower resonance frequency. 
     According to the planar antenna  80  of this modification, the same advantage as the planar antenna  30  can be obtained, and also the ground portion  820  of the planar antenna  80  can be manufactured integrally with the film, the conductor and the insulator. 
     The embodiment and the modifications described above are examples of the planar antenna and the electronic device, and the present invention is not limited to these embodiment and modifications. 
     In the above embodiment and the above modifications, the handy terminal is used as the electronic device. However, the present invention may be applied to PDA (Personal Digital Assistant), a cellular phone, a portable communication terminal, a portable device having a wireless communication function such as a portable type computer or the like, and other electronic devices. 
     In the above embodiment, the antenna element  311  may be oblong or the like, and a plurality of short stubs may be provided to connect the antenna element  311  and the antenna element  314 . The ground portion may be constructed by only a ground part which is not the antenna conductor portion such as the chassis portion  5  or the like. Furthermore, at least two of the above embodiment and the modifications may be properly combined with each other. 
     Furthermore, in the above embodiment and the modifications, the ground portion is rectangular, and the length L 1  in the horizontal direction corresponds to the higher resonance frequency f 1  while the length L 2  in the vertical direction corresponds to the lower resonance frequency f 2 . However, the present invention is not limited to this style. For example, the length L 1  in the horizontal direction may correspond to the resonance frequency f 2  while the length L 2  in the vertical direction corresponds to the resonance frequency f 1 . 
     In the above embodiment and the modifications, the planar antenna is the multiband antenna resonating at the two frequency bands, however, the present invention is not limited to this style. For example, the number of the antenna elements of the planar antenna may be set to three or more so that a multiband antenna resonating at three or more frequency bands corresponding to the lengths of the respective antenna elements is constructed. 
     Here, an example of the planar antenna having the three resonance frequency bands will be described. 
       FIG. 15  is a diagram showing the configuration of the planar antenna  90  having the three resonance frequency bands. 
     As shown in  FIG. 15 , the planar antenna  90  has an antenna portion  910  and a ground portion  930  as an antenna conductor portion. The antenna portion  910  has an antenna element  911  as a first antenna element, short stubs  912  and  913  as first short stubs, an antenna element  914  as a second antenna element, short stubs  915  and  916  as second short stubs, an antenna element  917  and a short stub  918 . The ground portion  930  contains the ground portion of the antenna conductor portion and the chassis portion  5 . The antenna element  911 , the short stubs  912  and  913 , the antenna element  914 , the short stubs  915  and  916  and the ground portion  930  are the same as the antenna element  311 , the short stubs  312  and  313 , the antenna element  314 , the short stubs  315  and  316  and the ground portion  330  of the planar antenna  30  in this order. 
     The antenna element  917  is connected to the antenna element  914  through the short stub  918 . An end portion of the antenna element  914  (the connection side to the short stub  915 ) and an end portion of the antenna element  917  are connected to each other through the short stub  918 . The antenna element  911  has a length at which it resonates at a frequency f 1 . The antenna element  914  has a length at which it resonates at a frequency f 2  lower than the frequency f 1 . The antenna element  917  has a length at which it resonates at a frequency f 3  between the frequency f 1  and the frequency f 2 . However, the resonance frequency is not limited to the relationship of f 2 &lt;f 3 &lt;f 1 , and it may satisfy the relationship of f 3 &lt;f 2 &lt;f 1 , for example. 
     If the number of the resonance frequencies is equal to three or more, the shape of the ground portion may be a shape having three or more sides different in length from one another, such as a trapezoid, and each side may have the length corresponding to each resonance frequency (the length of ¼ of the wavelength of the radio wave of the resonance frequency). 
     In the above embodiments and the above modifications, the insulating layer of the planar antenna is located at the case  2  side. However, the present invention is not limited to this style. The film of the planar antenna may be located at the case  2  side. 
     In the planar antennas of the above embodiment and the above modifications, the hole portion penetrating through the film and the insulating layer may be provided at a portion in which no antenna conductor portion exists. For example, another insulating part, for example, an insulating support part for the case  2  or the like may be made to penetrate through the hole portion. 
     With respect to the detailed configurations and operations of the respective elements of the planar antennas and the handy terminals as electronic devices in the above-described embodiments, it will be apparent to those skilled in the art that various modification and variations can be made without departing from the scope of the invention. 
     The entire disclosure of Japanese Patent Application No. 2008-140595 filed on May 29, 2008 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety. 
     Although various exemplary embodiments have been shown and described, the invention is not limited to the embodiments shown. Therefore, the scope of the invention is intended to be limited solely by the scope of the claims that follow.