Patent Publication Number: US-11030507-B2

Title: Antenna for RF tag, and RF tag

Description:
TECHNICAL FIELD 
     The present invention relates to an RF tag antenna and an RF tag. 
     BACKGROUND ART 
     In recent years, in various fields such as logistics, RFID systems using RFID (radio frequency identification) have been studied. 
     In an RF tag for use in an RFID system, an antenna and an IC chip are provided. The RF tag receives, using the antenna of the RF tag, radio waves (carrier wave) transmitted from a reading device. And the RF tag sends, as a response thereto, identification data and the like recorded in the IC chip to the reading device by carrying the data and the like on a reflected wave. By virtue of this, it is made possible to perform communications between the RF tag and the reading device without bringing the reading device into contact with the RF tag. Some reading devices such as a reader/writer have a writing function to write information to the RF tag. 
     Patent Literature 1 discloses, as an antenna of an RF tag, a patch antenna that includes a plate-like radiation conductor and a conductor ground plane (ground conductor) which are arranged on the front and back surfaces, respectively, of a dielectric substrate. Patent Literature 2 discloses a patch antenna that has a configuration in which a magnetic sheet sandwiched between an antenna section and a conductor ground plane. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 4379470 
     Patent Literature 2: Japanese Patent Application Laid-Open No. 2013-110685 
     SUMMARY OF INVENTION 
     Technical Problem 
     In RFID systems, there have been strong demands for extended communication distance between an RF tag and a reading device in terms of improved usability. 
     In view of this, an object of the present invention is to provide an RF tag antenna and an RF tag that make it possible to extend the communication distance with the reading device. 
     Solution to Problem 
     An RF tag antenna in accordance with the present invention includes an insulation member; a first waveguide element provided on one side of the insulation member; a second waveguide element arranged so as to face the first waveguide element, the second waveguide element being provided on another side of the insulation member; a power feeding section having one end electrically connected to the second waveguide element; and a short circuit section having one end electrically connected to the first waveguide element and another end electrically connected to the second waveguide element, in which the first waveguide element, the second waveguide element, the insulation member, the power feeding section, and the short circuit section constitute a planar inverted-F antenna configured to receive radio waves transmitted from a reading device; and a first insulating region and a second insulating region are defined within a region between the first waveguide element and the second waveguide element, the first insulating region faces the first waveguide element, the second insulating region faces the second waveguide element, a permittivity of the first insulating region and a permittivity of the second insulating region is different from one another. 
     Also, in the RF tag antenna, the permittivity of the first insulating region may be smaller than the permittivity of the second insulating region when the first waveguide element functions as a waveguide conductor that absorbs the radio waves and the second waveguide element functions as a ground conductor. 
     Also, in the RF tag antenna, the insulation member may have a first main surface on one side and a second main surface on the other side, and a plurality of bottomed holes may be provided in the first main surface. 
     Also, in the RF tag antenna, diameters of the bottomed holes may be gradually reduced from the first main surface toward the second main surface. 
     Also, in the RF tag antenna, the insulation member may have a first main surface on one side and a second main surface on the other side, and raised and depressed sections may be formed in the first main surface and/or second main surface. 
     Also, in the RF tag antenna, the shapes of raised and depressed sections may be different from each other on the first main surface and the second main surface. 
     Also, in the RF tag antenna, the insulation member may have a first main surface on one side and a second main surface on the other side, the insulation member may have a first insulating substrate including the first main surface and a second insulating substrate including the second main surface, and the permittivity of the first insulating substrate and the permittivity of the second insulating substrate may be different from each other. 
     An RF tag in accordance with the present invention is an RF tag that includes an RF tag antenna in accordance with the present invention and an IC chip that operates based on the radio waves transmitted from the reading device, in which the second waveguide element is arranged such that it is in contact with a conductor, and the permittivity of the second insulating region is larger than the permittivity of the first insulating region. 
     An RF tag in accordance with the present invention is an RF tag that includes an RF tag element including an RF tag antenna in accordance with the present invention and an IC chip that operates based on the radio waves transmitted from the reading device; and a case that accommodates the RF tag element, in which the case has a mounting unit for mounting the RF tag to a target object of mounting. 
     Also, in the RF tag, the case may be configured such that the position of the short circuit section is visible from the outside of the case. 
     Also, in the RF tag, a biasing member that presses the RF tag element toward to the second inner surface facing the first inner surface may be provided in the gap between the RF tag element and the first inner surface of the case. 
     Also, in the RF tag, the case may be made of a conductive material, and an opening through which radio waves transmitted from the reading device pass may be provided in the case. 
     Advantageous Effect of Invention 
     According to the present invention, it is made possible to provide an RF tag antenna and an RF tag that make it possible to extend the communication distance with the reading device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a perspective view of an RF tag element in accordance with an embodiment viewed from above. 
         FIG. 1B  is a perspective view of the RF tag element in accordance with the embodiment viewed from below. 
         FIG. 1C  is an exploded view of a sheet of the RF tag element in accordance with the embodiment. 
         FIG. 2  is a cross-sectional view taken along the line I-I of  FIG. 1A  in the embodiment. 
         FIG. 3  is a plan view of an insulation member in accordance with the embodiment. 
         FIG. 4  is a cross-sectional view taken along the line II-II of  FIG. 3 . 
         FIG. 5  is a diagram of an equivalent circuit of the RF tag element in accordance with the embodiment. 
         FIG. 6  is a cross-sectional view taken along the line I-I of  FIG. 1A  in the case of Modified Example 1 of the embodiment. 
         FIG. 7  is a perspective view of an insulation member in accordance with Modified Example 1 of the embodiment. 
         FIG. 8  is a cross-sectional view taken along the line I-I of  FIG. 1A  in the case of Modified Example 2 of the embodiment. 
         FIG. 9  is a perspective view illustrating a preferred example of arrangement of a plurality of RF tag elements on a conductor. 
         FIG. 10  is a longitudinal cross-sectional view of the RF tag in accordance with the embodiment. 
         FIG. 11  is a longitudinal cross-sectional view of an RF tag having a case in which a hole is provided as a mounting unit. 
         FIG. 12  is a longitudinal cross-sectional view of an RF tag having an adhesive layer as a biasing member. 
         FIG. 13  is a longitudinal cross-sectional view of an RF tag having a case in which a projection is provided as a biasing member. 
         FIG. 14A  is a perspective view of a conductive case in which an opening is provided. 
         FIG. 14B  is a perspective view of a conductive case in which an opening is provided. 
         FIG. 14C  is a perspective view of a conductive case in which an opening is provided. 
         FIG. 14D  is a perspective view of a conductive case in which an opening is provided. 
         FIG. 14E  is a perspective view of a conductive case in which an opening is provided. 
         FIG. 15  is a schematic diagram illustrating another example of how to assemble an RF tag  1 . 
         FIG. 16  is diagram illustrating a measurement system for measuring a minimum communication-possible gain of an RF tag element. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that, in each of the drawings, components having the same or equivalent functions are denoted by the same reference numerals. 
     First, an RF tag antenna  10  and an RF tag element  100  in accordance with the embodiment will be described. 
     As illustrated in  FIG. 1A  and  FIG. 1B , the RF tag element  100  includes an RF tag antenna  10  that receives radio waves transmitted from a reading device; and an IC chip  80  that operates based on the radio waves. 
     The RF tag antenna  10  includes a waveguide element  20  (first waveguide element), a waveguide element  30  (second waveguide element), an insulation member  40 , a power feeding section  50 , a short circuit section  60 , and an insulating sheet  70 . 
     The waveguide element  20 , the waveguide element  30 , the insulation member  40 , the power feeding section  50 , and the short circuit section  60  constitute a planar inverted-F antenna that receives radio waves transmitted from the reading device (not shown). When the waveguide element  20  functions as a waveguide conductor that absorbs radio waves, then the waveguide element  30  functions as a conductor ground plane. On the other hand, when the waveguide element  30  functions as a waveguide conductor, then the waveguide element  20  functions as the conductor ground plane. In other words, the waveguide elements  20  and  30  perform both of the functions of a waveguide conductor and a conductor ground plane depending on the mode of usage of the RF tag element  100 . 
     The IC chip  80  is arranged, as illustrated in  FIG. 1A , in the same plane as the waveguide element  20 , and is arranged between the waveguide element  20  and the power feeding section  50 . Note that the IC chip  80  may be arranged on a side surface of the insulation member  40  within the range where it functions as the planar inverted-F antenna. Also, an external power source may be connected to the IC chip  80  and the IC chip  80  may be configured to operate by the voltage supplied from the external power source. 
     The IC chip  80  is configured to operate based on the radio waves received by the planar inverted-F antenna of the RF tag antenna  10 . Specifically, the IC chip  80  first rectifies part of the carrier waves transmitted from the reading device and generates power supply voltage necessary for the operation. Then the IC chip  80  uses the generated power supply voltage to operate the control logic circuit in the IC chip  80  or a non-volatile memory unit that stores product-specific information or the like. Also, the IC chip  80  operates a communication circuit for performing transmission and reception of data to/from the reading device or any other relevant circuits. 
     Next, the individual components of the RF tag antenna  10  will be described in detail. 
     The waveguide element  20  is provided on one side of the insulation member  40 . The waveguide element  30  is arranged such that it faces the waveguide element  20 , and is provided on the other side of the insulation member  40 . 
     In this embodiment, the waveguide element  20  is provided on a first main surface (a main surface on one side) of a substantially rectangular parallelepiped insulation member  40 , and the waveguide element  30  is provided on a second main surface (a main surface on the other side) of the insulation member  40 . Note that an insulating coating may be formed on the surface of the waveguide element  20  and the waveguide element  30 . 
     As illustrated in  FIGS. 1A and 1B , the waveguide element  20  and the waveguide element  30  both have a substantially rectangular shape. In this manner, it is desirable that the waveguide element  20  and the waveguide element  30  have the same shape. Note that the “same shape” is not limited to identity in a strict sense but should be construed as covering cases where a slight difference may occur due to the antenna structure. 
     For example, if the IC chip  80  is provided in the same plane as the waveguide element  20  (see  FIG. 1A ), it is necessary to create a cutout section in part of the quadrangular waveguide element  20 . In this case, the waveguide element  20  and the waveguide element  30  in a strict sense do not have the same shape, but they are regarded as having the same shape in the context of this patent application. 
     The planar shape of the waveguide element  20  and the waveguide element  30  is not limited to a rectangular shape. For example, the waveguide element  20  and the waveguide element  30  may have a shape whose central portion is cut out (i.e., a hollow rectangle shape). 
     It is preferable that the sum of lengths of the sides of the waveguide element  20  be λ/4, λ/2, 3λ/4, or 5λ/8. Here, λ is the wavelength of the radio waves transmitted from the reading device. Note that the wavelength λ of the radio waves is not limited to a particular one as long as the wavelength λ is one which can be used in an RF tag. With regard to the sum of lengths of the sides of the waveguide element  30  as well, it is preferable that the sum be λ/4, λ/2, 3λ/4, or 5λ/8. 
     The insulation member  40  is, for example, a dielectric body having a relative permittivity of 1 or more and 20 or less (for example, synthetic resin such as ABS resin, ceramic, styrofoam, etc.). If a dielectric body having a large permittivity (e.g., ceramic) is used, then the capacitor (the capacitor  90  which will be described later) constituted by the waveguide element  20 , the waveguide element  30 , and the insulation member  40  will have a large capacitance, so that the area of the opening of the waveguide elements  20  and  30  becomes small, which makes it possible to reduce the size of the RF tag element  100 . 
     Meanwhile, since the gain of the RF tag antenna  10  becomes small, the distance of possible communications with the reading device (communication distance) becomes short. In the case where a relatively long communication distance in the order of several meters or more is necessary, it is preferable that a dielectric body having a small permittivity (e.g., relative permittivity of 5 or less, more preferably, for example, relative permittivity of 2 or less) be used as the insulation member  40 . 
     Also, as the insulation member  40 , materials such as non-woven fabric, a Teflon (registered trademark) foam member, a silicone foam member, PPS (polyphenylene sulfide), PP (polypropylene), any other suitable super engineering plastics, PES, PEI, PAI, PEEK, PTFE, PC, PA, PET, PBT, etc., or any other suitable composite materials thereof may be used. 
     The insulation member  40  has a substantially rectangular parallelepiped shape, and has a first main surface and a second main surface on the other side of the first main surface. As illustrated in  FIGS. 3 and 4 , the insulation member  40  has a plurality of bottomed holes  41  provided in the first main surface. These bottomed holes  41  are formed, for example, by cutting the insulation member  40  with a drill. 
     By providing the multiple bottomed holes  41  in the insulation member  40 , as illustrated in  FIG. 2 , it is ensured that the permittivity of the insulating region A 1  facing the waveguide element  20  (first insulating region) in the region between the waveguide element  20  and the waveguide element  30  and the permittivity of the insulating region A 2  facing the waveguide element  30  (second insulating region) in the same region are made different from each other. More specifically, since the inside of the bottomed hole  41  is filled with air, the permittivity of the insulating region A 1  becomes smaller than the permittivity of the insulating region A 2 . 
     Here, the insulating region A 1  is a region in contact with the waveguide element  20 , and includes at least a region in the vicinity of the waveguide element  20  within the region between the waveguide element  20  and the waveguide element  30 . Likewise, the insulating region A 2  is a region in contact with the waveguide element  30 , and includes at least a region in the vicinity of the waveguide element  30  within the region between the waveguide element  20  and the waveguide element  30 . 
     If the permittivity of the insulating region A 1  is smaller than the permittivity of the insulating region A 2 , then the waveguide element  20  has a larger opening area than the waveguide element  30 . As a result, the waveguide element  20  serves as the waveguide conductor (radiation conductor) that absorbs the radio waves radiated from the reading device and the waveguide element  30  serves as the ground conductor. On the other hand, if the permittivity of the insulating region A 1  is larger than the permittivity of the insulating region A 2 , then the waveguide element  20  serves as the ground conductor and the waveguide element  30  serves as the waveguide conductor. 
     Note that the shape of the bottomed hole  41  provided in the insulation member  40  is not limited to a circular shape as illustrated in  FIG. 3  and may be any other suitable shape (elliptical shape, polygonal shape, star shape, etc.). 
     Also, it is preferable that the bottomed holes  41  have, as illustrated in  FIG. 4 , a diameter that gradually reduces from the first main surface of the insulation member  40  toward the second main surface thereof. By virtue of this, the permittivity of the insulating region between the waveguide element  20  and the waveguide element  30  can be changed gradually and smoothly. 
     Also, the shape of the insulation member  40  is not limited to a rectangular parallelepiped shape described above and, for example, may be a disc-like shape, or a cross section thereof may be curved in the form of an arc. Also, the insulation member  40  may have a shape corresponding to the surface shape of the target object of mounting to which the RF tag element  100  should be mounted. 
     The power feeding section  50  is provided in the side surface of the insulation member  40  with one end thereof electrically connected to the waveguide element  30 , and the other end thereof electrically connected via the IC chip  80  to the waveguide element  20 . 
     The short circuit section  60  is provided in the side surface of the insulation member  40  with one end thereof electrically connected to the waveguide element  20  and with the other end thereof electrically connected to the waveguide element  30 . 
     The sheet  70  is made of, for example, flexible insulating materials such as PET, polyimide, and vinyl. The thickness of this sheet  70  is not particularly limited but is generally in the order of several tens of micrometers (μm). 
     Formed on the sheet  70 , as illustrated in  FIG. 1C , are the waveguide element  20 , the waveguide element  30 , the power feeding section  50 , and the short circuit section  60 . In addition, the waveguide element  20 , the waveguide element  30 , the power feeding section  50 , and the short circuit section  60  are attached to the insulation member  40  via the sheet  70  which is bent at a portion at a side of the insulation member  40 . 
     As illustrated in  FIG. 1A , the power feeding section  50  and the short circuit section  60  are provided on the sheet  70  in parallel with each other in the form of a bridge between the waveguide element  20  and the waveguide element  30 . Note that the power feeding section  50  and the short circuit section  60  do not always need to be provided in parallel with each other. 
     The waveguide element  20 , the waveguide element  30 , the power feeding section  50 , and the short circuit section  60  are formed by etching a thin metal film such as aluminum formed so as to cover the entire surface of the sheet  70 . 
     Alternatively, they may be formed, for example, by pattern printing using conductive ink, etc. 
     Note that the sheet  70  is not an essential feature and, the waveguide element  20 , the waveguide element  30 , the power feeding section  50 , and the short circuit section  60  may be provided on the insulation member  40  with no sheet  70  provided in between. For example, the waveguide element  20  and the waveguide element  30  may be separately formed and directly attached to the insulation member  40 . Alternatively, the waveguide element  20  and the waveguide element  30  may be formed on the sheet  70 , the sheet  70  may be detached therefrom, and then they may be directly attached to the insulation member  40 . 
     Next, the operation of the above-described RF tag element  100  will be described with reference to  FIG. 5 . In the RF tag element  100 , a resonance circuit is configured which resonates in the frequency band of the radio waves received by the planar inverted-F antenna. This resonance circuit is configured, as illustrated in  FIG. 5 , by an inductor pattern L and a capacitor  90 . Here, the inductor pattern L is configured, as illustrated in  FIG. 1C , by the waveguide element  20 , the short circuit section  60 , the waveguide element  30 , and the power feeding section  50 . The capacitor  90  is configured by the waveguide element  20 , the waveguide element  30 , and the insulation member  40 . 
     As illustrated in  FIG. 5 , in the equivalent circuit of the RF tag element  100 , the inductor pattern L, the capacitor  90 , and the IC chip  80  are connected in parallel with one another. The inductor pattern L, the capacitor  90 , and the IC chip  80  constitute the resonance circuit that resonates in the frequency band of the radio waves transmitted from the reading device. The resonance frequency f (Hz) of this resonance circuit is given by the expression (1). The value of the resonance frequency f is specified such that it is included in the frequency band of the radio waves transmitted from the reading device. 
     
       
         
           
             
               
                 
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     Here, La is the inductance of the inductor pattern L; Ca is the capacitance of the capacitor  90 ; and Cb is the equivalent capacitance inside the IC chip  80 . Note that, for example, a capacitance value disclosed as one of the specifications of the IC chip in use can be used as Cb. 
     By virtue of the above-described resonance circuit, it is made possible for the planar inverted-F antenna to receive with high sensitivity the radio waves transmitted from the reading device. Consequently, the reading performance of the RF tag element  100  can be enhanced. Further, the power supply voltage generated by the IC chip  80  can be increased. 
     Note that the IC chip  80  has a floating capacitance. Also, the IC chip  80  may be one which incorporates a capacitor. As a result, when the resonance frequency of the resonance circuit is to be specified, it is preferable that the equivalent capacitance inside the IC chip  80  be taken into account. In other words, it is preferable in terms of increased reception sensitivity that the resonance circuit has a resonance frequency specified taking into account the inductance of the inductor pattern L, the capacitance of the capacitor  90 , and the equivalent capacitance inside the IC chip  80 . 
     In the RF tag element  100  in accordance with this embodiment, as discussed in the foregoing, the waveguide elements  20  and  30  are capable of performing both functions of the waveguide conductor and the conductor ground plane depending on the mode of usage of the RF tag element  100 . In addition, in the case where the waveguide element  20  functions as the waveguide conductor and the waveguide element  30  functions as the ground conductor, the permittivity of the insulating region A 1  on the radiation side is smaller than the permittivity of the insulating region A 2  on the ground side. According to this embodiment, by providing a plurality of bottomed holes  41  in one main surface of the insulation member  40 , the permittivity of the insulating region A 1  facing the waveguide element  20  is made smaller than the permittivity of the insulating region A 2  facing the waveguide element  30 . By virtue of this, since the opening area on the radiation side is increased, the communication distance between the RF tag element  100  and the reading device can be extended without increase in the size of the RF tag element  100 . 
     Note that the configuration of the insulation member  40  is not limited to that which has been described in the foregoing and various configurations can be contemplated. While two modified examples will be described below, the same effect as that of the above-described embodiment can be obtained in either case. 
     Modified Example 1 
     Modified Example 1 will be described with reference to  FIGS. 6 and 7 .  FIG. 6  illustrates a cross-sectional view taken along the line I-I of  FIG. 1A  in the case of Modified Example 1.  FIG. 7  illustrates a perspective view of the insulation member  40  in Modified Example 1. 
     In Modified Example 1, a plurality of grooves  42  are provided, in the shape of stripes, in the first main surface of the insulation member  40 , and a plurality of grooves  43  are provided, in the shape of stripes, in the second main surface thereof. As illustrated in  FIG. 6 , the width of the grooves  42  is wider than the width of the grooves  43 . As a result, the permittivity of the insulating region A 1  becomes smaller than the permittivity of the insulating region A 2 . The waveguide element  20  and the waveguide element  30  are supported by the raised sections of the raised and depressed sections formed on the main surface of the insulation member (i.e., the portions other than the grooves). 
     Note that the permittivity may be changed by changing the number of the grooves. For example, five grooves  42  may be formed in the first main surface of the insulation member  40  and three grooves having the same width as the groove  42  may be formed in the second main surface. 
     Also, by forming raised and depressed sections other than stripe-shaped grooves in the first main surface and/or the second main surface, the permittivity of the insulating region A 1  and the permittivity of the insulating region A 2  may be made different from each other. The planar shape of the raised and depressed sections can take any suitable shape such as multiple concentric shapes, lattice-like shape, waveform-like shape, etc. By forming raised and depressed sections having shapes different from one another in the first main surface and the second main surface, the permittivity of the insulating region A 1  and the permittivity of the insulating region A 2  may be made different from each other. 
     The permittivity of the insulating region A 1  becomes smaller as the area of contact between the waveguide element  20  and the insulation member  40  becomes smaller. Consequently, by ensuring that the area of contact between the waveguide element  20  and the insulation member  40  is smaller than the area of contact between the waveguide element  30  and the insulation member  40 , it is made possible to make the permittivity of the insulating region A 1  smaller than the permittivity of the insulating region A 2 . 
     Modified Example 2 
     Modified Example 2 will be described with reference to  FIG. 8 .  FIG. 8  illustrates a cross-sectional view taken along the line I-I of  FIG. 1A  in the case of Modified Example 2. 
     In Modified Example 2, the insulation member  40  has an insulating substrate  40 A having a first main surface and an insulating substrate  40 B having a second main surface. The insulating substrate  40 A and the insulating substrate  40 B are overlaid on each other so as to constitute the insulation member  40  with a tow-layer structure. In addition, the permittivity of the insulating substrate  40 A and the permittivity of the insulating substrate  40 B are different from each other. For example, the insulating substrate  40 A is made of materials having lower insulation rate than the insulating substrate  40 B. Alternatively, the insulating substrate  40 A and the insulating substrate  40 B may be made of the same material and the insulation rates of them can be made different from each other by changing the number and size of the voids inside thereof. 
     Note that the insulation member  40  may be configured by three or more layers of insulating substrate. 
     In addition to the above-described Modified Examples 1 and 2, an insulation member may be used which has a flat-plate section and a plurality of pillar portions provided on an upper surface of this flat-plate section so as to protrude therefrom. In this case, the waveguide element  20  is provided on the plurality of pillar portions and the waveguide element  30  is provided on the lower surface of the flat-plate section. 
     &lt;Installation of the RF Tag Element&gt; 
     Next, installation of the RF tag element  100  will be described with reference to  FIG. 9 .  FIG. 9  illustrates a state where the RF tag element  100  is installed on an installation surface  201  of a conductor (target object of mounting)  200 . The RF tag element  100  is installed such that the waveguide element  30  is in contact with the conductor  200 . The RF tag element which is directly mounted to the target object of mounting such as the conductor  200  can be regarded as an RF tag that does not have a case (the cases  300 ,  500 , etc. which will be described later) that accommodates therein the RF tag element. 
     In this patent application, the state where “the waveguide element is in contact with the conductor” is not limited to a case where the waveguide element is in direct contact with the conductor and may encompass a state where the waveguide element can be regarded as being electrically connected to the conductor. In other words, a state where a certain substance (seal, adhesive, etc.) resides between the waveguide element and the conductor should also be encompassed by the state where the waveguide element is in contact with the conductor. Also, the “conductor” in this patent application is a “generic term for substances having relatively high electrical conductivity” in the same manner as in the general lexicographical meaning, a typical example of which is metal. Meanwhile, the “conductor” is not limited to metal and may be, for example, a human body, plants, water, ground, etc. 
     In the RF tag element  100  installed on a conductor  200 , the waveguide element  30  is electrically connected to the conductor  200  and the conductor  200  receives radio waves together with the waveguide element  30 . In other words, since the RF tag antenna  10  is a planar inverted-F antenna, the waveguide element  30  and the conductor  200 , as one single waveguide element having a large opening area, are capable of absorbing (receiving) the radio waves of the reading device. 
     Accordingly, it is made possible to ensure enhanced sensitivity of the planar inverted-F antenna. 
     It may also be ensured that the permittivity of the insulating region A 2  be larger than the permittivity of the insulating region A 1 . By virtue of this, the sensitivity of the planar inverted-F antenna can be further enhanced and the communication distance can be extended. 
     Note that, if a plurality of RF tag elements  100  are arranged on a rectangular conductor  200 , then, as illustrated in  FIG. 9 , it is preferable that the plurality of RF tag elements  100  be installed such that the short circuit section  60  of each RF tag element  100  is oriented toward the end (side) of the conductor  200  and that the short circuit section  60  is positioned on the inner side relative to the side of the conductor  200 . By arranging them in this manner, the return loss is reduced, as a result of which the operation efficiency of the RF tag element  100  is enhanced, which in turn makes it possible to efficiently emit the radio waves. 
     &lt;RF Tag&gt; 
     Next, an RF tag  1  will be described in which the RF tag element  100  that has been described in the foregoing is accommodated in the case. 
     The RF tag  1  includes, as illustrated in  FIG. 10 , an RF tag element  100  and a case  300  that accommodates this RF tag element  100 . The RF tag element  100  is fixed to the case  300  via an adhesive layer  400  provided on an inner surface  304  of the case  300 . The adhesive layer  400  is formed as a result of curing of an adhesive. By accommodating the RF tag element  100  in the case  300 , waterproof or dustproof property can be enhanced. 
     The case  300  is made, for example, from ABS resin and fiber reinforced plastics (FRP), but it is not limited thereto. Note that the case  300  is a rectangular parallelepiped shape in this embodiment but it is not limited thereto, and may have a disc-like shape. Alternatively, it may have a shape corresponding to the surface shape of the target object of mounting. 
     Also, materials such as non-woven fabric, a Teflon (registered trademark) foam member, a silicone foam member, PPS (polyphenylene sulfide), PP (polypropylene), any other suitable super engineering plastics, PES, PEI, PAI, PEEK, PTFE, PC, PA, PET, PBT, etc., or any other suitable composite materials thereof may be used as the case  300 . 
     The case  300  may have a mounting unit for mounting the RF tag  1  to the target object of mounting. This mounting unit is, for example, as illustrated in  FIG. 10 , an adhesive layer  401  formed as a result of curing of an adhesive. Also, as the mounting unit, a fixing hole may be provided in the case. The case  500  illustrated in  FIG. 11  has a housing  501  whose lower surface is an open surface and a lid  502  whose upper surface is an open surface. This lid  502  is accommodated in the housing  501  in the state where the RF tag element  100  is accommodated in the inside of the lid  502 . The housing  501  includes an outer edge section  501   a  extending from its left and right side surfaces. Provided in this outer edge section  501   a  is a hole  501   b . By using this hole  501   b , the case  500  can be fixed to the target object of mounting such as the conductor  200  by means of a fixing means such as a screw, a bolt and a nut, a peg, or the like. Note that the gap G 1  created between the inner surface of the housing  501  and the outer surface of the lid  502  may be filled with an adhesive. By virtue of this, the watertight and dustproof properties of the case  500  can be enhanced. 
     In addition, as the mounting unit, a suspension hole (not shown) may be provided in the case. In this case, a string is passed through the suspension hole and the RF tag  1  is suspended from the target object of mounting by means of the string. 
     Note that the case in which the RF tag element  100  is accommodated may be configured such that the position of the short circuit section  60  is visible from the outside of the case. For example, a marking or the like indicative of the position of the short circuit section  60  is printed on the outer surface of the case (e.g., the upper surface  301  or the side surface  302  and the like of the case  300 ). Alternatively, the position of the short circuit section  60  may be made visible from the outside by adjusting the outer shape of the case  300 . For example, a raised section or recessed section is provided at a portion of the outer surface of the case  300  corresponding to the position of the short circuit section  60 . 
     By configuring the case such that the position of the short circuit section  60  is visible from the outside of the case, when the multiple RF tags  1  are mounted to the conductor  200 , the RF tags  1  can be readily installed such that the short circuit section  60  is oriented toward the end of the conductor  200 . As a result, as has been discussed in the foregoing, operation efficiency of the RF tags  1  can be improved and radio waves can be efficiently radiated. 
     Also, a biasing member that presses the RF tag element  100  may be provided in the RF tag  1 . Specifically, a biasing member configured to press the RF tag element  100  toward the second inner surface facing the first inner surface may be provided in the gap between the RF tag element  100  and the first inner surface of the case. 
     For example, as illustrated in  FIG. 12 , an adhesive layer  601  is provided as the biasing member in the gap G 2  between the RF tag element  100  and the inner surface  303  of the case  300 . This adhesive layer  601  is a layer formed as a result of curing of an adhesive, the gap G 2  is filled with the adhesive layer  601 , and its thickness is larger than the thickness of the gap G 2 . As a result, the adhesive layer  601  presses the RF tag element  100  toward the inner surface  304  facing the inner surface  303 . Accordingly, the RF tag element  100  is firmly pressed against the side of the inner surface  304  by the adhesive layer  601 . 
     As another embodiment associated with the biasing member, as illustrated in  FIG. 13 , a projection (rib)  602  may be provided on the inner surface  303  of the case  300 . Specifically, the projection  602  configured to press the RF tag element  100  toward the inner surface  304  may be provided on the inner surface  303  of the case  300 . Note that the inner surface  303  may be the lower inner surface of the case  300 . 
     With regard to the biasing member, various modes other than those described above can be contemplated. For example, a member made of materials having stretching properties such as urethane, a spring or the like made of resin may be provided as the biasing member between the RF tag element and the case. 
     Note that the biasing member is preferably made of the same material as the insulating base material. 
     Also, it is desirable that the relative permittivity of the biasing member and the relative permittivity of the insulating base material are at the same or similar level. In particular, it is desirable that the relative permittivities agree with each other. 
     By providing a biasing member as described above, the antenna sensitivity of the RF tag  1  can be enhanced. Further, even when vibrations are applied to the case, it is made possible to avoid oscillation of the RF tag element  100  inside the case, which in turn makes it possible to achieve longer product life of the RF tag element  100 . 
     Also, the case in which the RF tag element  100  is accommodated may be made of conductive materials such as metal. In this case, an opening for passage of radio waves is provided in a case made of conductive material. As illustrated in  FIGS. 14A to 14E , an opening  701  for passage of the radio waves of the reading device is provided in the case  700  made of conductive material. For example, the opening  701  is provided in the upper surface of the case  700  (i.e., the surface facing the reading device). The radio waves that have passed through this opening  701  are received by the RF tag element  100  accommodated in the case  700 . 
     It is preferable that the opening  701  have a shape corresponding to the characteristics of the radio waves transmitted from the reading device. For example, the shape of the opening  701  can be modified as appropriate according to the characteristics of the radio waves such as a linear shape illustrated in  FIGS. 14A to 14C , a shape defined by two lines intersecting each other illustrated in  FIG. 14D , an elliptical shape illustrated in  FIG. 14E , etc. Note that the shape of the opening  701  is not limited to a rectangular shape, a cross shape, or a circular shape (elliptical shape) and may be any other suitable shape, for example, a star-like shape. Also, it is preferable that the area of the opening  701  is about 10% of the surface area of the case  700  (the sum of the area of the upper surface and the areas of the front, rear, left, and right side surfaces), but it may be adjusted as appropriate depending on the types of the radio waves and the installation location of the case. 
       FIG. 15  is a schematic diagram which illustrates another example of how to assemble the RF tag  1 . 
     In the RF tag  1  illustrated in  FIG. 15 , a rectangular hole  44  is provided on the side of the waveguide element  20  of the insulation member  40 . 
     Also, a plurality of cylindrical columns (a plurality of convex pillars)  45  are formed in the bottom surface of the hole  44 . Note that, in  FIG. 15 , the plurality of cylindrical columns  45  are formed with the same shape, but they are not limited to this, and any suitable shapes may be mixedly used such as a square prism, a triangular prism, and the like, or any suitable shape such as a square prism, a triangular prism, and the like, may be mixedly used only for some of them, and the plurality of cylindrical columns  45  may have different heights. 
     Further, the plurality of cylindrical columns  45  are regularly formed in a staggered pattern, but they are not limited to this, and they may be formed in arrangement according to random numbers. 
     Next, the insulation member  40  is, for example, a dielectric body having a relative permittivity of one or more and 20 or less (for example, synthetic resin such as ABS resin, ceramic, styrofoam, etc.). If a dielectric body having a large permittivity (e.g., ceramic) is used, then the capacitance of the capacitor constituted by the waveguide element  20 , the waveguide element  30 , and the insulation member  40  (the capacitor  90  which will be described later) becomes large, so that the opening area of the waveguide elements  20  and  30  is reduced, and the size of the RF tag element  100  can be reduced. 
     Meanwhile, as the gain of the RF tag antenna  10  becomes smaller, the distance within which communication can be performed with the reading device (communication distance) is shortened. In the case where a relatively long communication distance in the order of several meters or more is needed, it is preferable that a dielectric body having a small permittivity (e.g., relative permittivity of 5 or less, and more preferably, for example, relative permittivity of 2 or less) be used as the insulation member  40 . 
     Also, it may be made of materials such as non-woven fabric, a Teflon (registered trademark) foam member, a silicone foam member, PPS (polyphenylene sulfide), PP (polypropylene), any other suitable super engineering plastics, PES, PEI, PAI, PEEK, PTFE, PC, PA, PET, PBT, etc., or any other suitable composite materials thereof. 
     After that, as illustrated in  FIG. 15 , the RF antenna  10  is adhesively attached to the insulation member  40  and the IC chip  80  is mounted. As a result, the RF tag element  100  is formed. 
     Subsequently, as illustrated in  FIG. 15 , a hole  305  which is capable of accommodating the RF tag element  100  is formed in the case  300 . Also, a plurality of ribs  306  which are a support member are formed in the bottom surface of the hole  305 . In  FIG. 15 , three ribs  306  are formed. The rib  306  is for use in supporting the RF tag element  100  and, in addition to them, an additional rib may be provided in the side surface of the hole  305 . By virtue of this, it is made possible to create a gap between the RF tag element  100  and the case  300 . Also, the side of the RF tag element  100  where the IC chip  80  is mounted is arranged on the side of the rib  306  of the case. 
     Note that the case  300  and the lid  307  are preferably formed of the same material. Further, it is preferable that the case  307  and the insulation member  40  be formed of the same material. 
     The RF tag element  100  is accommodated in the hole  305  of the case  300 , and the hole  305  in which the RF tag element  100  has been accommodated is closed by the lid  307 . In this case, the case  300  and the lid  307  may be fixed to each other by adhesive bonding or may be fixed to each other by fusion, welding, or any other suitable method of fixing. 
     Also, the case  300  may be made of materials such as non-woven fabric, a Teflon (registered trademark) foam member, a silicone foam member, PPS (polyphenylene sulfide), PP (polypropylene), and any other suitable super engineering plastics, PES, PEI, PAI, PEEK, PTFE, PC, PA, PET, PBT, etc. or any other composite materials thereof. For example, if the case  300 , the insulation member  40 , and the lid  307  are formed of resin having a heat resistance property (materials such as PPS (polyphenylene sulfide), PP (polypropylene), any other suitable super engineering plastics, PES, PEI, PAI, PEEK, PTFE, PC, PA, PET, PBT, etc., or any other suitable composite materials thereof), then the RF tag element  100  accommodated in the case  300  having a heat resistance property, i.e., the RF tag  1  can be formed. 
     Note that the RF tag element  100  accommodated in the case  300  and the RF tag  1  have the feature that they are capable of communications regardless of their position of installation on a conductor such as a metal plate. 
     EXAMPLE 
       FIG. 16  illustrates a measurement system for measuring the minimum communication-possible gain of the RF tag element. Mounted on the acrylic plate  1000  is the RF tag element A or B. The RF tag element A is the RF tag element in accordance with the embodiment of the present invention, and, as the insulation member, ABS resin plate is used with a plurality of bottomed holes provided on its surface on the side of the standard antenna  1200 . Meanwhile, the RF tag element B is an RF tag element of a conventional type, and styrofoam plate is used as an insulation member. The styrofoam plate has a flat plate-like shape and no bottomed hole is provided therein. Note that, with regard to the RF tag elements A and B, features other than the insulation member and the sizes are identical with each other. 
     The specified distance D between the RF tag elements A and B and the standard antenna  1200  was set to 300 mm. 
     The reading device  1100  measures the minimum gain that allows communications with the RF tag elements A and B. The communication frequency was set to 920 MHz. Note that, when the minimum communication-possible gain is smaller, then the antenna sensitivity of the RF tag element becomes more favorable, making it possible to extend the communication distance. 
     As a result of the measurement in the above-described measurement system, it has been found that the minimum communication-possible gain of the RF tag element A is 16.5 dB. In contrast, the minimum communication-possible gain of the RF tag element B using a styrofoam plate was 19.8 dB. From this fact, it will be appreciated that the RF tag element A has a significantly enhanced antenna sensitivity as compared with the RF tag element B. 
     The permittivity of ABS resin (permittivity 3.1) is larger than the permittivity of styrofoam (permittivity 2.1), so that, if the insulation members have the same shape, then the gain of the RF tag antenna  10  becomes smaller and the communication distance becomes shorter in the RF tag element A than in the RF tag element B. However, in the RF tag element A, since the permittivity of the insulating region A 1  is lowered by providing a plurality of bottomed holes, antenna sensitivity is enhanced despite the fact that the RF tag element A has the same size as the RF tag element B. 
     In the present invention, the insulation member  40  corresponds to the “insulation member,” the waveguide element  20  corresponds to the “first waveguide element,” the waveguide element  30  corresponds to the “second waveguide element,” the power feeding section  50  corresponds to the “power feeding section,” the short circuit section  60  corresponds to the “short circuit section,” the planar inverted-F antenna or the F antenna corresponds to the “planar inverted-F antenna or the F antenna,” the insulating region A 1  on the side of the waveguide element  20  corresponds to the “permittivity of the first insulating region,” the insulating region A 2  on the side of the waveguide element  30  corresponds to the “permittivity of the second insulating region,” the RF tag antenna  10  corresponds to the “RF tag antenna,” the a plurality of bottomed holes  41  corresponds to the “a plurality of bottomed holes,” a plurality of cylindrical columns  45  correspond to “a plurality of convex pillars,” the IC chip  80  corresponds to the “IC chip,” the RF tag  1  corresponds to the “RF tag,” the RF tag element  100  corresponds to the “RF tag element,” the case  300 ,  700  corresponds to the “case,” the adhesive layer  601 , the rib  306 , and the rib  602  correspond to the “biasing member,” the opening  701  corresponds to the “opening,” the hole  305  corresponds to the “accommodating section,” and the rib  306  corresponds to the “support member.” 
     Whilst those skilled in the art will envisage additional effects of and various modifications to the present invention on the basis of the foregoing descriptions, aspects of the present invention are not limited to the above-described embodiments. Various additions, changes, and partial deletions can be made within the range where the conceptual idea and the purport of the present invention are not deviated which are derived from the content of the scope of claims and equivalents thereof. For example, although the explanations have been provided based on an inverted-F antenna, the present invention is not limited thereto and may be implemented on an F antenna, a slot antenna, a loop antenna, etc. 
     REFERENCE SIGNS LIST 
     
         
           1  RF tag 
           10  RF tag antenna 
           20 ,  30  waveguide element 
           40  insulation member 
           40 A,  40 B insulating substrate 
           41  bottomed hole 
           42 ,  43  groove 
           50  power feeding section 
           60  short circuit section 
           70  sheet 
           80  IC chip 
           90  capacitor 
           100  RF tag element 
           200  conductor 
           201  installation surface 
           202  non-installation surface 
           300 ,  500 ,  700  case 
           301  upper surface (of the case) 
           302  side surface (of the case) 
           303 ,  304  inner surface (of the case) 
           400 ,  401  adhesive layer 
           501  housing 
           501   a  outer edge section 
           501   b  hole 
           502  lid 
           601  adhesive layer 
           602  projection 
           701  opening 
           1000  acrylic plate 
           1100  reading device 
           1200  standard antenna 
         A, B RF tag element 
         A 1 , A 2  insulating region 
         D specified distance 
         G 1 , G 2  gap 
         L inductor pattern