JP2010183545A - Antenna device and rfid tag - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thinned antenna device including a combination of sheet-like structures having high surface impedance and antenna conductors, and to provide an RFID tag using the antenna device. <P>SOLUTION: The RFID tag includes the antenna device and an IC 60. The antenna device includes the sheet-like structure 20 having high surface impedance, a spacer 30 provided on the sheet-like structure 20, and the antenna conductor 50 supported by the spacer 30 via a PET sheet 40. In this case, the ratio of the thickness of the spacer 30 to that of the sheet-like structure 20 is not more than 0.5. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a sheet-like structure having a high surface impedance, an antenna device using the same, and an RFID (Radio Frequency Identification) tag.

  With recent miniaturization of devices, for example, the distance between a metal surface such as a ground plane and an antenna is shortened, and there is a concern about deterioration of antenna characteristics due to this. It is also known that when an RFID tag is attached to a metal surface, the characteristics of an antenna included in the RFID tag are remarkably deteriorated and the RFID tag does not function properly. These problems are all due to the low surface impedance of the metal surface. On the other hand, it is known that when an element having a high surface impedance is disposed in the vicinity of the antenna, the energy incident on the impedance surface is reflected in the same phase by the high surface impedance and the antenna performance is improved.

  As a sheet-like structure having a high impedance (also referred to as an EBG (Electromagnetic BandGap) structure), a sheet-like structure having a so-called mushroom-like conductor having vias is conventionally known (see, for example, Patent Document 1). On the other hand, a left-handed medium that is not limited to an antenna application and that does not require a via and has scalability exceeding the limit of via density has been proposed (for example, see Patent Document 2).

US Pat. No. 6,538,621 JP 2006-245984 A

  SUMMARY OF THE INVENTION An object of the present invention is to provide an antenna device that is a combination of a sheet-like structure and an antenna conductor and is made thinner, and an RFID tag using the antenna device.

  If the unit cell of the sheet-like structure is thinned in order to reduce the thickness of the antenna device, the common-mode reflection characteristics of the sheet-like structure deteriorate, so it is preferable to simply thin the sheet-like structure. Absent.

  On the other hand, when the distance between the sheet-like structure and the antenna conductor is shortened (that is, the spacer provided between the sheet-like structure and the antenna is thinned), the distance between the antenna conductor and the sheet-like structure is reduced. Therefore, it is considered that it is not preferable to simply make the spacer thin because the loss in the sheet-like structure appears as the loss of the antenna.

  However, as a result of further studies, the inventors have found that the sheet-like structure becomes a kind of leak antenna state in the vicinity of the in-phase reflection frequency. That is, when the spacer is thinned, the function of the antenna provided on the spacer is lost. On the other hand, the sheet-like structure itself has a certain performance as a leak antenna. Further, when the sheet-like structure functions as a leak antenna, the antenna conductor provided on the spacer functions as an exciter for the sheet-like structure. For this reason, even if the spacer is thinned, the deterioration of the antenna performance is saturated at a substantially constant level.

  Therefore, in order to obtain a thin antenna device, it is effective to make the sheet-like structure thicker while making the spacer thinner. The present invention has been made based on such knowledge, and specifically provides the antenna device and RFID tag described below.

That is, according to the present invention, as the first antenna device,
In an antenna device comprising a sheet-like structure having a high surface impedance, a spacer provided on the sheet-like structure, and an antenna conductor supported by the spacer,
An antenna device is obtained in which the ratio of the thickness of the spacer to the thickness of the sheet-like structure is 0.5 or less.

  Further, according to the present invention, as the second antenna device, an antenna device having a total thickness within 0.008 wavelengths in the first antenna device can be obtained.

  In addition, according to the present invention, as the third antenna device, an antenna device for the UHF band having a total thickness of 2.5 mm or less in the first or second antenna device can be obtained.

  Further, according to the present invention, as the fourth antenna device, in the third antenna device, an antenna device having a size of 86 × 54 mm or less and a total thickness of 2 mm or less can be obtained.

  Further, according to the present invention, as the fifth antenna device, in the first or second antenna device, an antenna device for 2.4 GHz band having a total thickness of 1 mm or less can be obtained.

  Furthermore, according to the present invention, an RFID tag including any one of the first to fifth antenna devices and an IC connected to the antenna conductor on the spacer can be obtained.

  According to the present invention, by setting the thickness ratio between the spacer and the sheet-like structure to an appropriate value, it is possible to reduce the thickness of the antenna device while suppressing the deterioration of the characteristics of the antenna device.

1 is a top view schematically showing an RFID tag according to an embodiment of the present invention. It is sectional drawing which shows the RFID tag of FIG. It is a perspective view which shows the unit cell of the sheet-like structure contained in the RFID tag of FIG. In FIG. 3, the dielectric is omitted. It is a perspective view which shows the relationship between the antenna conductor etc. in the RFID tag of FIG. 1, and a sheet-like structure. It is a perspective view which shows the example of the unit cell of another sheet-like structure.

  As shown in FIGS. 1 and 2, an RFID tag according to an embodiment of the present invention includes an insulating sheet 10, a sheet-like structure (EBG structure) 20 provided on the insulating sheet 10, and a sheet-like structure. 20, a spacer 30 provided on 20, a PET (Polyethylene Terephthalate) sheet 40 provided on the spacer 30, an antenna conductor 50 provided on the PET sheet 40, and an IC 60 connected to the antenna conductor 50. ing. That is, the RFID tag according to the present embodiment includes the antenna device according to the present embodiment and the IC 60.

  The insulating sheet 10 is for insulation between the sheet-like structure 20 and the metal surface when the RFID tag is attached to the metal surface. This insulating sheet 10 is a sheet with a glue layer in an actual RFID tag, and is used to attach the RFID tag to a metal. In this example, the paste layer is omitted for the purpose of making a prototype, and an insulating sheet is used.

  The sheet-like structure 20 has a structure in which unit cells as shown in FIG. 3 are arranged in the X direction and the Y direction (see FIG. 4). That is, the sheet-like structure 20 has a structure in which unit structures are periodically arranged. Specifically, the unit cell includes a ground conductor 21, a patch conductor 22, a via conductor 23 connecting the ground conductor 21 and the patch conductor 22, and a dielectric (not shown) provided between the ground conductor 21 and the patch conductor 22. ) And a floating capacitor conductor 24 provided so as to be electrically floating in the dielectric. The ground conductor 21 is connected to the ground conductor 21 of the adjacent unit cell. That is, the ground conductor 21 is a solid conductor provided on the bottom surface of the sheet-like structure 20. Each patch conductor 22 is insulated from the patch conductors 22 of adjacent unit cells. The floating capacitor conductor 24 serves to increase the capacitance between the patch conductors 22 adjacent in the X direction. Each floating capacitor conductor 24 is insulated from the floating capacitor conductor 24 of the unit cell adjacent in the Y direction, but is connected to the floating capacitor conductor 24 of the unit cell adjacent in the X direction. That is, each floating capacitor conductor 24 is provided so as to straddle two unit cells adjacent in the X direction. In the present embodiment, different dielectrics are filled above and below the floating capacitor conductor 24. These dielectrics may be made of, for example, a resin or a substrate material.

  The antenna conductor 50 in the present embodiment is made of an aluminum foil formed on the PET sheet 40, and has a shape extending in the X direction as shown in FIG. That is, the longitudinal direction of the antenna conductor 50 (the direction of the radiated electric field vector of the antenna) coincides with the direction in which the floating capacitor conductor 24 is connected to the floating capacitor conductor 24 of the adjacent cell.

  In the present embodiment, the ratio T1 / T2 of the thickness T1 of the spacer 30 to the thickness T2 of the sheet-like structure 20 is designed to be 0.5 or less. Since the radiation characteristics deteriorate when the sheet-like structure is made thin, it is better to make the spacer thinner without making the sheet-like structure thinner, and the ratio T1 / T2 should be 0.5 or less. good. In particular, the total thickness of the RFID tag is 0.008 times or less of the operating wavelength (that is, when the operating frequency is 953 MHz, the total thickness is 2.5 mm or less, and when the operating frequency is 2.4 GHz, the total thickness is If the ratio T1 / T2 is 0.5 or less, good antenna characteristics can be obtained.

  As an RFID tag in consideration of market needs, for example, a metal-compatible tag having a card size of 86 mm × 54 mm and a thickness of 1 mm can be cited. As an example of the RFID tag according to this embodiment corresponding to this, for example, the thickness of the insulating sheet 10 is 0.1 mm, the thickness of the sheet-like structure 20 is 0.6 mm, and the thickness of the spacer 30 is 0. 2 mm, the thickness of the PET sheet 40 is 0.05 mm, and the total thickness of the RFID tag is about 0.95 mm. Here, the ground conductor 21, the patch conductor 22, and the floating capacitor conductor 24 included in the sheet-like structure 20 are all made of copper foil and have a thickness of 18 μm. A dielectric with a thickness of 0.5 mm and a relative dielectric constant of 3.4 is filled between the conductor 24 and a space between the floating capacitor conductor 24 and the patch conductor 22 with a thickness of 0.05 mm and a relative dielectric constant of 3.35. The dielectric is filled. In this example, the ratio T1 / T2 is 1/3, which is 0.5 or less. By designing in this way, it is possible to configure a metal-compatible tag that is thin but has a long communication distance.

  A simulation was performed to verify the effect of the RFID tag according to the present embodiment. Specifically, the change in the communication distance of the RFID tag was calculated while changing the ratio T1 / T2 of the spacer 30 to the sheet-like structure 20 with the total thickness of the RFID tag being 0.95 mm. As a result, when the ratio T1 / T2 is 0.5 or less, the communication distance of the RFID tag is 4 m or more, but when the ratio T1 / T2 exceeds 0.5, the communication distance of the RFID tag Was less than 4m. From this, it is understood that when the thickness of the RFID tag is limited, it is effective to set the ratio T1 / T2 of the spacer 30 to the sheet-like structure 20 to 0.5 or less.

  As described above, the present invention has been described with specific examples. However, the present invention is not limited to this, and various application modifications are possible. For example, in the above-described embodiment, the sheet-like structure 20 having a unidirectional unit cell (see FIG. 3) is used, but the sheet-like structure is, for example, two as shown in FIG. Directional unit cells may be arranged in the X direction and the Y direction, or may not have via conductors like the sheet-like structure disclosed in Patent Document 2.

10 Insulating sheet 20 Sheet-like structure (EBG structure)
21 Ground conductor 22 Patch conductor 23 Via conductor 24, 24 'Floating capacitor conductor 30 Spacer 40 PET sheet 50 Antenna conductor 60 IC

Claims (6)

In an antenna device comprising a sheet-like structure having a high surface impedance, a spacer provided on the sheet-like structure, and an antenna conductor supported by the spacer,
The antenna device, wherein a ratio of a thickness of the spacer to a thickness of the sheet-like structure is 0.5 or less.   2. The antenna device according to claim 1, wherein the total thickness is within 0.008 wavelengths.   The antenna device according to claim 1 or 2, wherein the total thickness is 2.5 mm or less.   4. The antenna device according to claim 3, wherein the size is 86 × 54 mm or less, and the total thickness is 2 mm or less.   3. The antenna device for a 2.4 GHz band according to claim 1, wherein the total thickness is 1 mm or less. An RFID (Radio Frequency Identification) tag comprising the antenna device according to any one of claims 1 to 5 and an IC connected to the antenna conductor on the spacer.

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