JP5336874B2 - Antenna device and RFID tag - Google Patents

Description

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

  As a sheet-like structure having a high surface 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 device that does not require vias and has scalability exceeding the limit of via density has been proposed (for example, see Patent Document 2).

  Furthermore, it has also been proposed to use the above-described sheet-like structure as an in-phase reflector in an antenna device (see, for example, Patent Document 3). Specifically, in the antenna device of Patent Document 3, an antenna conductor is disposed on a sheet-like structure via a spacer.

US Pat. No. 6,538,621 JP 2006-245984 A JP 2008-131506 A

  However, the structure disclosed in Patent Document 3 has a problem that it is difficult to match the antenna conductor and the IC at the in-phase reflection frequency of the sheet-like structure.

  Therefore, an object of the present invention is to provide an antenna device that can solve the above-described problems and an RFID tag using the antenna device.

  As a result of research, the inventors of the present invention have found that a sheet-like structure and an antenna conductor provided on the sheet-like structure via a spacer are correlated by magnetic coupling. Since the in-phase reflection frequency has changed, it has been found that it is difficult to match the antenna conductor and the IC at the in-phase reflection frequency of the sheet-like structure.

  On the other hand, the inventors of the present invention have also found that the sheet-like structure operates as a leak antenna in the process of studying the antenna device using the sheet-like structure as the in-phase reflector.

  FIG. 1 shows the dispersion characteristics of the sheet-like structure. In FIG. 1, the vertical axis represents frequency (Hz) and the horizontal axis represents ka / π. Here, k is the magnitude of the wave vector, a is the period of the periodic structure of the sheet-like structure, and π is the circumference. The wave vector magnitude k is expressed as k = 2π / λ using the wavelength λ.

  Referring to FIG. 1, Light Line, which is a light dispersion characteristic in free space, is shown. In the region above this Light Line, the phase velocity exceeds the phase velocity of the light in vacuum, so that electromagnetic waves leak. On the other hand, the point where the magnitude of the wave vector is zero, that is, the vicinity of the Γ point is the operating point of the RFID tag configured using the sheet-like structure. From the above, it can be understood that, in an RFID tag configured using a sheet-like structure, the sheet-like structure theoretically operates as a leak antenna.

  Here, in order for the sheet-like structure to operate as a leak antenna, originally, it is desirable that many unit cells be arranged in the direction in which the energy advances inside the sheet-like structure. There are not many. However, in the case of a small sheet-like structure, energy reciprocates inside the sheet-like structure while being totally reflected at the end of the sheet-like structure, and leaks to the outside of the sheet-like structure during the reciprocating process. Therefore, it is presumed that the antenna operates as a leak antenna.

  Based on this estimation, it can be explained that the communication distance of the RFID tag can be greatly increased when a sheet-like structure is used for a small RFID tag. That is, in the case of a small RFID tag, since the size of the antenna conductor is shorter than the half wavelength of the operating frequency, there is a problem that the antenna characteristic by the antenna conductor is low and therefore the communication distance is short. When such an RFID tag is mounted on a sheet-like structure, the sheet-like structure functions as a leak antenna, and the antenna conductor is expected to function as an antenna by correlating with the leak antenna. For this reason, when a sheet-like structure is used for a small RFID tag, it seems that the communication distance can be extended.

  Furthermore, the inventors of the present invention can use an RFID tag without mounting an antenna conductor if a small RFID tag using a sheet-like structure is mainly operated as an antenna and is a sheet-like structure. A new RFID tag in which an IC is directly mounted on a sheet-like structure was constructed and the communication distance was verified. As a result of the verification, it was confirmed that it functions sufficiently as an RFID tag without mounting an antenna conductor. The present invention is based on such knowledge, and specifically provides the following antenna device and RFID tag.

That is, according to the present invention, as the first antenna device,
A sheet-like structure in which a plurality of unit cells including a top electrode are periodically arranged, and an IC land provided so as to extend over at least one set of adjacent top electrodes,
The adjacent top electrodes each have an IC connection provided in the IC land.
An antenna device is obtained.

Further, according to the present invention, as the second antenna device, in the first antenna device,
The unit cell includes at least a ground electrode, and a conductive via connecting the ground electrode and the top electrode,
The cell constituting the end of the sheet-like structure has a shape such that the unit cell is cut off so that the conductive via is located at or near the end face.
An antenna device is obtained.

Moreover, according to the present invention, as the third antenna device, in the first or second antenna device,
The sheet-like structure can correspond to at least an electric field in a predetermined direction,
One of the adjacent top electrodes is the top electrode located in the center in the predetermined direction, or the top electrode closest to the center when there is no top electrode located in the center,
An antenna device is obtained.

According to the present invention, as the fourth antenna device, in any of the first to third antenna devices,
An antenna device is obtained in which the operating frequency of the antenna device is 90% to 100% of the in-phase reflection frequency of the sheet-like structure.

  Furthermore, according to the present invention, an RFID (Radio Frequency Identification) tag including any one of the first to fourth antenna devices and an IC mounted on the IC land can be obtained.

  According to the present invention, since there is no antenna conductor, the sheet-like structure and the antenna conductor are not magnetically coupled. As a result, the in-phase reflection frequency of the sheet-like structure does not change. Therefore, the IC and the sheet-like structure can be matched at the in-phase reflection frequency of the sheet-like structure.

  Moreover, according to the present invention, the number of parts can be reduced and the thickness can be reduced in an antenna device using a sheet-like structure.

It is a figure which shows the dispersion characteristic of a sheet-like structure. 1 is a perspective view schematically showing an RFID tag according to an embodiment of the present invention. FIG. 3 is a cross-sectional view taken along line III--III of the sheet-like structure of FIG. 2. FIG. 4 is a cross-sectional view taken along line IV--IV of the sheet-like structure of FIG. 2. It is a perspective view which shows roughly the unit cell of the sheet-like structure of FIG. It is a top view which shows the sheet-like structure (antenna apparatus) of FIG. It is a top view which shows IC land vicinity in the sheet-like structure of FIG. It is a top view which shows the modification of IC land vicinity in the sheet-like structure of FIG. It is a figure which shows the radiation efficiency of the RFID tag by an Example and a comparative example. It is a figure which shows the analysis result of the directivity in the ZY surface of the RFID tag by an Example. It is a figure which shows the analysis result of the directivity in the ZX surface of the RFID tag by an Example. It is a figure which shows the position of IC chip. It is a figure which shows the radiation efficiency of the RFID tag at the time of mounting an IC chip in each of the positions shown in FIG. It is a perspective view which shows the modification of the unit cell of the sheet-like structure of FIG. It is a perspective view which shows roughly the sheet-like structure formed by arranging the unit cells of FIG. 14 periodically. It is a perspective view which shows typically the sheet-like structure which shortened the length of the edge part cell to about half. It is sectional drawing which shows the center vicinity of the sheet-like structure shown by FIG. 16 along a XVII-XVII line. It is sectional drawing which shows the edge part vicinity of the sheet-like structure shown by FIG. 16 along a XVIII-XVIII line.

  As shown in FIG. 2, the RFID tag 100 according to the embodiment of the present invention includes an antenna device including a sheet-like structure (EBG structure) 200 having a top electrode 30, and an IC 300 connected to the top electrode 30. It has.

  As shown in FIGS. 3 to 5, the sheet-like structure 200 in the present embodiment includes a ground electrode 10, a conductive via 20 connected to the ground electrode 10, and a top electrode 30 connected to the conductive via 20. And a dielectric 40 filled between the ground electrode 10 and the top electrode 30 and a floating capacitor electrode 50 held in a floating state in the dielectric 40.

  As understood from FIG. 5, the unit cell in the present embodiment has a first rectangular shape (same shape as the ground electrode 10 of the unit cell) when viewed from above (when viewed along the Z direction). The top electrode 30 has a second rectangular shape that is shorter in each side than the first rectangular shape, and thus has a smaller area than the first rectangular shape. Accordingly, the ground electrode 10 is connected to the ground electrode of the adjacent unit cell, but the top electrode 30 is separated from the top electrode 30 of the adjacent unit cell.

  The floating capacitor electrode 50 includes two parts in each unit cell. Two portions of the floating capacitor electrode 50 are arranged so as to sandwich the conductive via 20 in the X direction. Each part of the floating capacitor electrode 50 extends to the end of the unit cell in the X direction, and has the same length as the top electrode 30 in the Y direction. It has not reached the end.

  In the sheet-like structure in which such unit cells are periodically arranged in the X direction and the Y direction, each floating capacitor electrode 50 is located between two adjacent top electrodes 30 in the X direction, as shown in FIG. Therefore, it partially overlaps with the two adjacent top electrodes 30. On the other hand, in the Y direction, each of the floating capacitor electrodes 50 is provided so as to correspond to each top electrode 30 as shown in FIG. Not provided. For this reason, the sheet-like structure in the present embodiment has different in-phase reflection frequencies for the electric field in the X direction and the electric field in the Y direction. Specifically, the electric field in the X direction has a lower in-phase reflection frequency. In this structure, for miniaturization, in-phase reflection is obtained at the used frequency only for the electric field in the X direction.

  As shown in FIG. 6, the sheet-like structure 200 in the present embodiment includes IC lands 210 provided so as to extend over adjacent top electrodes 30 a, and the top electrodes 30 a are in the IC lands 210. The IC connection portion 31 is provided so as to extend. The sheet-like structure 200 itself functions as an antenna device that provides an antenna function to the IC 300 mounted on the IC land 210. Therefore, in this embodiment, the antenna conductor and the spacer on which it is placed can be omitted.

  Specifically, the top electrode 30 a is the one closest to the center in the X direction among the plurality of top electrodes 30. That is, in the present embodiment, the IC land 210 is provided so as to extend over the two top electrodes 30a closest to the center in the X direction. In the present embodiment, since the number of unit cells in the X direction (that is, the number of top electrodes 30, 30a) is an even number, the IC land extends between the top electrodes 30a closest to the center in the X direction. 210 is formed, but when the number of unit cells in the X direction is an odd number, the IC land 210 is arranged between the top electrode located in the center in the X direction and the top electrode adjacent in the X direction. Will be formed.

  In the IC land 210 according to the present embodiment, as shown in FIG. 7, an IC fixing portion 33 for fixing an IC 300 provided separately from the IC connection portion 31 is provided. As understood from FIGS. 6 and 7, the sheet-like structure 200 according to the present embodiment is provided with a plurality of IC lands 210. This is because impedance matching is adjusted between the IC 300 and the sheet-like structure 200 by selecting the IC land 210 as will be described later. The shape in the vicinity of the IC land 210 is not limited to that shown in FIGS. 6 and 7. For example, as shown in FIG. 8, the top electrode 30 b on which the IC connection portion 31 is provided is between the top electrodes 30 b. In order to make the distance between the IC lands 210 as large as possible, the portions 35 extending so as to protrude between the IC lands 210 may be provided.

  In order to verify the effect of the present invention, an analysis was performed using a simulator. The RFID tag 100 according to the embodiment of the present invention is a metal-compatible tag that operates in a 953 MHz band. The size of the sheet-like structure 200 is 86 mm × 54 mm × 0.7 mm. The size of the unit cell is 21 mm × 13.25 mm × 0.7 mm. The conductive via 20 has a via diameter φ of 1.4 mm, and the top electrode 30 has a size of 20.7 × 12.95 mm. The interval between the top electrodes 30 is 0.3 mm. The size of the floating capacitor electrode 50 is 9.6 × 12.95 mm per unit cell, and therefore, the size straddling two adjacent unit cells is 19.2 × 12.95 mm. The distance between the top electrode 30 and the floating capacitor electrode 50 is 0.1 mm, and the distance between the floating capacitor electrode 50 and the ground electrode 10 is 0.6 mm. Furthermore, the dielectric constant of the dielectric 40 is 3.5 and tan δ is 0.003. Each electrode is treated as a film for calculation, and the thickness is calculated as 0. As a comparative example, an antenna support (spacer) and a straight antenna supported by the antenna support are mounted on the sheet-like structure 200. Here, the thickness of the antenna support is 0.6 mm, and the size of the straight antenna is 6 mm × 63 mm.

  In order to evaluate the performance as a metal-compatible tag, the RFID tag 100 according to the above-described embodiment and the RFID tag according to the comparative example are respectively placed on an aluminum plate of 200 mm × 300 mm × 2 mm via an insulating spacer having a thickness of 0.1 mm. Arranged and verified the radiation efficiency of each. The verification result is shown in FIG. As is apparent from FIG. 9, it is understood that the RFID tag 100 according to the present embodiment has the same radiation efficiency as that of the RFID tag according to the comparative example in the vicinity of the use frequency band.

  The analysis results on directivity are shown in FIGS. From the calculation results of directivity, the directivity in the vertical direction is obtained on the metal surface or the sheet-like structure surface in the same way as the directivity of the antenna mounted on metal, so there is no particular problem even if the antenna conductor is not provided. Absent.

  Furthermore, the radiation efficiency when the IC 300 is mounted at each of the seven positions a to g shown in FIG. The examination results are shown in FIG. As is apparent from FIG. 13, when the IC 300 is mounted near the center in the X direction (that is, the electric field direction) (positions a to f), the radiation efficiency is greatly degraded even if the IC 300 is shifted from the vicinity in the Y direction. There is no. However, when the IC 300 is mounted at a position shifted from the vicinity of the center in the X direction (position f), the radiation efficiency is greatly deteriorated. That is, in consideration of radiation efficiency, it is preferable to mount the IC 300 near the center in the electric field direction. By selecting the positions a to f, the impedance of the sheet-like structure viewed from the IC can be adjusted to some extent.

  In the sheet-like structure 200, at the in-phase reflection frequency, the reactance of the input impedance of the feeding point (that is, the IC connection portion 31) becomes very large and approaches ∞. On the other hand, the reactance on the IC 300 side is usually around −j100Ω and does not become −∞. Matching is obtained in a state where the feed point impedance on the sheet-like structure side and the input impedance of the IC are complex conjugate. Therefore, the in-phase reflection frequency of the sheet-like structure 200 is preferably set slightly higher than the operating frequency of the RFID tag 100. However, since the radiation efficiency becomes the highest in the vicinity of the in-phase reflection frequency, the operating frequency of the RFID tag 100 is preferably between 90% and 100% of the in-phase reflection frequency of the sheet-like structure. With such a setting, the IC 300 and the sheet-like structure 200 can be easily matched with each other at the operating frequency of the RFID tag 100, and the radiation efficiency is increased.

  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, the sheet-like structure 200 (antenna device) in the above-described embodiment and examples corresponds to only an electric field in one direction (X direction). For example, as shown in FIGS. The floating capacitor electrode 50a may be arranged so as to straddle between adjacent unit cells in the X direction and the Y direction, thereby corresponding to electric fields in two directions of the X direction and the Y direction. As shown in FIG. 4, it may correspond to an electric field in three directions.

  Further, as shown in FIG. 16, the RFID tag 100a may be miniaturized by using a sheet-like structure 200a whose end is cut. The resonance unit inside the sheet-like structure 200a (same for the sheet-like structure 200) is a portion 60 sandwiched between the conductive vias 20 as shown in FIG. Yes. Therefore, as shown in FIG. 16 and FIG. 18, the half cycle of the end portion is cut (that is, the conductive via 20 is positioned so that the conductive via 20 is located at or near the end surface at the end of the sheet-like structure. Can be cut). As a result, the cell constituting the end portion of the sheet-like structure 200a is shortened to about half compared to the unit cell constituting the end portion, and the top electrode 30c of the cell constituting the end portion has a portion other than the end portion. It becomes shorter than the top electrode 30 of the unit cell which comprises. Therefore, the RFID tag 100a can be downsized.

DESCRIPTION OF SYMBOLS 10 Ground electrode 20 Conductive via 30 Top electrode 30a, 30b, 30c Top electrode 31 IC connection part 33 IC fixing | fixed part 35 part (resonance unit)
40 Dielectric 50, 50a Floating capacitor electrode 60 Site 100, 100a RFID tag 200, 200a Sheet-like structure (EBG structure: antenna device)
210 IC Land 300 IC

Claims (5)

A sheet-like structure in which a plurality of unit cells including a top electrode are periodically arranged, and a plurality of IC lands provided so as to extend over at least one set of adjacent top electrodes,
The adjacent top electrodes each have an IC connection provided in the IC land,
In the antenna device capable of adjusting impedance matching between the IC and the sheet-like structure by selecting the IC land ,
The top electrode having the IC connection portion has a portion extending so as to protrude between the IC lands.
Antenna device. The antenna device according to claim 1, wherein
The unit cell includes at least a ground electrode, and a conductive via connecting the ground electrode and the top electrode,
The cell constituting the end of the sheet-like structure has a shape such that the unit cell is cut off so that the conductive via is located at or near the end face.
Antenna device. The antenna device according to claim 1 or 2,
The sheet-like structure can correspond to at least an electric field in a predetermined direction,
One of the adjacent top electrodes is the top electrode located in the center in the predetermined direction, or the top electrode closest to the center when there is no top electrode located in the center,
Antenna device. The antenna device according to any one of claims 1 to 3,
The antenna device has a use frequency of 90% to 100% of the in-phase reflection frequency of the sheet-like structure. An RFID (Radio Frequency Identification) tag comprising the antenna device according to any one of claims 1 to 4 and an IC mounted on the IC land.

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