JP2008131505A - High surface impedance structure, antenna unit, and rfid tag - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high surface impedance structure which contributes to the thickness reduction of an antenna unit and an RFID tag. <P>SOLUTION: A capacity upper electrode 21 is formed on the top surface side of a dielectric layer 10. A ground electrode 31 in a rectangular frame shape, a center electrode 32 disposed in the center of the reverse surface, a capacity lower electrode 33 connected to the center electrode and disposed opposite the capacity upper electrode 21, an inductance portion 34 connecting the center electrode 32 and ground electrode 31 to each other, are formed on the reverse surface side of the dielectric layer 10. The inductance part 34 is in a meander shape and increased in inductance. <P>COPYRIGHT: (C)2008,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 the performance of the antenna is improved by placing a high surface impedance near the antenna.

  As this type of structure, a structure having a so-called mushroom-like conductor having a via is conventionally known (see, for example, Patent Document 1). On the other hand, a left-handed medium that is not limited to an antenna use and that does not require a via and has scalability exceeding the limit of via density (for example, see Patent Document 2) has been proposed.

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

  Whether it is placed near the antenna or applied to an RFID tag, if the thickness required to exhibit a high surface impedance is too thick, it will go against the trend of miniaturization of devices and the demand for RFID tags, which is a practical problem. There is. For example, when the technique described in Patent Document 1 is applied to the frequency band (2.4 GHz band) of an RFID tag, a thickness of 2 mm or more is required.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a high surface impedance structure that can contribute to thinning of an antenna device and an RFID tag, and an antenna device and an RFID tag using the structure.

  As a means for solving the above-mentioned problems, the present invention enhances the surface impedance for a specific frequency band by a periodic structure in which unit structures are repeatedly arranged in at least one direction, whereby the frequencies belonging to the specific frequency band are determined. A sheet-like structure that reflects the incident wave in phase with each other: the unit structure includes a dielectric layer having first and second main surfaces, and a first conductor formed on the first main surface And a second conductor layer formed on the second main surface; the first conductor layer being disposed so as to be spaced apart from each other, while the first main surface Four capacitor-shaped upper electrodes arranged in contact with the edges of the second conductive layer; the second conductor layer having a frame-shaped ground electrode having the same outer shape as the second main surface; Disposed in the center of the second main surface. A central electrode, four capacitive lower electrodes connected to the central electrode and opposed to the four capacitive upper electrodes, and the central electrode and the ground electrode between the four capacitive lower electrodes; A sheet-like structure is provided, each of which has a meander shape.

  According to the present invention, the inductance in the unit structure is increased. This means that the resonance frequency of the LC parallel resonance circuit, which is an equivalent circuit when the unit structure is viewed from the direction perpendicular to the main surface, is lowered. Therefore, the sheet-like structure can be miniaturized as understood by considering the scaling rule.

  In addition, when the resonance frequency is the same, the surface impedance increases as the inductance component increases and the capacitance component decreases.

  Furthermore, when a magnetic film is applied to at least one of the first conductor layer and the second conductor layer of the sheet-like structure, L can be increased and C can be reduced in the surface impedance of the sheet-like structure. Further, it is possible to further widen the frequency range exhibiting a high surface impedance.

  The sheet-like structure having a high surface impedance according to the first embodiment of the present invention has a surface impedance for a specific frequency band due to a periodic structure in which unit structures as shown in FIG. 1 are repeatedly arranged in at least one direction. In this way, incident waves having frequencies belonging to the specific frequency band are reflected in phase.

  As shown in FIG. 1 and FIGS. 2A and 2B, the unit structure according to the present embodiment includes a dielectric layer 10, and a first conductor layer 20 formed on the upper surface of the dielectric layer 10. And a second conductor layer 30 formed on the lower surface of the dielectric layer 10. Here, as understood from FIGS. 2A and 2B, the upper surface and the lower surface of the unit structure in the present embodiment are both square. Hereinafter, this square is referred to as a first square.

  As shown in FIG. 2A, the first conductor layer 20 includes four capacitive upper electrodes 21 arranged so as to be separated from each other. Each capacitor upper electrode 21 is arranged in contact with each side of the first square, that is, the edge of the unit structure. Thereby, when unit structures are juxtaposed, the capacitor upper electrodes 21 of adjacent unit structures are connected to each other.

  More specifically, the capacitor upper electrode 21 in the present embodiment has a right-angled isosceles triangle shape having a base shorter than one side of the first square. Hereinafter, this isosceles right triangle is referred to as a first triangle. The bottom side of the first triangular capacitor upper electrode 21 is arranged on each side of the first square, and the vertex of the electrode 21 is arranged to face the center point of the first square.

  As shown in FIG. 2B, the second conductor layer 30 includes a frame-shaped ground electrode 31 having the same outer shape as the first square, and a central electrode 32 disposed at the center of the first square. The four capacitive lower electrodes 33 respectively connected to the central electrode 32 and the four inductance portions 34 connecting the central electrode 32 and the ground electrode 31 between the four capacitive lower electrodes 33 are provided.

  Specifically, the ground electrode 31 is a rectangular frame designed so that the outer shape matches the first square and has a predetermined width. The center electrode 32 has a square shape having four sides shorter than the length of the bottom side of the capacitor upper electrode 21. Hereinafter, the square of the central electrode 32 is referred to as a second square. The center electrode 32 is formed by rotating the central electrode 32 around the center point of the first square on the center point of the first square. That is, the diagonal of the first square and the diagonal of the second square are shifted by 45 degrees. The capacitor lower electrode 33 is provided so as to face the capacitor upper electrode 21, and has a right isosceles triangle shape that is slightly smaller than the first triangle in the present embodiment. Hereinafter, the right isosceles triangle of the capacitor lower electrode 33 is referred to as a second triangle. The bottom side of the capacitor lower electrode 33 is located at a predetermined distance from the four inner sides of the square frame constituting the ground electrode 31, while the vertex of the second triangle is connected to the vertex of the second square on a one-to-one basis. Has been.

  Here, each of the inductance portions 34 in the present embodiment has a meander shape. More specifically, the inductance section 34 according to the present embodiment is once in the vicinity of the ground electrode 31 and the center electrode 32 with the direction parallel to the diagonal of the first square as the longitudinal direction on the diagonal of the first square. It has a meander shape that is folded. As a result, the inductance component can be dramatically increased. In the present embodiment, a meander shape is adopted in which the direction parallel to the diagonal of the first square is the longitudinal direction. For example, the direction perpendicular to the diagonal of the first square is the longitudinal direction. Such meander shape may be adopted.

  FIG. 3 shows the unit structure described above with an equivalent circuit. The unit structure described above may be expressed by an equivalent circuit as shown in FIG.

  When viewed from a plane wave incident on the unit structure from the upper surface of the first conductor layer 20, the equivalent circuits shown in FIGS. 3 and 4 can be simply approximated to an LC parallel resonant circuit in an appropriate frequency range. . This can be expected from the fact that the gap between the adjacent central electrodes 32 has a parallel connection between the gap and the continuous conductor.

Here, the surface admittance Ys (reciprocal of the surface impedance Zs) of the surface approximated by the LC parallel resonance circuit having the inductance L and the capacitance C can be expressed by the following formula (1). ω is an angular frequency, and j is an imaginary unit.

When the equation (1) is differentiated by ω, the following equation (2) is obtained.

Here, when the resonance angular frequency of the LC parallel circuit is ω ₀ and Equation (2) is modified using Equation (3) below, Equation (4) is obtained.

From the equation (4), the amount of change of the surface admittance Ys in the vicinity of ω _{0 with} respect to ω is obtained as in equation (5).

  As apparent from the equation (5), in the LC parallel resonance circuit, the amount of change with respect to the frequency change of the surface admittance at the resonance frequency is smaller as C is smaller (L is larger) and becomes j2C. That is, the surface impedance Zs, which is the reciprocal of the surface admittance Ys, also decreases with frequency as C decreases (as L increases) (see FIG. 5).

  Therefore, if the LC parallel resonant circuit is formed of a combination of a small capacitance and a large inductance as shown in FIG. 6, a broadband common-mode reflection effect can be obtained.

  As described above, the inductance portion 34 in the present embodiment is meander-shaped and increases the inductance, so that the frequency region exhibiting high surface impedance can be made relatively wide. In addition, when a magnetic layer is applied to at least one of the first conductor layer 20 and the second conductor layer 30, a wider-band in-phase reflection effect can be obtained.

  Referring to FIG. 7, an example in which the above-described sheet-like structure is applied to an RFID tag is shown. In FIG. 7, reference numerals 10 ′, 20 ′, and 30 ′ are formed by periodically connecting the dielectric layer 10, the first conductor layer 20, and the second conductor layer 30 having the unit structure described above. Is.

  The illustrated RFID tag is an example of a 2.4 GHz metal-compatible tag, and includes a sheet-like structure formed by connecting five unit structures each having a planar shape of 12 × 12 mm in a row. An antenna support layer 40 made of a dielectric is formed on the first conductor layer 20 ′, and an antenna layer 50 having a predetermined antenna pattern is connected to the antenna support layer 40 on the upper surface thereof. IC 60 is arranged. On the other hand, a shield support layer 70 is provided on the lower surface side of the second conductor layer 30 ′, and a shield layer 80 made of a solid pattern conductor is formed on the lower surface of the shield support layer 70. . Since the 2nd conductor layer 30 is not a solid electrode, when a tag is affixed on a metal surface, the performance may change under the influence of the metal. Therefore, in the tag of this example, a shield layer 80 made of a conductor is provided in advance so as to be separated from the second conductor layer 30 by a certain distance.

  In this example, the dielectric layer 10 ′, the antenna support layer 40, and the shield support layer 70 are made of PET (Polyethylene Terephthalate resin). On the other hand, the first conductor layer 20 ′, the second conductor layer 30 ′, the antenna phase 50, and the shield layer 80 are made of metal foil, more specifically, aluminum foil or copper foil.

  Here, the distance between the first conductor layer 20 ′ and the second conductor layer 30, that is, the thickness of the dielectric layer 10 is 0.2 mm, and the first conductor layer 20 ′ and the antenna layer 50 are The distance between them, that is, the thickness of the antenna support layer 40 is also 0.2 mm. The distance between the second conductor layer 30 and the shield layer 80, that is, the thickness of the shield support layer 70 is 0.5 mm.

  In order to reduce the correlation between the sheet-like structure and the antenna layer 50 and the correlation between the sheet-like structure and the shield layer 80, the thickness of the antenna support layer 40 or the shield support layer 70 is increased, or a low dielectric constant is set. It is necessary to form the antenna support layer 40 and the shield support layer 70 with the materials they have.

  Specifically, the material of the antenna support layer 40 and the shield support layer 70 is preferably a resin having a dielectric constant as low as possible, a foam material containing air, sponge, urethane, fiber, or the like. In particular, the air layer and the holes concentrate the electric field in the air and lower the electric field in the material, which is effective in reducing the loss.

  Referring to FIG. 8, there is shown a modification in which the magnetic layer 100 is provided on both surfaces of the first conductor layer 20 ′ and the second conductor layer 30 ′ and one surface of the shield layer 80. If the magnetic layer 100 is applied to the conductor layer as in this example, the inductance component in the LC parallel resonance equivalent circuit can be increased, and thus high impedance characteristics over a wide band can be obtained. The magnetic layer 100 is more effective when it is in close contact with the electrode, and the effect increases as the thickness increases. As the magnetic layer 100, for example, a composite magnetic material sheet made of ferrite plating or magnetic powder and a resin binder can be used.

  Ferrite plating has a higher electrical conductivity than an insulator, so it is desirable to plate only on metal parts. Further, in the example shown in FIG. 8, the magnetic film 100 is provided on both surfaces of the first conductor layer 20 ′ and the second conductor layer 30 ′. It is good also as giving to.

  For example, as shown in FIG. 9, the dielectric layer 10 in the sheet-like structure is replaced with a sheet 110 made of a composite magnetic material, or the shield support layer 70 is replaced with a sheet 170 made of a composite magnetic material. You can get the same effect.

It is a side view which shows roughly the unit structure in the sheet-like structure by the 1st Embodiment of this invention. 2A is a top view showing the dielectric layer and the first conductor layer shown in FIG. 1, and FIG. 2B shows the dielectric layer and the second conductor layer shown in FIG. FIG. It is a figure which shows the equivalent circuit at the time of seeing the unit structure shown by FIG. 1 from upper direction. FIG. 4 is a diagram illustrating an equivalent circuit that can be replaced with the equivalent circuit of FIG. 3. It is a figure for demonstrating that the broadband of a high impedance area | region can be achieved by increasing an inductance. It is a figure which shows the conditions on the equivalent circuit for making a high impedance area | region into a broadband. It is a figure which shows the example which applied the sheet-like structure shown by FIG. 1 to the RFID tag. It is a figure which shows the modification of the RFID tag shown by FIG. It is a figure which shows the further modification of the RFID tag shown by FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Dielectric board | substrate 20,20 '1st conductor layer 21 Capacity | capacitance upper electrode 30, 30' 2nd conductor layer 31 Ground electrode 32 Center electrode 33 Capacity | capacitance lower electrode 34 Inductance part 40 Antenna support layer 50 Antenna layer 60 IC
70 Shield Support Layer 80 Shield Layer 100, 110, 170 Magnetic Layer

Claims (7)

A sheet-like structure that enhances the surface impedance for a specific frequency band by a periodic structure in which unit structures are repeatedly arranged in at least one direction, thereby reflecting the incident wave having a frequency belonging to the specific frequency band in-phase,
The unit structure includes a dielectric layer having first and second main surfaces, a first conductor layer formed on the first main surface, and a first layer formed on the second main surface. 2 conductor layers,
The first conductor layer includes four capacitor-shaped upper electrodes of a predetermined shape that are arranged so as to be spaced apart from each other and are arranged so as to be in contact with an edge of the first main surface,
The second conductor layer includes a frame-shaped ground electrode having the same outer shape as the second main surface, a central electrode disposed in a central portion of the second main surface, and a connection to the central electrode. And four capacitive lower electrodes provided to face the four capacitive upper electrodes, and four inductance portions connecting the central electrode and the ground electrode between the four capacitive lower electrodes. And
Each of the inductance portions is a sheet-like structure having a meander shape. Each of the first and second main surfaces has a first square shape with one side having a first predetermined length;
Each of the capacitor upper electrodes has a first right isosceles triangle shape having a base of a second predetermined length shorter than the first predetermined length,
The four first right-angled isosceles triangles are arranged such that the base is arranged on each side of the first square while the vertex is directed to the center point of the first square,
The ground electrode is a square frame having an outer shape that matches the first square and has a predetermined width,
The center electrode has a second square shape smaller than the first square, and is arranged on the center point of the first square in a state rotated by 45 degrees around the center point. Yes,
Each of the capacitor lower electrodes has a second right-angled isosceles triangle shape arranged so as to be separated from any one of the four inner sides of the square frame by a predetermined distance, with the vertex at the center portion. The sheet-like structure according to claim 1, wherein the sheet-like structure has a second right-angled isosceles triangle shape connected to any one of the four vertices.   3. The meander shape of the inductance portion is formed by folding back at least once near the ground electrode and near the center electrode, with a direction parallel to the diagonal line of the first square shape as a longitudinal direction. Sheet-like structure.   The sheet-like structure according to any one of claims 1 to 3, further comprising a magnetic layer applied to at least one of the first conductor layer and the second conductor layer.   An antenna support comprising the sheet-like structure according to any one of claims 1 to 4, an antenna layer having an antenna pattern, and a dielectric formed on the first conductor layer of the sheet-like structure. An antenna device comprising a layer.   6. The antenna device according to claim 5, further comprising: a shield layer made of a solid pattern conductor; and a shield support layer made of a dielectric formed on the second conductor layer of the sheet-like structure.   An RFID (Radio Frequency Identification) tag comprising the antenna device according to claim 5 or 6 and an IC connected to the antenna pattern on the antenna support layer, wherein the sheet-like structure is a part of a tag base. .

Images