JP4843468B2 - High surface impedance structure, antenna device, and RFID tag - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high surface impedance structure which can be made thin and low in loss. <P>SOLUTION: A sheet type structure formed by repeatedly arranging unit structures in an (x) direction has a capacity upper electrode 21 formed on the top surface side of a dielectric layer 10. A ground electrode 31 in a rectangular frame shape, a capacity lower electrode 32 disposed opposite the capacity upper electrode 21, a center electrode 33 disposed in the center of the reverse surface to connect the capacity lower electrode 32, and an inductance portion 34 connecting the center electrode 33 and ground electrode 31 to each other in a (y) direction are formed on the reverse surface side of the dielectric layer 10. The inductance part 34 can suppress generation of an induced current by an antenna and suppress loss due to the induced current along with the length of the sheet structure and the length of the antenna in combination. <P>COPYRIGHT: (C)2008,JPO&amp;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 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 the RFID tag, if the thickness required to exhibit a high surface impedance is too thick, it will go against the trend of miniaturization of equipment and the demand for the RFID tag, 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. Therefore, there is a demand for thinning the high surface impedance structure.

  Further, if the loss in the high surface impedance structure can be kept low, a larger in-phase reflection amount can be obtained. That is, there is also a demand for low loss for the high surface impedance structure.

  Accordingly, an object of the present invention is to provide a high surface impedance structure that can be reduced in thickness and loss, and an antenna device and an RFID tag using the structure.

  As a means for solving the above-described problems, the present invention increases the surface impedance for a specific frequency band by a periodic structure in which unit structures are repeatedly arranged in the first predetermined direction, and thereby the frequencies belonging to the specific frequency band are increased. A sheet-like structure that reflects incident waves 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; while the first conductor layer is disposed so as to be separated from each other in the first predetermined direction. A capacitor upper electrode having two predetermined shapes disposed so as to be in contact with an edge of the first main surface intersecting with the first predetermined direction; and the second conductor layer includes the second main layer. A frame-shaped graph with the same surface and outer shape A second electrode, a central electrode disposed in a central portion of the second main surface, two capacitive lower electrodes connected to the central electrode and provided to face the two capacitive upper electrodes, Provided is a sheet-like structure that includes two inductance portions that extend in a second predetermined direction orthogonal to the first predetermined direction and connect the center electrode and the ground electrode.

  According to the present invention, since the loop through which current flows through the inductance is large, a large inductance can be obtained. 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, since the second predetermined direction orthogonal to the first predetermined direction is the extending direction of the inductance portion, the first predetermined direction is also formed on the sheet-like structure formed so as to have a length in the first predetermined direction. When an antenna having a predetermined direction as a longitudinal direction is formed, the longitudinal direction of the inductance portion and the longitudinal direction of the antenna are orthogonal to each other, so that generation of an induced current in the inductance portion can be suppressed and conductor loss can be reduced. .

  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.

A sheet-like structure having a high surface impedance according to a first embodiment of the present invention increases the surface impedance for a particular frequency band by repeatedly arranging comprising structural units structure in a first predetermined direction, as shown in FIG. 1 Thus, an incident wave having a frequency belonging to the specific frequency band is 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 is composed of two capacitor upper electrodes 21 arranged so as to be separated from each other in the x direction. Each capacitor upper electrode 21 has a rectangular shape having an area smaller than half of the area of the first square. Hereinafter, this square is referred to as a second square. Each of the second squares in the present embodiment is a rectangle in which the side in the y direction is approximately twice as long as the side in the x direction. The capacitor upper electrode 21 is arranged so as to share the long side of the second square, that is, one of the sides extending in the y direction, with the side extending in the y direction of the first square. When juxtaposed in the x direction, adjacent capacitor upper electrodes 21 having unit structures are connected to each other.

  As shown in FIG. 2B, the second conductor layer 30 includes a square frame-shaped ground electrode 31 having the same outer shape as the first square, and two capacitance lower portions corresponding to the two capacitance upper electrodes 21. The electrode 32, the center electrode 33 disposed at the center of the first square, and two inductance portions 34 extending in the y direction and connecting the center electrode 33 and the ground electrode 31 are provided. Here, the capacitive lower electrode 32 has a third rectangular shape that is slightly smaller than the second rectangular shape, and is connected to the central electrode 33 in the x direction. The center electrode 33 has a fourth rectangular shape that is smaller than the third square, and is disposed on the center point of the first square.

  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 adjacent central electrodes 33 has a parallel connection structure of a gap and a 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 equation (4), the amount of change of the surface admittance Ys in the vicinity of ω _{0 with} respect to the change in 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.

  According to the present embodiment, since the loop of the current flowing through the inductance section 34 is large, it is effectively equivalent to having a large inductance, and therefore a frequency region exhibiting a 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.

  7 to 10 show an example in which the above-described sheet-like structure is applied to an RFID tag. 7, 9, and 10, reference numerals 10 ′, 20 ′, and 30 ′ denote the dielectric layer 10, the first conductor layer 20, and the second conductor layer 30 having the unit structure described above, respectively. It is formed by periodically connecting in the direction. In addition, as the actual dielectric material, a relatively transparent material (which can be seen through at least inside) may be used. However, in FIGS. It is drawn.

  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 (in particular, FIG. 9 and FIG. 9). (See FIG. 10). As shown in FIGS. 7 and 8, an antenna support layer 40 made of a dielectric is formed on the first conductor layer 20 ′, and the antenna support layer 40 has a predetermined antenna pattern on the upper surface. An antenna layer 50 and an IC 60 connected to the antenna pattern are 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 layer 50, and the shield layer 80 are made of metal foil, more specifically, aluminum foil or copper foil. 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 , 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.

  As can be understood from FIG. 8, the antenna in this example is a dipole antenna having the x direction as the longitudinal direction and extending toward the object in the x direction around the IC 60. As can be understood from FIGS. 8 and 10, the longitudinal direction (x direction) of the dipole antenna and the longitudinal direction (y direction) of the inductance portion 34 are orthogonal to each other. Therefore, no induced current is generated in the inductance portion 34 by the antenna. Therefore, the conductor loss in the inductance part 34 is the minimum.

  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. It is necessary to configure the antenna support layer 40 and the shield support layer 70 with a material having a ratio.

  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. 11, 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 is shown. 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. In the example shown in FIG. 11, the magnetic film 100 is provided on both surfaces of the first conductor layer 20 ′ and the second conductor layer 30 ′, but either one or only one surface may be provided. It is good also as giving to.

  For example, as shown in FIG. 12, 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. It is a bottom view shown. 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. FIG. 8 is a top view of the RFID tag shown in FIG. 7. It is a top view of the sheet-like structure included in the RFID tag shown in FIG. It is a bottom view of the sheet-like structure contained in the RFID tag shown in FIG. 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 Capacity | capacitance lower electrode 33 Center 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 (8)

Enhance surface impedance by repeatedly arranging comprising structural units structure in a first predetermined direction for a specific frequency band, whereby a sheet-like structure for phase reflects incident wave having a frequency belonging to the particular frequency band,
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 layers are arranged so as to be separated from each other in the first predetermined direction, and are arranged so as to be in contact with an edge of the first main surface intersecting the first predetermined direction. With two upper electrodes of a predetermined shape,
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 two capacitor lower electrodes provided opposite to the two capacitor upper electrodes, and extending in a second predetermined direction orthogonal to the first predetermined direction to connect the center electrode and the ground electrode. A sheet-like structure having two inductance portions. Each of the first and second main surfaces has a first square shape including two sides extending in the first predetermined direction and two sides extending in the second predetermined direction.
Each of the capacitor upper electrodes has a second rectangular shape having an area smaller than half of the area of the first square, and shares one of the sides extending in the second predetermined direction with the first square. Are arranged to
The ground electrode is a square frame having an outer shape that matches the first square.
Each of the capacitor lower electrodes has a third rectangular shape having an area smaller than half of the area of the first square, and is disposed apart from the ground electrode.
The central electrode has a fourth rectangular shape smaller than the third rectangular shape, and is disposed on a center point of the first square;
Each of the inductance portions connects the center electrode and the ground electrode on a line that passes through the center point of the first square and extends in the second predetermined direction.
The sheet-like structure according to claim 1.   The sheet-like structure according to claim 1 or 2, 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 3, 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.   The antenna device according to claim 4, wherein the antenna pattern has a shape in which the first predetermined direction is a longitudinal direction.   The antenna device according to claim 5, wherein the antenna configured by the antenna pattern is a dipole antenna.   The 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. The antenna device according to 1.   An RFID (Radio) comprising the antenna device according to claim 5 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. Frequency Identification) tag.

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