Antenna and RFID tag with same mounted

A tag antenna which, within a limited area, resonates with an RFID LSI chip having a capacitance component, and an RFID tag on which such a tag antenna is mounted, and which has an antenna and an LSI chip connected in parallel to the antenna are disclosed; the antenna has a feed terminal connected to the LSI chip, a loop antenna connected to the feed terminal, and a bypass conducting path which bypasses the loop of the loop antenna.

TECHNICAL FIELD

The present invention relates to an antenna and a tag with such an antenna mounted, and in particular relates to an antenna comprising a loop antenna suited for use in a non-contact type tag which performs transmission and reception with an RFID reader/writer, and an RFID tag with such an antenna mounted.

BACKGROUND ART

In recent years, there has been widespread introduction of systems in which tags storing article information are attached to articles, including cattle and other living beings, and in which this information is read by non-contact means to manage data. An example of such a system is called an RF (radio frequency) ID system. In such a system, wireless signals of power approximately 1 W in the UHF band (860 to 960 MHz) are used to transmit signals from a reader/writer.

On the tag side, power to start processing is generated based on this signal, and a response signal is returned to the reader/writer from the tag according to instruction information comprised by the signal. By this means, information within the tag can be read by the reader/writer.

A tag attached to an article comprises an antenna, and a functional chip connected to the antenna. The communication distance between the tag and the reader/writer depends on the gain of the antenna incorporated within the tag, the chip operating voltage, and the surrounding environment, but is approximately 3 meters.

The interface portion of the LSI chip20mounted in a tag can be represented as equivalent to a parallel connection of a resistance Rc (for example, resistance value=1000Ω) and a capacitance C (for example, capacitance value=0.7 pF), as shown inFIG. 1explaining matching of the antenna and the LSI chip. On the other hand, the antenna mounted on the tag (called the tag antenna) can be represented as equivalent to a parallel connection of a resistance Ra (for example, resistance value=1000Ω) and an inductance L (for example, inductance value=40 nH).

As a result of parallel connection of the two, the capacitance value C and inductance value L are in resonance, and as indicated by equation (1), the imaginary component is substantially zero and matching is achieved, so that the power received by the tag antenna10is adequately supplied to the LSI chip20.

The most basic design for the tag antenna10is that of a resonant dipole antenna with total length equal to λ/2; in the UHF band, a length of approximately 150 mm is required. However, in consideration of cases in which such tags are affixed to the ears of cattle and managed, for example, there is a demand to form tags with a maximum diameter of approximately 35 mm.

However, it is substantially impossible to form a λ/2 resonant dipole antenna with such a required size.

The use of a loop antenna configured such as the loop antenna1connected to a feed terminal3shown inFIG. 2is conceivable. A loop antenna has characteristics such as those explained for example in Antenna Engineering Handbook, published by Ohmsha, Ltd., Mar. 5 1999, pp. 20-22. That is, it is explained that when a common-mode current flows uniformly in a circular loop1connected to a feed terminal3, as the loop radius is increased, the number of lobes of the power pattern increases.

DISCLOSURE OF THE INVENTION

As explained above, it is most important for matching between the chip20and the tag antenna10that in a tag used in an RFID system, the capacitance value C of the interface portion of the LSI chip20and the inductance value L of the antenna10be in resonance at the frequency used, as indicated inFIG. 1.

On the other hand, when the tag antenna10is configured with only a loop antenna structure such as that shown inFIG. 2, there is no inductance L portion such as shown inFIG. 1, and so satisfactory matching with the inductance portion of the LSI chip20is not obtained, power is not supplied adequately from the antenna10to the chip20, and so there is the problem that the communication distance is reduced drastically.

When, for convenience of handling, the tag antenna10is enclosed in a dielectric case and similar, the dielectric constant and thickness of the dielectric member used as the case must be considered when designing the shape of the antenna.

Hence an object of this invention is to provide a tag antenna which, while having limited size, resonates with an RFID LSI chip having a capacitance component, and an RFID tag on which this antenna is mounted.

A first aspect of an antenna of the invention which attains the above object is characterized in having a feed terminal, a loop antenna connected to the feed terminal, and a bypass conducting path which bypasses the loop of the loop antenna.

A second aspect of an antenna of the invention which attains the above object is the first aspect, characterized in that the bypass conducting path is arranged have a distance S from the center of the loop antenna, and that the size of the distance S is set such that the inductor which is the bypass conducting path has a prescribed inductance value.

A third aspect of an antenna of the invention which attains the above object is the second aspect, characterized in that the prescribed inductance value is a value resulting in resonance with the capacitance value of the interface portion of the LSI chip connected to the feed terminal.

A first aspect of an RFID tag of the invention which attains the above object is characterized in having an LSI chip connected in parallel to the antenna, and in that the antenna has a feed terminal connected to the LSI chip, a loop antenna connected to the feed terminal, and a bypass conducting path which bypasses the loop of the loop antenna.

A second aspect of an RFID tag of the invention which attains the above object is the RFID tag of the first aspect, characterized in that the bypass conducting path is arranged have a distance S from the center of the loop antenna, and that the size of the distance S is set such that the inductor which is the bypass conducting path has a prescribed inductance value.

A third aspect of an RFID tag of the invention which attains the above object is the RFID tag of the second aspect, characterized in that the prescribed inductance value is a value resulting in resonance with the capacitance value of the interface portion of the LSI chip connected to the feed terminal.

A fourth aspect of an RFID tag of the invention which attains the above object is the RFID tag of the first aspect, characterized in further having a conducting plate, which is arranged on the side of at least one face of the antenna, and which holds the antenna.

A fifth aspect of an RFID tag of the invention which attains the above object is the RFID tag of the fourth aspect, characterized in that a penetrating hole is provided in the center of the conducting plate.

A sixth aspect of an RFID tag of the invention which attains the above object is the RFID tag of the fifth aspect, characterized in that the bypass conducting path has an arc, along the periphery of the penetrating hole, which avoids the penetrating hole.

A seventh aspect of an RFID tag of the invention which attains the above object is the RFID tag of the fourth aspect, characterized in having, in the conducting plate, a ring-shape depression, corresponding to the loop antenna, having width and depth enabling accommodation at least of the LSI chip.

An eighth aspect of an RFID tag of the invention which attains the above object is the RFID tag of the fourth aspect, characterized in that the antenna is formed, on a sheet, from a conductor the main component of which is any one among Cu, Ag, and Al.

By means of this invention, a tag antenna which resonates with an RFID LSI chip having a capacitance component, and a tag on which such an antenna is mounted, can be provided within an extremely limited area of diameter approximately 35 mm.

PREFERRED EMBODIMENTS OF THE INVENTION

Below, embodiments of the invention are explained referring to the drawings. The aspects are provided to facilitate understanding of the invention, but the technical scope of the invention is not limited to these aspects.

Principle of the Invention

Prior to an explanation of the embodiments, the principle of an antenna configuration according to this invention is explained usingFIG. 3. InFIG. 3, a loop antenna1connected to a feed terminal3is formed. In addition, a bypass conducting line is formed within the loop antenna1, as a matching inductor L. InFIG. 3, S is the distance from the center of the loop antenna1to the bypass conducting line2; the parameter (S) of the inductance value L due to the bypass conducting line2is varied as this distance S changes.

FIG. 4is a graph showing the relation between the inductance value L and the distance S from the center of the loop antenna1to the bypass conducting line2. The numerical values shown inFIG. 4are calculated values obtained from an electromagnetic field simulator; it is seen that the inductance value L changes depending on the distance S from the center of the loop antenna1. Hence by appropriately choosing a distance S, an inductance value L can be set for the tag antenna10to match the capacitance value C of the interface portion of the LSI chip20.

For example, when the capacitance value of the capacitor C of the LSI chip20to be matched is C=0.7 pF, then from the above equation (1), near f=950 MHz, resonance occurs at L=40 nH. Hence fromFIG. 4, to obtain L=40 nH, when in the example ofFIG. 3the antenna is placed in air without modification, the dielectric constant ∈r is that of air (=0), and so from the characteristic curve A it is seen that matching of the tag antenna10and LSI chip20occurs when S=4.2 mm.

These inventors have confirmed in numerous prototype experiments that, for a tag antenna10with an outermost diameter of 33 mm and using Cu as the conducting material, with thickness 20 μm and wire width 2 mm, the resistance value of the parallel resistance Ra of the antenna is approximately 1000Ω. Hence adequate matching is possible even when the real-part resistance Rc of the LSI chip20is for example approximately 1000Ω to 2000Ω.

InFIG. 3, it is possible to hold the tag antenna10by forming a dielectric plate above and below, or on one face of, a planar antenna in which are formed a loop antenna1and bypass conducting line2. It is desirable that the parameter (S) be selected according to the dielectric constant and thickness of the dielectric plate.

Embodiments

FIG. 5shows a first embodiment of the principle of the invention shown inFIG. 3.FIG. 5Ais a plane view, andFIG. 5Bis a central cross-sectional view.

Dielectric plates4,5are arranged above and below the plane of a tag antenna10, comprising a loop antenna1and a bypass conducting line2, to enclose the tag antenna10and maintain the antenna shape. To this end, the dielectric plates4,5are fixed to and enclose the tag antenna10with for example a resin adhesive or similar.

This embodiment corresponds to the conceptual diagram ofFIG. 3; the bypass conducting line2is connected from midway in the loop antenna1connected to the feed terminal3, to form an inductance component L.

As the dimensions in the embodiment, the outermost diameter of the loop antenna1is 33 mm, and Cu is used as the conducting material, with a thickness of 20 μm and wire width of 2 mm. The thickness of the dielectric plates4,5is t=1 mm.

If the dielectric plates4,5are formed from plastic, rubber, or similar, then it is well known that the dielectric constant takes on a value of approximately ∈r=3 to 5. Hence when characteristics are determined using an electromagnetic field simulator assuming ∈r=3 and ∈r=5, the characteristic curves B, C shown inFIG. 4are obtained.

Similarly to characteristic curve A for the case of air with a dielectric constant ∈r=0, the inductance value L changes with the distance S; the distance from the loop center O of the loop antenna1to the bypass conducting line2is taken to be S. Based on calculated values obtained by the electromagnetic field simulator and shown inFIG. 4above, when the dielectric constant of the dielectric plates4,5is ∈r=3 in the configuration shown inFIG. 5, the characteristic curve B obtains, and when ∈r=5, the characteristic curve C obtains. Hence it is seen that in order to obtain resonance at L=40 nH, if ∈r=3 then S=6 mm, and if ∈r=5 then S=7.5 mm.

Here, the structure of the RFID tag for installation on an article must be considered.FIG. 6shows an embodiment which accommodates this need. A penetrating hole6is formed in the center portion of the RFID tag. This penetrating hole6can be used to install the tag on the article.

However, upon forming a penetrating hole7in the center, the penetrating hole6is superposed on the bypass conducting line2. Hence in the embodiment ofFIG. 6, the bypass conducting line2is formed having an arc2aalong the periphery of the penetrating hole6, so as to avoid the penetrating hole6. As a result of simulations in which this arc2awas fixed, and the distance S from the straight-line conducting line portion of the bypass conducting line2was taken as a parameter in electromagnetic field simulations, coincidence with the characteristic curves A and B for ∈r=3 and ∈r=5, shown inFIG. 4, was obtained.

InFIG. 6, when the radius of the penetrating hole6is r1=6.5 mm, the conductor width is 2 mm, and the distance S is greater than (radius of penetrating hole6+conductor width) (that is, when S>8.5 mm), then similarly to Embodiment 1, when for example the chip capacitance value is C=0.7 pF, from equation 1, resonance occurs in the vicinity of f=950 MHz when L=40 nH.

FromFIG. 4, L=40 nH when ∈r=3 and S=6.1 mm, or when ∈r=5 and S=7.5 mm; hence by selecting an appropriate value of S according to the relative dielectric constant for the antenna configuration shown inFIG. 6, matching of the chip20and antenna10is possible.

FIG. 7shows a further embodiment in which a penetrating hole6is provided; in contrast with the embodiment shown inFIG. 5, the size of the radius of the penetrating hole6is made smaller than the distance S from the center of the loop antenna1to the bypass conducting line2, so that a configuration is made possible in which there is no need to provide the arc2ashown inFIG. 6.

Here, it is desirable that the LSI chip20be formed integrally with the tag antenna10.FIG. 8shows the configuration of an embodiment which satisfies such a demand.

InFIG. 8, the LSI chip20is connected to the feed terminal of the tag antenna10comprising the loop antenna1and bypass conducting line2corresponding toFIG. 6.FIG. 8Ais a diagram in which the tag antenna10and LSI chip20are mounted on a dielectric plate5.

On the other hand,FIG. 8Bshows the dielectric plate4, having, as a characteristic, a ring-shape depression30corresponding to the loop antenna1. The depression30has a width and depth enabling accommodation of, at least, an LSI chip20having a chip thickness of approximately 0.5 mm. Hence by superposing, in order, the dielectric plate5, tag antenna10connected to the LSI chip20, and dielectric plate4, integral formation as shown inFIG. 8Cis possible.

As a characteristic, upon superpositioning, a space is secured such that the LSI chip20can be received in the depression30of the dielectric plate4, whatever the position of the LSI chip20, so that the upper and lower dielectric members4,5can be closely stacked. By this means, the number of processes required when enclosing the tag antenna10between the upper and lower dielectric members4,5is reduced, and an inexpensive antenna unit can be supplied.

FIG. 9shows the configuration of still another embodiment. In each of the above embodiments, as the method of fabrication of the tag antenna10comprising a loop antenna1and bypass conductor2, a thin conductor of such materials as Cu, Ag, Al or similar (of thickness 20 μm for example) can be formed on an extremely thin sheet40(of thickness for example 0.1 mm) of paper, PET (polyethylene sheet), or similar. Or, in place of a thin conductor, a wire-shape conductor may be used.

Also, in each of the above embodiments, configurations have been shown in which a tag antenna10comprising a loop antenna1and a bypass conductor2is enclosed between dielectric members4,5; but from the standpoint of holding the tag antenna10, a configuration is also possible in which a dielectric member4(5) is formed on only one side of the tag antenna10, as shown inFIG. 10.

FIG. 11shows antenna directional characteristics obtained in electromagnetic field simulations of an antenna of this invention. The three axis directions x, y, z are defined as shown inFIG. 12, and the angle from the x-axis in the x-y plane is φ, while the angle from the z axis is θ.

FromFIG. 11A, at φ=90°, that is, in the y-axis direction, when the angle θ from the z-axis is 90° and −90°, the maximum directional characteristic is exhibited. FromFIG. 11B, at φ=0°, that is, in the x-axis direction, when the angle θ from the z axis is 90° and −90°, the maximum directional characteristic is exhibited. In general, inFIG. 11AandFIG. 11Bthe same directional characteristics are shown, but more directionality is exhibited in general when θ=90° (FIG. 11A) than when θ=0° (FIG. 11B), and results specific to loop antennas are obtained.

Hence from these directional characteristics, it is seen that an antenna of this invention has satisfactory directional characteristics. Hence when using an RFID tag with an antenna of this invention mounted as an RFID tag for an article, application is possible to animals and to other articles that move as well, and the range of applicability is broad. By means of this invention, a tag antenna within an extremely limited area of diameter approximately 35 mm, and which resonates with an RFID LSI chip having a capacitance component, as well as an RFID tag on which this antenna is mounted, can be provided.

Further, by means of this invention, the positioning of a bypass conducting path forming an inductor having inductance L, which resonates with the capacitance C of the interface portion of an LSI chip and which is calculated in advance, as well as optimal dimensions according to the dielectric constant and thickness of dielectric plates, can easily be determined.

Further, in manufacturing processes, upper and lower dielectric members can easily be stacked regardless of the position of the LSI chip.