Radio tag

In a noncontact type radio tag communicating with an RFID reader/writer, a first planar antenna is arranged on one surface of a dielectric substrate, a ground plane with which a second planar antenna is integrally formed is arranged on the other surface of the dielectric substrate, and an IC chip is connected to the first and the second planar antenna with a feeder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio tag, and in particular to a noncontact type radio tag communicating with an RFID reader/writer.

2. Description of the Related Art

In recent years, an RFID (Radio Frequency IDentification) system identifying/managing objects by unique information stored in an IC chip has been actively developed. In the RFID system, an RFID reader/writer transmits a high-frequency electric wave signal. A radio tag provided with the IC chip having stored therein the unique information receives the electric wave signal, and then transmits the unique information to the RFID reader/writer.

For example, by attaching radio tags to commercial products such as books and clothes, it is made possible to read the unique information of the radio tag attached to the commercial products by using the RFID reader/writer, or reversely write the unique information in the radio tag.

The radio tag is generally composed of the IC chip and an antenna. When a high-frequency signal is received by the antenna, a rectifier portion embedded in the IC chip converts the high-frequency signal into a DC component on the order of 3 V, for example, so that the signal can be processed by the electric power thereof and further transmitted through the antenna.

As for the frequency of the signal, not only 13.56 MHz but also higher frequencies such as 900 MHz and 2.45 GHz have been used recently.

FIG. 5shows an example of an RFID tag as a generally used radio tag. An RFID tag300inFIG. 5is composed of a dipole antenna11of a plane circuit type having a length of λ/2 and an IC chip6.

Depending on the shape of the dipole antenna11and the chip power consumption in the IC chip6, if the transmission signal power from the RFID reader/writer (not shown) or the radio tag is on the order of 1 W, the RFID reader/writer and the radio tag have a communicable distance on the order of 1 m.

As for other prior art radio tags, there is one in which a semiconductor chip storing information of an object to be identified, a first antenna capable of receiving and transmitting the information of the semiconductor chip between a receiving/transmitting apparatus provided externally, and a second antenna operable by receiving an electric wave of a frequency different from that of the first antenna are integrated (see e.g. patent document 1).

This has integrated an identification tag and an antitheft tag, for example, to improve convenience in handling.

Also, there is another in which two radio tags whose axial directions of cylindrical antenna coils are arranged almost orthogonal with each other and are mutually fixed by a fixing means, thereby relieving directional restriction of transmission/reception sensitivity when the axial directions of the cylindrical antenna coils are arranged almost parallel to the affixing surface of the affixing member (see e.g. patent document 2).

This has enabled a signal transmission/reception by avoiding influences of a conductive member by utilizing a leaked electric wave even when an object to which a radio tag is attached is a conductive member such as a metal having influence on communication and power transfer of a radio tag by generating magnetic flux in the opposite direction for attenuating the original magnetic flux by an eddy current.

Moreover, there is a radio tag capable of transmitting/receiving a signal even if the radio tag is embedded in a conductive member such as a metal (see e.g. patent document 3).

The above-mentioned patent documents 2 and 3 resolve a problem in a case where an object to which the radio tag is attached is a conductive member such as a metal. Specifically, the above-mentioned patent document 3 discloses a radio tag capable of transmitting/receiving a signal even if it is embedded in a metal or the like.

However, it has been known that a radio tag using a patch antenna may be used for merely attaching a tag to a surface of a metal or the like.

FIG. 6shows an RFID tag400using a patch antenna2as such an example. The RFID tag400has the square patch antenna2whose side has a length of λ/2 on the top surface of a dielectric substrate1as shown inFIG. 6. Also, on the undersurface thereof, a ground8is formed all over the surface and functions as a ground of the patch antenna2.

The patch antenna2is connected to the IC chip6through a microstrip line3provided on the same surface, and is fed with electric power from the IC chip6through the microstrip line3. Also, the IC chip6is connected to the ground8on the undersurface through a through hole7.

For the RFID tag400inFIG. 6, if the surface to which the patch antenna2is not attached, i.e. the ground8of the undersurface is attached to a metallic object, the ground8and the metallic object assume the same electric potential, so that the electric potential of the patch antenna2itself does not change and therefore its input impedance does not change. Thus, the RFID tag400can be attached to the metallic object to be used.

FIGS. 7A and 7Bschematically show how an electric wave is received in this case. As shown inFIG. 7A, when the surface of the ground8is attached to a metallic object20, a signal S1arriving at the surface of the patch antenna2can be received by the patch antenna2. As a matter of course, since a signal S2arriving at the surface of the ground8is intercepted and reflected by the metallic object20, it cannot be received.

Contrarily, as shown inFIG. 7B, when the surface of the patch antenna2is attached to the metallic object20, the patch antenna2itself is connected to the electric potential of the metallic object20, so that not only its input impedance changes, but also the signal S1is reflected since the ground8is metallic. Namely, when the surface of the patch antenna2is attached to the metallic object20, neither the signal S2arriving at the side of the patch antenna2nor the signal S1arriving at the surface of the ground8can be received.

Therefore, in case of the RFID tag400, it is required that the side of the ground8is attached to the metallic object20without fail as shown inFIG. 7Aand that transmission and reception of an electric wave are enabled only in the direction of the top surface where the patch antenna2is attached (direction of signal S1inFIG. 7A).

Also, when the object to which the RFID tag400is attached is nonmetallic, the patch antenna2or the ground8may be attached to the object. However in this case, the electric wave from the direction incident on the ground8is reflected by the metallic ground8.

FIGS. 7C and 7Dschematically show how a signal is received in cases where the RFID tag400is attached so that the ground8and the patch antenna2may respectively touch a nonmetallic object30.

In either case, although both of the signals S1and S2have reached the RFID tag400, when the ground8is attached to the nonmetallic object30as shown inFIG. 7C, the signal S1shown can be received by the patch antenna2but the signal S2is reflected by the ground8and cannot be received. Contrarily, when the patch antenna2is attached to the nonmetallic object30as shown inFIG. 7D, the signal S2shown can be received by the patch antenna2but the signal S1is reflected by the ground8and cannot be received.

In contrast, the above-mentioned RFID tag300shown inFIG. 5has a problem that when it is attached to a metallic object the input impedance of the dipole antenna11is largely changed, so that the communication distance is extremely shortened or the communication is completely disabled. However, if the object to be attached is not metallic, the RFID tag300has a directivity in all directions except null directions A and B.

Therefore, the RFID tag300is more suitable than the RFID tag400when it is attached to a nonmetallic object.

Supposing a case where metallic objects and nonmetallic objects coexist as the objects to which the radio tags are attached, if it is not desired to limit the directivity of the radio tags to be attached to the nonmetallic objects to only one direction, the preparation of two types of radio tags, for example, the RFID tag400inFIG. 6and the RFID tag300inFIG. 5, respectively for the metallic objects and the nonmetallic objects is required.

In this case, it is required to identify whether or not the object to be attached is metallic at the time of attachment. If the RFID tag300for the nonmetallic object is attached to the metallic object by mistake, the communication distance will be extremely shortened or the communication will be completely disabled as mentioned above.

Contrarily, if the RFID tag400for the metallic object is attached to the nonmetallic object, the transmission/reception of the electric wave for only one surface can be performed as shown inFIGS. 7C and 7Dregardless of whether the attached surface is the patch antenna2or the ground8, so that there is a problem that the directivity in overall directions (except null direction) expected for the RFID tag300for the nonmetallic object which should have been originally attached cannot be obtained.

Also, it is possible to use e.g. the RFID tag400ofFIG. 6if one type of radio tag is commonly used for the metallic object and the nonmetallic object. However, there is a problem that in this case, the directivity of the RFID tag400when it is attached to the nonmetallic object is relatively weak compared to that of the RFID tag300ofFIG. 5as mentioned above.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a radio tag which improves the directivity of transmitting/receiving an electric wave when a radio tag for both of a metal and a nonmetal is attached to a nonmetallic object.

In order to achieve the above-mentioned object, a radio tag according to the present invention comprises: a first planar antenna arranged on one surface of a dielectric substrate; a second planar antenna formed integrally with a ground plane arranged on the other surface of the dielectric substrate; and an IC chip connected to the first and the second planar antenna through a feeder.

Namely, a first planar antenna is arranged on one surface of a dielectric substrate, a second planar antenna formed integrally with a ground plane is arranged on the other surface of the dielectric substrate, and an IC chip is connected to the first and the second planar antenna with a feeder.

Thus, when an object to which the radio tag is attached is a metal, by attaching the ground plane to the metal that is the object in the same way as in the prior art, an electric wave can be transmitted/received through the first planar antenna since there is no change in input impedance of the first planar antenna.

Also, when the object to which the radio tag is attached is a nonmetal, either of the first and the second planar antenna may be attached. The electric wave can be received through the first planar antenna when the arriving direction of the electric wave is in the first planar antenna or through the second planar antenna when the arriving direction of the electric wave is in the second planar antenna, thereby realizing the signal transmission/reception of the IC chip through the feeder.

Thus, by the above-mentioned radio tag for both of the metal and the nonmetal, the directivity of the electric wave transmission/reception when it is attached to the nonmetallic object is improved.

While the surface to be attached to the nonmetallic object may be either of the surfaces of the first or the second planar antenna as mentioned above, by deciding the surface to be attached to the side of the ground plane, it becomes unnecessary to check whether or not the object is the metal.

The above-mentioned first planar antenna may comprise a patch antenna and the above-mentioned second planar antenna may comprise a slot antenna.

Also, the above-mentioned feeder may comprise a microstrip line or a coplanar line.

The frequencies of electric wave signals transmitted and received by the above-mentioned first planar antenna and the above-mentioned second planar antenna may be the same.

The above-mentioned IC chip may be placed on the same surface as the first planar antenna and may be connected to the ground plane through a through hole provided in the dielectric substrate.

The above-mentioned feeder may be placed on the same surface as the first planar antenna and may be connected electromagnetically to the second planar antenna through a noncontact feeding portion.

DESCRIPTION OF THE EMBODIMENTS

Feeder Composed of Microstrip Line

FIG. 1shows an RFID tag100as an embodiment (1) of the present invention where a feeder is composed of a microstrip line3.

The RFID tag100ofFIG. 1has a square patch antenna2whose side has a length of λ/2 on the top surface of a dielectric substrate1in the same way as the prior art RFID tag400shown inFIG. 6. Also, on the undersurface thereof, a ground8is formed all over the surface, and functions as a ground of the patch antenna2.

The patch antenna2is connected to the IC chip6through the microstrip line3provided on the same surface. The patch antenna2is fed with electric power from the IC chip6through the microstrip line3. Also, the IC chip6is connected to the ground8on the undersurface through a through hole7.

The RFID tag100is different from the prior art RFID tag400in that a slot antenna5having the same transmission/reception frequency as that of the patch antenna2is formed within the ground8as shown in FIG. 1 Also, the microstrip line3that connects the IC chip6to the patch antenna2is branched at a branching point X as shown inFIG. 1and electromagnetically coupled to the slot antenna5on the undersurface by a junction4.

In a case where the RFID tag100is attached to a metallic object, by attaching the surface of the ground8to the metallic object an electric wave signal can be transmitted to and received from an RFID reader/writer (not shown) through the patch antenna2.

The manner how an electric wave signal is received in this case is the same as that in the case of the prior art RFID tag400. As having been shown inFIG. 7A, the signal S1arriving on the side of the patch antenna2can be received when the side of the ground8is attached to the metallic object20. Even when the RFID tag100is used, the signal S2arriving on the side of the ground8is reflected by the metallic object20, so that the signal S2cannot be received as in the prior art.

In a case where the RFID tag100is attached to a nonmetallic object, either surface may be attached, whereby the electric wave signal can be transmitted and received between the RFID reader/writer from either surface. Namely, when the RFID reader/writer is facing the patch antenna2, the communication with the RFID reader/writer can be performed through the patch antenna2. When the RFID reader/writer is contrarily facing the ground8, the communication with the RFID reader/writer can be performed through the slot antenna5.

The manner how the signal is received in this case will now be described referring toFIGS. 2A and 2B.

FIG. 2A, corresponding toFIG. 7Cshowing the prior art RFID tag400, shows a state in which both of the signals S1and S2are receivable when the ground8is attached to the nonmetallic object30. Namely, in the case where the prior art RFID tag400is used as having been shown inFIG. 7C, the signal S2is reflected by the ground8, and cannot be received. However, by using the RFID tag100as shown inFIG. 2A, the signal S2can be received from the slot antenna5formed on the ground8.

FIG. 2B, corresponding toFIG. 7Dshowing the prior art RFID tag400, shows a state in which both of the signals S1and S2are receivable when the patch antenna2is attached to the nonmetallic object30. Namely, in the case where the prior art RFID tag400is used as having been shown inFIG. 7D, the signal S1is reflected by the ground8, and cannot be received. However, by using the RFID tag100as shown inFIG. 2B, the signal S1can be received from the slot antenna5formed on the ground8.

Thus, the directivity of electric wave transmission/reception is improved compared with the prior art when the object to which the tag is attached is a nonmetal.

Feeder Composed of Coplanar Line

FIG. 3shows an RFID tag200as an embodiment (2) of the present invention where a feeder is composed of a coplanar line. This RFID tag200is different from the RFID tag100of the above-mentioned embodiment (1) in that the RFID tag200shown inFIG. 3uses a coplanar line composed of a central conductor9and a ground10substituted for the microstrip line3in the RFID tag100shown inFIG. 1. Also, the ground10of the coplanar line and the ground8corresponding to the patch antenna2are made the same electrical potential through the through hole7.

When the feeder is the microstrip line3as in the above-mentioned embodiment (1), the characteristic impedance of the feeder is determined by a relative permittivity ∈rand a thickness “t” of the dielectric substrate1, and a width of the microstrip line3is in a proportional relationship with the thickness “t” of the dielectric substrate1. Therefore, if the dielectric substrate1is thin, the width of the microstrip line3is required to be narrowed. However, there is a problem that when the width of the microstrip line3is too narrow, the feeding loss becomes large.

In contrast, when a coplanar line is used as in the embodiment (2), the characteristic impedance of the feeder is determined by a ratio of a width of the central conductor9to a distance between the ground10on both sides, independently of the thickness “t” of the dielectric substrate1. Therefore, even if the dielectric substrate is very thin, it is possible to reasonably set the line width of the central conductor9so that the feeding loss may not become too large. Thus, the design convenience is improved when the coplanar line is used.

As in the above-mentioned RFID tag100of the embodiment (1), the directivity of electric wave transmission/reception in the RFID tag200of the embodiment (2) is also improved compared with the prior art when the object to which the tag is attached is a nonmetal.

Comparison Between Prior Art Example and Present Invention

FIG. 4shows a comparison table summarizing attachability/non-attachability and directivity when the prior art RFID tag400and the RFID tags100and200of the present invention are respectively attached to a metal and a nonmetal, where the attached surface is made the surfaces of the ground8and the patch antenna2.

As is clear fromFIG. 4, there is no difference between the prior art RFID tag400and the RFID tags100,200of the present invention with regard to the attachability/non-attachability whether the tags are attached to the metal or the nonmetal.

Namely, to the metal, only the surface of the ground8can be attached, while to the nonmetal, both of the surfaces of the ground8and the patch antenna2are attachable.

Also, when the tags are attached to the metal, the directivities are the same for the prior art RFID tag400and the RFID tags100and200of the present invention.

Namely, the directivity when the surface of the ground8is attached to the metal is as shown inFIG. 7A, so that only the signal S1can be received. It is to be noted that when the patch antenna2is attached to the metal, both of the signals S1and S2cannot be received as shown inFIG. 7B.

As for the directivities when the tags are attached to the nonmetal, the prior art RFID tag400can receive only the signal S1as shown inFIG. 7Cwhen the surface of the ground8is attached thereto, while on the contrarily, it can receive only the signal S2as shown inFIG. 7Dwhen the surface of the patch antenna2is attached thereto.

In contrast, with the RFID tags100and200of the present invention, both of the signals S1and S2can be received as shown inFIGS. 2A and 2Beven when either of the surfaces of the ground8and the patch antenna2is attached to the nonmetal.

As described above, the radio tag according to the present invention is one for both of a metal and a nonmetal which can largely improve the directivity of the electric wave transmission/reception when attached to a nonmetallic object.