Antenna apparatus

An antenna apparatus includes a ground plane, a first dielectric layer disposed on the ground plane, a conductive line having a feeding point and an open end or a short end and disposed on the first dielectric layer, a second dielectric layer disposed on the first dielectric layer, a plurality of first conductive elements disposed on the second dielectric layer so that the first conductive elements intersect with the conductive line at a plurality of first positions corresponding to nodes of a standing wave of current flowing through the conductive line, and one or more second conductive elements disposed on the second dielectric layer so that the one or more second conductive elements intersect with the conductive line at second positions corresponding to antinodes of the standing wave between the second end and the first position closest to the second end among the the first positions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-006071 filed on Jan. 16, 2014, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an antenna apparatus.

BACKGROUND

Recently, Radio Frequency Identification (RFID) systems are widely used. Typically, some RFID systems utilize electromagnetic waves in a UHF band (900 MHz band) or a microwave (2.45 GHz) as a communication medium, and some RFID systems utilize a magnetic field generated by mutual induction. Among them, the RFID system utilizing electromagnetic waves in the UHF band is attracting attention since the RFID system can provide relatively long communication distance.

A micro-strip antenna is proposed as an antenna of a reader-writer which communicates with an RFID tag utilizing electromagnetic waves in the UHF band. The micro-strip antenna uses a micro-strip line as an antenna (see, for example, patent document 1 and non-patent documents 1 and 2).

There is a system which includes an antenna provided on a surface of a shelf. Merchandise to which an RFID tag is attached is arranged on the shelf. The system identifies that the merchandise is taken away from the shelf when the system becomes unable to detect the RFID tag. In such a system, it is preferable to use an antenna apparatus which can read the RFID tag attached to the merchandise provided in an area close to the surface of the antenna and can read the RFID tag over the entire surface of the shelf.

However, a communication distance of the conventional antenna is not sufficient and it is difficult to generate a uniform electric field over the entire surface of the antenna, particularly when size of the antenna becomes larger. Accordingly, it is difficult for the conventional antenna to provide uniform and sufficient communication distance.

Therefore, it is difficult to read all of the RFID tags uniformly in a case where a plurality of merchandise to which the RFID tags are attached is arranged on the shelf, in a case where the conventional antenna is used in the system as described above.

PRIOR ART REFERENCES

Patent References

SUMMARY

According to an aspect of an embodiment, there is provided an antenna apparatus including a first dielectric layer having a rectangular shape in plan view, a ground plane configured to be disposed on a first surface of the first dielectric layer, a conductive line configured to have a first end and a second end and to be disposed on a second surface of the first dielectric layer, the first end being a feeding point, the second end being an open end or a short end connected to the ground plane, a second dielectric layer configured to have a shape corresponding to the first dielectric layer and to be disposed on the second surface of the first dielectric layer in a state where the conductive line is sandwiched between the first dielectric layer and the second dielectric layer, the second dielectric layer having a first surface facing toward the first dielectric layer and a second surface opposite to the first surface, a plurality of first conductive elements configured to be disposed on the second surface of the second dielectric layer so that the first conductive elements intersect with the conductive line at a plurality of first positions corresponding to nodes of a standing wave of current flowing through the conductive line in plan view, respectively, and one or more second conductive elements configured to be disposed on the second surface of the second dielectric layer so that the one or more second conductive elements intersect with the conductive line in plan view at one or more second positions corresponding to one or more antinodes of the standing wave between the second end and the first position which is closest to the second end among the the first positions, respectively.

DESCRIPTION OF EMBODIMENTS

A description is given, with reference to the accompanying drawings, of embodiments of an antenna apparatus.

First Embodiment

FIG. 1is an oblique perspective diagram illustrating an antenna apparatus100of the first embodiment.FIG. 2is an oblique perspective diagram illustrating an antenna apparatus100of the first embodiment.FIG. 3is an enlarged diagram illustrating a part of the antenna apparatus100.FIG. 4is an enlarged diagram illustrating another part of the antenna apparatus100.FIG. 5is an oblique perspective exploded diagram illustrating an antenna apparatus100of the first embodiment.FIG. 6is a diagram illustrating an A-A cross section of the antenna apparatus100as illustrated inFIG. 1.

Hereinafter, the antenna apparatus100will be described by using a XYZ coordinate system as an orthogonal coordinate system. Hereinafter, for the purpose of illustration, a surface which is located on the negative side in Z axis direction will be referred to as a bottom surface, and a surface which is located on the positive side in Z axis direction will be referred to as a top surface. However, the top surface and the bottom surface are just exemplary names and do not mean universalistic relationship of upper and lower.

The antenna apparatus100includes dielectric layers110and120, a ground plane130, a meander conductive line140, conductive strips150and a conductive strip160. The antenna apparatus100includes eleven conductive strips150. In a case where the eleven conductive strips150are distinguished from each other, the eleven conductive strips150are referred to as conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3. In a case where the conductive strips150A1to150E3are not distinguished from each other, the conductive strips150A1to150E3will be described as the conductive strip(s)150.

The antenna apparatus100of the first embodiment is used for communicating electromagnetic waves in the UHF band, and a resonant frequency (central frequency) of the antenna apparatus100may be in a range from about 860 MHz to about 960 MHz, for example. In this embodiment, the antenna apparatus100having the resonant frequency (central frequency) of 919 MHz will be described, for example.

Since the antenna apparatus100communicates at the resonant frequency (central frequency), among lengths of configuration elements included in the antenna apparatus100, lengths of the meander conductive line140and the conductive strips150are set to lengths that correspond to wavelength at the resonant frequency.

Since the wavelength at the resonant frequency may be shortened by a shortening effect in a dielectric material (dielectric substance), the lengths of the meander conductive line140and the conductive strips150may be determined in the light of relative permittivity of the dielectric layers110and120.

For example, a practical wavelength of electromagnetic waves at 919 MHz is about 326 mm, while a wavelength λ used for designing the antenna apparatus100is about 180 mm. The wavelength λ is determined in consideration of relative permittivities of the dielectric layers110and120.

Hereinafter, designing the lengths of the meander conductive line140and the conductive strips150or the like based on the wavelength in light of the relative permittivity of the dielectric layers110and120or the like will be referred to as determining the lengths corresponding to the wavelength at the resonant frequency. A length of the wavelength obtained in a dielectric material (dielectric substance) will be referred to as a length corresponding to the wavelength at the resonant frequency.

The dielectric layers110and120are sheet-like substrates having rectangular shapes in plan view, respectively. A substrate of the antenna apparatus100is constituted by adhering the dielectric layers110and120to each other while the meander conductive line140is placed therebetween. The dielectric layer110is one example of a first dielectric layer, and the dielectric layer120is one example of a second dielectric layer.

Lengths of the dielectric layers110and120in X-axis direction are 730 mm, and lengths (widths) of the dielectric layers110and120in Y-axis direction are 200 mm, for example. Thickness of the dielectric layer110is 1.6 mm, and thickness of the dielectric layer120is 1.0 mm. For the purpose of illustration, thicknesses of the dielectric layers110and120are illustrated thicker than actual thicknesses inFIGS. 1 and 5.

In the first embodiment, the dielectric layers110and120are Flame Retardant type 4 (FR4) standardized substrate materials, for example. For example, glass-reinforced epoxy laminate sheets made of glass cloth dipped into epoxy resin may be used as the dielectric layers110and120. For example, relative permittivities Er of the dielectric layers110and120are 4.4, and dielectric tangents tan δ thereof are 0.02.

The ground plane130is disposed on the bottom surface of the dielectric layer110, and the meander conductive line140is disposed on the top surface of the dielectric layer110. The conductive strips150are disposed on the top surface of the dielectric layer120.

The ground plane130is disposed on the bottom surface of the dielectric layer110. The ground plane130is made of copper foil, for example, and constitutes a microstripline with the meander conductive line140.

The meander conductive line140is disposed on the top surface of the dielectric layer110. The meander conductive line140is one example of a conductive line. The meander conductive line140constitutes the microstripline with the ground plane130. The microstripline functions as a microstrip-antenna. The characteristic impedance of the micro-strip antenna is 50Ω or 75Ω, for example.

Since the meander conductive line140is disposed on the top surface of the dielectric layer110and is located under the dielectric layer120, the meander conductive line140is insulated from the conductive strips150that are disposed on the top surface of the dielectric layer120.

The meander conductive line140is made by patterning a copper foil, for example. The meander conductive line140is a type of a conductive pattern which extends along X-axis while snaking in Y-axis direction in a meander fashion. Line width of the meander conductive line140is 3 mm, for example.

The meander conductive line140includes a straight portion141, meander portions142and a straight portion143. The straight portion141extends in X-axis direction. An end portion of the straight portion141located on negative side in X-axis direction constitutes a first end of the meander conductive line140, and constitutes a feeding point141A.

The straight portion141is located on the central axis, parallel to X-axis, of the dielectric layers110and120. A cable core of a coaxial cable connected to the reader-writer is connected to the feeding point141A, for example.

Ten meander portions142are located on positive side of the straight portion141in X-axis direction and are connected in series with each other. The ten meander portions142have the same pattern which is illustrated inFIG. 3. Single unit of the meander portion142have a shape as illustrated inFIG. 3. The meander portion142includes straight portions142A,142B,142C,142D,142E,142F and142G. InFIG. 3, for the sake of indicating a positional relationship of the meander portion142and the conductive strips150in an easy-to-understand manner, the meander portion142and the conductive strips150are illustrated transparently.

As illustrated inFIG. 3, the meander portion142is located between a pair of the conductive strips150, i.e., two neighboring conductive strips. A trace length (line length) of the meander portion142is set to a length corresponding to the single wavelength (λ) at the resonant frequency. The trace length of the meander portion142is obtained between a crossover point of the straight portion142A and one conductive strip150and a crossover point of the straight portion142G and another conductive strip150.

InFIG. 3, a dashed line extending along X-axis direction is the centerline of the dielectric layers110and120which is parallel to X-axis. The straight portions142A and142G are located on the centerline. The meander portion142has a shape which is symmetrical with respect to a crossover point of the straight portion142D and the centerline.

The straight portion142A extends on the centerline from the negative side to the positive side in X-axis direction. The straight portion142B which extends toward positive Y-axis direction is connected to the end portion of the straight portion142A which is located on positive side in X-axis direction.

The straight portion142C which extends toward positive X-axis direction is connected to the end portion of the straight portion142B which is located on positive side in Y-axis direction. The straight portion142D which extends toward negative Y-axis direction is connected to the end portion of the straight portion142C which is located on positive side in X-axis direction. The straight portion142E which extends toward positive X-axis direction is connected to the end portion of the straight portion142D which is located on negative side in Y-axis direction.

The straight portion142F which extends toward positive Y-axis direction is connected to the end portion of the straight portion142E which is located on positive side in X-axis direction. The straight portion142G which extends toward positive X-axis direction is connected to the end portion of the straight portion142F which is located on positive side in Y-axis direction.

The meander portion142including the straight portions142A,142B,142C,142D,142E,142F and142G, as described above, extends along X-axis while snaking in Y-axis direction in the meander fashion. In the meander conductive line140, the ten meander portions142are connected in series between the straight portion141and the L-shaped portion143from negative side to positive side in X-axis direction.

The straight portion143(seeFIG. 2) is connected to an X-axis-positive-side-end-portion of the ten meander portions142. The straight portion143extends from the X-axis-positive-side-end-portion of the ten meander portions142to an X-axis-positive-side-end-portion of the dielectric layer110along X-axis. An end portion of the straight portion143constitutes an end portion of the meander conductive line140which is located on positive side in X-axis direction. The end portion constitutes a second end of the meander conductive line140and is a grounded point (grounded end)143A. The grounded point143A is one example of a short end.

As illustrated inFIG. 5, the grounded point143A is connected to the ground plane130via a through hole170which penetrates the dielectric layer110in thickness direction (Z-axis direction). The through hole170includes a conductive wall which electrically connects the grounded point143A and the ground plane130. Accordingly, the second end, i.e., the grounded point143A, of the meander conductive line140is shorted to the ground.

The length of the straight portion143is set to a length corresponding to quarter wavelength (λ/4) at the resonant frequency. In a case where the straight portion143is an open end, the length of the straight portion143may be set to a length corresponding to half wavelength (λ/2) at the resonant frequency.

As described above, the length of the straight portion143having the ground point143A is set to the length corresponding to the quarter wavelength (λ/4) at the resonant frequency. If the meander conductive line140is fed from the feeding point141A, a standing wave of current is formed on the meander conductive line140. Nodes of the standing wave occur at eleven locations that are λ/4, 3λ/4, 5λ/4, 7λ/4, 9λ/4, 11λ/4, 13λ/4, 15λ/4, 17λ/4, 19λ/4 and 21λ/4 away from the ground point143A, respectively. These lengths are obtained by subtracting quarter wavelength (λ/4) from multiplied result of half wavelength (λ/2) and integer number.

In other words, the eleven nodes occur at a boundary between the straight portion141and the meander portion142, nine boundaries between the ten meander portions142, and a boundary between the meander portion142and the straight portion143, respectively.

Each of the nodes of the standing wave of current is a point where current value becomes zero and electric field becomes the maximum value. In the antenna apparatus100of the first embodiment, the conductive strips150are disposed on the meander conductive line140via the dielectric layer120and intersect with the meander conductive line140at the locations of the nodes of the standing wave of the current, in order to electromagnetically couple the meander conductive line140and the conductive strips150and to maximize the electric field generated by the conductive strips150.

Each of antinodes of the standing wave of the current is a point where current value becomes the maximum value and electric field becomes zero. The antinodes appear at positions that are shifted by the quarter wavelength (λ/4) at the resonant frequency with respect to positions of the nodes. In other words, the antinodes appear at positions that are shifted by the quarter wavelength (λ/4) on the positive side or the negative side in X-axis direction with respect to the positions of the nodes as described above.

According to the antenna apparatus100of the first embodiment, the conductive strip160is disposed on the meander conductive line140via the dielectric layer120and intersect with the meander conductive line140at the location of the antinode located on the positive side of the conductive strip150E3in X-axis direction, in order to electromagnetically couple the meander conductive line140and the conductive strip160and to maximize the electromagnetic field generated by the conductive strip160.

The location of the antinode located on the positive side of the conductive strip150E3in X-axis direction is the location of the grounded point143A. The antinode is located the most positive side in X-axis direction among the antinodes of the standing wave of the current flowing through the meander conductive line140. This is because the length of the straight portion143is set to a length corresponding to the quarter wavelength (λ/4) at the resonant frequency.

Accordingly, in the first embodiment, the conductive strip160intersects with the straight portion143in a T-shaped fashion at the grounded point143A in plan view. The conductive strip160is placed so that the center point of the conductive strip160in a longitudinal direction overlaps with the grounded point143A and the conductive strip160and the straight portion143form a right angle in plan view.

The reason why the conductive strip160is provided on the positive side of the conductive strip150E3in X-axis direction is as follows. A distribution of the electric field in X-axis direction is uniformized by the conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3between the conductive strip150A1and the conductive strip150E3. Since the feeding point141A is located on negative side of the conductive strip150A1in X-axis direction and the grounded point143A is located on positive side of the conductive strip150E3in X-axis direction, there is an inclination that a distribution of the electric field in an area located on positive side of the conductive strip150E3in X-axis direction becomes weaker than that in an area located on negative side of the conductive strip150A1in X-axis direction.

Therefore, the conductive strip160is disposed on the positive side of the conductive strip150E3in X-axis direction for the sake of reinforcing a communication area in which the antenna apparatus100can communicate with an RFID tag on the positive side of the conductive strip150E3in X-axis direction.

It is possible to improve the distribution of the electromagnetic field on the positive side of the conductive strip150E3in X-axis direction by causing the conductive strip160to be coupled with the meander conductive line140by the magnetic field. The improvement of the distribution of the electromagnetic field is confirmed by measured results obtained from an experiment in which the RFID tags are attached to towels.

The electric field generated by the conductive strip150and the magnetic field generated by the conductive strip160leak in a near field on a top-surface-side of the micro-strip antenna including the meander conductive line140as described above. Accordingly, the micro-strip antenna makes it possible to communicate with the RFID tags by utilizing the leak electric field and the leak magnetic field in the near field of the micro-strip antenna.

The conductive strips150are constituted of eleven conductive patterns that are disposed on the top surface of the dielectric layer120. Each of the conductive strips150is one example of a first conductive element.

Since the conductive strips150are disposed on the top surface of the dielectric layer120, the conductive strips150are insulated from the meander conductive line140. The conductive strip150is made by patterning a copper foil, for example. The line width of the conductive strip150is set to 4 mm, for example.

As illustrated inFIG. 4, the conductive strip150includes straight portions151,152and153. InFIG. 4, for the sake of indicating a positional relationship of the conductive strip150and the straight portions142A and142G of the meander portion142in an easy-to-understand manner, the conductive strips150and the straight portions142A and142G are illustrated transparently.

The straight portion151extends in parallel with Y axis. Accordingly, the straight portion151intersects with the straight portions142A and142G at right angle. The straight portion152extends from an end portion of the straight portion151located on positive side in Y-axis direction. The straight portion153extends from an end portion of the straight portion151located on negative side in Y-axis direction.

The straight portions152and153are bent toward the feeding point141A with respect to the straight portion151. In other words, the straight portions152and153that extend in Y-axis direction are bent on negative side in X-axis direction. The straight portions152and153are bent on negative side in X-axis direction with respect to the straight portion151. Bend angles of the straight portions152and153with respect to the central axis of the straight portion151extending along the longitudinal direction are equal to each other. The bend angles are represented as angles θ. The bend angle is one example of a bend degree.

The conductive strip150is disposed so that the center point of the straight portion151in Y-axis direction overlaps with the position at which the node of the standing wave generated on the meander conductive line140occurs in plan view. Accordingly, the eleven conductive strips150are disposed on the dielectric layer120so that the eleven conductive strips150intersect with the meander conductive line140at the positions of the eleven nodes of the standing wave of the current formed on the meander conductive line140, respectively, in plan view.

In each of the conductive strips150, length from an end portion of the straight portion152to an end portion of the straight portion153along the straight portions152,151and153is set to a length corresponding to the single wavelength (λ) at the resonant frequency. Accordingly, each conductive strip150functions as a resonator (first resonator).

Thickness of the dielectric layer120is set to a thickness that does not suppress the electromagnetic coupling of the conductive strips150and the meander conductive line140. Therefore, the conductive strips150function as resonators that are electromagnetically coupled with the meander conductive line140. Each of the conductive strips150can radiate and receive electromagnetic waves via the meander conductive line140and can perform communications at the resonant frequency.

Each of the nodes of the standing wave of the current is a point where current value becomes zero and electric field becomes the maximum value. Accordingly, it becomes possible to increase electric field intensity on the positive side of the microstrip-antenna in Z-axis direction by utilizing the conductive strips150. The microstrip-antenna includes the meander conductive line140.

Since the conductive strips150are arranged on the top surface of the antenna apparatus100in a manner that the conductive strips150encompass the whole top surface of the antenna apparatus100in X axis direction and Y axis direction, it is possible to increase and uniformize the electric field intensity on the top surface side of the antenna apparatus100.

Next, the lengths and the angles θ of the conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3will be described.

The lengths of the conductive strips150A1and150A2are equal to each other and are set to 186 mm, for example. The lengths of the conductive strips150E1,150E2and150E3are equal to each other and are set to 202 mm, for example. The lengths of the conductive strips150A1,150A2,150E1,150E2and150E3, i.e. 186 mm and 202 mm, are lengths corresponding to the single wavelength at the resonant frequency.

The lengths of the conductive strips150B1and150B2are equal to each other. The lengths of the conductive strips150C1and150C2are equal to each other. The lengths of the conductive strips150D1and150D2are equal to each other. The lengths of the conductive strips150B1and150B2, the lengths of the conductive strips150C1and150C2and the lengths of the conductive strips150D1and150D2are longer than 186 mm and shorter than 202 mm. The lengths of the conductive strips150B1and150B2, the lengths of the conductive strips150C1and150C2and the lengths of the conductive strips150D1and150D2increase in this order. These three lengths correspond to the single wavelength at the resonant frequency as well.

In each of the conductive strips150, length of the straight portion151is 60 mm, and lengths of the straight portions152and153are equal to each other.

As illustrated inFIG. 2, in each of the conductive strips150A1and150A2, the bend angle θ of the straight portions152and153with respect to the central axis of the straight portion151is 30 degrees. In each of the conductive strips150B1and150B2, the bend angle θ of the straight portions152and153with respect to the central axis of the straight portion151is 35 degrees.

In each of the conductive strips150C1and150C2, the bend angle θ of the straight portions152and153with respect to the central axis of the straight portion151is 40 degrees. In each of the conductive strips150D1and150D2, the bend angle θ of the straight portions152and153with respect to the central axis of the straight portion151is 45 degrees.

In each of the conductive strips150E1,150E2and150E3, the bend angle θ of the straight portions152and153with respect to the central axis of the straight portion151is 50 degrees.

The lengths and the angles θ were derived by an electromagnetic field simulation utilizing a Finite Element Method. The simulation result will be described later. More enhanced S11 parameter characteristics were obtained in a case where the lengths of the eleven conductive strips150are different as described above than in a case where the lengths of the eleven conductive strips150are the same.

A more uniformized field distribution was obtained in a case where the lengths of the eleven conductive strips150are different as described above than in a case where the lengths of the eleven conductive strips150are the same. As illustrated inFIG. 2, the electric field Ed generated by the conductive strips150is divided into X component Ex obtained in X axis direction and Y component Ey obtained in Y axis direction. The reason why the more uniform field distribution is obtained in a case where the lengths of the eleven conductive strips150are different is because the Y component Ey is increased compared with the case where the lengths of the eleven conductive strips150are the same.

If the conductive strips150have straight-line-shapes extending along Y axis, the electric field Ed generated by the conductive strips150only have the X component Ex. In other words, in this case, Y component Ey is not generated by the conductive strips150.

Accordingly, it is important for each of the conductive strips150that the straight portions152and153are bent at the bend angle θ with respect to the straight portion151in order to obtain the Y component Ey. By setting the bend angles θ of the eleven conductive strips150to various angles as illustrated inFIG. 2, it becomes possible to obtain the Y components Ey with various intensities and to obtain more uniformized field distribution.

The conductive strip160is one example of a second conductive element. The conductive strip160is disposed on the top surface of the dielectric layer120so that the conductive strip160intersects with the straight portion143in a T-shaped fashion at the location of the antinode of the standing wave of the current flowing through the meander conductive line140on the positive side of the conductive strip150E3in X-axis direction in plan view.

The conductive strip160is placed at the location of the antinode of the standing wave of the current flowing through the meander conductive line140for the sake of improving the distribution of the electromagnetic field on the positive side of the conductive strip150E3in X-axis direction.

Each of antinodes of the standing wave of the current is the point where the current value and the magnetic field become the maximum values and the electric field becomes zero. The antinodes appear at the positions that are shifted by the quarter wavelength (λ/4) at the resonant frequency with respect to the positions of the nodes.

According to the antenna apparatus100of the first embodiment, the conductive strip160is disposed on the meander conductive line140via the dielectric layer120and intersects with the meander conductive line140at the location of the antinode located on the positive side of the conductive strip150E3in X-axis direction, in order to electromagnetically couple the meander conductive line140and the conductive strip160and to maximize the electromagnetic field generated by the conductive strip160.

The length of the conductive strip160is a half wavelength (λ/2) at the resonant frequency. The conductive strip160is a straight-shaped conductive pattern extending in Y-axis direction and is disposed on the top surface of the dielectric layer120so that the center point of the conductive strip160in the longitudinal direction overlaps with the grounded point143A in plan view.

The conductive strip160is made by patterning a copper foil, for example. The line width of the conductive strip150is set to 4 mm, for example. The copper foil used for forming the conductive strip160may be the same as the copper foil used for forming the conductive strip150.

Since the length of the conductive strip160is set to half wavelength (λ/2) at the resonant frequency, the conductive strip160functions as the resonator (second resonator). In the conductive strip160, the current becomes the maximum value at the center point in the longitudinal direction and becomes zero at the end portions in the longitudinal direction. Accordingly, the magnetic field generated by the conductive strip160becomes the maximum value at the center point of the conductive strip160in the longitudinal direction.

It is possible to improve the distribution of the electromagnetic field on the positive side of the conductive strip150E3in X-axis direction by causing the conductive strip160to be coupled with the meander conductive line140by the magnetic field.

Thickness of the dielectric layer120is set to a thickness that does not suppress the electromagnetic coupling of the conductive strip160and the meander conductive line140. Therefore, the conductive strip160functions as the resonator which is electromagnetically coupled with the meander conductive line140. The conductive strip160can radiate and receive electromagnetic waves via the meander conductive line140and can perform communications at the resonant frequency.

Each of the antinodes of the standing wave of the current is a point where current value becomes the maximum value and electric field becomes zero. Accordingly, it becomes possible to increase the intensity of the magnetic field on the positive side of the microstrip-antenna in Z-axis direction at around the end portion located on positive side of the microstrip-antenna in X-axis direction by utilizing the conductive strip160. The microstrip-antenna includes the meander conductive line140.

According to the first embodiment, it is possible to provide the antenna apparatus100which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the conductive strips150with the microstrip-antenna. The microstrip-antenna includes the meander conductive line140and the ground plane130. Further, it is possible to provide the antenna apparatus100which can generate the magnetic field having sufficient intensity around the grounded point143A in the near field by electromagnetically coupling the conductive strip160with the microstrip-antenna.

Since the feeding point141A is located on negative side of the conductive strip150A1in X-axis direction and the grounded point143A is located on positive side of the conductive strip150E3in X-axis direction, there is an inclination that the distribution of the electric field in the area located on positive side of the conductive strip150E3in X-axis direction becomes weaker than that in the area located on negative side of the conductive strip150A1in X-axis direction and the area between the conductive strips150A1and150E3, if the antenna apparatus100does not include the conductive strip160. Decrease or ununiformity of the distribution of the electric field makes the communication area narrow.

Therefore, the conductive strip160is disposed on the positive side of the conductive strip150E3in X-axis direction for the sake of reinforcing or broadening the communication area on the positive side of the conductive strip150E3in X-axis direction. The communication area is provided by the magnetic field generated by the conductive strip160.

According to the antenna apparatus100, the electric field generated by the conductive strip150and the magnetic field generated by the conductive strip160leak in the near field on the top-surface-side of the micro-strip antenna including the meander conductive line140. Accordingly, the antenna apparatus100makes it possible to communicate with the RFID tags in the near field located on the whole surface area of the the antenna apparatus100.

According to the embodiment as described above, the conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3are disposed at positions that are located designated distances away from the ground point143A, respectively.

Accordingly, the length between the conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3are lengths corresponding to λ/2 at the resonant frequency.

Accordingly, currents flowing through the two neighboring conductive strips150among the conductive strips150A1,150A2,150B1,150B2,150C1,150C2,150D1,150D2,150E1,150E2and150E3have opposite phases with each other.

According to a variation example of the first embodiment, the antenna apparatus100may include only the conductive strips150A1,150B1,150C1,150D1,150E1and150E3. In this case, the currents flowing through the two neighboring conductive strips150have the same phases with each other. Accordingly, it is possible to provide a configuration in that the electric fields generated by the conductive strips150A1,150B1,150C1,150D1,150E1and150E3strengthen one another.

It is possible to manufacture the antenna apparatus100as described above as follows. First of all, prepare a sheeted substrate material to which two copper foils are attached on both surfaces of the substrate material. Form the meander conductive line140by patterning one of the copper foils and keep the other copper foil as the ground plane130. Accordingly, a first structural body which includes the dielectric layer110, the ground plane130and the meander conductive line140is obtained.

Next, prepare another sheeted substrate material to which one copper foil is attached on a surface of the substrate material. Form the conductive strips150and160by patterning the copper foil. Accordingly, a second structural body which includes the dielectric layer120and the conductive strips150and160is obtained.

Then, put the top surface of the first structural body and the bottom surface of the second structural body together. Accordingly, the antenna apparatus100is completed. The dielectric layer110and the dielectric layer120may be put together by thermo-compression bonding, adhesive bonding or the like.

According to the embodiment as described above, the ground plane130, the meander conductive line140and the conductive strips150and160are made of copper. However, the ground plane130, the meander conductive line140and the conductive strips150and160may be made of metal such as gold, silver, nickel or the like, or alloy of these metals.

A cover member which covers the bottom surface of the ground plane130may be attached to the antenna apparatus100. The cover member may be made of resin, for example, and may have dimensions in X axis direction and Y axis direction similar to that of the dielectric layer110. Similarly, a cover member which covers the conductive strips150and160and the top surface of the dielectric layer120may be attached to the antenna apparatus100. The cover member may be made of resin, for example, and may have dimensions in X axis direction and Y axis direction similar to that of the dielectric layer120.

According to the embodiment as described above, the conductive strips150and160are placed on the top surface of the dielectric layer120. The conductive strips150or the conductive strip160may be placed on a top surface of another dielectric layer disposed on the dielectric layer120. In this case, the conductive strips150and the conductive strip160are placed on the top surfaces of the different dielectric layers with respect to each other.

Next, another variation example of the antenna apparatus100according to the first embodiment will be described with reference toFIGS. 7A to 7E.

FIGS. 7A to 7Eare diagrams illustrating conductive strips171to175of the variation example of the antenna apparatus100according to the first embodiment. The conductive strips171to175as illustrated inFIGS. 7A to 7Emay be used instead of the conductive strips150as illustrated inFIGS. 1 to 6.

As illustrated inFIG. 7A, the conductive strip171includes straight portions171A and171B. The straight portions171A and171B are bent at angles θ1with respect to the central axes of the dielectric layers110and120that are described by dashed line and are parallel with X axis. The angles θ1may be greater than 0 degrees and less than 90 degrees.

As illustrated inFIG. 7B, the conductive strip172includes straight portions172A,172B,172C and172D. The straight portions172A and172B are bent at angles θ2with respect to the central axes of the dielectric layers110and120that are described by dashed line and are parallel with X axis. The angles θ2may be greater than 0 degrees and less than 90 degrees.

The straight portions172C and172D are formed from end portions of the straight portions172A and172B, respectively, in a continuous fashion. The straight portions172C and172D are bent with respect to the straight portions172A and172B so that the straight portions172C and172D face toward the feeding point141A (seeFIGS. 1 and 2) more than the straight portions172A and172B.

As illustrated inFIG. 7C, the conductive strip173includes straight portions173A,173B,173C,173D and173E. The straight portion173A extends in Y axis direction in a manner similar to that of the straight portion151of the conductive strips150as illustrated inFIGS. 1 to 6.

The straight portions173B and173C are formed from both end portions of the straight portion173A, respectively, in a continuous fashion. The straight portions173B and173C are bent with respect to the straight portion173A so that the straight portions173B and173C face toward the feeding point141A (seeFIGS. 1 and 2).

The straight portions173C and173E are formed from end portions of the straight portions173B and173C, respectively, in a continuous fashion. The straight portions173D and173E are bent with respect to the straight portions173B and173C so that the straight portions173D and173E face toward the feeding point141A (seeFIGS. 1 and 2) more than the straight portions173B and173C.

As illustrated inFIG. 7D, the conductive strip174includes straight portion174A and tapered portions174B and174C. The straight portion174A extends in Y axis direction in a manner similar to that of the straight portion151of the conductive strips150as illustrated inFIGS. 1 to 6.

The tapered portions174B and173C are formed from both end portions of the straight portion174A, respectively, in a continuous fashion. The tapered portions174B and173C are bent with respect to the straight portion174A so that centerlines of the tapered portions174B and173C face toward the feeding point141A (seeFIGS. 1 and 2).

As illustrated inFIG. 7E, the conductive strip175includes straight portions175A,175B and175C and branch portions175D1,175D2,175E1and175E2. The straight portions175A,175B and175C are similar to the straight portion173A,173B and173C as illustrated inFIG. 7C.

The branch portions175D1and175D2and the branch portions175E1and175E2are formed from end portions of the straight portions173B and173C in a continuous fashion and branch into two portions, respectively. The branch portions175D1and175D2and the branch portions175E1and175E2are bent with respect to the straight portions175B and175C so that central axes of the branch portions175D1and175D2and the branch portions175E1and175E2face toward the feeding point141A (seeFIGS. 1 and 2) more than the straight portions175B and175C.

The conductive strips171to175as illustrated inFIGS. 7A to 7Emay be used instead of the conductive strips150as illustrated inFIGS. 1 to 6. The bend angles or number of the branches may not be limited to that illustrated inFIGS. 7A to 7E, and may be changed in various way. However, it is preferable for the conductive strips171to175to be bent toward the feeding point141A (seeFIGS. 1 and 2).

Accordingly, the conductive strips150may be bent or rounded with respect to Y axis direction in non-linear fashion.

According to the embodiment as described above, the conductive strips150are bent or rounded with respect to Y axis direction in non-linear fashion, the conductive strips150may extend along Y axis direction in a linear fashion as long as sufficient electric field in the near field can be obtained.

Next, the locations of the antinodes of the standing wave of the current flowing through the meander conductive line140will be described with reference toFIG. 8.

FIGS. 8A and 8Bare diagrams illustrating the locations of the antinodes of the standing wave of the current flowing through the meander conductive line140. InFIG. 8A, for the purpose of illustration, the meander conductive line140is described as a straight-shape in plan view, the feeding point141A is placed at an end portion located on negative side of the straight-shape in X-axis direction and the grounded point143A is placed at an end portion located on positive side of the straight-shape in X-axis direction. Further, inFIG. 8A, a rectangular-shaped outline represents the dielectric layers110and120. Furthermore, for the purpose of illustration, length of the meander conductive line140as illustrated inFIG. 8Ais different from that along the meander conductive line140as illustrated inFIGS. 1, 2 and 5. InFIG. 8B, a plurality of positions at which a plurality of conductive strips160are placed is added compared withFIG. 8A.

As illustrated inFIG. 8A, the antinodes145appear at positions placed at the half wavelength length (λ/2) intervals from the grounded point143A on the meander conductive line140. The antinodes145appear at the positions that are shifted by the quarter wavelength (λ/4) at the resonant frequency with respect to the positions of the nodes.

Distances between the adjacent two antinodes145are the half wavelength (λ/2) at the resonant frequency. The antinodes145having the same phases alternately. InFIG. 8A, the antinodes145indicated by white circles have the same phases with each other and the antinodes145indicated by black circles have the same phases with each other. The phases of the antinodes145indicated by white circles and the phases of the antinodes145indicated by black circles have opposite phases with each other. The antinodes145indicated by the white circles and the antinode145indicated by the black circles appear alternately.

InFIG. 8A, the conductive strip160located on the grounded point143A corresponds to the conductive strip160as illustrated inFIGS. 1, 2 and 5. The conductive strip160is located on the antinode indicated by the white circle. In addition to the conductive strip160located on the grounded point143A, the antenna apparatus100may further include one or more conductive strip(s)160located on the antinode(s)145beside the antinode145located on the grounded point143A.

In this case, the antenna apparatus100may further include the one or more conductive strip(s)160located on the antinode(s)145indicated by white circle(s).

In a case where the antenna apparatus100further include one or more conductive strip(s)160located on the antinode(s)145beside the antinode145located on the grounded point143A, it is possible to further reinforce the communication area provided by the conductive strips150.

In this case, if the antenna apparatus100further include the one or more conductive strip(s)160located on the antinode(s)145indicated by white circle(s), it is possible to reinforce the communication area provided by the conductive strips150furthermore and more effectively.

FIG. 8Billustrates positional relationship of the conductive strips160in a case where the antenna apparatus100further include one or more conductive strip(s)160located on the antinode(s)145beside the antinode145located on the grounded point143A. InFIG. 8B, for the purpose of illustration, the positions of the conductive strips160are indicated by dashed lines. These conductive strips160are disposed on the top surface of the dielectric layer120with the conductive strips150as illustrated inFIGS. 1, 2 and 5in a practical sense. Accordingly, the conductive patterns of the conductive strips150and the conductive strips160are designed so that the conductive strips160do not contact with the conductive strips150and that the conductive strips150and the conductive strips160are insulated with each other.

According to the embodiment as described above, the length of the conductive strip160is the half wavelength (λ/2) at the resonant frequency. The length of the conductive strip160may set to length obtained by multiplying an integer number to the half wavelength (λ/2). If the conductive strip160have such a length, the conductive strip160which is placed on the antinode of the standing wave of the current functions as the resonator.

Next, the conductive patterns of the conductive strips160in plan view are described with reference toFIGS. 9A to 9E.

FIGS. 9A to 9Eare diagrams illustrating the conductive patterns of the conductive strips160in plan view.FIGS. 9A to 9Eillustrate five exemplary conductive patterns. InFIGS. 9A to 9E, the conductive strips160are indicated by solid lines and the meander conductive lines140are indicated by dashed lines. InFIGS. 9A to 9E, configuration elements other than the conductive strips160and the meander conductive lines140are omitted.

For example, as illustrated inFIG. 9A, the conductive strip160having the straight-shape may intersect with the meander conductive line140at an apex of a rounded portion of the meander conductive line140. The conductive strip160having the straight-shape may extend in a direction along X-axis as illustrated inFIGS. 1 and 2, for example.

As illustrated inFIG. 9B, the conductive strip160may have a bent-shape in plan view.

As illustrated inFIG. 9C, the conductive strip160may include a crank shape having a straight portion located in a center portion of the conductive strip160, and the straight portion may be overlapped with the straight portion of the meander conductive line140.

As illustrated inFIG. 9D, the conductive strip160having the straight-shape may intersect with the straight portion of the meander conductive line140at a designated angle.

As illustrated inFIG. 9E, the conductive strip160having the straight-shape may be placed in parallel with the straight portion of the meander conductive line140. In this case, the conductive strip160may be overlapped with the straight portion of the meander conductive line140.

As described above, the conductive strips160having various conductive patterns may be used. Since the electric field generated by the conductive strip160is increased in two directions in which the both ends of the conductive strip160extend, respectively, i.e., in directions as indicated by arrows as illustrated inFIGS. 9A to 9E, the conductive pattern of the conductive strip160may be designed in accordance with the positions and the shapes of the conductive strips150and with the distribution characteristics of the communication area of the antenna apparatus100.

Next, a shelf antenna system900utilizing the antenna apparatus100according to the first embodiment will be described with reference toFIG. 10.

FIG. 10is a diagram illustrating the shelf antenna system900utilizing the antenna apparatuses100according to the first embodiment. In the shelf antenna system as illustrated inFIG. 10, four antenna apparatuses100are connected to a reader-writer910and are disposed on each level of four-level shelf901. Since the antenna apparatuses100can perform communications in the near fields, readable areas902are formed at each level of the shelf901.

In the shelf antenna system900, merchandises to which RFID tags are attached are arranged on the antenna apparatuses100provided on each of the shelf910. In this condition, the reader-writer901reads the RFID tags. The shelf antenna system900identifies that at least one of the merchandise is taken away from the shelf901when the shelf antenna system900becomes unable to detect any of the RFID tags. The reader-writer910cannot read the RFID tag when the merchandise is taken away from the readable areas902.

Second Embodiment

FIG. 11is an oblique perspective diagram illustrating an antenna apparatus200of the second embodiment.FIG. 12is an oblique perspective diagram illustrating an antenna apparatus200of the second embodiment. In the antenna apparatus200according to the second embodiment, configuration elements corresponding to the meander conductive line140and the conductive strips150of the antenna apparatus100according to the first embodiment are changed.

Accordingly, the same elements as or elements similar to those of the antenna apparatus100of the first embodiment are referred to by the same reference numerals, and a description thereof is omitted. InFIG. 12, principal dimensions are illustrated.

The antenna apparatus200includes dielectric layers110and120, a ground plane130, a meander conductive line240, conductive strips250and a conductive strip260. The antenna apparatus200includes eleven conductive strips250. In a case where the eleven conductive strips250are distinguished from each other, the eleven conductive strips250are referred to as conductive strips250A1,250A2,250B1,250B2,250C1,250C2,250D1,250D2,250E1,250E2and250E3. In a case where the conductive strips250A1to250E3are not distinguished from each other, the conductive strips250A1to250E3will be described as the conductive strip(s)250.

In the meander conductive line240, a meander shape is rounded whereas a meander shape of the meander conductive line140of the first embodiment is bent at a right angle. The meander conductive line240includes an open end243A instead of the grounded point143A of the meander conductive line140of the first embodiment.

The meander conductive line240is disposed on the top surface of the dielectric layer110. The meander conductive line240is one example of a conductive line. The meander conductive line240constitutes the microstripline with the ground plane130. The microstripline functions as a microstrip-antenna.

The meander conductive line240includes a straight portion241, meander portions242and an adjust portion243. The straight portion241extends in X axis direction. An end portion of the straight portion241located on negative side in X-axis direction constitutes a first end of the meander conductive line240and constitutes a feeding point241A. This is similar to the straight portion141of the first embodiment. Length of the straight portion241is 60 mm, for example.

Ten meander portions242are connected in series with each other between the straight portion241and the adjust portion243in a similar manner to that of the ten meander portions142of the first embodiment. Since the ten meander portions242have similar configuration to each other, the meander portion242will be described with reference toFIG. 13. The adjust portion243will be described with reference toFIG. 14.

FIG. 13is a diagram illustrating the meander portion242of the second embodiment in plan view. InFIG. 13, the meander portion242located between the conductive strips250B1and250B2is illustrated.

The meander portion242includes line portions242A,242B,242C,242D,242E,242F and242G. As illustrated inFIG. 13, connecting portions of the line portions242A,242B,242C,242D,242E,242F and242G are rounded in plan view.

Straight portions and rounded portions included in the line portions242A,242B,242C,242D,242E,242F and242G have the same width. The width is 3 mm, for example. Radius of curvature of the rounded portions is 9 mm, for example. The radius of curvature is one example of the rounded degree. The line portions242A,242B,242C,242D,242E,242F and242G have dimensions as illustrated inFIG. 13other than the dimensions as described above, for example. Unit of dimensions as illustrated inFIG. 13is mm.

Trace length from a first end at which the meander portion242intersects with the conductive strip250B1to a second end at which the meander portion242intersects with the conductive strip250B2is set to a length corresponding to the single wavelength (λ) at the resonant frequency. A gap between the conductive strips250B1and250B2in X-axis direction is 63 mm, for example.

FIG. 14is a diagram illustrating the adjust portion243and the conductive strip260of the second embodiment in plan view.

A first end of the adjust portion243is connected to a second end of the meander portion242farthest from the feeding point241A, and a second end of the adjust portion243is the open end243A. The open end243A is opened and is not electrically connected to anything.

The adjust portion243extends from the first end located on positive side in X-axis direction, is rounded in circular arc shape, extends in positive Y-axis direction, is rounded in circular arc shape, extends in negative Y-axis direction, is rounded in circular arc shape and extends in positive X-axis direction to the open end243A in plan view.

Length of the adjust portion243between the first end and the second end is set to a length corresponding to the half wavelength (λ/2) at the resonant frequency. Width of the adjust portion243is constant from the first end to the second end, and is 3 mm, for example. The adjust portion243has dimensions as illustrated inFIG. 14. Unit of dimensions as illustrated inFIG. 14is mm.

The trace length of the adjust portion243including the open end243A is set to the half wavelength (λ/2) at the resonant frequency. Accordingly, if electrical power is fed into the meander conductive line240from the feeding point241A, current flowing through the meander conductive line240is reflected at the open end243A and a standing wave of the current is generated on the meander conductive line240.

Nodes of the standing wave occur at eleven locations that are λ/2, λ, 3λ/2, 2λ, 5λ/2, 3λ, 7λ/2, 4λ, 9λ/2, 5λ and 11λ/2 away from the open end243A, respectively. These lengths are obtained by multiplying integer numbers by the half wavelength at the resonant frequency, respectively.

In other words, the eleven nodes occur at a boundary between the straight portion241and the meander portion242, nine boundaries between the ten meander portions242, and a boundary between the meander portion242and the adjust portion243, respectively.

Each of the nodes of the standing wave of current is a point where current value becomes zero and electric field becomes the maximum value. In the antenna apparatus200of the second embodiment, the conductive strips250are disposed on the meander conductive line240via the dielectric layer120and intersect with the meander conductive line240at the locations of the nodes of the standing wave of the current, in order to electromagnetically couple the meander conductive line240and the conductive strips250and to maximize the electric field generated by the conductive strips250.

The microstrip-antenna including the meander conductive line240makes it possible to perform a communication in the near field by utilizing electric field which leaks from the top surface of the microstrip-antenna. Herein, the electric field which leaks from the top surface of the microstrip-antenna is referred to as leak electric field.

Although the eleven conductive strips250as illustrated inFIG. 11have three straight portions, respectively, in a manner similar to that of the conductive strips150as illustrated inFIG. 3, lengths and angles θ of the eleven conductive strips250are different from the lengths and the angles θ of the conductive strips150.

Herein, the conductive strip260is described before describing the lengths and the angles θ of the eleven conductive strip250.

The conductive strip260is one example of a second conductive element. The conductive strip260is disposed on the top surface of the dielectric layer120so that the conductive strip260intersects with the adjust portion243at a location corresponding to the antinode of the standing wave of the current flowing through the meander conductive line240on the positive side of the conductive strip250E3in X-axis direction in plan view.

Herein, the location corresponding to the antinode is not limited to the location of the antinode of the standing wave of the current, but also include the location where magnetic field coupling similar to that obtained in a case where the conductive strip260is placed at the location of the antinode is obtained. The location corresponding to the antinode is placed nearer the antinode than a mid-point between the antinode and the node. The reason why the location corresponding to the antinode is not limited to the exact location of the antinode is because there may be a case where the antenna apparatus200does not include enough space to dispose the conductive strip260.

The conductive strip260is placed at the location corresponding to the antinode for the sake of improving the distribution of the electromagnetic field on the positive side of the conductive strip250E3in X-axis direction. This principle is similar to that of the conductive strip160of the second embodiment.

The length of the conductive strip260is the half wavelength (λ/2) at the resonant frequency. The conductive strip260is a straight-shaped conductive pattern and is disposed on the top surface of the dielectric layer120so that the center point of the conductive strip260in the longitudinal direction is placed on a central axis indicated by dashed line in plan view. The conductive strip260extends obliquely with respect to X-axis and Y-axis. The central axis is a central axis of the top surface of the dielectric layer120which extends along X-axis direction.

The conductive strip260is made by patterning a copper foil, for example. The line width of the conductive strip250is set to 4 mm, for example. The copper foil used for forming the conductive strip260may be the same as the copper foil used for forming the conductive strip250.

Since the length of the conductive strip260is set to the half wavelength (λ/2) at the resonant frequency, the conductive strip260functions as the resonator (second resonator). In the conductive strip260, the current becomes the maximum value at the center point in the longitudinal direction and becomes zero at the end portions in the longitudinal direction. Accordingly, the magnetic field generated by the conductive strip260becomes the maximum value at the center point of the conductive strip260in the longitudinal direction.

It is possible to improve the distribution of the electromagnetic field on the positive side of the conductive strip250E3in X-axis direction by causing the conductive strip260to be coupled with the meander conductive line240by the magnetic field.

According to the second embodiment, it is possible to provide the antenna apparatus200which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the conductive strips250with the microstrip-antenna. The microstrip-antenna includes the meander conductive line240and the ground plane130. Further, it is possible to provide the antenna apparatus200which can generate the magnetic field having sufficient intensity on the positive side of the conductive strip250E3in X-axis direction in the near field by electromagnetically coupling the conductive strip260with the microstrip-antenna.

Since the feeding point241A is located on negative side of the conductive strip250A1in X-axis direction and the open end243A is located on positive side of the conductive strip250E3in X-axis direction, there is an inclination that the distribution of the electric field in the area located on positive side of the conductive strip250E3in X-axis direction becomes weaker than that in the area located on negative side of the conductive strip250A1in X-axis direction and the area between the conductive strips250A1and250E3. Decrease or ununiformity of the distribution of the electric field makes the communication area narrow.

Therefore, the conductive strip260is disposed on the positive side of the conductive strip250E3in X-axis direction for the sake of reinforcing or broadening the communication area on the positive side of the conductive strip250E3in X-axis direction. The communication area is provided by the magnetic field generated by the conductive strip260.

The angles θ included in the conductive strips250A1,250A2,250B1,250B2,250C1,250C2,250D1,250D2,250E1,250E2and250E3will be referred to as angles θ21, θ22, θ23, θ24, θ25, θ26, θ27, θ28, θ29, θ30and θ31, respectively. Each of the angles θ21to θ31is formed by three straight portions included in each of the conductive strips250A1to250E3as illustrated inFIG. 12.

As described above, according to the antenna apparatus200of the second embodiment, the lengths L25and L26of the conductive strips250C1and250C2that are disposed in the middle in X-axis direction are the longest. On the other hand, the lengths L21, L22, L29, L30and L31of the conductive strips250A1,250A2,250E1,250E2and250E3that are disposed on both ends in X-axis direction are the shortest.

Herein, the lengths L21to L31are lengths corresponding to the single wavelength (λ) at the resonant frequency.

As described above, the angles θ21to θ31included in the conductive strips250A1to250E3becomes smaller in an area closer to the feeding point241A and becomes larger in an area closer to the open end243A.

The lengths L21to L31and the angles θ21to θ31are optimum values derived from the electromagnetic field simulation utilizing the Finite Element Method.

Herein, for the sake of validating an effect of the different lengths L21to L31of the conductive strips250A1to250E3as described above, a comparison result of S11 parameter of the antenna apparatus200according to the second embodiment and S11 parameter of an antenna apparatus of a comparative example will be described with reference toFIG. 15. In the antenna apparatus of the comparative example, the lengths L21to L31are set to 186 mm.

FIG. 15is a diagram illustrating frequency characteristics of the S11 parameter of the antenna apparatus200according to the second embodiment and the S11 parameter of the antenna apparatus of the comparative example. The frequency characteristics of S11 parameter of the antenna apparatus200is obtained in a condition where the antenna apparatus200does not include the conductive strip260. It is experimentally verified that the antenna apparatus200which includes the conductive strip260has wider communication area than that of the antenna apparatus200which does not include the conductive strip260and that the antenna apparatus200which includes the conductive strip260can read the RFID tags in a broader area than the antenna apparatus200which does not include the conductive strip260. However, the frequency characteristics of S11 parameter of the antenna apparatus200which does not include the conductive strip260will be described.

InFIG. 15, a solid line represents the frequency characteristics of the S11 parameter obtained from the antenna apparatus200. A dashed line represents the frequency characteristics of the S11 parameter obtained from the antenna apparatus of the comparative example. In the antenna apparatus of the comparative example, the lengths L21to L31of the eleven conductive strips250A1to250E3are set to 186 mm.

Both S11 parameters are calculated under a condition where values of S11 parameter of the antenna apparatus200and the S11 parameter of antenna apparatus of the comparative example take almost the same values at 935 MHz. A criterion value of S11 parameter is −10 dB.

As illustrated inFIG. 15, a bandwidth in which the value of S11 parameter of the antenna apparatus200is less than or equal to −10 dB is wider than that of the antenna apparatus of the comparative example.

Accordingly, it becomes possible to widen the bandwidth by setting the lengths L21to L31of the conductive strips250A1to250E3to the different lengths as described above. Similar results will be obtained with the antenna apparatus200including the conductive strip260.

Next, a simulation result of electric field vector was obtained at a point 400 mm high from the top surface of the dielectric layer110of the antenna apparatus200while varying a phase φ of the input signal fed into the feeding point241A of the antenna apparatus200.

FIGS. 16 to 18are diagrams illustrating the simulation results of the electric field vector of the antenna apparatus200. Similar to the frequency characteristics as illustrated inFIG. 15, the simulation results as illustrated inFIGS. 16 to 18are obtained from the antenna apparatus200which does not include the conductive strip260. It is experimentally verified that the antenna apparatus200which includes the conductive strip260has wider communication area than that of the antenna apparatus200which does not include the conductive strip260and that the antenna apparatus200which includes the conductive strip260can read the RFID tags in a broader area than the antenna apparatus200which does not include the conductive strip260. However, the simulation results of the electric field vector of the antenna apparatus200which does not include the conductive strip260will be described.

FIGS. 16 to 18illustrate the simulation results of the electric field vector of the antenna apparatus200to which the input signals of 919 MHz, 910 MHz and 930 MHz are fed into the feeding point241A, respectively.

Each ofFIGS. 16 to 18illustrates the five simulation results of the electric field vector of the antenna apparatus200at moments when the phase φ becomes 0 degrees, 40 degrees, 80 degrees, 120 degrees and 160 degrees, respectively. In these FIGS., distributions and directions of the electric field vector are illustrated. The phase φ of the input signal represents a phase during one cycle (360 degrees) at 919 MHz, 910 MHz and 930 MHz.

In actual simulation results, the electric field intensities are represented in full color, i.e. 0 V/m is indicated by blue (see the bottom of legend isFIGS. 16 to 18) and 5 V/m is indicated by red (see the bottom of legend isFIGS. 16 to 18). Since the electric field intensities are represented by achromatic color inFIGS. 16 to 18, it is not possible to distinguish 5 V/m and 0 V/m.

However, the strong electric fields that are represented in red in the actual simulation result are located in a central portion of the antenna apparatus200in plan view, and the weak electric fields that are represented in blue in the actual simulation result are located in the peripheral portion of the antenna apparatus200in plan view.

Accordingly, large arrows that represent principal directions of the strong electric field are added to the central portions inFIGS. 16 to 18.

As illustrated inFIG. 16, when the phase φ of the input signal of 919 MHz is 0 degrees, the principal directions of the strong electric fields that occur in the central portion of the antenna apparatus200are negative X axis direction.

As the phase φ of the input signal of 919 MHz varies to 40 degrees, 80 degrees, 120 degrees and 160 degrees, the principal directions of the strong electric fields vary in counterclockwise direction. When the phase φ of the input signal of 919 MHz is 160 degrees, the principal directions of the strong electric fields are positive X axis direction.

This means that the principal directions of the strong electric fields that occur on the top surface of the antenna apparatus200rotate in a circular polarization manner as the phase φ of the input signal varies.

An inclination such as this can be seen in a case where the input signals of 910 MHz and 930 MHz are input to the feeding point241A of the antenna apparatus200as illustrated inFIGS. 17 and 18.

According to the second embodiment, it is possible to provide the antenna apparatus200which generates the electric field of which the direction rotates in a circular polarization manner as the phase φ of the input signal of 919 MHz, 910 MHz and 930 MHz varies.

As described above, the direction of the electric field generated on the surface of the antenna apparatus200varies in response to the phase φ of the input signal. Accordingly, it is possible to read the identification information of the RFID tag which is attached to the merchandise arranged on the shelf501in a state where the antenna apparatus200is provided on the shelf501, even if the merchandise is disposed on the shelf501in any direction.

As described above, it is experimentally verified that the antenna apparatus200which includes the conductive strip260has wider communication area than that of the antenna apparatus200which does not include the conductive strip260and that the antenna apparatus200which includes the conductive strip260can read the RFID tags in a broader area than the antenna apparatus200which does not include the conductive strip260. Such an improvement of the antenna apparatus200is confirmed by the measured results obtained from the experiment in which the RFID tags are attached to towels.

According to the second embodiment, it is possible to provide the antenna apparatus200which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the conductive strips250with the microstrip-antenna. The microstrip-antenna includes the meander conductive line240and the ground plane130. Further, it is possible to provide the antenna apparatus200which can generate the magnetic field having sufficient intensity on the positive side of the conductive strip250E3in X-axis direction in the near field by electromagnetically coupling the conductive strip260with the microstrip-antenna.

Although the simulation results as illustrated inFIGS. 15 to 18were derived with respect to the antenna apparatus200of the second embodiment, it is presumed that similar results can be obtained with respect to the antenna apparatus100of the first embodiment.

Third Embodiment

Antenna apparatuses according to third to sixth embodiments use a micro-strip line, one end of which is connected to a power feeding port and the other end of which is an open end, as a micro-strip antenna in a manner similar to the first and the second embodiments. Therefore, in the antenna apparatuses, a current which flows through the micro-strip antenna is reflected by the open end, whereby the current forms a standing wave. The antenna apparatuses include at least one conductor for resonance in a region where electromagnetic coupling is possible with the micro-strip antenna. The region is in the vicinity of any of nodal points of the standing wave, i.e., any of positions where flowing current is minimum and the intensity of the electric field around the position is maximum. This causes an improvement of the uniformity and intensity of the electric field in the vicinity of an antenna surface.

In each embodiment described below, each antenna apparatus disclosed in this specification is formed as an antenna apparatus. However, the antenna apparatuses disclosed in this specification may be utilized for purposes other than the antenna apparatus.

FIG. 19is a perspective view of an antenna apparatus according to a third embodiment.FIG. 20is a side cross-sectional view of the antenna apparatus along a line B-B seen from a direction of an arrow inFIG. 19.FIG. 21is a plan view of the antenna apparatus as illustrated inFIG. 19.

The antenna apparatus300includes a substrate10which includes two dielectric layers, a ground electrode11provided under the substrate10, a conductor12provided between the two dielectric layers of the substrate10, a plurality of resonators13-1to13-5provided on an upper surface of the substrate10, and a conductive strip360.

The conductive strip360is disposed on a top surface of an upper layer10-2and intersects with the conductive line12in plan view. Otherwise, the conductive strip360is similar to the conductive strip260of the second embodiment. The conductive line12corresponds to the meander conductive line240of the second embodiment. The conductive line12has a straight-shape in plan view. The conductive line12is obtained by modifying the meander conductive line240into the straight-shape in plan view. The resonators13-1to13-5correspond to the conductive strips250of the second embodiment. The substrate10includes a lower layer10-1and the upper layer10-2. The lower layer10-1and the upper layer10-2correspond to the dielectric layers110and120of the second embodiment, respectively. The grounded electrode11corresponds to the ground plane130of the second embodiment.

The substrate10supports the ground electrode11, the conductor12, and the resonators13-1to13-5. The substrate10includes a lower layer10-1located in relatively lower side, and an upper layer10-2arranged above the lower layer10-1. Both of the lower layer10-1and the upper layer10-2of the substrate10are formed with a dielectric, and accordingly, the ground electrode11, the conductor12, and the resonator13-1to13-5are isolated each other. For example, the lower layer10-1and the upper layer10-2may be formed of a glass epoxy resin such as FR-4, respectively. Alternatively, the lower layer10-1and the upper layer10-2may be formed of other dielectric which can be layered. Moreover, the lower layer10-1and the upper layer10-2may be formed with the same dielectric, or may be formed with different dielectrics.

The ground electrode11is a grounded plain-plate-like conductor, and is provided so that the overall lower surface of the substrate10may be covered.

The conductor12is a conductor in the form of a line provided between the lower layer10-1and the upper layer10-2of the substrate10, and one end of the conductor12is a power feeding port12a. On the other hand, the other end12bof the conductor12is an open end. The conductor12, the ground electrode11, and the lower layer10-1of the substrate10forms the micro-strip antenna.

Since the end12bof the conductor12is the open end, the current which flows through the conductor12due to the electric wave radiated from the micro-strip antenna, or due to the electric wave received by the micro-strip antenna forms a standing wave. Therefore, nodal points of the standing wave are formed at a distance of half-wavelength of the current and distances of integral multiples thereof from the end12bof the conductor12, i.e., from the open end of the micro-strip antenna. It is noted that, since the conductor12is located between the lower layer10-1and the upper layer10-2, the wavelength of the electric wave shortens depending on the relative permittivity of the lower layer10-1and the relative permittivity of the upper layer10-2. At each nodal point of the standing wave, the current has a local minimum value, and a relatively strong electric field is formed around the nodal point. Hereinafter, the wavelength of the electric wave radiated from the micro-strip antenna or received by the micro-strip antenna in the substrate10is referred to as a design wavelength for the sake of simplicity.

Each of the resonators13-1to13-5is formed with a conductor in the form of a line which has a length substantially equal to the design wavelength or integral multiples thereof, and is provided on the surface of the upper layer10-2of the substrate10. In the present embodiment, the length of each resonator is substantially equal to the design wavelength.

As described above, the relatively strong electric field is formed around the conductor12at the distance of half design wavelength and distances of integral multiples thereof from the open end12bof the micro-strip antenna along with the conductor12. Therefore, each of the resonators13-1to13-5is arranged, along with the conductor12, at the distance of half design wavelength and distances of substantially integral multiples thereof from the end12bof the conductor12, so that the resonator crosses the conductor12perpendicular thereto. In the present embodiment, the resonators13-1to13-5are arranged in the vicinity of positions where are spaced from the open end12bat distances of .lamda./2, .lamda., 3.lamda./2, 2.lamda. and 5.lamda./2, respectively (.lamda. denotes the design wavelength). Accordingly, each resonator13-1to13-5is electromagnetically coupled with the micro-strip antenna with respect to an electric wave which has the design wavelength. Therefore, each resonator13-1to13-5can radiate or receive the electric wave with the design wavelength. Furthermore, since the resonators13-1to13-5are arranged so that the resonators cross the conductor12perpendicular thereto, each of the resonators13-1to13-5can form the electric field which expands in a different direction from the electric field due to the micro-strip antenna. As a result, uniformity and intensity of the electric field in the vicinity of the surface of the antenna apparatus300are improved in comparison with the electric field produced by only the micro-strip antenna.

In addition, for example, based on a result of an electric field simulation using a finite element method, the exact arrangement position of each resonator13-1to13-5is adjusted so that the electromagnetic coupling between each resonator13-1to13-5and the micro-strip antenna may be the strongest. Moreover, the length of each resonator may be determined, based on the result of the electric field simulation using the finite element method, so that the electric field radiated from each resonator13-1to13-5may be the strongest.

The ground electrode11, the conductor12, and the resonators13-1to13-5are made of, for example, metal such as copper, gold, silver and nickel, alloys thereof, or other material which has conductivity. The ground electrode11, the conductor12, and the resonators13-1to13-5are fixed to the lower layer10-1or the upper layer10-2of the substrate10by etching or adhesion, for example. Moreover, the lower layer10-1and the upper layer10-2are also fixed to each other by adhesion, for example.

The thickness of the upper layer10-2is optimized by a simulation using a finite element method so that the micro-strip antenna and each resonator13-1to13-5are electromagnetically coupled. On the other hand, the thickness of the lower layer10-1is determined so that the characteristic impedance of the micro-strip antenna is a certain value, for example, 50Ω or 75Ω.

The conductive strip360is one example of a second conductive element. The conductive strip360is disposed on the top surface of the upper layer10-2so that the conductive strip360intersects with the conductive line12at right angle at a location corresponding to the antinode of the standing wave of the current flowing through the conductive line12on the positive side of the resonator13-1in X-axis direction in plan view. Since a length of the conductive line12between the resonator13-1and the open end12B is a half wavelength (λ/2) at the resonant frequency, the conductive strip360is placed at a location shifted by a quarter wavelength (λ/4) on the positive side in X-axis direction, i.e., on the side of the open end12B. The length of the conductive line12between the conductive strip360and the open end12B is the quarter wavelength (λ/4) at the resonant frequency.

Next, simulation results of the frequency characteristics of the S11 parameter of the antenna apparatus300is described with reference toFIG. 22. The frequency characteristics of S11 parameter of the antenna apparatus300is obtained in a condition where the antenna apparatus300does not include the conductive strip360. It is experimentally verified that the antenna apparatus300which includes the conductive strip360has wider communication area than that of the antenna apparatus300which does not include the conductive strip360and that the antenna apparatus300which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus300which does not include the conductive strip360. However, the frequency characteristics of S11 parameter of the antenna apparatus300which does not include the conductive strip360will be described.

FIG. 22is a drawing illustrating a simulation result of frequency characteristic of S parameter with respect to the antenna apparatus300.FIG. 23is drawings illustrating simulation results of the electric field formed in surface's vicinity of the antenna apparatus300. With respect to the simulation results of which are illustrated inFIG. 22andFIG. 23, the condition of the simulation is defined as follows. Both of the lower layer10-1and the upper layer10-2of the substrate10are formed by FR4 (relative permittivity .di-elect cons.r=4.4, and dielectric tangent tan .delta.=0.02). The length of the substrate10along a longitudinal direction of the conductor12is 550 mm, and the length of the substrate10along a direction perpendicular to the longitudinal direction of the conductor12is 200 mm. The thickness of the lower layer10-1is 1.6 mm so that the characteristic impedance of the micro-strip line formed with the lower layer10-1, the ground electrode11and the conductor12is 50Ω. The thickness of the upper layer10-2is 1.0 mm.

The ground electrode11, the conductor12and the resonators13-1to13-5are formed with copper (electric conductivity .sigma.=5.8.times.10.sup.7). Furthermore, the width of the conductor12is 3 mm. On the other hand, the width of each resonator13-1to13-5is 4 mm, and the length of each resonator is 161 mm. The distance from the open end12bof the conductor12to the center line of the resonator13-1is 84 mm. Furthermore, the distance between the center line of the resonator13-1and the center line of the resonator13-2is 85 mm. Similarly, the distance between the center line of the resonator13-2and the center line of the resonator13-3, the distance between the center line of the resonator13-3and the center line of the resonator13-4, and the distance between the center line of the resonator13-4and the center line of the resonator13-5are 82 mm, 85 mm and 85 mm, respectively.

InFIG. 22, a horizontal axis represents frequency [GHz] and a vertical axis represents value [dB] of S.sub.11 parameter. The characteristics401represent the frequency characteristics of the S.sub.11 parameter of the antenna apparatus300, obtained by a simulation of electromagnetic field according to a finite element method. As illustrated in the characteristics401, it is found that the S.sub.11 parameter of the antenna apparatus300is −10 dB or less, the range providing an indication of excellent antenna characteristics, in 950 MHz to 960 MHz utilized for an RFID system.

Next, a distribution of an intensity of the electric field parallel to the surface of the antenna apparatus300is described with reference toFIG. 23. The the distribution of the intensity of the electric field as illustrated inFIG. 23is obtained in a condition where the antenna apparatus300does not include the conductive strip360. It is experimentally verified that the antenna apparatus300which includes the conductive strip360has wider communication area than that of the antenna apparatus300which does not include the conductive strip360and that the antenna apparatus300which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus300which does not include the conductive strip360. However, the distribution of the intensity of the electric field of the antenna apparatus300which does not include the conductive strip360will be described.

InFIG. 23, the distribution chart501represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus300at a distance of 50 mm above from the surface of the antenna apparatus300. The distribution chart502represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus300at a distance of 100 mm above from the surface of the antenna apparatus300. The distribution chart503represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus300at a distance of 200 mm above from the surface of the antenna apparatus300. In each distribution charts, the frequency of the electric wave is 950 MHz. In each distribution chart, the area which has deeper color represents a stronger electric field. As illustrated in the distribution charts501to503, it is found that the electric field is strong not only in the vicinity of the conductor12but also in the vicinity of each resonator13-1to13-5. Therefore, it is found that the uniformity of the electric field in the vicinity of the surface of the antenna apparatus300is improved in comparison with the uniformity of the electric field formed by the micro-strip antenna itself. The maximum values of the intensities of the electric field at the distances 50 mm, 100 mm, and 200 mm above from the surface of the antenna apparatus300are 9.7 V/m, 2.9 V/m and 1.2 V/m, respectively.

According to above-described configuration, in this antenna apparatus, the current which flows through the micro-strip antenna forms a standing wave by forming one end of the micro-strip antenna as the open end. Then, the micro-strip antenna and the resonators are electromagnetically coupled by arranging the resonators in the vicinity of the nodal points of the standing wave. Therefore, this antenna apparatus can radiate the electric wave from both of the micro-strip antenna and the resonators, and can receive the electric wave by both of them, whereby it is possible to improve the uniformity of the electric field in the vicinity of the surface of the antenna apparatus, and to achieve stronger intensity of the electric field.

As described above, it is experimentally verified that the antenna apparatus300which includes the conductive strip360has wider communication area than that of the antenna apparatus300which does not include the conductive strip360and that the antenna apparatus300which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus300which does not include the conductive strip360. Such an improvement of the antenna apparatus300is confirmed by the measured results obtained from the experiment in which the RFID tags are attached to towels.

According to the third embodiment, it is possible to provide the antenna apparatus300which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the resonators13-1to13-5with the microstrip-antenna. The microstrip-antenna includes the conductive line12and the grounded electrode11. Further, it is possible to provide the antenna apparatus300which can generate the magnetic field having sufficient intensity around the open end12B in the near field by electromagnetically coupling the conductive strip360with the microstrip-antenna.

Fourth Embodiment

Next, an antenna apparatus according to a fourth embodiment will be described. The antenna apparatus according to the fourth embodiment is different from the antenna apparatus according to the third embodiment in a position of the resonators. Accordingly, hereinafter, the description related to the resonators will be made. The explanation for other components of the antenna apparatus according to the fourth embodiment can be referred to the explanation of corresponding components of the antenna apparatus according to the third embodiment.

FIG. 24is a plan view of the antenna apparatus according to the fourth embodiment. InFIG. 24, the same reference numbers are provided to each component of the antenna apparatus400according to the fourth embodiment as the reference number of the corresponding component of the antenna apparatus300as illustrated inFIG. 19throughFIG. 21.

In the antenna apparatus400according to the fourth embodiment, each of the three resonators13-1,13-3, and13-5is formed with a conductor in the form of a line which has a length substantially equal to the design wavelength, and each of resonators is provided on the surface of the upper layer10-2of the substrate. However, it is different from the antenna apparatus300according to the third embodiment in that the resonators13-2and the resonator13-4located at distances of integral multiples of the design wavelength from the open end12bare omitted in the antenna apparatus400. In other words, the resonators13-1,13-3, and13-5are provided in positions at the distances of sum of the half design wavelength and the integral multiples of the design wavelength from the open end12bof the micro-strip antenna, respectively. Therefore, the distance between two resonators which are adjacent each other along the conductor12is substantially equal to the design wavelength.

In the antenna apparatus300according to the third embodiment, each of the resonators13-1to13-5is separated from the other resonators adjacent thereto at the distance of substantially half design wavelength along with the conductor12. Therefore, the phases of the current which flows through two adjacent resonators are reversed with each other.

On the other hand, in the antenna apparatus400according to the fourth embodiment, since each of the resonators13-1,13-3, and13-5is separated from the other resonators adjacent thereto at the distance of substantially design wavelength along with the conductor12, the phases of the current which flows through two adjacent resonators are in-phase. Accordingly, the electric field formed by each resonator can also be strengthened one another.

FIG. 25is a drawing illustrating a simulation result of frequency characteristic of S parameter with respect to the antenna apparatus400.FIG. 26is drawings illustrating simulation results of electric field formed in surface's vicinity of the antenna apparatus400. In this simulation, it is assumed that the size and the position of each component of the antenna apparatus400are the same as the size and position of corresponding component of the antenna apparatus300.

Similar to the third embodiment, the frequency characteristics of S11 parameter of the antenna apparatus400as illustrated inFIG. 25are obtained in a condition where the antenna apparatus400does not include the conductive strip360. The distribution of the intensity of the electric field as illustrated inFIG. 26is obtained in a condition where the antenna apparatus400does not include the conductive strip360.

InFIG. 25, a horizontal axis represents frequency [GHz] and a vertical axis represents value [dB] of S.sub.11 parameter. The characteristics700represent the frequency characteristics of the S.sub.11 parameter of the antenna apparatus400, obtained by a simulation of electromagnetic field according to a finite element method. As illustrated in the characteristics700, it is found that the S.sub.11 parameter of the antenna apparatus400is −6 dB or less, the range providing an indication of antenna characteristics for operation without any difficulty, in 950 MHz to 960 MHz utilized for an RFID system.

InFIG. 26, the distribution chart801represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus400at a distance of 50 mm above from the surface of the antenna apparatus400. The distribution chart802represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus400at a distance of 100 mm above from the surface of the antenna apparatus400. The distribution chart803represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus400at a distance of 200 mm above from the surface of the antenna apparatus400. In each distribution chart, the frequency of the electric wave is 950 MHz. In each distribution chart, the area which has deeper color represents a stronger electric field. As illustrated in the distribution charts801to803, it is found that the electric field is strong not only in the vicinity of the conductor12but also in the vicinity of each resonator13-1,13-3and13-5. Furthermore, it is found that, in the position of 100 mm above from the surface of the antenna apparatus400, the intensity distribution of the electric field is more uniform in comparison with the electric field formed by the antenna apparatus300.

Furthermore, the maximum values of the intensities of the electric field at the distances 50 mm, 100 mm, and 200 mm above from the surface of the antenna apparatus400are 11.6 V/m, 5.6 V/m and 4.2 V/m, respectively. Those values in respective positions are stronger than the maximum values of the intensities of the electric field about the antenna apparatus300.

As described above, in the antenna apparatus according to the fourth embodiment, the distance between two resonators adjacent to each other is substantially equal to the design wavelength. Accordingly, the phases of the current which flows through each resonator is in-phase. As a result, the electric fields radiated from respective resonators are strengthened one another, whereby it is possible to improve the uniformity of the electric field in the vicinity of the surface of the antenna apparatus, and to strengthen the intensity of the electric field.

Similar to the third embodiment, it is experimentally verified that the antenna apparatus400which includes the conductive strip360has wider communication area than that of the antenna apparatus400which does not include the conductive strip360and that the antenna apparatus400which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus400which does not include the conductive strip360. Such an improvement of the antenna apparatus400is confirmed by the measured results obtained from the experiment in which the RFID tags are attached to towels.

According to the fourth embodiment, it is possible to provide the antenna apparatus400which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the resonators13-1,13-3and13-5with the microstrip-antenna. The microstrip-antenna includes the conductive line12and the grounded electrode11. Further, it is possible to provide the antenna apparatus400which can generate the magnetic field having sufficient intensity around the open end12B in the near field by electromagnetically coupling the conductive strip360with the microstrip-antenna.

Fifth Embodiment

Next, an antenna apparatus according to a fifth embodiment will be described. In the antenna apparatus according to the fifth embodiment, the conductor which forms the micro-strip antenna is bent, for example, the conductor may meander, whereby the interval between resonators adjacent to each other is narrowed, in comparison with the antenna apparatus according to the third embodiment. Accordingly, hereinafter, the description related to the conductor and the resonators will be made. The explanation for other components of the antenna apparatus according to the fifth embodiment can be referred to the explanation of corresponding components of the antenna apparatus according to the third embodiment.

FIG. 27is a plan view of the antenna apparatus according to the fifth embodiment. InFIG. 27, the same reference numbers are provided to each component of the antenna apparatus500according to the fifth embodiment as the reference number of the corresponding component of the antenna apparatus300as illustrated inFIG. 19throughFIG. 21.

In the antenna apparatus500according to the fifth embodiment, a conductor12′ which forms a part of the micro-strip antenna includes a meander shape, in which the conductor is bent to a right angle, at a plurality of positions between the two resonators adjacent to each other.

In the antenna apparatus500according to the fifth embodiment, each of the five resonators13-1to13-5is formed with a conductor in the form of a line which has a length substantially equal to the design wavelength, and each of resonators is provided on the surface of the upper layer10-2of the substrate. Each of the resonators13-1to13-5is arranged so that the distance along the conductor12′ between two resonators adjacent to each other is substantially equal to the design wavelength. Therefore, the distance in a straight line between two resonators adjacent to each other is shorter than the design wavelength. As a result, the electric waves radiated from respective resonators can be strengthened by one another. Moreover, in this embodiment, the resonator13-1which is the nearest to the open end12bof the micro-strip antenna among the resonators13-1to13-5is preferably arranged at the distance of substantially half design wavelength from the open end12balong the conductor12′, i.e., in the vicinity of the nodal point of the standing wave nearest to the open end12b.

FIG. 28is a drawing illustrating a simulation result of frequency characteristic of S parameter with respect to the antenna apparatus500.FIG. 29is drawings illustrating simulation results of electric field formed in surface's vicinity of the antenna apparatus500. The condition of this simulation is defined as follows. As illustrated inFIG. 27, with respect to sections formed by bending the conductor12′, the length of the longest section which is perpendicular to a longitudinal direction of the conductor12′ is 50 mm, and the length of the each section which is parallel to the longitudinal direction of the conductor12′ and is adjacent to the longest section is 20 mm. Moreover, the distance in a straight line between the center lines of two adjacent resonators is 86 mm so that the length along the conductor12′ between the two adjacent resonators is substantially equal to the design wavelength. Further, the length of the substrate10along a longitudinal direction of the conductor12′ is 505 mm. The sizes and the materials of respective components of the antenna apparatus500other than those described above are the same as the sizes and the materials which are set in the simulation of the antenna apparatus300according to the third embodiment.

Similar to the third and fourth embodiments, the frequency characteristics of S11 parameter of the antenna apparatus500as illustrated inFIG. 28is obtained in a condition where the antenna apparatus500does not include the conductive strip360. The distribution of the intensity of the electric field as illustrated inFIG. 29is obtained in a condition where the antenna apparatus500does not include the conductive strip360.

InFIG. 28, a horizontal axis represents frequency [GHz] and a vertical axis represents value [dB] of S.sub.11 parameter. The characteristic 1000 represent the frequency characteristic of the S.sub.11 parameter of the antenna apparatus500, obtained by a simulation of electromagnetic field according to a finite element method.

As illustrated in the characteristics1000, it is found that the S.sub.11 parameter of the antenna apparatus500is −10 dB or less, the range providing an indication of excellent antenna characteristics, in 950 MHz to 960 MHz utilized for an RFID system.

InFIG. 29, the distribution chart1101represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus500at a distance of 50 mm above from the surface of the antenna apparatus500. The distribution chart1102represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus500at a distance of 100 mm above from the surface of the antenna apparatus500. The distribution chart1103represents the intensity distribution of the electric field on a plane which is parallel to the surface of the antenna apparatus500at a distance of 200 mm above from the surface of the antenna apparatus500. In each distribution chart, the frequency of the electric wave is 950 MHz. In each distribution chart, the area which has deeper color represents a stronger electric field. As illustrated in the distribution charts1101to1103, it is found that the electric field is strong not only in the vicinity of the conductor12′ but also in the vicinity of each of resonators13-1to13-5. Furthermore, it is found that, in the position of 100 mm above from the surface of the antenna apparatus500and in the position of 200 mm above from the surface thereof, the intensity distribution of the electric field is more uniform in comparison with the electric field formed by the antenna apparatus300.

Furthermore, the maximum values of the intensities of the electric field at the distances 50 mm, 100 mm, and 200 mm above from the surface of the antenna apparatus500are 17.3 V/m, 11.3 V/m and 7.8 V/m, respectively. Those values in respective positions are stronger than the maximum values of the intensities of the electric field about the antenna apparatus300or the antenna apparatus400.

As described above, in the antenna apparatus according to the fifth embodiment, since the conductor12′ includes the meander shape, the length along the conductor12′ between the two adjacent resonators is substantially equal to the design wavelength whereas the distance in a straight line between the two resonators is narrower than the design wavelength. Therefore, in this antenna apparatus, the electric fields radiated from respective resonators can be strengthened by one another. As a result, this antenna apparatus can improve the uniformity of the electric field in the vicinity of the surface of the antenna apparatus, and strengthen the intensity of the electric field in the vicinity of the surface of the antenna apparatus.

Similar to the third and fourth embodiments, it is experimentally verified that the antenna apparatus500which includes the conductive strip360has wider communication area than that of the antenna apparatus500which does not include the conductive strip360and that the antenna apparatus500which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus500which does not include the conductive strip360. Such an improvement of the antenna apparatus500is confirmed by the measured results obtained from the experiment in which the RFID tags are attached to towels.

According to the third embodiment, it is possible to provide the antenna apparatus500which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the resonators13-1to13-5with the microstrip-antenna. The microstrip-antenna includes the conductive line12′ and the grounded electrode11. Further, it is possible to provide the antenna apparatus300which can generate the magnetic field having sufficient intensity around the open end12B in the near field by electromagnetically coupling the conductive strip360with the microstrip-antenna.

According to modifications of the fifth embodiment, the conductor12′ may be bent in any manner between two adjacent resonators. For example, the conductor12′ may be formed between the two adjacent resonators in a shape of a sine wave, or in a shape of a sawtooth.

Moreover, according to another modification of the fifth embodiment, each resonator may be arranged so that the distance along the conductor which is a part of the micro-strip antenna between the two adjacent resonators is substantially half design wavelength, and the distance in a straight line between the two adjacent resonators is shorter than the half design wavelength.

Sixth Embodiment

Next, an antenna apparatus according to a sixth embodiment will be described. The antenna apparatus according to the sixth embodiment is different from the antenna apparatus according to the fifth embodiment in that each resonator is formed in a convex toward an open end of the conductor, and at least a part of each resonator and a line extending from the power feeding port to the open end of the conductor makes an acute angle. By this means, the antenna apparatus produces the electric field components along a long side direction and the electric field components along a short side direction of the antenna apparatus, respectively. This results in a uniform intensity of the electric field in a plane parallel to the surface of the antenna apparatus, regardless of the direction of the electric field. Hereinafter, a description of the conductor and the resonators will be made. An explanation of other components of the antenna apparatus according to the sixth embodiment can be referred to in the explanation of corresponding components of the antenna apparatus according to the first to fifth embodiments.

FIG. 30is a plan view of the antenna apparatus according to the sixth embodiment. InFIG. 30, the same reference numbers are provided to each component of the antenna apparatus600according to the sixth embodiment as the reference number of the corresponding component of the antenna apparatus500as illustrated inFIG. 27.

In the antenna apparatus600according to the sixth embodiment, as is the case in the fifth embodiment, a conductor12′ which forms a part of the micro-strip antenna includes a meander shape, in which the conductor is bent to a right angle at a plurality of positions between the two resonators adjacent to each other. Therefore, the conductor12′ includes a part121along the long side direction of the antenna apparatus600, and a part122parallel to the short side direction of the antenna apparatus600. Therefore, the conductor12′ produces the electric field component parallel to the long side direction of the antenna apparatus600, and the electric field component parallel to the short side direction of the antenna apparatus600.

For the sake of simplicity, the long side direction along the surface of the antenna apparatus600is referred to as an X-axis direction, and the short side direction along the surface of the antenna apparatus600is referred to as a Y-axis direction.

Each of the resonators13-1to13-7is formed with a conductor in the form of a line which has a length substantially equal to the design wavelength, and is provided on the surface of the upper layer10-2of the substrate10. The resonator13-1is arranged at the distance of substantially half design wavelength from the open end12bof the conductor12′ so that the resonator13-1is arranged in the vicinity of the nodal point of the standing wave of the current which flows through the conductor12′. Furthermore, the resonators13-2to13-7are also arranged so that the distances between two resonators which are adjacent to each other along the conductor12′ are substantially equal to the design wavelength, and the resonators13-2to13-7are arranged in the vicinity of the nodal point of the standing wave of the current which flows through the conductor12′.

In the present embodiment, the resonators13-1to13-7include three elements13ato13cwhich have straight line shape, respectively. The central element13acrosses a line (hereinafter, referred to as center line for the sake of simplicity), extending from the open end12bto the power feeding port12aof the conductor12′, perpendicular thereto in the middle of the element13a. On the other hand, the elements13band13cwhich are located on both sides of the central element13aapproach the open end12bof the conductor12′ as approaching the center line, and approach the power feeding port12aas separating from the center line, so that an acute angle may be made with the center line, respectively. As a result, each resonator is formed in a convex form toward the open end12bof the conductor12′.

Therefore, the electric field produced by each resonator also has, as is the case with an electric field produced by the conductor12′, a component (i.e., a component parallel to the center line) along the X-axis direction, and a component (i.e., a component perpendicular to the center line) along the Y-axis direction. Therefore, in the vicinity of the surface of the antenna apparatus600, a combination of the intensity of the instant electric field component in the X-axis direction and the intensity of the instant electric field component in the Y-axis direction is also changed according to a change of the phase of the current which flows through the conductor12′ and each resonator, and this results in a change of a direction of the instant electric field. Therefore, the antenna apparatus600can uniform the intensity of the electric field regardless of the direction of the electric field. Moreover, forming each resonator in a convex shape toward the open end12ballows a coincidence of the wavelength at which the antenna apparatus600resonates, and the wavelength of impedance matching.

Angles made by the elements13band13con the both sides of the resonators13-1to13-7with the center line are preferably determined so that the elements13band13cdo not overlap with the conductor12′. If the resonators13-1to13-7overlap with the conductor12′ in a position other than the center line, electromagnetic coupling occurs between the resonators and the conductor12′ at the overlapped position. This results in uneven distribution of the current in the resonators and uneven electric field produced by the resonators.

On the other hand, the larger the angles between the elements13band13con the both sides of the resonators13-1to13-7and the center line are, the stronger the electric field component parallel to the Y-axis direction produced by each resonator is relatively, and the weaker the electric field component parallel to the X-axis direction is relatively. Therefore, the angles between the elements13band13con the both sides of the resonators13-1to13-7and the center line are preferably set so that the intensity of the electric field parallel to the Y-axis direction is substantially equal to the intensity of the electric field parallel to the X-axis direction.

Moreover, the shorter the elements13band13con both sides are, the weaker the electric field component parallel to the X-axis direction produced by the resonators13-1to13-7is. Therefore, the length of the elements13band13cis also preferably set so that the intensity of the electric field parallel to the Y-axis direction is substantially equal to the intensity of the electric field parallel to the X-axis direction. In the present embodiment, the length of the elements13band13cis set to substantially one third or more of the design wavelength.

FIG. 31Ais a drawing illustrating a simulation result of an intensity of an electric field component parallel to X-axis direction formed in surface's vicinity of the antenna apparatus.FIG. 31Bis a drawing illustrating a simulation result of an intensity of an electric field component parallel to Y-axis direction formed in surface's vicinity of the antenna apparatus.

The condition of the simulation is defined as follows. The length of the substrate10along the X-axis direction is 500 mm, and the length thereof along the Y-axis direction is 200 mm. Moreover, the width of the conductor12′ is 3 mm. The length of the longest part parallel to the Y-axis direction is 61 mm among the bent part of the conductor12′, and the length of the parts which are located on front side and back side of the longest part and are parallel to the X-axis direction are 18 mm, respectively. The distances between two adjacent resonators on the center line are 63 mm so that the length between the two adjacent resonators along the conductor12′ is substantially equal to the design wavelength.

On the other hand, the width of each of resonators13-1to13-7is 4 mm, and length thereof is 182 mm. The length of the central element13aof the each of resonators13-1to13-7is 60 mm, and the lengths of the elements13band13care 61 mm, respectively. Furthermore, the angle between the elements13b,13c, and the center line is 55 degrees (i.e., the angle between the elements13b,13cand the central element13ais 35 degrees). The size and the material of each component of the antenna apparatus500other than the above-described components are the same as the size and the material which are set in the simulation of the antenna apparatus300according to the third embodiment.

Similar to the third to fifth embodiments, the frequency characteristics of S11 parameter of the antenna apparatus600as illustrated inFIG. 31is obtained in a condition where the antenna apparatus600does not include the conductive strip360. The distribution of the intensity of the electric field as illustrated inFIG. 32is obtained in a condition where the antenna apparatus600does not include the conductive strip360.

InFIG. 31AandFIG. 31B, a horizontal axis expresses the distance from the power feeding port12aalong the X-axis direction. On the other hand, a vertical axis expresses the intensity of the electric field. Characteristics1301to1305represent relations of the distance from the power feeding port12ain the X-axis direction and the intensity of the electric field component parallel to the x-axis, at a distance of 400 mm above from the surface of the antenna apparatus600, respectively. Among them, the characteristics1301represent a relation of the distance from the power feeding port12aand the intensity of electric field component parallel to the X-axis direction, at a position where the distance from the center line in the Y-axis direction is 0 mm. Moreover, the characteristics1302and1303represent the relationship of the distance from the power feeding port12aand the intensity of electric field component parallel to the X-axis direction, at positions where the distances from the center line in the Y-axis direction are 50 mm and −50 mm, respectively. Furthermore, the characteristics1304and1305represent the relationship of the distance from the power feeding port12aand the intensity of electric field component parallel to the X-axis direction, at positions where the distances from the center line in the Y-axis direction are 100 mm and −100 mm, respectively. Note that, inFIG. 30, the distance from the center line in the Y-axis direction is represented as positive for upper side of the center line, and is represented as negative for lower side of the center line.

On the other hand, characteristics1311to1315represent relations of the distance from the power feeding port12aalong the X-axis direction and the intensity of the electric field component parallel to the y-axis, at a distance of 400 mm above from the surface of the antenna apparatus600, respectively. Among them, the characteristics1311represent a relation of the distance from the power feeding port12aand the intensity of electric field component parallel to the Y-axis direction at a position where the distance from the center line in the Y-axis direction is 0 mm. Moreover, the characteristics1312and1313represent the relations of the distance from the power feeding port12aand the intensity of electric field component parallel to the Y-axis direction, at positions where the distances from the center line in the Y-axis direction are 50 mm and −50 mm, respectively. Furthermore, the characteristics1314and1315represent the relations of the distance from the power feeding port12aand the intensity of electric field component parallel to the Y-axis direction, at positions where the distances from the center line in the Y-axis direction are 100 mm and −100 mm, respectively.

As illustrated in the characteristics1301to1305and1311to1315, it is found that, at the distance of 400 mm above from the surface of the antenna apparatus600, the difference between the intensity distribution of the electric field component parallel to the X-axis direction and the intensity distribution of the electric field component parallel to the Y-axis direction is small.

FIG. 32is a drawing illustrating a simulation result of frequency characteristic of the S parameter with respect to the antenna apparatus600. In this simulation, it is assumed that the size and the electrical characteristic of each part of the antenna apparatus600is the same as the size and the electrical characteristic in the simulation results as illustrated inFIG. 31AandFIG. 31B. InFIG. 32, a horizontal axis expresses frequency [GHz] and a vertical axis expresses value of S11 parameter [dB]. A characteristics1400represent the frequency characteristic of the S11 parameter of the antenna apparatus600obtained by a simulation of the electromagnetic field according to the finite element method. As illustrated in the characteristics1400, it was found that, in the antenna apparatus600, the S11 parameter is equal to or less than −10 dB, the range providing an indication of excellent antenna characteristics, in 912 MHz to 934 MHz utilized for the RFID system.

When the antenna apparatus600communicates with other communication apparatus, such as an RFID tag attached to an article placed on the antenna apparatus600, the other communication apparatus may face various directions against the antenna apparatus600. However, according to the present embodiment, the antenna apparatus600can make the intensity of the electric field uniform regardless of the direction of the electric field. Accordingly, the antenna apparatus600can perform good communication with the other communication apparatus regardless the direction of the antenna of the other communication apparatus.

Similar to the third to fifth embodiments, it is experimentally verified that the antenna apparatus600which includes the conductive strip360has wider communication area than that of the antenna apparatus600which does not include the conductive strip360and that the antenna apparatus600which includes the conductive strip360can read the RFID tags in a broader area than the antenna apparatus600which does not include the conductive strip360. Such an improvement of the antenna apparatus600is confirmed by the measured results obtained from the experiment in which the RFID tags are attached to towels.

According to the fourth embodiment, it is possible to provide the antenna apparatus600which can generate the electric field having sufficient uniformity and intensity in the near field by electromagnetically coupling the resonators13-1to13-7with the microstrip-antenna. The microstrip-antenna includes the conductive line12′ and the grounded electrode11. Further, it is possible to provide the antenna apparatus600which can generate the magnetic field having sufficient intensity around the open end12B in the near field by electromagnetically coupling the conductive strip360with the microstrip-antenna.

FIG. 33is a plan view of an antenna apparatus according to a modification of the sixth embodiment.

Also in the present modification, each of the resonators13-1to13-5is formed with a conductor in the form of a line which has a length substantially equal to the design wavelength, and each of resonators is provided on the surface of the upper layer10-2of the substrate. Each of the resonators13-1to13-5is arranged so that the distance between two resonators adjacent to each other along the conductor12′ is substantially equal to the design wavelength.

In the present modification, each of the resonators13-1to13-5includes two elements13aand13bwhich have straight line shape, and are connected at a position in which the elements overlap with the center line extending from the open end12bto the power feeding port12aof the conductor12′. The elements13aand13bare symmetrical to the center line, and are formed to make an acute angle with the center line so that the elements are most close to the open end12bat the position in which the elements overlap with the center line and approach the power feeding port12aas separating from the center line. Therefore, also in the present modification, the resonators13-1to13-5are formed in the convex shape toward the open end12bof the conductor12′, respectively.

FIG. 34AandFIG. 34Bare plan views of the antenna apparatus according to further modification of the sixth embodiment, respectively. In the modification as illustrated inFIG. 34AandFIG. 34B, the shape and direction of each resonator are different from the antenna apparatus600as illustrated inFIG. 30. In the modification as illustrated inFIG. 34A, respective angles made by the two elements13band13con the both sides of the resonators13-1to13-7and the central element13aare different each other. Specifically, the angle between the element13band the central element13ais larger than the angle between the element13cand the central element13a. Accordingly, each resonator is asymmetrical to the center line extending from the power feeding port12ato the open end12b.

Moreover, in the modification as illustrated inFIG. 34B, each resonator is arranged with a tilt to the center line so that the angle between the centerline extending from the power feeding port12ato the open end12b, and the central element13aof each resonator13-1to13-7is the acute angle. Accordingly, in the present modification, an angle between the element13bon one side of the resonator and the center line is smaller than an angle between the element13con another side and the center line. Accordingly, each resonator is asymmetrical to the center line. In both modifications, each resonator is formed in the convex shape toward the open end12bof the conductor12′, and the angle between at least a part of the resonator and the center line is made an acute angle. Therefore, as illustrated inFIG. 34AandFIG. 34B, even when each resonator is formed so as to be asymmetrical to the center line, each resonator can produce the electric field component of the X-axis direction, and the electric field component of the Y-axis direction.

FIG. 35is a plan view of an antenna apparatus according to a further modification of the sixth embodiment. In this modification, the shape of each resonator is different from the resonator in the antenna apparatus600as illustrated inFIG. 30. In this modification, the resonators13-1to13-7are formed in arc-like shape. Also in this modification, each resonator is formed in the convex shape toward the open end12bof the conductor12′, and is arranged so that the middle point of each resonator crosses the conductor12′. Therefore, since the angle between the resonator and the line extending from the power feeding port12ato the open end12bis an acute angle except for the middle point of the resonator, each resonator can produce the electric field component parallel to the X-axis direction, and the electric field component parallel to the Y-axis direction. Therefore, the antenna apparatus600according to this modification can uniform the intensity of the electric field regardless of the direction of the electric field. Consequently, this antenna apparatus enables communication with the other communication apparatus, regardless the direction of the antenna of the other communication apparatus, such as the RFID tag.

Note that, also in these modifications of the sixth embodiment, it is preferable that each resonator does not overlap with the meandering shape part of the conductor, in order to avoid uneven distribution of the current in the resonator.

Furthermore, according to modifications of each of the above-described embodiments, each resonator may have a shape other than the form of a line.FIG. 36AtoFIG. 36Care drawings illustrating shapes of the resonators according to other embodiments, respectively. In each modification, each resonator is arranged in the vicinity of the nodal point of the standing wave of the current which flows through the micro-strip antenna, i.e., in the vicinity of any of the positions at the distance of half design wavelength and distances of integral multiples thereof from the open end.

In an example as illustrated inFIG. 36A, the resonators14-1to14-3include the two conductors in the form of lines arranged in the shape of an X character. In this example, the two conductors which form the resonator also have a length substantially equal to the design wavelength. Each resonator is arranged so that an intersection of the two conductors which form the resonator is located right above the conductor12.

In an example as illustrated inFIG. 36B, the resonators15-1to15-3have a bow-tie-like shape. Each of the resonators15-1to15-3is arranged so that the part at which the width along the longitudinal direction of the conductor12is minimum is located above the conductor12.

In an example as illustrated inFIG. 36C, the resonators16-1to16-3include a meandering shape, respectively. In this case, each of the resonators16-1to16-3is designed so that the length along the meandering conductor is substantially equal to the design wavelength. Each resonator is arranged so that a middle point of the resonator is located above the conductor12. Moreover, the shapes of respective resonators may differ with each other. For example, if an antenna apparatus includes three resonators, one of the resonators may be a conductor in the form of a line as the resonator13-1as illustrated inFIG. 21, another may be a conductor in the form of X character as illustrated inFIG. 36A, and still another may be a conductor in the form of a bow tie as illustrated inFIG. 36B.

According to yet another modifications, each resonator may be arranged so that the longitudinal direction of the resonator and the longitudinal direction of the conductor which is a part of the micro-strip antenna take an acute angle.

In any of the embodiments and their modifications, one of resonators is preferably arranged in the vicinity of the position at the distance of the half design wavelength from the open end of the micro-strip antenna, i.e., in the vicinity of the nodal point nearest to the open end among the nodal points of the standing wave of current which flows through the micro-strip antenna. This is because the electric field in the vicinity of the nodal point which is nearest to the open end is stronger than the electric field in the vicinity of other nodal points, and therefore the resonator arranged in the vicinity of the nodal point can be strongly electromagnetically coupled with the micro-strip antenna.

The descriptions of the antenna apparatus of exemplary embodiments have been provided heretofore. The present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.

So far, the preferred embodiments and modification of the antenna apparatuses are described. However, the invention is not limited to those specifically described embodiments and the modification thereof, and various modifications and alteration may be made within the scope of the inventions described in the claims.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention.

Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.