JP5647528B2 - Antenna device - Google Patents

Description

  The present invention relates to a sheet-like antenna device suitable for performing communication using weak radio waves that do not require a radio station license, for example.

  In recent years, as a method for managing articles such as documents, books, office supplies, commodities, and medicines in offices, stores, laboratories, etc., a predetermined communication device has been developed from a small wireless tag (responder) storing predetermined information. There is known an RFID (Radio Frequency Identification) system for reading information by an (interrogator).

  In such an RFID system, as an antenna on the interrogator side, for example, an upper conductor of a sheet-like parallel plate has a mesh shape, and a structure that slightly leaks electromagnetic waves transmitted in the parallel plate is used. Used (see, for example, Patent Document 1). Further, Patent Document 1 discloses a configuration of an interface device for satisfactorily performing wireless communication between an interrogator antenna and a responder.

  When the electromagnetic wave is propagating through the parallel plate of the interrogator antenna and the interface device is brought close to the mesh-shaped upper conductor side, the interface device and the interrogator antenna are capacitively coupled. As a result, a part of the electromagnetic wave propagating in the parallel plate is sucked out from the mesh-like upper conductor to the interface device.

  The electromagnetic wave sucked out by the interface device is radiated into the space and received by the responder. The response signal radiated from the transponder that has received the electromagnetic wave is input to the interrogator antenna via the interface device and received by the interrogator.

  The interrogator antenna and the interface device configured as described above can be applied not only to the RFID system but also to a wireless LAN system or the like.

JP 2007-150652 A

  Here, it is assumed that the interrogator antenna disclosed in Patent Document 1 is installed on a table. In general, a license is required to transmit radio waves, but if the frequency used is 322 MHz or less, if the electric field strength at a distance of 3 m from the antenna device is 500 μV / m or less, a license for a radio station is applied. There is no need. For this reason, the request | requirement of the antenna device which can be used on said use conditions is high.

  However, in the interrogator antenna disclosed in Patent Document 1, when power is fed from the end of the flat plate, the far-field directivity that radiates strongly in a specific direction is generated in the same manner as the leakage wave antenna of traveling wave excitation. The maximum value of the electric field strength of weak radio waves is limited in the maximum directionality. That is, there is a problem in that the electric field strength is lowered and the communication area is narrowed when the specific direction is deviated.

  Further, the interrogator antenna disclosed in Patent Document 1 has a problem that the transmission mode in the sheet-like parallel plate is the same as that of a waveguide having a thin wavelength, and the propagation loss is large.

  The present invention has been made to solve the conventional problems, and provides an antenna device that has a small propagation loss, uniform radiation directivity, and can widen a communication area in a two-dimensional plane. For the purpose.

The antenna device of the present invention includes a first conductor plate, a first dielectric layer provided on the first conductor plate, a center conductor provided on the first dielectric layer, An antenna device comprising: a second dielectric layer provided on a central conductor; and a second conductor plate provided on the second dielectric layer, wherein the central conductor is in one direction An even number of strip portions arranged in parallel with each other at a predetermined interval, an end portion of one strip portion of two adjacent strip portions, and another strip portion on the same side as the end portion A connecting portion for connecting the end portion of the strip portion to the end portion of the two outermost strip portions having an end portion not connected by the connecting portion. It has an input unit for inputting an output unit that outputs a signal, respectively, prior to The second conductive plate at intervals predetermined along said strip portion, that having a plurality of slots which are parallel to each other in a position straddling the strip portion.

With this configuration, the antenna device of the present invention has a stripline structure in which a slot is formed in a conductor plate. Therefore, propagation loss can be reduced as compared with a conventional sheet-like antenna device. In addition, since the antenna device of the present invention has a structure in which the central conductor of the strip line is folded, the communication area can be expanded in a two-dimensional plane.
Furthermore, since the antenna device of the present invention has an even number of the plurality of strip portions, the traveling wave excitation in the reverse direction is realized by the pair of adjacent strip portions, so that the radiation directivity can be made uniform. .

Furthermore, the antenna device of the present invention has a configuration in which the distance between the centers of the two adjacent strip portions is 1/25 or less of the wavelength of the operating frequency.
With this configuration, the antenna device of the present invention can perform good communication particularly when the impedance Z of the strip line is 50Ω and the use frequency is around 300 MHz.
In the antenna device of the present invention, the interval between the slots adjacent to each other in the one direction is 1/50 or less of the wavelength of the used frequency.
In the antenna device of the present invention, the distance between the first conductor plate and the second conductor plate is 1/200 or less of the wavelength of the used frequency.

  The present invention can provide an antenna device that has the effects that propagation loss is small, radiation directivity is uniform, and a communication area can be expanded in a two-dimensional plane.

It is the figure which showed the leaky coaxial cable typically. It is explanatory drawing of the electric field strength at the time of using a leaky coaxial cable as an infinite array antenna. It is explanatory drawing in case the radiation from each wave source forms an equiphase wave front. It is a graph which shows the relationship between P / (lambda) and radiation angle (theta). It is a block diagram in 1st Embodiment of the antenna apparatus which concerns on this invention. It is a fragmentary sectional view of the stripline of the antenna device concerning the present invention. It is a graph which shows the relationship between line | wire width and a dielectric constant. It is a graph which shows the relationship between the space | interval of a parallel plate, and line | wire width. It is a graph which shows the simulation result of a far field directivity. It is a block diagram of the comparative example of this invention. It is a block diagram in 2nd Embodiment of the antenna apparatus which concerns on this invention.

  Before describing embodiments of the present invention, the basic characteristics of a leaky coaxial cable closely related to the theoretical background of the present invention will be described.

  The leaky coaxial cable generally has a structure in which slots are periodically provided in the length direction on the outer conductor of the coaxial cable to leak radio waves. FIG. 1 schematically shows a leaky coaxial cable.

  In FIG. 1, a leaky coaxial cable 1 has a center conductor 2 having a diameter d, an outer conductor 3 having a diameter D, and a slot 4 provided in the outer conductor 3. In the drawing, P indicates a slot interval. Here, the slot interval P is the distance between the centers of adjacent slots. When the diameter D of the outer conductor 3 is sufficiently small (D / λ <0.12) compared with the wavelength λ of the frequency to be used, the slot 4 is less likely to resonate and has a wide frequency characteristic.

Here, the leaky coaxial cable 1 shown in FIG. 1 is considered as an infinite array antenna as shown in FIG. The electric field strength E generated on the slot surface is expressed by [Expression 1] where I is the current flowing through the outer conductor in the cable axial direction (Z-axis direction).

Further, the cable axis direction component E _Z of the electric field strength E is expressed by [Equation 2]. If the cables circumferential component E _phi is sufficiently wide in the Z-axis direction of the slot 4 is smaller becomes substantially zero. In this case, the surface acoustic wave mode is dominant in the leaky coaxial cable 1, and the radiation wave mode can be ignored. In the following description, it is assumed that this condition is satisfied.

Further, the electric field of each slot 4a, 4b, 4c, 4d... When the position of the leftmost slot 4a is set to z = 0 is expressed by the following equation. Here, β _g = 2π / λ _g (λ _g : wavelength in cable).

Next, in order to investigate the propagation direction of the electromagnetic wave in the infinite array antenna shown in FIG. 2, consider the case where the electromagnetic wave radiated from each wave source having the same intensity propagates in the θ direction (FIG. 3). As shown in FIG. 3, in order for the radiation from each wave source to create an equiphase wavefront, the relationship shown in [Equation 4] needs to be established. However, n = 0, ± 1, ± 2,..., K = 2π / λ (λ: free space wavelength). W _i and W _{i + 1} indicate points where a certain equiphase wavefront and a straight line indicating the propagation direction of the electromagnetic wave radiated from the wave sources S _i and S _{i + 1} intersect. In [Equation 4], the lengths of S _i W _i and S _{i + 1} W _{i + 1} with an upper line are the lengths of the line segment S _i W _i and the line segment S _{i + 1} W _{i + 1} , respectively.

That is, when the relationship of [Equation 4] is applied to the infinite array antenna of FIG. Where ν is the wavelength shortening rate (λ _g = νλ) of the cable. Further, n = 0, ± 1, ± 2,.

When the condition shown in the following equation is satisfied in [Equation 5], θ _n is not a real number, and the infinite array antenna operates in the surface wave mode.

Here, there is a relationship represented by the following equation between the wavelength shortening rate ν and the relative dielectric constant ε _r in the cable.

When using a low-loss dielectric substrate in the high frequency band, the relative dielectric constant ε _r is about 2.0 to 3.5, and the wavelength shortening rate ν in that case is 0.535 to 0.707.

As an example, the graph of FIG. 4 shows the result of calculating the slot interval P normalized by the wavelength λ with the wavelength shortening rate ν of 0.5 and n of −1 in [Equation 5]. As can be seen from this graph, the range of the value of P in which θ _n is a real number and is in the range of −90 degrees to 90 degrees is 0.333λ to 1.0λ. That is, when the value of P is in this range, the infinite array antenna operates in the radiated wave mode, and when the value of P is in the range of 0.0 to 0.333λ, θ _n does not become a real number, and the infinite array antenna Will operate in surface wave mode.

(First embodiment)
Next, embodiments of the present invention will be described with reference to the drawings. The antenna device according to the present invention extends the above-described leaky coaxial cable in a two-dimensional plane. In addition, the dimensional ratio of each structure on each drawing does not necessarily correspond with the actual dimensional ratio.

First, the configuration of the antenna device according to the first embodiment of the present invention will be described.
As shown in FIG. 5, the antenna device 10 according to the present embodiment includes a lower conductor 11, a first dielectric layer 12, a central conductor 13, a second dielectric layer 14, and a second conductor plate as a first conductor plate. The upper conductor 15 as a conductive plate is formed in a so-called triplate structure in which the conductors 15 are laminated in this order.

  The central conductor 13 extends in the Y-axis direction and is arranged in parallel with each other at a predetermined interval, the end of the strip 16a, and the same side as the end It has a meandering shape having a connecting portion 17a that connects the end portion of the strip portion 16b.

  Further, the center conductor 13 includes an input unit 13a for inputting a signal and an output unit 13b for outputting a signal. A signal generator for generating an electric signal having a predetermined frequency is connected to the input unit 13a, and a termination resistor is usually connected to the output unit 13b.

  The upper layer conductor 15 is provided with a plurality of slots 18 that emit electromagnetic waves at a predetermined slot interval P along the Y-axis direction in which the strip portions 16a and 16b of the center conductor 13 extend. Here, the slot interval P is the distance between the centers of slots adjacent in the Y-axis direction.

  The plurality of slots 18 are formed in parallel to each other at a position straddling the strip portions 16a and 16b. Hereinafter, the two-dimensional array of the plurality of slots 18 as a whole is referred to as a slot array, and the one-dimensional array of slots 18 formed above the strip portions 16a and 16b is referred to as a slot row as appropriate.

  In the manufacturing stage of the antenna device 10, a thermosetting resin material (prepreg) (not shown) is disposed on the first dielectric layer 12 and the center conductor 13. Further, the second dielectric layer 14 is disposed on the prepreg, and the first dielectric layer 12 and the second dielectric layer 14 are integrated through the prepreg by hot pressing. As a result, as shown in a partial cross-sectional view perpendicular to the Y axis of the antenna device 10 (FIG. 6), the periphery of the center conductor 13 is covered with the dielectric material.

(Design example)
A design example of the dimensions of each component of the antenna device 10 according to the present embodiment shown in FIG. 5 is shown below.
(1) The dimension of the antenna device 10 in the X-axis direction: 500 mm
(2) Dimensions of the antenna device 10 in the Y-axis direction: 1000 mm to 2000 mm
(3) Each thickness of dielectric layers 12 and 14: 1 mm
(4) Dielectric constants ε _{r of the} dielectric layers 12 and 14: 2 to 3.5
(5) Line width W of the central conductor 13: 1.0 mm (Note: 50Ω)
(6) Length L _{X of the} slot 18 in the X-axis direction: 6.0 mm
(7) The width L _{Y of the} slot 18 in the Y-axis direction: 2.0 mm
(8) Slot interval P (distance between centers of slots adjacent in the Y-axis direction): 20 mm
(9) Turn-back interval Q (distance between the centers of the strip portions 16a and 16b): 40 mm

Next, an example of a method for determining the slot interval P of the antenna device 10 according to the present embodiment will be described below. In the following description, it is assumed that the impedance Z of the strip line of the antenna device 10 is 50Ω and the use frequency is 300 MHz (use wavelength λ ₀ is about 1 m).

The value of the guide wavelength λ _g is obtained as a value depending on the relative dielectric constant ε _r of the strip line according to the following equation determined from the relationship of λ _g = νλ _{0 and} the relationship of [Equation 7].

Further, assuming that the phase difference (β _g · P = 2πP / λ _g ) between two slots adjacent in the Y-axis direction is about 10 degrees, the value of the slot interval P is obtained as shown in [Table 1]. It is done.

Here, the relative dielectric constant ε _r includes a relative dielectric constant (about 4.6) of FR-4 (Flame Retardant Type 4), which is a general substrate material, and a relative dielectric constant of a low-loss substrate material for high frequencies. The rate (about 2 to 3.5) is taken as an example.

  From the value of the slot interval P shown in [Table 1], the slot interval P is 20 mm or less, that is, 1 of the wavelength of the used frequency under the condition that the phase difference between adjacent slots in the Y-axis direction is allowed about 10 degrees. / 50 or less is desirable.

Next, an example of a method for determining the distance between the lower layer conductor 11 and the upper layer conductor 15, the length L _X and the width L _{Y of the} slot 18, and the line width W of the antenna device 10 according to the present embodiment will be described.

As already introduced, FIG. 6 is a partial cross-sectional view perpendicular to the Y axis of the strip line of the antenna device 10. That is, the strip line shown in FIG. 6 includes a lower conductor 11, first and second dielectric layers 12, 14, a strip portion 16a (16b), and an upper conductor 15. Here, the line width of the strip portion 16a (16b) is W, and the distance between the lower layer conductor 11 and the upper layer conductor 15 (that is, the thickness of the integrated first and second dielectric layers 12, 14) is H. , and the relative dielectric constant of the first and second dielectric layers 12, 14 and epsilon _r.

The impedance Z of the strip line configured as described above is expressed by the following equation.

For example, when the impedance Z of the strip line is 50Ω, the result of calculating the relative permittivity ε _r with respect to the line width W normalized by the interval H from [Equation 9] is shown in FIG.

For example, when the relative dielectric constant ε _r is 4.6, the possible W / H value is about 0.4 from [Equation 9]. When the relative dielectric constant ε _r is 3.5, the possible W / H value is about 0.5 from [Equation 9]. When the relative dielectric constant ε _r is 2, the possible W / H value is about 0.8 from [Equation 9]. These relationships are shown in the graph of FIG. 8 with the horizontal axis as the interval H and the vertical axis as the line width W.

Meanwhile, in order to avoid interference of the slot adjacent to each other in the Y-axis direction, (a value obtained by subtracting the width L _Y from slot interval P of the slot Y-axis direction) of the gap between the slots is approximately X-axis direction of the slot 18 it is desirable for at least about three times the length L _X. Here, it is assumed that the width L _Y of the slot 18 in the Y-axis direction is 2 mm, and as already discussed, the slot interval P is 20 mm or less, which is a value of 1/50 or less of the wavelength of the used frequency (300 MHz). Then, the preferred slot length L _X is about 6.0 mm or less.

Further, the line width W is desirable for not more than 1/3 of the length L _X of the slot 18 across the strip portion 16 emits electromagnetic waves. Accordingly, the line width W is a value of about 2.0 mm or less.

Here, it can be seen from FIG. 8 that when the value of the relative dielectric constant ε _r is 2, the interval H at which the line width W becomes 2.0 mm or less is about 2.5 mm or less. When the relative dielectric constant ε _r is 3.5, the distance H at which the line width W is 2.0 mm or less is about 4.0 mm or less, and the relative dielectric constant ε _r is 4.6. When the line width W is 2.0 mm or less, the distance H is about 5.0 mm or less.

Therefore, when the value of the relative permittivity ε _r is in the range of 2 to 4.6, it can be seen that the interval H is 1/200 or less of the wavelength (about 1 m) of the operating frequency.

  Next, an example of a method for determining the folding interval Q for folding the center conductor 13 of the antenna device 10 according to the present embodiment will be described below.

In folded central slot sequence formed over the conductor 13, the gap of the slot adjacent to each other in X-axis direction (a value obtained by subtracting the length L _X of the X-axis direction from the folded interval Q slots) are too close Slots interfere with each other. In order to avoid interference between the slots, it is desirable that the gap between the slots adjacent in the X-axis direction is approximately three times or more the length L _X of the slot 18.

Taking the above matters into consideration, when the length L _{X of the} slot 18 straddling the strip portion 16 of the central conductor 13 is about 6 mm as already considered, the gap between adjacent slots in the X-axis direction is 18 mm or more. It becomes. Accordingly, the preferred folding interval Q is 24 mm or more. That is, the turn-back interval Q is larger than 24 / 1000≈1 / 42 of the wavelength of the used frequency.

  However, if the folding interval Q is too large, the radiation near field created by the two-dimensional array of slots 18 will not be uniform. As a result of obtaining a suitable value of the folding interval Q by electromagnetic field analysis, about twice the slot interval P is the upper limit value. For example, as already discussed, when the slot interval P is 20 mm or less, 40 mm corresponding to 1/25 of the wavelength of the used frequency is the upper limit value of the folding interval Q. Therefore, the folding interval Q is preferably within a range of 1/42 to 1/25 of the wavelength.

  The antenna device 10 according to the present embodiment configured as described above has a thin planar structure in which the distance H between the lower layer conductor 11 and the upper layer conductor 15 is 1/200 wavelength or less. It can be easily installed on the front or back surface, floor, or attached to the wall.

  FIG. 9 shows a simulation result (solid line) of the far-field directivity on the YZ plane of the antenna device 10 configured as described above. At the same time, a simulation result of the antenna device 30 having the one-dimensional configuration shown in FIG. 10 is indicated by a broken line as a comparative example. Here, the antenna device 30 is formed as a triplate structure in which the lower conductor 11, the first dielectric layer 12, the central conductor 33, the second dielectric layer 14, and the upper conductor 35 are laminated in this order. It is. The center conductor 33 has a strip shape extending in the Y-axis direction, and includes an input unit 33a for inputting a signal and an output unit 33b for outputting a signal. The upper layer conductor 35 is provided with a plurality of slots 18 that radiate electromagnetic waves at predetermined slot intervals P along the Y-axis direction in which the central conductor 33 extends. The plurality of slots 18 are formed in parallel to each other at a position straddling the central conductor 33.

  From FIG. 9, the radiation directivity of the antenna device 30 according to the present embodiment, while the radiation directivity of the antenna device 30 is strong in a specific direction (particularly the direction of 60 degrees to 90 degrees), It can be seen that it is uniform in the YZ plane (Z ≧ 0).

  As already mentioned, if the use frequency is 322 MHz or less, the allowable value of the electric field strength at a distance of 3 m of the weak radio station is 500 μV / m, and it is not necessary to apply for a license for the radio station. It is preferable to operate the antenna device below.

  However, when the antenna device 30 has directivity that radiates strongly in a specific direction, assuming that the electric field strength in the maximum directionality direction (near 90 degrees) is 500 μV / m, for example, near 320 degrees. Thus, the electric field strength becomes 28 μV / m, which is about 25 dB lower than that.

  On the other hand, since the antenna device 10 according to the present embodiment has uniform radiation directivity, the antenna device 10 substantially covers the range of 0 to 90 degrees and 270 to 360 degrees in the YZ plane (Z ≧ 0). A constant field strength of 500 μV / m can be achieved.

  As the shape of the interface device for the antenna device 10, an antenna having a simple configuration such as a minute dipole antenna or a minute loop antenna can be used. At this time, the longitudinal direction of the minute dipole antenna is the same as the Z axis. Further, the minute loop antenna is configured such that its loop surface is in a plane perpendicular to the XY plane (in a plane including the Z axis).

  As described above, according to the antenna device 10 according to the present embodiment, since the central conductor 13 with the slot array formed upward is folded back at the folding interval Q, the slot array adjacent in the X-axis direction is reversed. Directional traveling wave excitation can be realized. As a result, the radiation directivity is uniform in the YZ plane and does not radiate strongly only in a specific direction. Furthermore, the antenna device 10 according to the present embodiment can form a good communication area only in the near field region within 1 / (2π) wavelength from the surface of the upper layer conductor without emitting unnecessary radio waves in the external space. it can.

  Moreover, since the antenna device 10 according to the present embodiment is formed of a strip line, it is possible to reduce the propagation loss as compared with the conventional sheet-like antenna device.

(Second Embodiment)
A configuration of the antenna device according to the second embodiment of the present invention will be described. Note that description of matters similar to those in the first embodiment will be omitted as appropriate.

  As shown in FIG. 11, the antenna device 20 according to the present embodiment includes a lower conductor 11, a first dielectric layer 12, a central conductor 23, a second dielectric layer 14, and a second conductor plate as a first conductor plate. The upper layer conductor 25 as a conductor plate is formed in a triplate structure in which these are laminated in this order.

  The center conductor 23 extends in the Y-axis direction and is arranged in parallel with each other at a predetermined interval, and the end of one of the two adjacent strip portions. The connection part 27 (27a-27c) which connects the edge part of the other strip part on the same side as the said edge part has comprised the meander shape. Furthermore, the center conductor 23 includes an input unit 23a for inputting a signal and an output unit 23b for outputting a signal.

  The upper layer conductor 25 is provided with a plurality of slots 18 that emit electromagnetic waves at predetermined slot intervals P along the Y-axis direction in which the strip portion 26 extends. The plurality of slots 18 are formed in parallel to each other at a position straddling the strip portion 26.

Moreover, it is desirable to satisfy the following items as in the first embodiment.
Slot interval P is 1/50 or less of the wavelength of the used frequency. Spacing H is 1/200 or less of the wavelength of the used frequency (relative permittivity ε _r is 2 to 3.5).
-The folding interval Q (the distance between the centers of two adjacent strips) is 1/25 or less of the wavelength of the operating frequency.

  Furthermore, it is important that the number of strip portions 26, that is, the number of slot rows is an even number. This is because, as described in the first embodiment, the traveling wave excitation in the reverse direction is realized by the pair of adjacent slot rows, and the radiation directivity becomes uniform in the YZ plane. If the number of strip portions 26 is an odd number and the input portion 23a and the output portion 23b are formed at opposite ends of the antenna device, respectively, it is shown as a comparative example in FIG. Similarly to the one-dimensional antenna device, the radiation directivity is increased in a specific direction.

  The antenna device 20 configured as described above is further expanded in a two-dimensional plane shape than the antenna device 10 of the first embodiment, so that the communication area can be further expanded. In addition, the length and the number (however, even number) of the strip part of the center conductor 23 in the Y-axis direction can be designed to arbitrary values.

  As described above, the antenna device according to the present invention has effects that the propagation loss is small, the radiation directivity is uniform, and the communication area can be expanded in a two-dimensional plane. It is useful as an antenna device for an interrogator or an antenna device for a wireless LAN system.

10, 20 Antenna device 11 Lower layer conductor (first conductor plate)
12 Dielectric layer (first dielectric layer)
13, 23 Center conductors 13a, 23a Input portion 13b, 23b Output portion 14 Dielectric layer (second dielectric layer)
15, 25 Upper layer conductor (second conductor plate)
16a, 16b, 26a, 26b, 26c, 26d Strip portion 17a, 27a, 27b, 27c Connection portion 18 slot

Claims (4)

A first conductor plate, a first dielectric layer provided on the first conductor plate, a central conductor provided on the first dielectric layer, and provided on the central conductor; An antenna device comprising: a second dielectric layer; and a second conductor plate provided on the second dielectric layer,
The center conductor includes an even number of strip portions extending in one direction and arranged in parallel with each other at a predetermined interval, an end portion of one strip portion of two adjacent strip portions, and the end portion The outermost two strips having a meander-like shape having a connecting portion that connects the end of the other strip portion on the same side as the other, and having an end portion that is not connected by the connecting portion An input unit for inputting a signal and an output unit for outputting a signal, respectively, at the end of the unit;
The antenna device, wherein the second conductor plate has a plurality of slots formed in parallel to each other at a position straddling the strip portion at a predetermined interval along the strip portion.   The antenna device according to claim 1, wherein a distance between centers of two adjacent strip portions is 1/25 or less of a wavelength of a use frequency. The antenna apparatus according to claim 1 or 2, wherein an interval between the slots adjacent to each other in the one direction is 1/50 or less of a wavelength of a use frequency. The antenna device according to any one of claims 1 to 3, wherein an interval between the first conductor plate and the second conductor plate is 1/200 or less of a wavelength of a use frequency.

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