JP2013125998A - Loop antenna - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a loop antenna which can create sharp magnetic field distribution so that borders of a communication area is unambiguously set.SOLUTION: A loop antenna 1 comprises a first conductor loop 11, and a second conductor loop 12 encompassing the first conductor loop 11. An electric current Iin the first conductor loop 11 and an electric current Iin the second conductor loop 12 flow in directions different from each other.

Description

  The present invention relates to a loop antenna that can create a sharp magnetic field distribution that can clearly set the boundary of a communication area.

  In recent years, there has been an increasing need for wireless communication (area limited wireless) in which a communication area is intentionally limited. For example, the “electric field communication system” disclosed in Patent Document 1 below is one means for realizing area-limited radio.

JP 2007-174570 A

  In electric field communication, only a terminal device existing in an area near an access point device installed in the environment can communicate with the access point device. However, since the electric field distribution in the vicinity of the access point device largely depends on the installation environment, the user's posture, and the like, it has been difficult to realize a clear nearby area with an electric field. Therefore, the terminal device existing at the position to be communicated cannot communicate, or vice versa, and a stable and highly reliable area limited wireless system cannot be constructed.

  One of the causes of such difficulty is considered to be the use of an electric field as a communication medium. This is because the electric field distribution is strongly influenced by conductors and dielectrics existing around it.

  On the other hand, a low-frequency (approximately 10 MHz or less) magnetic field has a feature that the interaction with the human body and the surrounding environment is significantly lower than the electric field. Therefore, it is conceivable to use a low-frequency magnetic field as a communication medium as one means for solving the problem.

  If a “sharp magnetic field distribution” in which the magnetic field strength sharply attenuates at the boundary of the communication area can be created, a highly reliable area limited wireless system can be constructed.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a loop antenna that can create a sharp magnetic field distribution that can clearly set the boundary of a communication area.

  In order to solve the above-described problem, the first aspect of the present invention includes a first conductor loop and a second conductor loop that includes the first conductor loop, and flows through the first conductor loop. The solution means is a loop antenna characterized in that the direction of the current is different from the direction of the current flowing through the second conductor loop.

  For example, in the first aspect of the present invention, the absolute value of the magnetic moment that is the product of the current value of the current flowing through the first conductor loop and the area of the portion surrounded by the first conductor loop is the second value. It is equal to the absolute value of the magnetic moment, which is the product of the current value of the current flowing through the conductor loop and the area of the portion surrounded by the second conductor loop.

  For example, in the first aspect of the present invention, the absolute value of the magnetic moment that is the product of the current value of the current flowing through the second conductor loop and the area of the portion surrounded by the second conductor loop is the first value. It is larger than the absolute value of the magnetic moment, which is the product of the current value of the current flowing through the conductor loop and the area of the portion surrounded by the first conductor loop.

  The second aspect of the present invention has a first conductor loop and a plurality of second conductor loops arranged so as to surround the first conductor loop, and a current flowing through the first conductor loop. And a direction of the current flowing through the second conductor loop is different from each other.

  For example, in the second aspect of the present invention, the absolute value of the magnetic moment that is the product of the current value of the current flowing through the first conductor loop and the area of the portion surrounded by the first conductor loop is the second value. It is equal to the sum of absolute values of magnetic moments, which is the product of the current value of the current flowing through the conductor loop and the area of the portion surrounded by the second conductor loop.

  For example, in the second aspect of the present invention, the total sum of absolute values of magnetic moments, which is the product of the current value of the current flowing through the second conductor loop and the area of the portion surrounded by the second conductor loop, is It is greater than the absolute value of the magnetic moment, which is the product of the current value of the current flowing through one conductor loop and the area of the portion surrounded by the first conductor loop.

  For example, in the first and second present inventions, the shape of the first conductor loop is any one of a circle, a regular polygon, and a rectangle, and the shape of the second conductor loop is a circle, a regular polygon. , One of the rectangles.

  With the loop antenna according to the present invention, it is possible to create a sharp magnetic field distribution that can clearly set the boundaries of the communication area.

It is a figure which shows the loop antenna which concerns on 1st Embodiment. It is a figure which shows the loop antenna of a single loop used for description of the loop antenna which concerns on 1st Embodiment. It is a figure which shows the coordinate system used for description of the loop antenna which concerns on 1st Embodiment. It is the graph which compared the magnetic field intensity on az axis with the loop antenna (double loop) which concerns on 1st Embodiment. It is the graph which compared the magnetic field intensity on az axis with the loop antenna (double loop) concerning a 2nd embodiment. It is a figure which shows the loop antenna which concerns on 3rd Embodiment. It is a figure which shows the loop antenna which concerns on 4th Embodiment. It is a figure which shows the loop antenna which increased the number of the 2nd conductive loops 22 of the loop antenna which concerns on 4th Embodiment. It is a figure which shows the loop antenna which made the distance of the 1st conductive loop 21 and the 2nd conductive loop 22 of the loop antenna which concerns on 4th Embodiment not constant.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a diagram illustrating a loop antenna according to a first embodiment.

The loop antenna 1 has a first conductor loop 11 and a second conductor loop 12 that encloses the first conductor loop 11, and the direction of the current I ₁ flowing through the first conductor loop 11 and the second conductor loop 11. wherein the direction of the current I ₂ flowing in the conductor loop 12 is different.

In the first embodiment, the absolute value of the magnetic moment m ₁ that is the product of the current value of the current I ₁ and the area of the portion surrounded by the first conductor loop 11 is the current value of the current I ₂ 2 is equal to the absolute value of the magnetic moment m ₂ , which is the product of the area surrounded by the two conductor loops 12.

  The shapes of the first and second conductor loops 11 and 12 are, for example, circular.

In general, a magnetic field is generated around a current I ₁ flowing through a single-loop antenna as shown in FIG. The current I ₁ may be direct current or alternating current, and the amplitude of the generated magnetic field is proportional to the current value of the current I 1. If the wavelength corresponding to the frequency of the alternating current I ₁ is sufficiently larger than the radius a ₁ of the loop, the magnetic field distribution near the loop antenna does not depend on the frequency and matches the magnetic field distribution of the static magnetic field.

The loop antenna has a property as a magnetic dipole, and has a magnetic moment m given by Equation (1) where I is the current value of the current I ₁ and S is the area of the loop.

The magnetic field H generated from the loop antenna is approximately represented by the following equation (2).

  Here, bold indicates a vector. R is the position vector of the observation point, and n is the unit normal vector of the loop plane. The center of the loop is assumed to be located at the origin.

Expression (2) is an approximate expression, but is sufficiently accurate if the condition of the following expression (3) is satisfied.

  As can be seen from the equations (2) and (3), the magnetic field attenuates in proportion to the cube of the distance R in a region sufficiently away from the loop antenna. If it is a decibel display, it will attenuate at a rate of -60db / dec.

  On the other hand, it is known that in a region where Expression (3) is not satisfied, that is, a region that is not sufficiently distant from the loop antenna, the magnetic field is not attenuated by the cube of the distance R, and attenuation is moderate.

  Considering the above conditions, the magnetic field distribution when a current is passed through the loop antenna will be considered.

  First, the coordinate system of FIG. 3 is set. That is, the xy axis is taken in a plane parallel to the paper surface, and the z axis is taken in a direction perpendicular to the paper surface. The (x, y, z) coordinate system is a right-handed system, that is, the direction from the back to the front of the page is the positive direction of the z axis.

As shown in FIG. 2, the center considers the radius a ₁ of the conductor loop to match the coordinate origin. Orientation of the looped arrow represents the direction of the current I _1. For convenience, this will be referred to as a single loop. Further, as shown in FIG. 1, a structure in which a _second conductor loop 12 having a radius a ₂ is added to the outside of a first conductor loop 11 that is a single loop is referred to as a double loop. Magnitude of the current flowing through the second conductor loop 12 of the outer is I _2, the orientation of the I ₂ thereof will be different from the I _1.

Here, for example, in a double loop, the absolute value of the magnetic moment m ₁ of the first conductor loop 11 provided inside is equal to the absolute value of the magnetic moment m ₂ of the second conductor loop 12 provided outside. to set the I _2. However, since I ₁ and I ₂ are in opposite directions, the directions of the magnetic moments are always opposite. In the case of FIG. 1, I ₂ may be set so as to satisfy Expression (4).

When set to the condition of Equation (4), in the distant loop antenna 1, a magnetic moment m ₁ of the first conductor loop 11, there cancellation and the magnetic moment m ₂ of the outer second conductor loop 12 Thus, the magnetic field strength is considered to be extremely small. On the other hand, when focusing on the magnetic field intensity near the origin, the contribution of the inner first conductor loop 11 is considered to be superior to the contribution of the outer second conductor loop 12. Therefore, when the condition of the expression (4) is set in the double loop of FIG. 1, the magnetic field distribution near the origin of the double loop is similar to that of the single loop, but the far field strength of the double loop is that of the single loop. It is expected to be significantly smaller than that.

FIG. 4 is a graph comparing the magnetic field strength on the z-axis between a single loop and a double loop. However, the double loop was plotted under the condition of equation (5).

In the region of z <a _1, the magnetic field distribution generated by the single-loop and double loop are substantially equal. On the other hand, in the region of z> a ₁ , it can be seen that the magnetic field distribution of the double loop attenuates rapidly (−100 dB / dec). Therefore, if the condition of Expression (5) is adopted in the double loop, a sharper magnetic field distribution than that in the single loop can be created.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. In the second embodiment, the same or similar configuration is used in the first embodiment, and the same or similar components are denoted by the same reference numerals used in the first embodiment, and redundant description is omitted. The description will focus on matters different from those of the first embodiment.

In the second embodiment, the absolute value of the magnetic moment, which is the product of the current value of the current I ₂ and the area of the portion surrounded by the second conductor loop 12, is the current value of the current I ₁ and the first value. The absolute value of the magnetic moment, which is the product of the area of the portion surrounded by the conductor loop 11, is larger.

In the first embodiment, the condition of Expression (5) is used, but in the second embodiment, the condition of Expression (6) is used as an example.

That is, the absolute value m ₂ of the magnetic moment of the second conductor loop 12 is twice the absolute value m ₁ of the magnetic moment of the first conductor loop 11.

  FIG. 5 is a graph comparing the magnetic field strength on the z-axis under the condition of equation (6).

In this case, although almost exhausted difference between the z >> a _1, the double loop, the magnetic field strength takes a minimum value (depression appears), in part, a double of the first embodiment A magnetic field distribution sharper than the loop can be generated. The position at which the depression appears can be adjusted appropriately by changing the current value of the current I ₂ and the radius a ₂ .

The absolute value m ₂ of the magnetic moment is not limited to twice the absolute value m ₁ of the magnetic moment, and the same effect can be obtained if it is larger than the absolute value m ₁ of the magnetic moment.

[Third Embodiment]
Next, a loop antenna according to a third embodiment will be described. In the third embodiment, description will be made centering on differences from the first and second embodiments.

  In the first and second embodiments, the shapes of the first and second conductor loops 11 and 12 are circular. However, the shape is not limited to a circle, and various shapes such as an ellipse, a rectangle, and a polygon can be used. Can be used. As an example, as shown in FIG. 6, the first and second conductor loops 11 and 12 can be square.

[Fourth Embodiment]
FIG. 7 is a diagram illustrating a loop antenna according to the fourth embodiment.

  The loop antenna 2 includes a first conductor loop 21 and a plurality (four) of second conductor loops 22 arranged so as to surround the first conductor loop 21, and the first conductor loop 21. The direction of the current flowing through 21 and the direction of the current flowing through the second conductor loop 22 are different.

  In the fourth embodiment, the absolute value of the magnetic moment, which is the product of the current value of the current flowing through the first conductor loop 21 and the area of the portion surrounded by the first conductor loop 21, is the second conductor. This is equal to the sum of absolute values of magnetic moments, which is the product of the current value of the current flowing through the loop 22 and the area of the portion surrounded by the second conductor loop 22.

  The shape of the first and second conductor loops 21 and 22 is, for example, a circle.

  Here, current flows through the first and second conductor loops 21 and 22, but the current values are different.

The magnetic moment of the first and second conductor loops 21 and 22 depends on the loop area and current, but the absolute value of the magnetic moment of the first conductor loop 21 is m ₁ . A current in the direction opposite to that of the first conductor loop 21 flows through the second conductor loop 12. The absolute value of the magnetic moment of each second conductor loop 22 is m _{2 to} m ₅ . However, the following conditions shall be satisfied.

  By setting so as to satisfy the condition of Expression (7), the same effect as that of the first embodiment can be obtained. That is, since the vector sum of the magnetic moments of the first and second conductor loops 21 and 22 becomes zero, the far magnetic field of the loop antenna 2 is rapidly attenuated. On the other hand, the magnetic field distribution in the vicinity of the first conductor loop 21 is not substantially affected by the second conductor loop 22 and is therefore almost equal to the single-loop magnetic field distribution. Therefore, the same effect as the first embodiment can be obtained, and a sharp magnetic field distribution can be formed.

  In FIG. 7, the number of second conductor loops 22 is four, but any number is effective as long as the relationship of Expression (7) is satisfied. For example, as shown in FIG. 8, an arbitrary number of second conductor loops 22 are arranged on the circumference centered on the first conductor loop 21 or the apex of a regular polygon centered on the first conductor loop 21. It may be arranged above.

  In FIGS. 7 and 8, the distance from the first conductor loop 21 to the second conductor loop 22 is constant, but it is not necessarily constant as shown in FIG.

  Moreover, although the shape of the 1st, 2nd conductor loops 21 and 22 was circular, not only circular but not only circular but various shapes, such as an ellipse, a rectangle, and a polygon, can be utilized. .

[Fifth Embodiment]
Next, a fifth embodiment of the present invention will be described. In the fifth embodiment, the same or similar configuration is used in the fourth embodiment, and the same or similar components are denoted by the same reference numerals used in the fourth embodiment, and the redundant description is omitted. Description will be made mainly on matters different from the fourth embodiment.

  In the fourth embodiment, the sum of the absolute values of the magnetic moments, which is the product of the current value of the current flowing through the second conductor loop 22 and the area of the portion surrounded by the second conductor loop 22, is the first value. The absolute value of the magnetic moment, which is the product of the current value of the current flowing through the conductor loop 21 and the area of the portion surrounded by the first conductor loop 11, is larger.

That is, the magnetic moment is set so as to satisfy the condition of Expression (8) in the multi-loop of FIG.

  Thereby, as the loop antenna 2 of the fourth embodiment has the same effect as the loop antenna 1 of the first embodiment, the loop antenna of the fifth embodiment is the same as the second embodiment. The same effect as that of the loop antenna of the form is shown. That is, a partially sharp magnetic field distribution can be formed.

  In FIG. 7, the number of the second conductor loops 22 is four, but any number is effective as long as the relationship of Expression (8) is satisfied. For example, as shown in FIG. 8, an arbitrary number of second conductor loops 22 are arranged on the circumference centered on the first conductor loop 21 or the apex of a regular polygon centered on the first conductor loop 21. It may be arranged above.

  In FIGS. 7 and 8, the distance from the first conductor loop 21 to the second conductor loop 22 is constant, but it is not necessarily constant as shown in FIG.

  Moreover, although the shape of the 1st, 2nd conductor loops 21 and 22 was circular, not only circular but not only circular but various shapes, such as an ellipse, a rectangle, and a polygon, can be utilized. .

1, 2 Loop antennas 11, 12 First conductor loops 21, 22 Second conductor loops

Claims (7)

A first conductor loop;
A second conductor loop enclosing the first conductor loop;
The direction of the electric current which flows through the said 1st conductor loop, and the direction of the electric current which flows through the said 2nd conductor loop differ. The loop antenna characterized by the above-mentioned. The absolute value of the magnetic moment, which is the product of the current value of the current flowing through the first conductor loop and the area of the portion surrounded by the first conductor loop, is
2. The loop antenna according to claim 1, wherein the loop antenna is equal to an absolute value of a magnetic moment that is a product of a current value of a current flowing through the second conductor loop and an area of a portion surrounded by the second conductor loop. The absolute value of the magnetic moment, which is the product of the current value of the current flowing through the second conductor loop and the area of the portion surrounded by the second conductor loop,
2. The loop antenna according to claim 1, wherein the loop antenna is greater than an absolute value of a magnetic moment that is a product of a current value of a current flowing through the first conductor loop and an area of a portion surrounded by the first conductor loop. A first conductor loop;
A plurality of second conductor loops disposed so as to surround the first conductor loop;
The direction of the electric current which flows through the said 1st conductor loop, and the direction of the electric current which flows through the said 2nd conductor loop differ. The loop antenna characterized by the above-mentioned. The absolute value of the magnetic moment, which is the product of the current value of the current flowing through the first conductor loop and the area of the portion surrounded by the first conductor loop, is
5. The loop according to claim 4, wherein the loop is equal to a sum of absolute values of magnetic moments, which is a product of a current value of a current flowing through the second conductor loop and an area of a portion surrounded by the second conductor loop. antenna. The sum of absolute values of magnetic moments, which is the product of the current value of the current flowing through the second conductor loop and the area of the portion surrounded by the second conductor loop,
5. The loop antenna according to claim 4, wherein the loop antenna is larger than an absolute value of a magnetic moment that is a product of a current value of a current flowing through the first conductor loop and an area of a portion surrounded by the first conductor loop. The shape of the first conductor loop is any one of a circle, a regular polygon, and a rectangle,
The loop antenna according to any one of claims 1 to 6, wherein the shape of the second conductor loop is any one of a circle, a regular polygon, and a rectangle.

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