JP5856000B2 - Loop antenna - Google Patents

Description

  The present invention relates to a loop antenna that generates a magnetic field by passing a current through a current loop.

  In recent years, communication devices that perform non-contact communication such as non-contact type IC cards and RFID (Radio Frequency IDentification) have become widespread. This communication apparatus has a loop antenna which is, for example, a coiled antenna, and performs non-contact communication by magnetic field coupling using an alternating magnetic field in the loop antenna. The magnetic field coupling by such a communication device is suitable for non-contact communication in a short distance because the communication distance is short compared to long-distance wireless communication used for communication such as a radio or a mobile phone.

JP 2000-113142 A

Tokyo Denki University "Introductory Electromagnetism" Tokyo Denki University Press, 2006

Conventionally, as shown in FIG. 11, when a steady current I (A) is passed through a circular coil having a radius r (m), the magnetic flux density generated at a point P on the central axis z (m) away from the center is Is represented by the following equation.

  According to Equation 1, the magnetic flux density B attenuates in inverse proportion to the third power of z when a condition sufficiently away from the circular coil, that is, when r >> z is satisfied.

  Expression 1 is an expression that is established when a steady current is passed through the circular coil. However, when the frequency is low, it is also approximately established when an alternating current is passed. The attenuation characteristic of the magnetic flux density generated from the loop antenna used in the magnetic field communication performed using the alternating current also follows the above equation.

  One means for ensuring security in wireless communication is to reduce the possibility of eavesdropping from a distance by narrowing the communication area. However, the magnetic flux density formed by the loop antenna is inversely proportional to the cube of the distance from the loop antenna according to Equation 1, and it is not possible to obtain an attenuation characteristic having a gradient higher than that. In addition, in order to narrow the communication area, there are means of reducing the number of turns of the loop antenna or reducing the current value flowing through the loop antenna, but both means communicate the magnetic flux density radiated from the loop antenna. Since it is uniformly reduced in the power space, there is a possibility that communication quality is impaired.

  The present invention has been made in view of the above-mentioned problems, and the object of the present invention is to ensure stable communication quality by realizing a large magnetic flux density in a specific space, and in other spaces. Is to provide a loop antenna that limits the transmission of information by remarkably attenuating the magnetic flux density and realizes high-performance space limitation.

In order to solve the above-described problem, the first aspect of the present invention is composed of a plurality of N circular current loops having the same center and arranged in the same plane, and k is an integer of 1 to N. for current loop current _{I k} flowing, the number of turns _{M k,} when the radius _{r k,} the following condition
and
A loop antenna characterized by satisfying

The second aspect of the present invention comprises a plurality of N circular current loops having the same center arranged in the same plane,
Let k be an integer between 1 and N, and let the flowing current be I _k , M turns _k , Radius r _k And the following conditions
And N = 4, comprising the first current loop and the second current loop.
A first loop pair that satisfies the following conditions, a third current loop, and a fourth current loop:
The second loop pair that satisfies the above is a solution means having a loop antenna that is arranged concentrically.
The third aspect of the present invention comprises a plurality of N polygonal current loops having the same center arranged on the same plane and having the same length from the center to each vertex, and k is an integer of 1 to N. , For the k th current loop, let the flowing current be I _k , M turns _k , The length from the center to each vertex is r _k And the following conditions
and
A loop antenna characterized by satisfying
The fourth aspect of the present invention comprises a plurality of N polygonal current loops having the same center arranged in the same plane and the same length from the center to each vertex, and k is an integer of 1 to N. , For the k th current loop, let the flowing current be I _k , M turns _k , The length from the center to each vertex is r _k And the following conditions
And N = 4, comprising the first current loop and the second current loop.
A first loop pair that satisfies the following conditions, a third current loop, and a fourth current loop:
The solution is a loop antenna characterized in that a second loop pair satisfying the above is arranged.

  According to the present invention, a multi-loop antenna exhibiting an attenuation characteristic inversely proportional to the (2N + 1) th power is realized by flowing an appropriate amount of current through N loop antennas in an appropriate direction. A steep magnetic flux density attenuation characteristic in which a large magnetic flux density is ensured and the power of the inverse proportion exceeds the third power can be realized in other areas.

It is a figure which shows the structure of the loop antenna ((2N + 1) power attenuation | damping loop antenna) which concerns on 1st Embodiment. It is a figure which shows an example (5th power attenuation | damping loop antenna) of the structure of the loop antenna which concerns on 1st Embodiment. It is a figure which shows an example (7th power attenuation | damping loop antenna) of the structure of the loop antenna which concerns on 1st Embodiment. It is a figure which shows the characteristic of the magnetic flux density produced | generated from the loop antenna ((2N + 1) power attenuation | damping loop antenna) which concerns on 1st Embodiment. It is a figure which shows the structure of the loop antenna ((2N + 1) power attenuation | damping loop antenna) which concerns on 2nd Embodiment. It is a figure which shows the structure of the loop antenna (5th attenuation | damping loop pair) which concerns on 3rd Embodiment. It is a figure which shows the calculation result of the attenuation characteristic of the loop antenna which concerns on 3rd Embodiment. It is a figure which shows the structure of the loop antenna (polygonal loop antenna) which concerns on 4th Embodiment. It is a figure which shows the calculation result of the magnetic flux density produced from the loop antenna ((2N + 1) power attenuation | damping polygonal loop antenna) which concerns on 4th Embodiment. It is a figure which shows the calculation result of the magnetic flux density which arises from the loop antenna (5th power attenuation | damping polygonal loop pair) which concerns on 4th Embodiment. It is a figure which shows the positional relationship of a current loop and a coordinate.

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

[First Embodiment]
FIG. 1 is a diagram showing a configuration of a loop antenna according to the first embodiment. FIG. 2 is a diagram illustrating an example of the configuration of the loop antenna according to the first embodiment. FIG. 3 is a diagram illustrating an example of the configuration of the loop antenna according to the first embodiment. FIG. 4 is a diagram showing characteristics of magnetic flux density generated from the loop antenna according to the first embodiment.

  The loop antenna shown in FIG. 1 is used, for example, for authentication when entering and exiting a gate. Similarly, in other embodiments, the loop antenna is used for authentication and the like.

The loop antenna includes a first current loop A1, a second current loop A2,..., A kth current loop A3,. Current loops A1 to A4 are arranged to have the same center. Here, since the magnetic flux density generated in each of the current loops A1 to A4 follows Formula 1, the magnetic flux density generated from the loop antenna can be expressed by the sum of the magnetic flux densities generated from all loops, that is, the sum of Formula 1 as shown in Formula 2. It is represented by

Here, from the expansion formula of Taylor

When equation 2 is rewritten using equation 3, equation 4 is obtained.

  The right side in Equation 4 is a polynomial, and by setting a condition that makes some of these terms zero, the fifth power, seventh power,..., (2N + 1) power that exceeds the third power attenuation of the distance z. Attenuation can be realized.

As an example of (2N + 1) th power attenuation, conditions for realizing fifth power attenuation are shown. From equation 4, when setting the condition that the first term is 0, that is, when setting to satisfy the condition of equation 5,

In Equation 4, the term inversely proportional to the cube of the distance z disappears, and the magnetic flux density shows an attenuation characteristic inversely proportional to the fifth power of the distance z. As an example of realizing the fifth power attenuation, in the case of a loop antenna constituted by double current loops (N = 2), for example, as shown in FIG. 2, I ₁ = 1 [A], I ₂ = −4 [A], r ₁ = 1 [m], r ₂ = 0.5 [m]
By setting as follows, the following Expression 6 corresponding to Expression 5 can be satisfied.

When the above set value is used, the magnetic flux density generated from the loop antenna composed of the double current loop is rewritten as in Equation 7, and the attenuation characteristic depends on the term of the highest order of the distance z. An attenuation characteristic inversely proportional to the fifth power can be realized.

Similarly, an example of realizing a seventh power attenuation will be described. When setting conditions for the first and second terms in Equation 4 to be 0, that is, when setting the following conditions:

The term inversely proportional to the third and fifth powers of the distance disappears on the right side of Equation 4, and the magnetic flux density shows an attenuation characteristic that is inversely proportional to the seventh power of the distance. As an example of realizing an attenuation characteristic that is inversely proportional to the seventh power of the distance, in the case of a loop antenna constituted by triple current loops (N = 3), for example, I ₁ = 1 [A as shown in FIG. ], I ₂ = −50 [A], I ₃ = 175 [A], r ₁ = 5 [m], r ₂ = 2 [m], r ₃ = 1 [m] And the following formulas 9 and 10 corresponding to formula 8 can be satisfied.

When the above set value is used, the magnetic flux density generated from the loop antenna formed by the triple current loop is rewritten as shown in Equation 11, and an attenuation characteristic inversely proportional to the seventh power of the distance z can be realized.

Generalization of the fifth power attenuation and seventh power attenuation of the above conditions, it is possible I _k, r _k set conditions for realizing the loop antenna (2N + 1) th power attenuation of N current loops. I _k of its condition setting from the first term in the right side of the equation 4 to the N term becomes _zero, is to select the r _k, it can be expressed as shown in Equation 12.

  By satisfying Expression 12, an attenuation characteristic of (2N + 1) power can be selected and realized arbitrarily.

  As an effect of the present embodiment, the calculation result of the attenuation characteristic is shown in FIG. As N = 2, 3 and 4 in Z >> r, the fifth, seventh and ninth power attenuation characteristics are obtained.

  In the conventional method of reducing the number of turns of the loop antenna or reducing the value of the flowing current, the magnetic flux density is uniformly reduced in the space to be communicated. Abruptly attenuates at Z >> r.

  With this attenuation characteristic, stable communication quality is provided by securing a large magnetic flux density in a specific area, and high safety is provided by making the magnetic flux density undetectable by steep attenuation in other areas. be able to.

[Second Embodiment]
The second embodiment is a modification of the first embodiment.

  FIG. 5 is a diagram showing a configuration of a loop antenna according to the second embodiment.

  In such a modification, each current loop in the first embodiment is wound one or more times. In other words, the first embodiment is a case where each current loop in the second embodiment has one turn.

Due to the difference that each current loop is wound a plurality of times, the conditional expression for realizing (2N + 1) power attenuation is also changed from Expression 12, and the current flowing through each current loop is I _k , and the number of turns of each current loop is M _{When k} and the radius of each current loop are r _{k, the condition} is that the positive integer k satisfies Expression 13 for all possible numbers from 1 to N.

  Others are the same as those in the first embodiment.

As an example of (2N + 1) power attenuation, conditions for realizing 7th power attenuation are shown. Since this is the case of N = 3 in Equation 13, a magnetic flux density that attenuates to the seventh power can be realized by satisfying Equation 14 and Equation 15 below.

Expressions 14 and 15 are, for example, I ₁ = 1 [A], I ₂ = −5 [A], I ₃ = 7 [A], r ₁ = 5 [m], r ₂ = 2 [m], r _{If 3} = 1 [m], M ₁ = 1, M ₂ = 10, and M ₃ = 25, this is established.

According to such a modification, the current value flowing through the current loop can be reduced by setting the number of turns of the current loop to a plurality of times. For example, in the form of the first embodiment, I ₃ = 175 [A], but by reducing the number of turns to 25 in the form of the second embodiment, it is reduced to I ₃ = 7 [A]. is made of.

  By reducing the current value, the power consumption of the communication device can be reduced and the electricity bill can be saved.

  In addition, when a battery is used for the communication device, the operation time can be extended.

[Third Embodiment]
FIG. 6 is a diagram showing a configuration of a loop antenna according to the third embodiment. This loop antenna uses two current loop pairs composed of two current loops, and is composed of a total of four current loops.

One of the two pairs is composed of a first current loop and a second current loop that have the same center, and the current flowing through the first current loop and the radius of the current loop are I ₁ and r _1. When the current flowing in the second current loop and the radius of the current loop are I ₂ and r ₂ , the following conditions

  Current loop pair A001. The magnetic flux density of the magnetic field generated from the current loop pair A001 exhibits an attenuation characteristic that is inversely proportional to the fifth power of the distance z according to the principle of the first embodiment.

The remaining one of the two pairs is composed of a third current loop and a fourth current loop that have the same center, and the current flowing through the third current loop and the radius of the current loop are defined as I ₃ , r ₃ and when the current flowing in the fourth current loop and the radius of the current loop are I ₄ and r ₄ ,

  The current loop pair A002 satisfying The magnetic flux density of the magnetic field generated from the current loop pair A002 exhibits an attenuation characteristic that is inversely proportional to the fifth power of the distance z according to the principle of the first embodiment.

  The loop antenna of the present embodiment is a loop antenna characterized in that a current loop pair A001 and a current loop pair A002 are configured concentrically.

As an example of the present embodiment, for example, as shown in FIG. 6, I ₁ = 1 [A], I ₂ = -4 [A], r ₁ = 1 [m], r ₂ = 0.5 [m], Set I ₃ = −2 [A], I ₃ = 8 [A], r ₃ = 0.8 [m], and r ₄ = 0.4 [m].

  FIG. 7 shows the calculation results of the attenuation characteristics under these setting conditions.

  According to the present embodiment, the magnetic flux density generated from the loop antenna has an attenuation characteristic that is inversely proportional to the fifth power at z >> r, and also has a depression characteristic at an arbitrary spatial point, so that it is locally inversely proportional. Since it exhibits a rapid attenuation characteristic exceeding the fifth power, it is possible to limit the communication area more strictly and with high operability.

Each current loop may be wound a plurality of times. In that case, if the number of turns of each current loop is M _k , the conditions to be satisfied are as follows.

[Fourth Embodiment]
The fourth embodiment is a modification of the first to third embodiments.

  This modification is characterized in that each current loop in the first to third embodiments is not a circle but a polygon (a shape having three or more corners or a rectangle). Since a polygonal antenna is easier to manufacture than a circular antenna, there is an advantage that the manufacturing cost is reduced and the product price can be reduced.

FIG. 8 is a diagram showing a configuration of a loop antenna according to the fourth embodiment. Symbols in FIG. 8, the current flowing through each current loop I _k, the length of the apex and r _k the number of turns of each current loop M _k, from the center of the current loop.

  Even if the loop antenna is polygonal, it can be treated as equivalent in nature to a circular loop antenna in the region of z >> r, that is, sufficiently far from the loop antenna.

  As an example of the present embodiment, calculation results of normalized attenuation characteristics of an octagonal loop antenna are shown in FIGS. FIG. 9 shows the magnetic flux density generated from the (2N + 1) power attenuated polygon loop antenna, and FIG. 10 shows the magnetic flux density generated from the fifth power attenuated polygon loop pair.

  9 and 10, the black dotted line is the attenuation characteristic of the octagonal loop antenna, and the gray solid line is the attenuation characteristic of the circular loop antenna. The gray solid line in FIG. 9 is the same as the calculation result of FIG. 4, and the gray solid line in FIG. 10 is the same as the calculation result of FIG.

  In FIG. 9, both lines overlap, and it can be seen that the (2N + 1) power attenuation characteristics generated from the polygonal and circular loop antennas are equal. Also, in FIG. 10, both lines are overlapped, and it can be seen that the (2N + 1) th power attenuation characteristic that produces a depression characteristic at an arbitrary spatial point resulting from a polygonal and circular loop pair is equal.

I _k current M _k number of turns r _k radius

Claims (4)

Consists of a plurality of N circular current loops having the same center arranged in the same plane,
The k as one or more N an integer, for the current loop of the k, current I _{k flowing,} the number of turns M _k, when the radius r _k, the following condition
and
A loop antenna characterized by satisfying Consists of a plurality of N circular current loops having the same center arranged in the same plane,
The k as one or more N an integer, for the current loop of the k, current I _{k flowing,} the number of turns M _k, when the radius r _k, the following condition
The filling,
N = 4
It consists of a first current loop and a second current loop, and the following conditions
A first loop pair satisfying
Consists of a third current loop and a fourth current loop, with the following conditions
A second loop pair satisfying
A loop antenna characterized by being arranged concentrically. It is made up of a plurality of N polygonal current loops having the same center arranged in the same plane and having the same length from the center to each vertex,
Let k be an integer between 1 and N, and let the flowing current be I _k , M turns _k , The length from the center to each vertex is r _k And the following conditions
and
A loop antenna characterized by satisfying It is made up of a plurality of N polygonal current loops having the same center arranged in the same plane and having the same length from the center to each vertex,
Let k be an integer between 1 and N, and let the flowing current be I _k , M turns _k , The length from the center to each vertex is r _k And the following conditions
The filling,
N = 4
It consists of a first current loop and a second current loop, and the following conditions
A first loop pair satisfying
Consists of a third current loop and a fourth current loop, with the following conditions
A second loop pair satisfying
A loop antenna characterized by being arranged.