JPH04248704A - Antenna for near distance communication - Google Patents

Abstract

PURPOSE:To limit a communication area by arranging loop coils close to each other and setting a magnetic field generated by each coil to a specific value. CONSTITUTION:Plural loop coils 1, 2 are arranged concentrically or close to each other, and let an ampere-turn between a current flowing to the coil 1 and the number of turns of the coil 1 be IT1 and an ampere-turn between a current flowing to the coil 2 and the number of turns of the coil 2 be IT2, then a magnetic field H1 at a point P generated in the coil 1 and a magnetic field H2 at a point P generated in the coil 2 are in the state expressed respectively in equation I. When the two coils are wound opposite to each other, the magnetic field at the pint P is a specific value and the state at a remote location is expressed in equation II. When the ampere-turns of both the coils are expressed in equation III, the magnetic field at a remote location is zero.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antenna that performs communication over short distances using a magnetic field.

0002

2. Description of the Related Art Conventionally, a loop coil has been used as an antenna for short-range communication using a magnetic field.

0003

[Problems to be Solved by the Invention] However, conventional loop coils have the following problems.

(1) Obtaining a magnetic field above a certain level at a certain distance is impossible due to legal restrictions. That is,
The electric field strength at the position of λ/2π must be 15 mV or less, but if the frequency f is increased to increase the transmission speed, it becomes impossible to output an output according to this formula. Therefore, it becomes necessary to provide a high gain amplifier on the receiving side.
Problems arise, such as the need for additional power for this purpose, which impedes miniaturization of the receiving side. Additionally, there is a problem of lowering the S/N ratio.

(2) Since there is only one loop antenna, it is difficult to limit the reception area, which may cause interference.

[0006] An object of the present invention is to provide an antenna for short-distance communication in which the magnetic field decreases rapidly outside a predetermined area by using a plurality of loop coils.

[0007]

[Means for Solving the Problem] Consisting of n loop coils (n>1) arranged close to each other, the magnitude, direction, coil diameter, and number of turns of the current of each coil are generated by each loop coil. When the magnetic field at an arbitrary position (r, θ) is defined as Hn(r, θ), the magnetic field is set to a value such that ΣHn = 0 at the position r>>.

[0008]

[Operation] When n loop coils are placed close to each other, any position (r
, θ) is Hn (r, θ), then
The magnitude and direction of the current in each coil, the coil diameter, and the number of turns are selected so that ΣHn=0 at the position r>>. That is, the magnetic fields generated from each loop coil cancel each other over a long distance, so the magnetic field suddenly becomes smaller at a certain distance. In other words, even if you relatively increase the current applied to the coil or the number of turns in the coil, the possibility of exceeding the legal limit is reduced, and it is possible to relatively increase the reception level as long as it is within a certain distance. Become. It also becomes possible to limit the communication area.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an antenna according to an embodiment of the present invention. In this embodiment, an antenna is constructed using two concentric loop coils 1 and 2. In this embodiment, the current flowing through each coil, the coil diameter, and the number of turns are set so that a2 * IT1 = b2 * (-IT2). Note that IT1 is the (number of turns *
IT2 represents (number of turns * current) of coil 2. The above equation is derived below.

FIG. 2 is a diagram for explaining the magnetic field strength on the central axis.

In FIG. 2, the magnetic field strength at point p on the central axis is

0012

[Math 1]

[0013]

Considering the double loop shown in FIG. 1, let IT1 be the current flowing through coil 1 (current * number of turns) and IT2 be the current flowing through coil 2 (current * number of turns), then the current at point p generated by coil 1 is Magnetic field H1, p generated by coil 2
The magnetic field H2 at each point is

[0015]

[Math 2]

[0016] If these two coils are wound in opposite directions, the magnetic field at point p will be H(p) = H1 - H2. And in the far distance (x>>a, b),

00
17]

[Math 3]

[0018] here,

[0019]

[Math 4]

[0020] Then, the far magnetic field becomes 0. For the above reasons, the double loop allows an area-limited antenna with large attenuation over distance to be constructed.

[0021] Besides the central axis, according to Biot-Savart's law, with reference to FIGS. 3(A) and (B),

[0022]

[Math 5]

[0023] If we integrate this along the circumference, we get

002
4]

[Math 6]

[0025] If this is a double loop (the two coils are reversely wound),

[0026]

[Math 7]

It is expressed as follows. FIG. 4 shows the distance and the z-direction in the case of 2a=b in the embodiment shown in FIG. The relationship between magnetic fields is shown (the magnetic field when distance = a is set to 1). As shown in the figure, within a distance of 2a, the magnetic field strength in each of the three cases is almost the same, but as the distance increases beyond this, the magnetic field sharply decreases in this embodiment.

FIG. 5 shows another embodiment of the invention. In this embodiment, the two coils 1 and 2 are placed a little apart. In this example, each value is determined so that a2 *IT1 = b2 *IT2. For example, if a=b, IT1=-IT2.

FIG. 6 shows yet another embodiment of the invention. In this example, four coils are placed in close proximity. The diameter of each coil is a1, a2, a3, a4,
(Coil current * Number of turns) are respectively IT1, IT2,
Let it be IT3 and IT4.

[0030] If x>>a1, a2, a3, a4, the time H(p) at point p is

[0031]

[Math. 8]

##EQU00003## and this H(p)=0, a1 ~
By selecting a4, IT1 to IT4, the far magnetic field can be made zero. Further, in this embodiment, it is possible to appropriately change the magnetic field distribution by adjusting the number of coils.

FIG. 7 shows an example of a drive circuit for driving the coil shown in FIG. 1 above.

Further, FIG. 8 shows a case where the above antenna is applied to a commuter pass gate (pass gate). This pass gate uses magnetic fields to exchange information with the commuter pass held by the user.

[0035]

Effects of the Invention In the present invention, since the magnetic fields generated from each of the plurality of coils cancel each other out over a long distance, the following effects can be obtained.

(1) Since the magnetic field at a long distance becomes smaller, it is possible to increase the magnetic field at a short distance by increasing the current flowing through the coil and the number of turns of the coil. This improves reliability.

(2) Since the magnetic field suddenly decreases with distance, the communication area can be limited. Therefore, there is no interference.

(3) Since the short-range magnetic field can be increased within legal limits, the configuration of the receiving side can be simplified, leading to cost reduction.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the invention.

FIG. 2 is a diagram for explaining the principle of the above embodiment.

FIGS. 3A and 3B are diagrams for explaining the operation of the embodiment of the present invention.

FIG. 4 is a diagram illustrating the effects of the above embodiment by comparing it with a conventional example.

FIG. 5 shows another embodiment of the invention.

FIG. 6 shows yet another embodiment of the invention.

FIG. 7 shows a drive circuit for driving the coil of the embodiment of FIG. 1;

FIG. 8 is a diagram showing the antenna arrangement position when the short-range communication antenna according to the present invention is applied to a pass gate.

[Explanation of symbols]

1, 2 Loop coil

Claims (1)

[Claims] Claim 1: n pieces (n>1
), and the magnitude of the current in each coil,
The magnetic field at any position (r, θ) generated by each loop coil is expressed as Hn (
r, θ), at the position r>>, ΣHn
An antenna for short-range communication, characterized in that the antenna is set to a value such that =0.