JP4684152B2 - Magnetic loop antenna - Google Patents

Description

  The present invention relates to a magnetic field loop antenna for use in a reader / writer that communicates with a wireless tag.

  As an antenna for a wireless tag reader / writer, a magnetic loop antenna in which a copper wire is wound in a coil shape on a plane is used for the LF band (124, 135 KHz) and HF (13.56 MHz).

  On the other hand, since the transmission output of the wireless tag reader / writer is restricted by regulations of the Radio Law, the communication distance between these antennas and the wireless tag (Radio Frequency ID: RFID) is naturally limited.

  For this reason, in order to simultaneously detect wireless tags arranged in a wide range, it is necessary to arrange a plurality of wireless tag reader / writers and antennas.

  In addition, there are roughly the following four types of wireless tags that can be used in Japan at present.

(1) 124 KHz, 135 KHz band (2) 13.56 MHz band (3) 900 MHz band (950 MHz band)
(4) 2.45 GHz band (wavelength of about 13 m)
(5) 300 MHz band (weak radio wave)
The wireless tag of such a frequency band is currently used for various purposes.

Of these, (1) and (2) are relatively inexpensive, but can only search for a short distance of about several centimeters to several tens of centimeters. Therefore, it is impossible to detect the tag from a distance of several meters or more.

  On the other hand, (3), (4), and (5) are suitable when searching at a long distance of several tens cm to several m or more. In the case of these wireless tags, since the use frequency is high, the wavelength is short, so that a highly efficient one with a practical antenna size can be used.

  Among these, the frequencies (3) and (4) are suitable for detection applications only for the line-of-sight distance because the propagation characteristics are remarkably straight.

  However, if the tag cannot be seen from the antenna and is hidden behind a metal or concrete that absorbs radio waves, it cannot be detected.

  On the other hand, in the 300 MHz band of (5), the reflected wave propagates efficiently and the wavelength is about 1 m due to the radio wave propagation characteristics, and therefore various types of antennas that are theoretically 100% efficient are practical. Can be used in size. Therefore, it is possible to extend the detection distance of the tag by 10 m or more by performing various ideas. In addition, it is possible to detect a tag that is blocked by an object such as metal or concrete with respect to the antenna.

  A wireless tag detection system using the 300 MHz band wireless tag will be described with reference to FIGS. 11, 12, 13, and 14. FIG.

  In the wireless tag detection system of FIG. 11, the dipole antenna 2 and the wireless tag receiver 3 are connected by the cable 4, the radio wave from the wireless tag 2 is received by the dipole antenna 3, and the received signal is transmitted via the cable 4 to the wireless tag. Send to receiver 3. The wireless tag receiver 3 transmits the received signal to the monitoring system 7 via the network 6.

  As a wireless tag detection system using the dipole antenna 2, there is a system that detects the wireless tag 5 placed in the school bag by attaching the dipole antenna 2 to a school gate, for example.

  The wireless tag detection system of FIG. 12 connects the Yagi antenna 8 and the wireless tag receiver 3 with a cable 4, receives a radio wave from the wireless tag 2 with the Yagi antenna 8, and receives the received signal via the cable 4 with the wireless tag. Send to receiver 3. The wireless tag receiver 3 transmits the received signal to the monitoring system 7 via the network 6.

  In the wireless tag detection system of FIG. 13, the whip antenna 9 and the wireless tag receiver 3 are connected by the cable 4, the radio wave from the wireless tag 2 is received by the whip antenna 9, and the received signal is transmitted via the cable 4 to the wireless tag. Send to receiver 3. The wireless tag receiver 3 transmits the received signal to the monitoring system 7 via the network 6.

  In the wireless tag detection system of FIG. 14, a planar antenna (patch antenna) 10 and a wireless tag receiver 3 are connected by a cable 4, and radio waves from the wireless tag 2 are received by the planar antenna (patch antenna) 10. Is sent to the wireless tag receiver 3 via the cable 4. The wireless tag receiver 3 transmits the received signal to the monitoring system 7 via the network 6.

  That is, the receiving antenna used in the 300 MHz band UHF tag is a dipole antenna (FIG. 11), a Yagi antenna (FIG. 12), a whip antenna (FIG. 13), a patch antenna, a planar antenna (FIG. 14), etc. These are all called electric field antennas and are the principle of operation for detecting electric field components in radio waves.

On the other hand, as described in Japanese Patent Application Laid-Open No. 2004-85277 (Patent Document 1), the above-described wireless tag is attached to clothing, and the detection device receives the radio wave from the wireless tag at the antenna portion and detects that the clothing has become wet. There is also. The antenna unit has a transmission coil and a reception coil.
JP 2004-85277 A

  However, the dipole antenna used in the 300 MHz band UHF tag in FIG. 11 requires a half wavelength (50 cm), and an actual mounting area of 1 m or more is required.

  In addition, when the Yagi antenna of FIG. 12 is used, at least 50 cm × 1 m or more is necessary, and the area for mounting becomes large.

  Further, when the whip antenna of FIG. 13 is used, a length of 50 cm is inevitably required if grounding is not sufficient.

  Furthermore, the patch antenna (planar antenna) of FIG. 14 is A4 size, which also increases the mounting area.

  That is, the various receiving antennas referred to as electric field antennas have a problem that they require work for mounting and a relatively large area for mounting.

  On the other hand, these antennas are called electric field antennas, and are moved by an electric field. Therefore, there is a problem that detection cannot be performed if there is a metal nearby.

  For example, if the antenna connected to the wireless tag receiver is installed close to an object that absorbs radio waves, such as metal or concrete, its characteristics cannot be demonstrated even in the above 300 MHz band, and the wireless tag is detected. The distance will be very short.

  The reason for this is that the receiving antenna generally used in the 300 MHz band and UHF tag is an electric field antenna, so if there is metal or concrete that absorbs and reflects radio waves within a few wavelengths around the antenna, The original performance of the antenna cannot be exhibited, the directivity is disturbed, and the efficiency is significantly reduced.

  For this reason, radio waves cannot be transmitted / received to / from the wireless tag (no radio waves reach), and the detection distance of the wireless tag is significantly reduced.

  Such physical properties of the electric field antenna are extremely inconvenient when viewed from the practical use of the wireless tag system.

  As an example, when a wireless tag is detected inside a building, it is necessary to install an antenna inside the building.

  In this case, in order to prioritize aesthetics and people's activities in the building, the tag detection antenna is as inconspicuous as possible and limited space such as under the floor, walls, and ceilings so that it does not get in the way. It is required to be installed so as to be pushed into.

  However, from the viewpoint of the radio wave characteristics of the antenna as described above, it is desirable that no object that affects the radio wave be present around the antenna and in an area of several wavelengths.

  In other words, the installation conditions that come from the system requirements and the installation conditions that come from the antenna technology are inconsistent, and there is a trade-off between each other, looking for a compromise, and deciding the appropriate place while performing on-site adjustments and physical tests. .

  Moreover, since recent buildings have a reinforced concrete structure, the building slab is made of concrete, and the floor has a free access structure to enable intelligent electronic wiring, with a small space of about several tens of centimeters from the slab. In many cases, it is forced to install a wireless floor board and install electronic wiring between the small spaces or install an antenna for a wireless tag.

  Alternatively, there is a case where an RFID tag antenna is installed on the wall surface of the building, but it is also necessary to embed the RFID tag antenna in a gap of only about a few tens of centimeters between the concrete slab of the building and the wall decorative board.

  The same applies to the case where a RFID tag antenna is installed on the ceiling, and it is not allowed to go down from the ceiling decorative board, and the RFID tag antenna is placed behind the ceiling between the concrete building slab and the ceiling decorative board. Must be installed.

  These spaces are only about 1/10 wavelength or less with respect to the wavelength of 300 MHZ, and a much insufficient space is allowed as compared with the free space of several wavelengths required by the various electric field antennas described above. For this reason, the performance as an antenna must be sacrificed very much.

  In other words, because the performance is not as designed in advance, there is a problem that it is wasteful work and cost in the field, which makes the system more expensive than necessary or difficult to use.

  Furthermore, in order to avoid this, when installed in a building space where the RFID tag antenna can perform without problems, it will affect the aesthetics and the workability and comfort of human beings working in the building. Given this, there was also a problem that such a system that should originally support human activities had an adverse effect because of the presence of antennas.

  The present invention was made in order to solve the above problems, and can detect a shielded tag such as metal or concrete by making full use of the characteristics of the antenna, without compromising the aesthetics of the building, and large-scale construction An object of the present invention is to obtain a magnetic field loop antenna that does not need to be used.

The magnetic field loop antenna of the present invention is
A first substrate on which a loop-shaped first copper foil having an entire circumference of 0.5 wavelength or less is attached with a capacitor of 10 PF or less;
A loop-shaped second copper foil having a diameter of 1/10 of the loop-shaped first copper foil was attached to the first substrate with a constant interval. A second substrate;

An insulating member provided with a thickness of 10 mm or less between the first substrate and the second substrate;
  A case containing the first and second substrates and the insulating member;

And a connector attached to the case so as to be connectable to both ends of the loop-shaped second copper foil of the second substrate .

  As described above, according to the present invention, the reflected wave propagates efficiently in the 300 MHz band due to the radio wave propagation characteristics, and the wavelength is about 1 m. Therefore, the antenna is theoretically 100% efficient in various types of antennas. Can be used in a practical size, and an effect of detecting a shielded tag such as metal or concrete with respect to the antenna is obtained.

  In addition, there is an effect that the performance as an antenna can be sufficiently exerted without deteriorating the aesthetics of the building, the need for extensive construction, and the presence of metal and concrete in the vicinity.

  FIG. 1 is a schematic configuration diagram of a wireless tag detection system using a 300 MHz band magnetic loop antenna according to the present embodiment.

  As shown in FIG. 1, the wireless tag detection system according to the present embodiment uses a magnetic field loop antenna 15 in which a capacitor 17 and a loop coil 16 are connected in series connected to a cable 4.

  In this system, the magnetic field loop antenna 15 radiates radio waves based on the output signal from the radio tag receiver 3 and receives radio waves from the radio tag 5. At this time, the magnetic field loop antenna 15 reacts only to the magnetic field component of the radio wave radiated from the wireless tag 5 and outputs the obtained signal to the wireless tag receiver 3 via the cable 4.

  The wireless tag receiver 3 decodes the ID code from the signal from the magnetic field loop antenna 15 and outputs the ID code to the monitoring system 7 via the network 6.

  The magnetic loop antenna 15 will be described with reference to FIG. As shown in FIG. 2, a conductor (hereinafter referred to as a loop coil 16) having a length of 0.5 wavelengths or less (50 cm or less: for example, 20 cm) around the entire circumference and a capacitor 17 (10 PF or less) are connected in series and incorporated in the case 18. is doing. That is, a series resonance current flows. The loop coil 16 is fixed to the bottom of the case 18.

  Further, the detection conductor 20 is coiled (also referred to as a link coil), and one end thereof is connected to the core wire of the connect 19 connected to the cable 4 (feed line), and the other is connected to the shield side of the connector 19. ing.

  The space between the above-described detection conductor 20 and the loop coil 16 is, for example, about 5 mm. A signal of a magnetic field component detected by the loop coil 16 is output by the detection conductor 20.

  In order for such a 300 MHz band magnetic loop antenna 15 to exhibit optimum performance in a building or the like, it is necessary to define optimum design parameters.

  The design parameters (including capacitor capacity, loop coil length, width, etc.) will be described below.

  First, the relationship between the distance from the source of the electromagnetic wave radiated from the antenna and the intensity of the electromagnetic wave will be shown below.

Assuming that the amount of charge at a place P that is a distance r away from an electric dipole having a length of 1 at the origin of a certain space is q (t) (t is time), the rate of change in the amount of charge is current.

And can be expressed in the form of differentiation.

When there is a minute dipole facing the z-axis direction at the origin, the electric field E (t) and magnetic field H (t) of the minute dipole at the point P shown in FIG.

It becomes. These expressions are expressed based on polar coordinates, and er, eθ, and eΦ are e-direction, θ-direction, and Φ-direction unit vectors, respectively. C is the propagation speed of electromagnetic waves in space.

  In these equations, the term proportional to r-3 is a term that creates an electrostatic magnetic field, and in the case of an electric dipole, it exists only in the electric field.

The term r-2 is a term that generates an induction electromagnetic field. The r-1 term is a term that creates a radiation field. If it is far enough, the electrostatic term and the induction term are much smaller than the radiation term. Therefore, the electric field / magnetic field (far field) in a place sufficiently away from the dipole is

It becomes. Since the direction unit vectors are orthogonal, the electric field and the magnetic field are orthogonal in the far field, and there is no electric / magnetic field component in the wave traveling direction.

In the frequency domain display, the far-field electric and magnetic field components are

It becomes.

Here, the ratio ξ between the far electric field and the magnetic field is called wave impedance, and is expressed by the following equation.

  From [Equation 6] and [Equation 7], since the wave impedance ξ increases as the distance r from the wave field decreases in the near field region, the magnetic field component of the electromagnetic field is strong in the near field region (Fresnel region). It can be said that the electric field component becomes stronger in the far field region (Fraun for Far region).

  From the above, in order to detect only the magnetic field component near the antenna, the antenna itself is operated in the current mode, generating a large current to the antenna conductor (loop coil), and the magnetic field component is first radiated into the space from the current. The physical configuration of the antenna may be determined as described.

  In order to realize this, the antenna itself may be formed as a circuit that is in series resonance in the 300 MHz band that is the use frequency so that a large series resonance current flows through the antenna conductor and the structure including the antenna conductor.

  Specifically, since the inductance is constituted by the antenna conductive wire, the value of the capacitor that resonates with the inductance is determined using itself as a closed loop, and they are electrically connected in series.

  As a method for transmitting and receiving energy to the resonance loop and the coaxial cable for feeding, any method such as a link coil method or an impedance tap method may be adopted.

  By doing so, a magnetic loop antenna is formed, and its operating impedance becomes the resonant impedance of the series resonant circuit.

  That is, the inductive reactance due to the inductance of the conductor and the electrostatic reactance due to the capacitance of the capacitor cancel each other out to zero, and the closed-loop impedance is theoretically very low, less than 1Ω, which is only the net resistance of the conductor. Value.

  Therefore, a large current flows in the closed loop. And it reacts only with the magnetic field component of the area | region of the vicinity of an antenna, The energy can be efficiently transmitted / received between space and an antenna. In other words, since the antenna operates in a current mode with a very low impedance, even if a substance such as metal or concrete that disturbs the electric field component of the electromagnetic wave is present in the immediate vicinity of the antenna, the characteristics of the magnetic loop antenna are almost affected. There is nothing.

    A quantitative numerical example is shown below.

About the size of conductors close to each other N: Number of turns of loop conductor (times)
W: Length of one side of the conductor (m) (assuming a square shape)
a: Radius of conductor (m) (assuming conductor is linear)
Assuming μs: relative permeability, this inductance is defined by the following equation.

  In consideration of various wireless tag sizes applied to the actual system, an appropriate value may be substituted into this equation to determine the conductor size and the capacitor value.

For example,
L = about 40 (nH)
If a conductor structure of the size is given, if a capacitor of C = about 7 (PF) is connected,
f = 1 / 2π√LC
Therefore, resonance occurs at about 300 MHZ.

  As to how efficiently the electromagnetic wave energy in space is transmitted to and received from the antenna, the gain of the antenna increases as the loop area increases.

That is,
h: Relative gain of antenna (effective height)
A: loop area N: number of loop turns
h = (2π / λ) NA
It becomes.

  Therefore, if the loop is made larger and larger in order to increase the gain of the antenna, the inductance of the loop will also increase. Therefore, in order to cancel the large inductance and cause the loop to resonate in the 300 MHz band, the value of the capacitor must be made smaller and smaller.

  According to calculations and experiments, when copper is used as the material for the loop, which is cheap and easy to obtain and easy to process, the total length of the loop is about 20 cm (loop diameter is about 5 to 6 cm), and the value of the resonance capacitor is several PF. It is. The antenna gain in this case is about −several to −10 dB with respect to a normal half-wave dipole antenna, and a sufficiently practical gain can be obtained.

  When the loop length is increased to further increase the antenna gain, the value of the capacitor becomes 1PF or less in calculation, and even if an antenna is actually manufactured, the capacitance value of the capacitor that cannot be physically realized. End up.

  Furthermore, the theoretical limit for the loop to operate in the magnetic field mode is 0.5 wavelength or less in the entire circumference (50 cm or less in the case of 300 MHZ band), and if the length is longer than this, the current distribution on the loop conductor is not constant, Standing waves are generated.

  This standing wave inevitably changes the mode to operate as an electric field antenna, and is no longer a magnetic field loop antenna.

  On the other hand, if the value of the capacitor is made larger than about 10 PF so as to facilitate manufacture and adjustment, the loop length (loop area) becomes smaller and the antenna gain decreases.

  Moreover, the smaller the loop length, the lower the conductor impedance of the loop, and the DC resistance of the material forming the loop (in this case copper) cannot be ignored, combined with the skin effect due to the high frequency of 300 MHZ, The decrease in antenna efficiency due to conductor resistance loss becomes significant.

  In order to prevent this, it is necessary to take measures such as applying gold plating, which is a material having a low high-frequency resistance, to the surface of copper to reduce the ohmic loss due to the skin effect, resulting in a very high cost.

  Therefore, the practical size of the magnetic loop magnetic field loop antenna for the 300 MHZ band is 0.5 wavelength or less on the entire circumference, and the value of the resonance capacitor is 10 PF or less.

  According to experiments, when a dipole antenna as shown in FIG. 11 is installed under the floor as shown in FIG. 6 and communicates with a wireless tag, the detection distance in the room is reduced to only about 1 to 2 m. End up.

  Only by installing the antenna in the magnetic field loop antenna of the present embodiment under exactly the same conditions, the detection distance can be improved to about 7 to 8 m.

  The loop coil 16 and the conductor 20 (hereinafter simply referred to as a magnetic loop antenna) of the magnetic field loop antenna 15 are used in a storage case 21 as shown in FIGS. 4 (a) and 4 (b). Is preferred.

  FIG. 4 is an external view of a storage case incorporating a magnetic field loop antenna used in the 300 MHz band.

  As shown in FIGS. 4A and 4B, a connector 19 connected to the cable 4 is attached to a case 21 having a built-in magnetic field loop antenna, and jigs 23 and 24 for fixing the cable 19 to an internal wall or the like. Is attached. Thus, since the case 21 is a general-purpose wall-embedded box (AC outlet type, switch type), when mounted on an actual structure, its workability is high due to its size and structure.

  Thus, since the magnetic field loop antenna of the present embodiment is housed in a general-purpose power supply BOX-type storage case, it is very inexpensive.

  Moreover, the antenna itself can be installed in a narrow space very close to a structure made of a material that is extremely disadvantageous in terms of radio wave characteristics such as metal and concrete.

  Therefore, it is freed from the problem of selecting the antenna installation location in the wireless tag system, and it is possible to install the antenna in any place that is most convenient for the end user on a case-by-case basis, regardless of the antenna installation location.

  As a result, an inexpensive and easy-to-use system can be realized, construction on the site and adjustment work can be simplified, and an economical wireless tag system can be constructed in a short time.

  And by using the magnetic field loop antenna as described above built in the case as shown in FIG. 4, even if the distance between the wall 27 of the room 26 of the building 26 made of reinforcing steel, concrete, etc. is about 5 cm to 15 m, The magnetic field component of the radio wave from the wireless tag 5 can be detected without affecting the characteristics. For this reason, while being able to take the size of a room large, even if it attaches, aesthetics are not spoiled.

  That is, as shown in FIG. 5, even if a magnetic loop antenna is installed at a distance within one wavelength from a building or other structure, it can operate without significant deterioration from the original performance of the antenna.

  Similarly, when an electric field type antenna (dipole antenna, whip antenna, Yagi antenna, etc.) is installed within one wavelength from the structure, the electric field near the antenna is affected by the structure, and the antenna itself Due to the disturbance of the operating impedance, the original performance (gain, directivity, etc.) of the antenna was not exhibited.

  However, even when the magnetic field loop antenna of the present invention is installed at a distance within a wavelength very close to the structure in this way, it operates in the low impedance current mode and operates in the magnetic field mode. The main feature is that it is hardly affected.

  In addition, the fact that the system can be installed at a close distance of one wavelength or less from the structure allows the system to be constructed extremely compactly in practice, which is very advantageous in practice.

  In FIG. 6, the floor surface, ceiling surface, wall surface, etc. of the building are formed of metal or concrete, and the vicinity of the space is very narrow and the space surrounded by the structure of such members is This is an example in which a magnetic loop antenna is installed.

  In this way, even if a magnetic field loop antenna is installed in an extremely narrow and severe radio environment environment made of a material that absorbs radio waves, such as metal or concrete, the above-mentioned characteristics of this antenna are exhibited. In order to operate without greatly degrading the original performance of the antenna, the electromagnetic energy between the space in the building and the antenna is reduced by a slight gap between the ceiling and floor and the eddy current effect generated on the metal surface. Communication is possible.

  Therefore, when viewed from inside the building, the antenna does not appear at a position where it can be seen. Therefore, the antenna does not get in the way of visual aesthetics and activities in the building, which is extremely convenient.

  If a conventional electric field antenna is used in such a system, normal operation as an antenna will not be possible in spaces such as the back of the ceiling, under the floor, and walls. In addition, when these surfaces are made of metal or the like, there is a big problem that energy transmission / reception of electromagnetic waves between the antenna and space is not sufficiently performed due to the electric field shielding effect.

  Therefore, when the electric field antenna is installed at a position where it can operate normally, it is necessary to install it at an appropriate distance from the ceiling, floor, or wall as judged from the radio wave characteristics. In that case, it becomes an aesthetic problem, and it becomes a big problem that it interferes with various activities in the building.

  The magnetic field loop antenna of the present embodiment can construct a wireless tag system without causing any such problems.

  FIG. 7 shows a similar case in which the magnetic field loop antenna is housed in something smaller than a building, such as a locker or a storage box for various items.

  In this case, the characteristics of the magnetic field loop antenna are exhibited for the same reason as described above.

<Other embodiments>
FIG. 8 is a configuration diagram of a magnetic field loop antenna according to another embodiment. FIG. 9 is a side view when viewed from the A direction. 8 includes a copper foil loop coil 31 (50 cm or less: 20 cm) and a capacitor 30 of 10 PF or less provided on a printed circuit board 32, and a copper foil detection coil 33 (the size of which is the same as that of the loop coil 31). About 1/10) is provided on the printed circuit board 34, and is formed of a member 35 (glass epoxy) in which the distance between the printed circuit board 32 and the printed circuit board 34 is about 5 mm. A magnetic field loop antenna 28 composed of the printed board 34, the printed board 32, and the member 35 is built in a case 29. That is, the loop coil 31 and the detection coil 33 are overlapped with each other with a member 35 of about 5 mm.

  One of the detection coils 33 is connected to the core of the connector 36 (50Ω), and the other is connected to the shield side of the connector 36.

  FIG. 10 shows the relationship between frequency and SWR. As shown in FIG. 10, the SWR curve shown in FIG. 10 moves in the horizontal direction depending on the size of the loop coil L and C, and the SWR curve moves up and down in the interval between the detection coil and the loop coil.

  In the present embodiment, the deepest SWR curve can be obtained when the interval is 5 mm.

  That is, since the magnetic field loop antenna is made of the printed circuit board and the member, mass production becomes possible.

It is a schematic block diagram of the radio | wireless tag detection system using the magnetic field loop antenna of 300 MHz band of this Embodiment. It is a block diagram of the magnetic field loop antenna of 300 MHz band of this Embodiment. It is explanatory drawing explaining the relationship between the distance from the transmission source of the electromagnetic waves transmitted from an antenna, and the intensity | strength of electromagnetic waves. It is an external view of the storage case which incorporated the magnetic field loop antenna used in a 300MHZ band. It is a schematic block diagram of the radio | wireless tag detection system which installed and used the magnetic field loop antenna of this Embodiment within one wavelength from other structures. It is a schematic block diagram of the radio | wireless tag detection system which used the magnetic field loop antenna of this Embodiment installed in another structure, a floor, and a ceiling. It is a schematic block diagram of the radio | wireless tag detection system which uses the magnetic field loop antenna of this Embodiment for the locker. It is a schematic block diagram of the magnetic field loop antenna of other embodiment. It is a side view of the magnetic field loop antenna of FIG. It is explanatory drawing explaining the SWR characteristic of the magnetic field loop antenna of this Embodiment. It is a schematic block diagram of the conventional radio | wireless tag detection system (use of a dipole antenna). It is a schematic block diagram of the conventional wireless tag detection system (using Yagi antenna). It is a schematic block diagram of the conventional radio | wireless tag detection system (use of a whip antenna). It is a schematic block diagram of the conventional radio | wireless tag detection system (planar antenna use).

Explanation of symbols

4 Case 15 Magnetic loop antenna 16 Loop coil 17 Capacitor

Claims (1)

A first substrate on which a loop-shaped first copper foil having an entire circumference of 0.5 wavelength or less is attached with a capacitor of 10 PF or less;
A loop-shaped second copper foil having a diameter of 1/10 of the loop-shaped first copper foil was attached to the first substrate with a constant interval. A second substrate;
An insulating member provided with a thickness of 10 mm or less between the first substrate and the second substrate;
A case containing the first and second substrates and the insulating member;
A magnetic field loop antenna comprising: a connector attached to the case so as to be connectable to both ends of a loop-shaped second copper foil of the second substrate .

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