JP2011222761A - Helmholtz coil type magnetic field generator using shielded loop - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field generator that can generate a magnetic field having the strength in agreement with a calculated value based on a current value by reducing a leak of an electromagnetic field from a feeder cable and a feeding end.SOLUTION: A Helmholtz coil type magnetic field generator with a shielded loop comprises: a coil part with two circular shielded loops having the same diameter arranged in the same manner as a Helmholtz coil structure, a feeding system for the coil part, and a coaxial feeder line for use in feeding. In-phase feeding is provided if the shielded loops are directed in the same direction as each other, and anti-phase feeding is provided if they are opposite to each other. The feeding is provided through either a unidirectional coupler or a bidirectional coupler, and feeding power and reflected wave power are measured using a branch output.

Description

  This invention has a large volume in a constant electromagnetic field strength region, mainly in the long wave to short wave band region, and depending on miniaturization, it is used for calibration of various devices and irradiation of a uniform magnetic field to a device under test. The present invention relates to a magnetic field generator that can be used and has no leakage electromagnetic field from a feeder line, and more particularly to a Helmholtz coil type magnetic field generator using a shielded loop.

  In general, calibration of a magnetic field measuring device is performed using a measurement value in a standard magnetic field. For this reason, there is a need for a magnetic field generator that can generate a magnetic field having a large volume in a constant electromagnetic field strength region and a strength that matches the theoretical value. As an example, Helmholtz coils are known in the frequency band from direct current to 100 kHz. The Helmholtz coil is composed of two coils having the same diameter and is arranged so that the distance between the coils is equal to the radius. Usually, the coil portion of the Helmholtz coil is a wire wound, and as will be described later, the magnetic field strength generated in the space between the two coils can be determined by measuring the current value flowing through the wire.

  However, as the frequency increases, the impedance increases and it becomes difficult to pass a current through the conductor. In addition, when a high-frequency current is passed through a coil made of a conductive wire, the current cannot be measured accurately. The present invention solves such a problem, and can accurately generate a magnetic field having a higher frequency than a conventional Helmholtz coil.

  As described in Patent Document 1 (Japanese Patent Laid-Open No. 2006-184264), Helmholtz coils are used in magnetic field generators in a range from DC to a frequency of about 100 kHz. As shown in FIG. 8, this consists of a structure in which two coils of diameter 2a wound with a conducting wire are arranged so that the planes that the coils make are parallel. When the distance between the two coils is d = a, N is the number of turns of the coil, and I is the current value flowing through the coil, the magnetic field Hz generated at the center (reference point) between the coils is given by the following equation.

  The Helmholtz coil can create a uniform magnetic field strength distribution in the plane S including the reference point between the two coils, and its strength can be easily obtained by using Equation 1, and an electric field is generated at the reference point. There is a feature that does not.

  However, in the Helmholtz coil, as the frequency increases, the reactance component of the coil impedance increases, so that it becomes difficult for current to flow. In order to flow a large current to such a load with a large impedance, there is a problem that a voltage must be increased, and as a result, a huge power supply device is required.

  In addition, since the coil is wound with a conducting wire, handling as a distributed constant circuit becomes difficult when the frequency is high, and so-called common mode current flows between the floor and the conducting wire. In other words, there is a problem that the generated magnetic field strength cannot be accurately obtained using Equation 1.

JP 2006-184264 A

  As described above, in the Helmholtz coil, as the frequency increases, the reactance component of the coil impedance increases, so that the current does not easily flow. In order to flow a large current to such a load with a large impedance, there is a problem that a voltage must be increased, and as a result, a huge power supply device is required.

  In addition, since the coil is wound with a conducting wire, handling as a distributed constant circuit becomes difficult when the frequency is high, and so-called common mode current flows between the floor and the conducting wire. In other words, there is a problem that the generated magnetic field strength cannot be accurately obtained using Equation 1.

  When a high-frequency current is passed through the Helmholtz coil, balanced power feeding is performed. However, an electromagnetic field is likely to leak at the connection portion of the power feeding cable, particularly with the coil. Therefore, the leakage electromagnetic field can be eliminated by using the coaxial cable, but as is well known, the power supply using the coaxial cable is a non-balanced power supply, and a balun (balanced-unbalanced converter) is used. It is necessary to convert to balanced power supply. The balun is generally housed in a metal case to complete the shield and is placed very close to the Helmholtz coil, so that the electromagnetic field may be disturbed by the balun. For this reason, the structure which does not need to use a balun is calculated | required.

  The Helmholtz coil type magnetic field generator according to the present invention includes a coil portion in which two circular sealed dead loops having the same diameter are arranged in the same manner as the configuration of the Helmholtz coil, a power feeding system that feeds power to the coil portion, and the coil from the power feeding portion to the coil A coaxial line for supplying power to the power source.

  The coaxial line portion of the shielded loop, the shield portion of the coaxial line, and the central conductor portion are each electrically continuous. This is possible by using a shielded loop, and it is possible to suppress common mode current and leakage electromagnetic fields from the feed line and the feed portion.

  Half of the number of turns of the sealed dead loop is a shield part of the coaxial line, and the other half is a conductor part connected to the central conductor part. Therefore, since the shielded loop is not symmetric with respect to the straight line passing through the feeding portion, using this provides a choice in arrangement. As an option, the two circular sealed dead loops may be arranged to have translational symmetry in the magnetic field direction at the center.

  In this case, the distributor that feeds the two circular shielded loops feeds alternating currents of the same phase. In this case, a power splitter having a simple structure can be used as the distributor.

  In addition to the above arrangement, for the two circular sealed dead loops, one of the two circular sealed dead loops is rotated halfway around the line connecting the connector portion and the power feeding portion, thereby the magnetic field at the center. It can be set as the arrangement which has translational symmetry in a direction. That is, each shield part and the center conductor part can be arranged to face each other.

  In this case, the distributor that supplies power to the two circular shielded loops supplies alternating currents of opposite phases. The distributor in this case can use a 180 ° hybrid circuit with a simple structure. As a result, a part of the magnetic field due to the leakage magnetic field or the leakage current or common mode current can be canceled.

In measuring the power supplied to the coil unit, particularly when supplying a large amount of power, the coil unit is fed through a unidirectional coupler or a bidirectional coupler, and the unidirectional coupler or A part of the power supply to the coil part is taken out by a bidirectional coupler and the electric power is measured. Further, particularly in the case of a bidirectional coupler, a reflected signal from the shielded loop can be further output, so that the reflected power intensity can be measured as necessary. The current flowing through the shielded loop can be determined from the obtained electric power, and the strength of the generated magnetic field can be known. In general, to determine the current flowing through the shielded loop,
1) Insert an ammeter in series on the wiring.
2) Insert a resistor (resistance value is known) in series on the wiring, and measure and determine the voltage generated at both ends of the resistor.
3) Measure the current by covering the wiring with a current probe (current probe).
Such a method can be adopted. In this case, the current flowing through the position where the ammeter is inserted is equal to the current flowing through the loop coil. However, it is known that these current values deviate as the frequency increases. Also,
4) Insert a thermocouple into the loop coil,
However, as the frequency increases, the influence on the thermocouple measurement cannot be ignored.

  In addition, a display means for displaying the magnetic field intensity calculated using the measured power supply power and the previously measured S parameter is provided. With this method, there is no problem even if the current value flowing through the directional coupler differs from the current value flowing through the loop coil. Further, the measurement can be performed at a location away from the loop coil, and the distribution of the generated magnetic field is not disturbed, and it is possible to cope with higher frequencies.

  It is possible to realize a magnetic field generator that can generate a magnetic field having a strength that matches the calculated value from the current value by reducing leakage of the electromagnetic field from the power supply cable and the power supply end.

It is a figure which shows the example of a Helmholtz coil type magnetic field generator using the shielded loop by this invention. It is a figure which shows the detail of a shield dead loop. It is a figure which shows the example which attaches a thermocouple to the electric power feeding part of a shield dead loop, and performs an electric current measurement. It is a figure which shows arrangement | positioning of two circular shield dead loops. It is a figure which shows distribution of the magnetic field in the case of a shield dead loop single-piece | unit, and the case of the magnetic field generator by this invention. It is a graph which shows the relationship of magnetic field intensity distribution with respect to the distance from a center. It is a graph which shows the relationship between the direction of the magnetic field with respect to the distance from a center, and the angle which a Z-axis makes. It is a figure which shows the normal magnetic field generator using a Helmholtz coil. It is a figure which shows the example of the shield dead loop which can be used for this invention.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  FIG. 1 shows an example of a Helmholtz coil type magnetic field generator using a shielded loop according to the present invention. This is because a coil part 100 in which two circular sealed dead loops 1 and 2 having the same diameter are arranged in the same manner as the Helmholtz coil configuration, a power feeding system 200 that feeds power to the coil part, and power feeding from the power feeding part to the coil. For this purpose, coaxial lines 3 and 4 are provided.

  This example uses a coaxial line to enable generation of a high-power uniform magnetic field in a high-frequency region, and can be used as a standard magnetic field generator. As a feature, first, by making a structure similar to the Helmholtz coil, it was possible to generate a uniform plane magnetic field. The two coils 1 and 2 constituting the Helmholtz coil each have a sealed dead loop structure known as a shielded loop antenna, and the two coils are fed from the feeding system 200. Was performed on the coaxial lines 3 and 4 by using a distributor 5 that bisects. As a result, it is possible to cope with high frequencies.

  Details of the sealed dead loop 1 or 2 are shown in FIG. FIG. 2 (a) shows only one (radius a) of two sealed dead loops having 1 winding. In the connection between the connector 12 and the coaxial line portion 11, the shields and the core wires of the connector 12 and the coaxial line portion 11 are electrically continuous. Connection between the coaxial line portion 11 and the conductive wire portion 10 is performed by the power feeding portion 13. In the power supply unit 13, the core wire of the coaxial line portion 11 and the conductor portion 10 are connected, and this core wire is equivalent to an extension of the shield diameter. Moreover, the conducting wire portion 10 is connected to the shield of the connector, and the loop is completed. The lengths of the coaxial line portion 11 and the conductor portion 10 are assumed to be equal. The diameter of the shield of the coaxial line portion 11 and the diameter of the conductor portion 10 are the same. If conductors of different lengths and diameters are used, the magnitude of the current and the amount of phase change flowing through the shield of the coaxial line portion 11 and the conducting wire portion 10 should be equalized by attaching a dielectric sheath to the outside, for example. Is desirable.

  The shielded loop 1 or 2 supplies power using a coaxial line. The power input from the connector 12 is fed by the zenith feeder 13 and the current I flows through the outer surface of the coaxial line portion 11 and the surface of the conductor, which is the same as in the case of a so-called shielded loop antenna. As shown in FIG. 2B, in the case of multiple turns, the number of turns of the coaxial line portion 11 and the number of turns of the conductor portion 10 are the same. In FIG. 2 (b), the winding method is such that the radius is not fixed, but it is desirable to have an orderly winding structure.

  In FIG. 1, the power output from the signal generator 8 is amplified to a desired power by the amplifier 7 and enters the port 1 of the directional coupler 6. In the directional (or bidirectional) coupler 6, some of the signal enters the wattmeter 9 attached to the port 2, but most of the power reaches the distributor 5. In the distributor 5, the electric power is equally divided into two and output from the ports 3 and 4. The output power enters the coil and reaches the power feeding unit. A current flows through the coil due to the reached electric power and a magnetic field is generated.

  As described above, the loop coil 1 or 2 having the shielded loop structure may be wound multiple times. If the number of turns is N (an integer greater than or equal to 1), the generated magnetic field intensity at the same current is N times that of one turn (FIG. 2). However, the number of turns in each of the coaxial line portion and the conductor portion must be the same, and it is necessary to prevent short-circuiting by providing an insulator coating. It is clear that the amount of current for generating the same magnetic field can be reduced to 1 / N times.

  As shown in FIG. 3, it is possible to measure the current directly by attaching a thermocouple to the feeding part of the shielded loop, but the thermocouple does not operate correctly in the high frequency region, and a large current flows. It will break. Therefore, when a magnetic field is generated in a high frequency region and at a high output, the current is indirectly measured using a directional (or bidirectional) coupler 6 and a wattmeter 9 as shown in FIG. To. As the directional (or bidirectional) coupler 6, a unidirectional coupler that outputs feed power or a bidirectional coupler that can output feed power and reflected wave power, respectively, is used. As a result, a large current can flow, and a strong magnetic field can be generated.

FIG. 9 shows another example of a shielded loop that can be used in the present invention.
FIG. 9A shows a structure in which the coaxial line portions are arranged symmetrically, and the feeding portion has a structure in which the shield portion of the coaxial line structure is removed. The two coaxial line portions are fed through a 180-degree hybrid fed by the coaxial line. The 180-degree hybrid is connected to the shielded loop of FIG. 9A by a coaxial line. The coaxial line for connection is desirably provided with a sufficient distance so that the 180-degree hybrid sufficiently suppresses the influence on the magnetic field at the center of the coil.
In FIG. 9B, the contact point between the coaxial line portion and the conductive wire portion serves as a feeding point, but two feeding points are arranged symmetrically. The 180-degree hybrid is connected through the coaxial line and the 180-degree hybrid at the midpoint of the coaxial line portion. Also in this case, it is desirable that the coaxial line for connection is provided with a sufficient distance so that the 180-degree hybrid sufficiently suppresses the influence on the magnetic field at the center of the coil.
FIG. 9C shows a configuration in which the 180-degree hybrid can be removed from the configuration of FIG. 9A, and one end that is not one feeding point of the two coaxial line portions is terminated with a non-reflective end.

  In the configuration of FIG. 1, the following formula 2 is used to obtain the generated magnetic field at the reference point from the current value.

Here, S _31, S _21, the port 3 from the port 1, respectively, transmission S parameter from port 1 to port 2, gamma ₃ is the reflection coefficient of the shielded loop, Z ₀ is the characteristic impedance of the coaxial line, P _M is This is the power measured by the power meter connected to port 2. Measurement may be performed using a spectrum analyzer or the like instead of the wattmeter. Even if the frequency is continuously changed, a magnetic field having a known strength can be generated.

In the case of an ideal system shown in FIG. 1, a magnetic field having the intensity given by Equation 2 is generated for the power P _M measured by the wattmeter. There are imperfections, such as losses occurring inside the coupler. Now, consider the above system as a 4-port circuit from port 1 to port 4. Since this system has a coaxial connector at each port, it is possible to measure S-parameters using a measuring instrument such as a vector network analyzer. Further, since the coil is also provided with a connector for connecting to port 3 to port 4, the reflection coefficient can be measured. When the S parameter obtained by these measurements is used, the above formula 2 in an actual system becomes the following formula 3.

However, the following abbreviations were used.

(1) When a 180 ° hybrid circuit is used as a distributor Here, a case where a 180 ° hybrid circuit is used as a distributor is considered. In this case, as shown in FIG. 4B, the two circular sealed dead loops 1 and 2 are rotated halfway around the line A connecting the connector portion 12 and the power feeding portion 13 of the circular sealed dead loop. The arrangement has translational symmetry in the magnetic field direction at the center. When a hybrid circuit having no problem with balance is used, the S parameter can be expressed as follows.

このとき、数４、数５は以下の様に書き直すことができる。

At this time, Equations 4 and 5 can be rewritten as follows.

Here, assuming that the reflections at the ports 3 and 4 of the hybrid circuit are equal (that is, S ₃₃ = S ₄₄ ) and the reflection coefficient of the coil is equal (that is, Γ ₃ = Γ ₄ ), the approximation can be made as follows.

Approximately, A ≒ 1, D ≒ 0.1 , When a gamma ₂ = 0.01, in the end, becomes as follows. The S parameter used here is a value measured in advance.

ただし、Ｍは不確かさの要因で以下に表す値である。

However, M is a value expressed below as a factor of uncertainty.

  Eventually, by using Equations 11 and 12, the current value is estimated from the value measured by the wattmeter connected to port 2, and the magnetic field strength at the reference point can be determined. Although this determination is not shown in the figure, it is performed by a PC (personal computer), and the determined magnetic field strength value is also displayed on the display screen of the PC.

(2) When a power splitter is used as a distributor Consider a case where a power splitter is used as a distributor. In this case, as shown in FIG. 4A, the two circular shielded loops 1 and 2 are arranged to have translational symmetry. In this case, the following relationship is established.

このとき、数４、数５は、次の様に書き直すことができる。

At this time, Equations 4 and 5 can be rewritten as follows.

Here, assuming that A≈1, D≈0.1, and Γ ₂ = 0.01, the following is finally obtained. The S parameter used here is a value measured in advance.

However, M is a value expressed below as a factor of uncertainty.

  Eventually, by using the equations (18) and (19), the current is estimated from the value measured by the wattmeter connected to the port 2, and the magnetic field strength at the reference point can be determined. Although this determination is also not shown in the figure, it is performed by a PC (personal computer), and the determined magnetic field strength value is also displayed on the display screen of the PC.

A specific explanation will be given using the results obtained by numerical simulation.
FIG. 5 shows the distribution of the magnetic field generated by the sealed dead loop having a diameter of 42 cm and the magnetic field generator according to the present invention. FIG. 5A shows the distribution of the magnetic field created at a location 10.5 cm away from the plane created by the loop in the case of a shielded loop alone. FIG. 5 (b) shows the distribution of the magnetic field produced on the central plane separated by 10.5 cm from the two sealed dead loops by the magnetic field generator according to the present invention. It changes from black to ash from a strong place to a weak place. In the case of FIG. 5A, it can be seen that the magnetic field on the central axis (Z-axis) is weaker than the surrounding magnetic field. In the case of FIG. 5B, the tendency is not seen. Moreover, the arrow shown in the figure has shown the direction of the magnetic field. In the case of FIG. 7B, there is only a magnetic field in the Z-axis direction, but in FIG. 5A, a magnetic field is also generated in the radial direction of the shielded loop, and as a result, in a direction inclined from the Z-axis. It can be seen that the magnetic field is suitable. FIG. 6 shows a graph of the magnetic field strength distribution with respect to the distance from the center. FIG. 7 shows a graph of the direction of the magnetic field and the angle formed by the Z axis with respect to the distance from the center. From this figure, it can be seen that the magnetic field has a directional component other than the Z-axis direction as the distance from the Z-axis increases.

  Conventionally, a relatively high frequency magnetic field is generated using a single sealed dead loop. However, as shown in FIGS. 6 and 7, the magnetic field has the same radial direction (radial direction of the loop) in addition to the z-direction component. With ingredients. On the other hand, the present invention has an advantage that the magnetic field is not generated except for the z-direction component as described above.

  Further, it can be seen from FIGS. 6 and 7 that the magnetic field generated by the Helmholtz coil type magnetic field generator using the shielded loop of the present invention can increase the volume of the region where the magnetic field is constant. For this reason, it is possible to easily place the entire sample having a relatively large volume in a constant magnetic field environment. Because of this characteristic, for example, by applying the present invention to calibration of a three-dimensional magnetic field sensor or the like, it becomes possible to perform calibration more accurately than in the conventional case.

  The present invention can be used as a standard magnetic field generator capable of generating a magnetic field as a theoretical value in a relatively high frequency region. Such a standard magnetic field generator is necessary when investigating the influence of electromagnetic fields on living organisms on relatively high frequency magnetic fields and the influence on communication devices such as mobile phones. In addition, a standard magnetic field generator capable of generating a magnetic field having a known intensity is indispensable for calibrating a probe for measuring a magnetic field. Calibration can be performed in the frequency band.

DESCRIPTION OF SYMBOLS 1, 2 Shield dead loop 3, 4 Coaxial line 5 Divider 6 Directional (or bidirectional) coupler 7 Amplifier 8 Signal generator 9 Wattmeter 10 Conductor part 11 Coaxial line part 12 Connector 13 Feed part 100 Coil part 200 Power supply system

Claims (8)

A coil portion in which two circular sealed dead loops having the same diameter are arranged in the same manner as the Helmholtz coil;
A power feeding system for feeding power to the coil section;
A coaxial feeder for feeding the coil from the feeder,
A Helmholtz coil type magnetic field generator using a sealed dead loop.   The Helmholtz using a sealed dead loop according to claim 1, wherein the coaxial line portion of the sealed dead loop, the shield portion of the coaxial feed line, and the central conductor portion are electrically continuous. Coil type magnetic field generator.   The Helmholtz coil type using a sealed dead loop according to any one of claims 1 and 2, wherein the two circular sealed dead loops are arranged to have translational symmetry in the magnetic field direction at the center thereof. Magnetic field generator.   4. The Helmholtz coil type magnetic field generator using a sealed dead loop according to claim 3, wherein the feeding system includes a distributor for feeding alternating currents having the same phase to the two circular sealed dead loops.   The two circular sealed dead loops are arranged to have translational symmetry in the magnetic field direction at the center by half-rotating around a line connecting the connector part and the power feeding part of the circular sealed dead loop. A Helmholtz coil type magnetic field generator using the sealed dead loop according to any one of claims 1 and 2.   6. The Helmholtz coil type magnetic field generator using a sealed dead loop according to claim 5, wherein a distributor for feeding alternating currents of opposite phases to the two circular sealed dead loops is provided in the feeding system.   Power is supplied to the coil unit from the power supply system through a unidirectional coupler or a bidirectional coupler, and power supplied to the coil unit is supplied using an output from the unidirectional coupler or bidirectional coupler. The Helmholtz coil type magnetic field generator using the shielded loop according to any one of claims 1 to 6, wherein the measurement is performed.   8. The Helmholtz coil type magnetic field generator using a sealed dead loop according to claim 7, further comprising display means for displaying a magnetic field intensity calculated using the measured feeding power and a previously measured S parameter.

Images