How to isolate a magnetic field. Magnetic shielding

Two methods are used to shield the magnetic field:

shunting method;

Screen magnetic field method.

Let's take a closer look at each of these methods.

The method of shunting the magnetic field with a screen.

The method of shunting the magnetic field with a screen is used to protect against a constant and slowly changing alternating magnetic field. Screens are made of ferromagnetic materials with high relative magnetic permeability (steel, permalloy). In the presence of a screen, the lines of magnetic induction pass mainly along its walls (Figure 8.15), which have low magnetic resistance compared to the air space inside the screen. The shielding quality depends on the magnetic permeability of the shield and the resistance of the magnetic circuit, i.e. the thicker the shield and the fewer seams, joints running across the direction of the magnetic induction lines, the shielding efficiency will be higher.

Screen displacement method.

The screen displacement method is used to shield alternating high-frequency magnetic fields. In this case, screens made of non-magnetic metals are used. Shielding is based on the phenomenon of induction. Here the phenomenon of induction is useful.

Let's put a copper cylinder on the path of a uniform alternating magnetic field (Figure 8.16, a). Variable ED will be excited in it, which, in turn, will create variable induction eddy currents (Foucault currents). The magnetic field of these currents (Figure 8.16, b) will be closed; inside the cylinder, it will be directed towards the exciting field, and outside it, in the same direction as the exciting field. The resulting field (Figure 8.16, c) is weakened near the cylinder and strengthened outside it, i.e. there is a displacement of the field from the space occupied by the cylinder, which is its screening effect, which will be the more effective, the lower the electrical resistance of the cylinder, i.e. the more eddy currents flowing through it.

Due to the surface effect ("skin effect"), the density of eddy currents and the intensity of the alternating magnetic field, as they go deeper into the metal, fall according to an exponential law

, (8.5)

where (8.6)

- an indicator of the decrease in the field and current, which is called equivalent penetration depth.

Here, is the relative magnetic permeability of the material;

– vacuum magnetic permeability equal to 1.25*10 8 gn*cm -1 ;

– resistivity of the material, Ohm*cm;

- frequency Hz.

It is convenient to characterize the screening effect of eddy currents by the value of the equivalent penetration depth. The smaller x 0 , the greater the magnetic field they create, which displaces the external field of the pickup source from the space occupied by the screen.

For a non-magnetic material in formula (8.6) =1, the screening effect is determined only by and . And if the screen is made of ferromagnetic material?

If equal, the effect will be better, since >1 (50..100) and x 0 will be less.

So, x 0 is a criterion for the screening effect of eddy currents. It is of interest to estimate how many times the current density and magnetic field strength become smaller at a depth x 0 compared to that at the surface. To do this, we substitute x \u003d x 0 into formula (8.5), then

whence it can be seen that at a depth x 0 the current density and the magnetic field strength decrease by a factor of e, i.e. up to a value of 1/2.72, which is 0.37 of the density and tension on the surface. Since the field weakening is only 2.72 times at depth x 0 not enough to characterize the shielding material, then two more values ​​of the penetration depth x 0.1 and x 0.01 are used, characterizing the drop in current density and field voltage by 10 and 100 times from their values ​​on the surface.

We express the values ​​x 0.1 and x 0.01 through the value x 0, for this, on the basis of expression (8.5), we compose the equation

And ,

deciding which we get

x 0.1 \u003d x 0 ln10 \u003d 2.3x 0; (8.7)

x 0.01 = x 0 ln100=4.6x 0

Based on formulas (8.6) and (8.7) for various shielding materials, the values ​​of penetration depths are given in the literature. For the sake of clarity, we present the same data in the form of Table 8.1.

The table shows that for all high frequencies, starting from the medium wave range, a screen made of any metal with a thickness of 0.5..1.5 mm acts very effectively. When choosing the thickness and material of the screen, one should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, rigidity, resistance to corrosion, ease of joining individual parts and the implementation of transitional contacts between them with low resistance, ease of soldering, welding, etc.

It follows from the data in the table that for frequencies greater than 10 MHz, a film of copper, and even more so of silver, with a thickness of less than 0.1 mm gives a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil getinaks or other insulating material coated with copper or silver.

Steel can be used as screens, but you need to remember that due to the high resistivity and the hysteresis phenomenon, a steel screen can introduce significant losses into the screening circuits.

Shielding of magnetic fields can be done in two ways:

Shielding with ferromagnetic materials.

Shielding with eddy currents.

The first method is usually used for screening constant MF and low frequency fields. The second method provides significant efficiency in shielding high frequency MF. Due to the surface effect, the density of eddy currents and the intensity of the alternating magnetic field, as they go deeper into the metal, fall according to an exponential law:

A measure of the reduction in field and current, which is called the equivalent penetration depth.

The smaller the penetration depth, the greater the current flows in the surface layers of the screen, the greater the reverse MF created by it, which displaces the external field of the pickup source from the space occupied by the screen. If the shield is made of a non-magnetic material, then the shielding effect will depend only on the specific conductivity of the material and the frequency of the shielding field. If the screen is made of a ferromagnetic material, then, ceteris paribus, a large e will be induced in it by an external field. d.s. due to the greater concentration of magnetic field lines. With the same conductivity of the material, eddy currents will increase, resulting in a smaller penetration depth and a better shielding effect.

When choosing the thickness and material of the screen, one should proceed not from the electrical properties of the material, but be guided by considerations of mechanical strength, weight, rigidity, resistance to corrosion, ease of joining individual parts and making transitional contacts between them with low resistance, ease of soldering, welding, and so on.

It can be seen from the data in the table that for frequencies above 10 MHz, copper and even more so silver films with a thickness of about 0.1 mm give a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil-coated getinax or fiberglass. At high frequencies, steel gives a greater shielding effect than non-magnetic metals. However, it should be borne in mind that such screens can introduce significant losses into the shielded circuits due to high resistivity and hysteresis. Therefore, such screens are applicable only in cases where insertion loss can be ignored. Also, for greater shielding efficiency, the screen must have less magnetic resistance than air, then the magnetic field lines tend to pass along the walls of the screen and penetrate into the space outside the screen in a smaller number. Such a screen is equally suitable for protection against the effects of a magnetic field and for protecting the external space from the influence of a magnetic field created by a source inside the screen.

There are many grades of steel and permalloy with different values ​​of magnetic permeability, so for each material it is necessary to calculate the value of the penetration depth. The calculation is made according to the approximate equation:

1) Protection against external magnetic field

The magnetic lines of force of the external magnetic field (the lines of induction of the magnetic interference field) will pass mainly through the thickness of the walls of the screen, which has a low magnetic resistance compared to the resistance of the space inside the screen. As a result, the external magnetic interference field will not affect the operation of the electrical circuit.

2) Shielding of own magnetic field

Such craneing is used if the task is to protect external electrical circuits from the effects of a magnetic field created by the coil current. Inductance L, i.e., when it is required to practically localize the interference created by the inductance L, then such a problem is solved using a magnetic screen, as shown schematically in the figure. Here, almost all field lines of the field of the inductor will be closed through the thickness of the screen walls, without going beyond them due to the fact that the magnetic resistance of the screen is much less than the resistance of the surrounding space.

3) Dual screen

In a double magnetic screen, one can imagine that part of the magnetic lines of force, which go beyond the thickness of the walls of one screen, will close through the thickness of the walls of the second screen. In the same way, one can imagine the action of a double magnetic screen when localizing magnetic interference created by an electrical circuit element located inside the first (inner) screen: the bulk of the magnetic field lines (magnetic stray lines) will close through the walls of the outer screen. Of course, in double screens, the wall thicknesses and the distance between them must be rationally chosen.

The overall shielding coefficient reaches its greatest value in cases where the wall thickness and the gap between the screens increase in proportion to the distance from the center of the screen, and the gap is the geometric mean of the wall thicknesses of the screens adjacent to it. In this case, the shielding factor:

L = 20lg (H/Ne)

The manufacture of double screens in accordance with this recommendation is practically difficult for technological reasons. It is much more expedient to choose the distance between the shells adjacent to the air gap of the screens, greater than the thickness of the first screen, approximately equal to the distance between the steak of the first screen and the edge of the shielded circuit element (for example, coils and inductors). The choice of one or another wall thickness of the magnetic screen cannot be made unambiguous. Rational wall thickness is determined. shield material, interference frequency and specified shielding factor. It is useful to take into account the following.

1. With an increase in the frequency of interference (frequency of an alternating magnetic field of interference), the magnetic permeability of materials decreases and causes a decrease in the shielding properties of these materials, since as the magnetic permeability decreases, the resistance to magnetic flux exerted by the screen increases. As a rule, the decrease in magnetic permeability with increasing frequency is most intense for those magnetic materials that have the highest initial magnetic permeability. For example, sheet electrical steel with a low initial magnetic permeability changes the value of jx little with increasing frequency, and permalloy, which has large initial values ​​of the magnetic permeability, is very sensitive to an increase in the frequency of the magnetic field; its magnetic permeability drops sharply with frequency.

2. In magnetic materials exposed to a high-frequency interference magnetic field, the surface effect is noticeably manifested, i.e., the displacement of the magnetic flux to the surface of the screen walls, causing an increase in the magnetic resistance of the screen. Under such conditions, it seems almost useless to increase the thickness of the screen walls beyond the limits occupied by the magnetic flux at a given frequency. Such a conclusion is incorrect, because an increase in the wall thickness leads to a decrease in the magnetic resistance of the screen even in the presence of a surface effect. At the same time, the change in magnetic permeability should also be taken into account. Since the phenomenon of the skin effect in magnetic materials usually becomes more noticeable than the decrease in magnetic permeability in the low-frequency region, the influence of both factors on the choice of screen wall thickness will be different in different ranges of magnetic interference frequencies. As a rule, the decrease in shielding properties with increasing interference frequency is more pronounced in shields made of materials with a high initial magnetic permeability. The above features of magnetic materials provide the basis for recommendations on the choice of materials and wall thicknesses of magnetic screens. These recommendations can be summarized as follows:

A) screens made of ordinary electrical (transformer) steel, which have a low initial magnetic permeability, can be used, if necessary, to provide small screening coefficients (Ke 10); such screens provide an almost constant screening factor in a fairly wide frequency band, up to several tens of kilohertz; the thickness of such screens depends on the frequency of interference, and the lower the frequency, the greater the thickness of the screen required; for example, at a frequency of a magnetic interference field of 50-100 Hz, the thickness of the screen walls should be approximately equal to 2 mm; if an increase in the shielding factor or a greater thickness of the shield is required, then it is advisable to use several shielding layers (double or triple shields) of smaller thickness;

B) it is advisable to use screens made of magnetic materials with high initial permeability (for example, permalloy) if it is necessary to provide a large screening factor (Ke > 10) in a relatively narrow frequency band, and it is not advisable to choose a thickness of each magnetic screen shell greater than 0.3-0.4 mm; the shielding effect of such screens begins to drop noticeably at frequencies above several hundred or thousand hertz, depending on the initial permeability of these materials.

Everything said above about magnetic shields is true for weak magnetic interference fields. If the shield is located close to powerful sources of interference and magnetic fluxes with high magnetic induction arise in it, then, as is known, it is necessary to take into account the change in magnetic dynamic permeability depending on the induction; it is also necessary to take into account the losses in the thickness of the screen. In practice, such strong sources of magnetic interference fields, in which one would have to reckon with their effect on screens, are not encountered, with the exception of some special cases that do not provide for amateur radio practice and normal operating conditions for radio engineering devices of wide application.

Test

1. With magnetic shielding, the shield must:
1) Possess less magnetic resistance than air
2) have magnetic resistance equal to air
3) have greater magnetic resistance than air

2. When shielding the magnetic field Grounding the shield:
1) Does not affect shielding efficiency
2) Increases the effectiveness of magnetic shielding
3) Reduces the effectiveness of magnetic shielding

3. At low frequencies (<100кГц) эффективность магнитного экранирования зависит от:
a) Shield thickness, b) Magnetic permeability of the material, c) Distance between the shield and other magnetic circuits.
1) Only a and b are true
2) Only b and c are true
3) Only a and b are true
4) All options are correct

4. Magnetic shielding at low frequencies uses:
1) Copper
2) Aluminum
3) Permalloy.

5. Magnetic shielding at high frequencies uses:
1) Iron
2) Permalloy
3) Copper

6. At high frequencies (>100 kHz), the effectiveness of magnetic shielding does not depend on:
1) Screen thickness

2) Magnetic permeability of the material
3) Distances between the screen and other magnetic circuits.

Used literature:

2. Semenenko, V. A. Information security / V. A. Semenenko - Moscow, 2008.

3. Yarochkin, V. I. Information security / V. I. Yarochkin - Moscow, 2000.

4. Demirchan, K. S. Theoretical Foundations of Electrical Engineering Volume III / K. S. Demirchan S.-P, 2003.

Magnetic field shielding.

shunt method. - Magnetic field screen method.

Magnetic field shunting method It is applied to protection against the constant and slowly changing alternating magnetic field. Screens are made of ferromagnetic materials with high relative magnetic permeability (steel, permalloy). In the presence of a screen, the lines of magnetic induction pass mainly along its walls, which have low magnetic resistance compared to the air space inside the screen. The thicker the screen and the fewer seams, joints, the more effective the shielding. Screen displacement method used for shielding alternating high-frequency magnetic fields. In this case, screens made of non-magnetic metals are used. Shielding is based on the phenomenon of induction.

If you put a copper cylinder in the path of an equally variable magnetic mole, in which alternating eddy induction currents (Foucault currents) are excited. The magnetic field of these currents will be closed; inside the cylinder, it will be directed towards the exciting field, and outside it, in the same direction as the exciting field. The resulting field is weakened near the cylinder and strengthened outside it, i.e. there is a displacement of the field from the space occupied by the cylinder, which is its screening effect, which will be the more effective, the lower the electrical resistance of the cylinder, i.e. the more eddy currents flowing through it.

Due to the surface effect ("skin effect"), the density of eddy currents and the intensity of the alternating magnetic field, as they go deeper into the metal, fall according to an exponential law

Where

μ is the relative magnetic permeability of the material; μ˳ – vacuum magnetic permeability equal to 1.25*108 h*cm-1; ρ is the resistivity of the material, Ohm*cm; ƒ – frequency, Hz.

For a non-magnetic material, μ = 1. And the shielding effect is determined only by ƒ and ρ.

Shielding is an active method of protecting information. Shielding of the magnetic field (magnetostatic shielding) is used when it is necessary to suppress pickups at low frequencies from 0 to 3..10 kHz. The efficiency of magnetostatic shielding increases with the use of multilayer shields.

The effectiveness of magnetic shielding depends on the frequency and electrical properties of the shield material. The lower the frequency, the weaker the screen acts, the thicker it has to be made to achieve the same screening effect. For high frequencies, starting from the medium wave range, a screen made of any metal with a thickness of 0.5 ... 1.5 mm is very effective. When choosing the thickness and material of the screen, one should take into account the mechanical strength, rigidity, resistance to corrosion, the convenience of joining individual parts and making transitional contacts with low resistance between them, the convenience of soldering, welding, etc. For frequencies above 10 MHz, copper and even more so silver film is thick more than 0.1 mm gives a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil getinaks or other insulating material coated with copper or silver. For the manufacture of screens, the following are used: metal materials, dielectric materials, glasses with a conductive coating, special metallized fabrics, conductive paints. Metallic materials (steel, copper, aluminium, zinc, brass) used for shielding are made in the form of sheets, meshes and foils.

All of these materials meet the requirement for corrosion resistance when used with appropriate protective coatings. The most technologically advanced are the designs of steel screens, since welding or soldering can be widely used in their manufacture and installation. Metal sheets must be electrically connected to each other around the entire perimeter. The seam of electric welding or soldering must be continuous in order to obtain an all-welded shield construction. The thickness of the steel is selected based on the purpose of the screen design and the conditions of its assembly, as well as on the possibility of providing continuous welds during manufacture. Steel screens provide attenuation of electromagnetic radiation by more than 100 dB. Mesh screens are easier to manufacture, easy to assemble and operate. To protect against corrosion, it is advisable to cover the mesh with an anti-corrosion varnish. The disadvantages of mesh screens include low mechanical strength and lower shielding efficiency compared to sheet ones. For mesh screens, any seam design is suitable that provides good electrical contact between adjacent mesh panels at least every 10-15 mm. Soldering or spot welding can be used for this purpose. A screen made of tinned low-carbon steel mesh with a cell of 2.5-3 mm gives an attenuation of about 55-60 dB, and from the same double (with a distance between the outer and inner meshes of 100 mm) about 90 dB. The screen, made of a single copper mesh with a cell of 2.5 mm, has an attenuation of the order of 65-70 dB