Shielding of electromagnetic waves.

Shielding of electromagnetic waves.

Shielding of electromagnetic waves

Shielding of electromagnetic waves is the basis of environmental safety and one of the most effective means of protecting an object from information leakage through technical channels. In the absence of the necessary literature on the issue under consideration, this article and the recommendations set forth in it will provide practical assistance to entities of various forms of ownership and employees of special units.

Industrial espionage sooner or later forces an entrepreneur to study the aspects of protecting commercial secrets. The pace of development of market relations in the country turns the issue of protection from industrial espionage into a complex problem for an entrepreneur, to solve which he is often not ready.

Based on generally accepted formulations, the concept of «protection of commercial secrets» can be defined as a set of organizational and technical measures carried out by an entrepreneur in order to prevent theft, deliberate transfer, destruction and unauthorized access to information or leakage of data to a competitor. The problem of protecting commercial secrets is closely linked to such concepts as «information leakage», «leak source», «leak channel», «blocking the leak channel».

In the modern world, along with rapidly developing technology, the problem of forming an electromagnetic environment that ensures the normal functioning of electronic devices and environmental safety is becoming increasingly acute. The electromagnetic environment is a set of electromagnetic fields in a given area of ​​space that can affect the functioning of a specific electronic device or biological object.

To create a favorable electromagnetic environment and to ensure the requirements for electromagnetic safety of an object, which includes counteracting unauthorized access to information using special technical means, electromagnetic waves are shielded.

The use of high-quality screens allows solving many problems, including information protection in rooms and technical channels, problems of electromagnetic compatibility of equipment and devices when used together, problems of protecting personnel from increased levels of electromagnetic fields and ensuring a favorable environmental situation around operating electrical installations and microwave devices.

Shielding is generally understood as both the protection of devices from the effects of external fields and the localization of radiation of any means, preventing the manifestation of this radiation in the environment. In any case, the shielding efficiency is the degree of attenuation of the field components (electric or magnetic), defined as the ratio of the effective values ​​of the field strength at a given point in space in the absence and presence of a shield. Since the ratio of these quantities reaches large values, it is more convenient to use a logarithmic representation of the shielding efficiency: where Ke is the attenuation (shielding) coefficient for the electric component, Kn is the attenuation (shielding) coefficient for the magnetic component, Eo (Ho) is the strength of the electric (magnetic) component of the field in the absence of a shield, E1 (H1) is the strength of the electric (magnetic) component of the field in the presence of a shield at the same point in space.

The theoretical solution to the shielding problem, determining the field strength values ​​in the general case is extremely difficult, therefore, depending on the type of problem being solved, it seems convenient to consider individual types of shielding: electrical, magnetostatic and electromagnetic. The latter is the most general and frequently used, since in most cases of shielding one has to deal either with variable or fluctuating fields and, less often, with truly static fields.

Theoretical and experimental studies by a number of authors have shown that the shape of the screen has little effect on its efficiency. The main factor determining the quality of the screen is the radiophysical properties of the material and design features. This allows for the simplest representation of the screen to be used when calculating the efficiency of the screen in real conditions: a sphere, a cylinder, a plane-parallel sheet, etc. Such a replacement of the real design does not lead to any significant deviations in the actual efficiency from the calculated one, since the main reason limiting the achievement of high values ​​of shielding efficiency is the presence of technological openings in the screen (input-output devices, ventilation), and in shielded rooms — life support devices connecting the room with the external environment.

A plane-parallel screen in the electromagnetic case can be characterized by the normal impedance of the screen material, which is defined as the ratio of the tangential components of the electric and magnetic fields. The transmission coefficient through the layer is the efficiency of the screening, since it is equal to the ratio of the amplitudes of the transmitted and incident waves on the screen. If the medium on both sides of the screen is a vacuum, then the transmission coefficient D can be represented as where is the wavelength in free space, and and relative permittivity and magnetic permeability of the screen material.

In the general case — with complex permittivity and magnetic permeability of the material — theoretical analysis of the given expression is extremely difficult, therefore most researchers resort to separate consideration of the shielding efficiency — by absorption and reflection of the incident wave by the screen.

Since the analytical evaluation of the shielding efficiency from the general formula for the transmission coefficient for a plane-parallel infinite shield is difficult in the general case, a simpler, approximate analysis can be used, based on the representation of the shield efficiency as the sum of individual components:

K=Kabs+Kotr+Kn.otr,
where Кпогла — shielding efficiency due to absorption of electric energy by the screen, Котре — shielding efficiency due to reflection of electromagnetic wave by the screen, Кн.отр — correction factor taking into account multiple internal reflections of the wave from the screen surfaces.

If the loss of wave energy in the screen, that is, its absorption, exceeds 10 dB, then the last factor in the given expression can be neglected. Shielding efficiency due to absorption of energy in the thickness of the screen can be calculated from a simple relationship: obtained on the basis of the representation of the electric and magnetic components of the field in a material on the surface of which the Leontovich boundary conditions are satisfied.

It is obvious that at low frequencies a steel screen, the magnetic permeability of which can be quite high (or a screen made of another conductive material with significant magnetic permeability), is more effective than copper in absorption. However, to increase its efficiency, it is necessary to increase the thickness of the shielding sheet. In addition, with increasing frequency, the magnetic permeability of all materials decreases rapidly, and the more significantly, the greater its initial value. Therefore, materials with a high value of initial magnetic permeability (104 H/m) are advisable to use only up to frequencies of about 1 kHz. At high values ​​of magnetic field strength, due to saturation of the ferromagnetic material, its magnetic permeability drops the more sharply, the greater the initial value of permeability.

Fig. 1. Dependence of the penetration depth of the electromagnetic field for various materials

To avoid the saturation effect, the screen is made multilayered, and it is desirable that each subsequent (in relation to the shielded radiation) layer has a higher initial value of magnetic permeability than the previous one, since the equivalent depth of penetration of the electromagnetic field into the thickness of the material is inversely proportional to the product of its magnetic permeability and conductivity. The thickness of the screen required to ensure a given value of its efficiency is easily determined from . The dependences of the penetration depth on the frequency for various materials often used in the manufacture of screens are shown in Fig. 1.

The second component of the shielding efficiency Kotr is due to the reflection of the electromagnetic wave at the free space-shield interface due to the difference in the wave impedances of the vacuum (Z for near fields — electric or magnetic and Z for far-field fields).

The shielding efficiency due to reflection can be simply defined as , where Z for metallic materials can be represented as: A significantly greater shielding effect can be achieved by using multilayer screens of the same total thickness rather than uniform ones. This is explained by the presence of several surface interfaces in multilayer screens, on each of which the electromagnetic wave is reflected due to the difference in the wave impedances of the layers. The efficiency of a multilayer screen depends not only on the number of layers, but also on the order in which they are arranged. The most efficient screens are those that combine magnetic and non-magnetic layers, with the outer layer relative to the radiation source preferably made of a material with magnetic properties.

Calculation of the shielding efficiency of two-layer screens made of various materials shows that the most appropriate combination in the frequency range of 10 kHz to 100 MHz is a copper and steel layer. The thickness of the magnetic layer should be greater than that of the non-magnetic layer (steel — 82% of the total thickness, copper — 18%).

An additional increase in the shield thickness by one layer leads to a not very noticeable increase in the shielding efficiency.

When designing electromagnetic screens in general, it is necessary to keep in mind that at relatively low frequencies it is most difficult to ensure effective screening of the magnetic component of the field, while screening of the electrical component does not present any particular difficulties even when using perforated or mesh screens.

Despite the fact that at low frequencies highly conductive materials can provide very high values ​​of shielding efficiency, in a number of cases (for technological, design, economic reasons) it is more expedient to use (especially when shielding static and fluctuating magnetic fields with a low intensity value) magnetic materials with high values ​​of initial magnetic permeability. For a single-layer cylinder, the length of which significantly exceeds its diameter D, the shielding efficiency of the magnetic field intensity component: perpendicular to the axis of the cylinder, can be approximately estimated as

As in the electromagnetic case, multilayer shells are more effective than a single-layer shield, and their efficiency increases almost proportionally to the number of layers.

A special place among the materials used for shielding static and quasi-static magnetic fields is occupied by amorphous ferromagnets. Magnetic shields are made of alloys such as permalloy containing 20% ​​at. Fe and 80% at. Ni. High magnetic properties (large value and the shielding coefficient) are achieved after complex and expensive heat treatment. However, the properties of screens made of such materials change under the influence of mechanical effects. Screens made of amorphous alloys are not sensitive to impacts and bends. The magnetic properties of amorphous alloys are high enough that they can be used as a screen material. They have high initial magnetic permeability, which maintains its level up to frequencies of the order of hundreds of megahertz. For example, to shield cables in equipment installed on board the Voyager-class spacecraft, Metshild fabric was used, made of amorphous alloy Fe40Ni40P14B6 in the form of a tape 1.5 mm wide and 58 μm thick. The results of the studies showed that the shielding ability of such fabric reaches 11 dB at a magnetic field strength of 40 A/m and 24 dB at a field strength of 200 A/m at a frequency of 60 Hz. These values ​​exceed the characteristics of similar permalloy screens by 1.5-2 times and do not change after mechanical impacts.

To date, our specialists have managed to create screens with shielding factors of up to 60 dB from amorphous alloys for industrial interference and the radio frequency range. Magnetic screens for quasi-static fields (the earth's magnetic field) have also been developed from amorphous ferromagnets. Amorphous ferromagnetic microwire can now be used for magnetic shielding of small volumes.

Thus, by shielding electromagnetic waves it is possible to fully ensure the electromagnetic safety of an object. However, ensuring the requirements for the electromagnetic safety of an object, especially in the part concerning the protection of information from leakage through technical channels created using special equipment (electroacoustic channel, radio channel, channel of side electromagnetic radiation and interference, etc.), must be provided for at the stage of developing the project of the object.

For example, when designing within the facility, it is necessary to allocate areas of increased confidentiality — meeting rooms, technological rooms in which information intended for official use circulates, etc. There should be no windows in such rooms, they should have an independent power supply system, shielded doors. When constructing such a facility, it is possible to use shielding materials — shungite concrete or concrete with an electrically conductive filler. The walls of the room are finished with flexible screens, for example, woven carpets made of amorphous materials or electrically conductive fabrics. Various carbon fabrics or metallized films can be used as shielding fabric.

The interior of the room is lined with structural radio-absorbing material to prevent the formation of standing electromagnetic waves with frequencies greater than 1 GHz and to create a more comfortable environmental environment. Specialized foam glass of various brands or cellular structures can be used as radio-absorbing materials. The shielding coefficient of such a room can exceed 60 dB in a wide frequency range.

Our technologies allow us to produce high-quality shielding of existing premises that were not originally intended for special use. Wall finishing with multilayer flexible screens is applicable in most cases. If there are windows, they are covered with metallized films and curtains made of shielding fabrics. Flexible broad-range radio-absorbing materials can be used in premises of this class. Filled foam glass is used for facing the ceilings of the premises. The shielding coefficient reaches 20 dB and more.

The specific shielding value depends on the area of ​​the windows, the configuration of the room, its volume and the material of the walls. Also, in existing rooms, to mask existing sources of electromagnetic radiation, it is proposed to use broadband noise generators, which can simultaneously be used to counter bugs with data exchange via a radio beam.

In addition, the use of flexible shielding and radio-absorbing materials makes it possible to create small temporary shielded volumes with a shielding factor of 10-20 dB, which, in combination with a portable broadband noise generator, is sufficient to solve a number of problems.

Based on the above, it should be noted that electromagnetic wave shielding is a multifaceted and unique topic. The importance and significance of shielding is also supported by the fact that in the United States, more than 1% of the cost of all industrial production is spent annually on developing this problem. The same issues are dealt with by the Special International Committee on Radio Interference, which operates within the International Electrotechnical Commission (IEC). At the same time, in the United States, companies spend an average of 10-15 billion dollars annually on measures to protect confidential information.

In general, American entrepreneurs have to spend up to 20% of their total expenses on research and development work on such measures. Most of these expenses are spent on measures to protect information from leakage through technical channels, because everything changes quickly in the world of special equipment. Information interception equipment is being developed and improved.

Today, no thrifty foreign company will begin financing a new expensive project without guarantees of the safety of commercial secrets,