Optimization of the Emitting Coil of a Hardware-Software Complex for Study of Low-Frequency Electromagnetic Radiation’s Shielding Effectiveness

Optimization of the radiation coil of the hardware-software complex for studying the effectiveness of shielding of low-frequency electromagnetic radiation will make it possible to assess the effectiveness of shielding coatings at a higher level. This fact will make it possible to develop coatings with improved characteristics. The purpose of this work was to determine the optimal characteristics of the emitting coil which will ensure its stable operation and magnetic field strength in the frequency range up to 100 kHz. The parameters of the manufactured samples, such as inductance (L), active (R) and total resistance (Z), were obtained using an MNIPI E7-20 emittance meter. In practice, the coils with the optimal parameters calculated theoretically were connected to a current source and amplifier. To detect electromagnetic radiation, a multilayer inductor connected to a UTB-TREND 722-050-5 oscilloscope was used as a signal receiver. The results of measurements showed that the resistance of multilayer coils is approximately 1000 times higher than that of single-layer coils. Also, for multilayer coils, an avalanche-like increase in total resistance is observed starting from a frequency of 10 kHz, while for single-layer coils there is a uniform increase in total resistance over the entire frequency range up to 100 kHz. The paper presents results of research on the correlation of the performance of single-layer and multilayer inductors depending on their parameters in the frequency range from 20 Hz to 100 kHz. Values of the voltage required to provide the magnetic field strength of 1, 5, 20 Oe at 25 Hz and 100 kHz have been calculated. After analyzing the data obtained, the optimal parameters of the inductor were found which ensure stable performance in the frequency range up to 100 kHz.

Ключевые слова: катушка индуктивности, экранирование, электромагнитное излучение. DOI: 10.21122/2220-9506-2021-12-1-30-37 Introduction A great threat to the security of information on electronic devices, sensitive elements of various devices, biological objects is created by electromagnetic radiation of various nature. Sources of electromagnetic radiation in everyday life are household appliances, electric vehicles, mobile phones and other electronic devices [1,2]. To protect against electromagnetic radiation, there are active and passive methods [3]. The passive method is to use screens that absorb or reflect electromagnetic radiation (EMR). For passive protection materials with high magnetic permeability are used. With the help of passive screens it is possible to protect certain sensitive elements of the device, and these screens are easy to use and more profitable in economic terms. However, at the moment there is no unified method for analyzing the effectiveness of shielding. Most scientific papers consider the effect of low-frequency EMR on living organisms and plants without affecting the analysis of the effectiveness of screens.
Analysis of information on the topic of this work shows that there are no standardized generally accepted methods for assessing the effectiveness of shielding [5][6][7]. To assess the shielding properties of various materials a device or apparatus with a sufficiently powerful emitter of electromagnetic radiation is required which stably operates both with direct and alternating currents of various frequencies.
Inductors [8] can be used as such as an emitter, varying the parameters of which the required result can be achieved.
The purpose of this work was to manufacture inductor coils with different parameters, study their characteristics, and determine a sample that provides stable electromagnetic radiation in the frequency range from 25 Hz to 100 kHz. The sample coil with optimal parameters will be used in the hardwaresoftware complex as an emitter and will allow to obtain new, unique data on the shielding efficiency of magnetic coatings and multilayer structures based on them, as well as to compare the previously obtained data on the shielding efficiency in a constant magnetic field with new data that will be obtained with alternating electromagnetic radiation.

Measurement technique
Single-layer and multi-layer inductors were manufactured as prototypes. For the manufacture of single-layer inductors copper wire with a cross section of 1 and 2 mm was used. The inner diameter (d ) of the coils was 7 and 15 mm, the number of turns (N ) was from 6 to 41, and the length (l ) was from 15 to 65 mm. Multilayer coils were made from copper wire with a diameter of 0.1; 0.18 and 0.35 mm. The coils diameter was 6 mm, the number of turns was from 1000 to 6000, and the length was from 20 to 120 mm. Detailed information about the samples′ parameters is presented in Table 1. The manufactured samples′ parameters, such as inductance (L), active (R ) and total resistance (Z ) were obtained using an MNIPI E7-20 emittance meter. In practice, the coils with the optimal parameters calculated theoretically were connected to a current source and amplifier. To detect electromagnetic radiation a multilayer inductor connected to a UTB-TREND 722-050-5 oscilloscope was used as a signal receiver.

Results and discussion
As can be seen from the graphs (Figure 1), the SL_N6_t2_d7 coil has the lowest inductance among the coils with an inner diameter of 7 mm. Among the coils with an inner diameter of 15 mm, the SL_N6_t2_d15 coil has the smallest inductance. The inductance of the first sample increases from 0.1 to 0.2 μH in the frequency range from 25 Hz to 100 kHz, while the inductance of the second sample is stable over the entire frequency range and equals 0.5 μH. Despite the large inductance the SL_N6_t2_d15 coil looks preferable to the SL_N6_t2_d7 one, because it has a constant inductance value over the entire frequency range.  Figure 2 shows that all samples with an inner diameter of 7 and 15 mm have constant resistance values R at frequencies from 25 Hz to 5 kHz. After 5 kHz a sharp increase in resistance is observed in almost all samples, with the exception of samples SL_N6_t2_d7, SL_N11_t2_d7 and SL_N6_t2_d15. This is most likely due to the skin effect [9] which consists in the fact that the currents flowing on the surface of the wire screen the inner layers, as a result of which almost all current is concentrated in a thin surface layer. The proximity effect also contributes, the essence of which is that eddy currents arise in the coil wire under the influence of a magnetic field from adjacent turns [10].  Figure 3 shows that the total resistance of all coils increases linearly. Coils SL_N6_t2_d7 and SL_N6_t2_d15 have the lowest total resistance. This is probably due to the fact that these coils have fewer turns, respectively, in their manufacture, a shorter wire length is used compared to the rest of the samples.
These graphs show that the inductance L for coils with a diameter of 7 mm and a wire thickness a b a b Devices andMethods of Measurements 2021, vol. 12, no. 1, pp. 30-37 O.D. Kanafyev et al. Приборы и методы измерений 2021. -Т. 12, № 1. -С. 30-37 O.D. Kanafyev et al. 34 of 2 mm does not change significantly at 100 Hz. For coils with a diameter of 15 mm and a wire thickness of 2 mm, as well as for coils with a diameter of 7 mm and a wire thickness of 1 mm, the inductance L practically does not change at frequencies up to 100 kHz. Coils with a diameter of 7 and 15 mm and a wire thickness of 2 mm have less resistance than coils with a diameter of 7 mm and a wire thickness of 1 mm. At the same time, for all coils a slight increase in resistance is observed over the entire frequency range up to 100 kHz. The total resistance of all single-layer coils increases linearly with increasing emitting frequency. Figure 4 shows that some of the multilayer coils have constant inductance values over the entire frequency sample. The second part of the samples shows unpredictable inductance values. It can be assumed that this is due to the large number of turns in the coils, some of which may have insulation defects. As seen in Figure 5, the resistance of multilayer coils is constant from 25 Hz to 10 kHz. After 10 kHz, an avalanche-like increase in resistance occurs. Coils N4-t.1-d6 and N1-t.18-d6 come into resonance due to the presence of skin and proximity effects. Parasitic capacitances also increase strongly at high frequencies. These capacitances, in combination with the high inductance of the coil, transform it into a whole chain of oscillatory circuits with resonances falling within the operating range or even below it, and then the coil acquires capacitive resistance.  Figure 6 presents that the total resistance of all samples has a constant value from 25 Hz to 5 kHz, after 5 kHz there is a rapid increase in the total resistance. This is due to the presence of a skin effect and a proximity effect. These effects are much more a b pronounced with multilayer coils, due to the longer winding length and also due to the large number of turns. Analyzing the graphs, we can conclude that all multilayer coils for the entire frequency sample have a constant inductance, except for two samples. At an electromagnetic field frequency of up to 10 kHz, multilayer coils have constant resistance. At a current frequency above 50 kHz, the resistance of all samples begins to increase sharply. The total resistance of the coils begins to increase when the frequency of the electromagnetic field is over 10 kHz.
The values of the magnetic field strength inside the cylindrical coil H (E) for the manufactured samples of inductance coils were calculated by the formula: where I -the current in the coil; n -the number of turns of the coil; l -the length of the coil; where U -the voltage applied to the circuit section; Z -impedance measured with an RLC meter.

I
In l = , For the stable operation of the electromagnetic radiation at different frequencies, it is required to vary the values of the emitting supplied to the coils. The tables above show the Devices andMethods of Measurements 2021, vol. 12, no. 1, pp. 30-37 O.D. Kanafyev et al. Приборы и методы измерений 2021. -Т. 12, № 1. -С. 30-37 O.D. Kanafyev et al.  voltage values calculated to provide magnetic field strengths of 1,5,20 Oe in the frequency range from 25 Hz to 100 kHz, for single-layer and multilayer coils.
Analyzing the data obtained, we can say that to ensure the operation of multilayer coils, a much higher voltage is required compared to single-layer coils. This is due to the high values of the inductance of multilayer coils as well as the presence of skin and proximity effects. Single-layer coils are also subject to these disadvantages but to a much lesser extent. In addition the voltage values required to operate at 20 Oe of multilayer coils are likely to be critical. Difficulties also arise with the selection of a voltage source operating in such a wide range of values.

Conclusion
Investigations of single-layer and multilayer inductors with different parameters have been carried out.
It was found that it is preferable to use single-layer inductors as a source of electromagnetic radiation operating in the frequency range up to 100 kHz. At a frequency from 10 to 100 kHz, an avalanchelike increase in total resistance begins in multilayer inductors; when measuring the active resistance, some samples enter into resonance. This is due to the presence of skin and proximity effects, which are pronounced in multilayer samples, due to a much larger number of turns, as well as their tightness. The inductance of multilayer coils is 1000 times higher than single-layer coils, as a result of which parasitic capacitances increase significantly. The combination of the factors given above transform a multilayer coil into a chain of oscillatory circuits with resonances often falling into the operating range or below it.
Due to the use of a wire with a cross section of 2 mm for winding single-layer coils, the active resistance of these samples is approximately 1000 times less than that of multilayer coils. Also, due to this fact, much lower values of the applied voltage are required to provide a power of electromagnetic radiation similar to multilayer samples.
Measurements have shown that when the current frequency reaches 5 kHz, a sharp increase in resistance is observed in almost all samples of single-layer coils, with the exception of samples with 6 turns and inner diameters of 7 and 15 mm. It can be assumed that these coils are less susceptible to negative effects due to the fewer turns. A coil with 6 turns and a 7 mm diameter has the lowest inductance value at a current frequency of 100 kHz. However, a sample with 6 turns and a 15 mm diameter has a constant inductance value over the entire frequency range from 25 Hz to 100 kHz. This fact makes the choice in favor of this coil preferable, because the difference in the voltage values required to provide a magnetic field strength of 20 Oe is insignificant.
Reel with optimal parameters planned use as a source of electromagnetic radiation in the hardware and software complex to study the effectiveness of shielding electromagnetic radiation. This hardware and software complex will have a unique structure and characteristics and will allow high-precision studies of the correlation of the chemical composition, structure, and shielding efficiency of a wide range of materials.