Electromagnetic pulse shielding effectiveness of circular multi-waveguides for fluids

Results in Physics 16 (2020) 102946

Contents lists available at ScienceDirect

Results in Physics journal homepage: www.elsevier.com/locate/rinp

Electromagnetic pulse shielding effectiveness of circular multi-waveguides for fluids Sangho Yoona, Kwan Jeonga, Sohail Mumtazb, Eun Ha Choib, a b

T

⁎

Department of Physics and Chemistry, Korea Military Academy, Seoul 01805, Republic of Korea Department of Electrical and Biological Physics, Kwangwoon University, Seoul 01897, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Dielectric Electromagnetic pulses HEMP Nuclear explosions Multi-waveguides Shielding effectiveness

Generally, large-scale plumbing work is necessary to facilitate continuous supply of air, oil, and water to ensure a sustained operation for more than two weeks under unusual conditions during which no necessary supplies can be procured from external sources. A multi-waveguides can be used in large-scale plumbing to provide the required shielding effectiveness (SE) against high-altitude electromagnetic pulses (HEMP), which are generated by high-altitude nuclear explosions. Therefore, this study analyzes the SE of a circular multi-waveguide using experimental values along with theoretical calculations with waveguides installed in large pipes instead of single pipes studied so far. This is to find a large multi-waveguides that can be used in large protective facilities. To validate the results, the theoretical values were compared with experimental data on the SE of waveguides with diameters of 250 mm, which were filled with air, oil, and water. The results revealed that the air-filled and oilfilled multi-waveguides with diameters of 30 mm and 50 mm satisfy the requirements of MIL-STD-188-125-1. However, the water-filled multi-waveguides with diameters of 30 mm and 50 mm did not achieve the required SE. Accordingly, in this study, the appropriate diameter and length of multi-waveguides for transmitting water are suggested through theoretical calculations.

Introduction Generation of high-power electromagnetic waves and their effects have been extensively studied over the past few decades [1–3]. Further, HEMP has become known through the 1962 US and former Soviet Union nuclear tests [4], in addition to HEMP, E-bombs and directional weapons using emp generators have been found to have a profound effect on sensitive electronic equipment. To respond to the threats of HEMP, EMP protection measures for national and military major command and control facilities are discussed in various ways, and studies on the characteristics of HEMP penetration and evaluation of protection facilities [5,6] have been conducted by researchers. Generally permanent protection facilities for the protection of sensitive electronic equipment typically use metal barriers to shield part or all of the facility, and to provide points of entry for these internal needs. For example, there should be facilities for access of personnel and equipment, inflow of electric power and communication, air-conditioning facilities, ventilation, oil, water supply and drainage, etc. In this paper, we tried to find out about pipe ventilation, oil and water supply. In the existing studies, most of the facilities for ventilation were hexagonal structure, which is a hub comb structure, and a single pipe of

⁎

different diameters for oil and water was studied [7,8]. Currently, because of the changes in the nature of warfare, at least two weeks of operational continuity within national and military major command and control facilities [9] is required. Therefore, to ensure the sustainability of operations, the major command and control facilities must contain adequate water and energy resources, in case the supply of water and energy resources from outside the facility is limited. In the case of large major command and control facilities, large-scale piping facilities capable of supplying air, water, and oil are generally installed. In addition, EMP protection measures must be established for such large piping facilities. In the case of large-scale protection facilities, special EMP protection measures are required for large-scale air, water and sewage pipes, and smoke passages of boilers, generators, and cooking utensils. Generally, in the case of large-scale protection facilities that can accommodate 200–300 people, air ventilation facilities of honeycomb structure of size 600 mm × 600 mm are installed in a minimum of 140–150 places, and pipes with diameters of 200–250 mm are normally installed as water inlet and outlet pipes. A small pipe with a diameter of 50 mm is typically installed to supply oil, but larger pipes could be used in the future. Typical EMP shielding methods for air supply devices are

Corresponding author. E-mail address: [email protected] (E.H. Choi).

https://doi.org/10.1016/j.rinp.2020.102946 Received 27 November 2019; Received in revised form 13 January 2020; Accepted 13 January 2020 Available online 27 January 2020 2211-3797/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Results in Physics 16 (2020) 102946

S. Yoon, et al.

When the frequency of the electromagnetic wave function, including e−jβz , is lower than the cutoff frequency, the propagation constant β becomes an imaginary number, and the electromagnetic wave propagate along with attenuation. Generally, because conductor and dielectric losses occur in the waveguide, e−jβz is replaced by e−γz , where γ is a complex propagation constant defined by,

to use multiple waveguides of square structures [10] or honeycomb structures [11]. Although multiple waveguides of square or honeycomb structures can be used in water or oil supply devices, a circular pipe, which is cheap and easy to manufacture, is generally used in water and oil supply equipment. Accordingly, this study is focused on studying the shielding effect using a circular multiple waveguide. As specified in MIL-STD-188-125-1(hereinafter MIL-STD), waveguides used in EMP protection facilities should have a cut-off frequency of at least 1.5 GHz and shielding effect of at least 80 dB at 1 GHz [12]. For this purpose, the diameter of the circular waveguide for air supply must be less than 10 cm and the length must be at least 5 times higher than the diameter. The cutoff frequency of the circular waveguide is inversely proportional to the square root of the dielectric constant of the fluid in the waveguide. Considering that the dielectric constant of air, oil, and water is approximately 1, 2, and 78, respectively, the cutoff frequency of a water-filled circular waveguide is 78 times lower than that of a air-filled circular waveguide of the same diameter. Therefore, the circular waveguide for water supplies that satisfies the criteria of MIL-STD will have a diameter of less than 10 cm. In this study, we investigate the appropriate diameter and length of circular multi-waveguides required to satisfy the minimum shielding criterion, when it is applied to a certain section of water and oil supply pipes from outside the military command facility. For this purpose, the shielding effectiveness (SE) equation of the circular multi-waveguides was obtained by a theoretical approach, and the EMP shielding effect was measured for three types (air, oil, water) of the 250 mm diameter pipe to validate the theory. Further, the experimental and theoretical values were compared and analyzed. Subsequently, we analyzed the experimental values of the SE of the 30 mm and 50 mm diameter airfilled circular multi-waveguides. In addition, using the theoretical values, we confirmed that the 30 mm and 50 mm diameter circular multiwaveguides satisfy the minimum shielding criteria. Furthermore, we propose a circular multi-waveguides of appropriate diameter and length that satisfy the criteria. Only oil- and water-filled circular multi-waveguides with diameters of 30 mm and 50 mm (theoretical) are suggested because of the limitations of measurement and failure of the experimental method.

γ=

kc2 − k 2 = α c + αd + jβ

(2)

where kc is the intercept wave number, k is the wave number, α c is the conductor attenuation constant, and αd is the dielectric attenuation constant. The conductor attenuation constant α c in Eq. (2) varies based on the mode. The attenuation constant αCTE11 of the electromagnetic wave propagating from the TE11 mode to the frequency f is given by Eq. (3) [13], TE

αCTE11 =

2

⎡ ⎛ fc 11 ⎞ ⎤ πfε ⎟ + 0.4184⎥ ⎢⎜ TE11 2 2 σa [1 − (fc / f ) ] ⎝ f ⎠ ⎣ ⎦

(3)

where σ is the conductivity of the waveguide surface. The permittivity of the dielectric can be expressed by the complex permittivity as shown in Eq. (4) using the relative permittivity εr , permittivity ε0 of the free space, and loss tangent tanϕ .

ε = εr ε0 (1 − jtanϕ)

(4)

If the loss tangent tanϕ is very small, i.e., tanϕ ≪ 1, the dielectric constant αd in Eq. (2) can be obtained as follows [14]:

αd =

k 2tanϕ 2β

(5)

The SE of a waveguide refers to the rate of electromagnetic wave attenuation by a waveguide at a given frequency. If the electric field entering the waveguide is Ei and the electric field exiting the waveguide is Et , the SEdB of the waveguide in decibels (dB) is defined as follows:

SEdB = 20log(Ei/ Et )

(6)

Therefore, at frequencies higher than the cut-off frequency, the SEdB of the waveguide can be obtained using Eq. (3) and Eq. (5) as follows:

Shielding effectiveness in waveguide

SEdB = 20loge (αc + αd ) d = 8.6859(α c + αd ) d

Circular multi-waveguides shielding effect

On the contrary, at a frequency lower than the cutoff frequency, the SEdB of the waveguide is given by,

As shown in Fig. 1, the TE11 mode is the main mode of the electromagnetic waves propagating in the +z-axis direction in the circular waveguide (radius a , length d ) filled with the medium having the permeability μ and the permittivity ε ; the cutoff frequency fCTE11 of the mode is given by,

f CTE11 =

1.8412 2πa με

SEdB = 20loge γd = 8.6859 kc2 − k 2 d

(7)

(8)

In mode TE11, kc = 1.8412/ a ; thus, SEdB for a frequency lower than the cutoff frequency fCTE11 in a single circular waveguide of radius a and length d is as follows: 2

(1)

SEdB = 31.98

f d 1 − ⎛⎜ TE11 ⎞⎟ . 2a f c ⎝ ⎠

(9)

The SE of circular multi-waveguides composed of multiple waveguides can be obtained by adding the additional term Eq. (10) to Eq. (9), which was obtained by approximating the infinite arrayed waveguide to a flat plate waveguide, and then using the Wiener-Hopf method [15]:

4ka cosθ⎞ − 20log ⎛ ⎝ π ⎠

(10)

where θ is the incident angle of the electromagnetic wave to the waveguide section. The simulation results of a honeycomb multiple waveguide obtained using the commercial software HFSS [11] indicated that the SE in multiple waveguides is constant regardless of the number of waveguides. Therefore, the SEdB of the circular multi-waveguides with respect to a frequency lower than the cut-off frequency fCTE11 can be obtained by,

Fig. 1. Circular waveguide. 2

Results in Physics 16 (2020) 102946

S. Yoon, et al.

2

SEdB = 31.98

f 4ka d 1 − ⎛⎜ TE11 ⎞⎟ − 20log ⎛ cosθ⎞. 2a π f ⎝ ⎠ c ⎝ ⎠

(11)

In summary, the SEdB for a frequency higher than the cutoff frequency is obtained by Eq. (3), Eq. (5), and Eq. (7). The SEdB for a frequency lower than the cutoff frequency is obtained by Eq. (9) for a circular single waveguide, and by Eq. (11) for a circular multi-waveguides. Effect of dielectric on SE Because Eq. (3), Eq. (5), Eq. (9), and Eq. (11) include the wave number k = 2πf με or the cutoff frequency fCTE11 given by Eq. (1), the SE of the waveguide is affected by the permittivity and the permeability of the fluid. Because the relative permeability of water and oil used in this experiment is close to 1, we consider only the permittivity of water and oil. The permittivity depends not only on the frequency but also on temperature and pressure. In Eq. (4), εr (1 − jtanϕ) is the complex di− electric constant, and the complex dielectric constant εr can be obtained using the Debye relaxation model [16] as follows: − ε − ε∞ ⎤ σ −j εr = ε∞ + ⎡ s ⎢ 2πfε0 ⎣ 1 + j (2πfτ ) ⎥ ⎦

Fig. 2. Three types of circular waveguides of diameters (a) 250 mm, (b) 50 mm, and (c) 30 mm, used in the test.

(12)

whereεs is the relative permittivity at low frequency (electrostatic region), ε∞ is the relative permittivity at high frequency (optical), τ is the relaxation time, and σ is the conductivity of the dielectric. The conductivity in Eq. (12) represents the degree of current flow. The conductivity of pure water using the Debye model is given by [17],

σ = σdc +

(εs − ε∞ )(2πf )2ε0 τ 1 + (2πfτ )2

were bonded to the test specimen, and the inlet of each multi-waveguides was sealed with a plug made of transparent plastic. In the inlet and the outlet ports, a rubber stopper was used to prevent the medium from leaking. To meet the requirements of MIL-STD, which is the test standard for evaluating electromagnetic shielding performance of HEMP protection facilities, experimental measurement equipment was selected as shown in Table 2, and the measurement program KTISE103 developed by the KTI company was used.

(13)

where σdc is the dc conductivity. In the case of water, the experimental data are in good agreement with the Debye model. Therefore, in this paper, the dielectric constant required to calculate the SE of the waveguide is obtained using Eq. (12) and Eq. (13). We used the Debye parameter [17] mentioned in Table 1, which was obtained by matching the theoretical and the experimental values using the data fitting method in an existing study. Because the research data for complex permittivity of oil were insufficient, the generally used value of 2 was applied in the calculation of SE for the relative permittivity of oil.

Experimental method To meet the installation requirements of the measuring equipment mandated by MIL-STD, the test specimen was installed at 2.90 m above the ground as shown in Fig. 4, and the transmitting antenna was installed at 1.50 m from the shielding wall inside the shield room. The receiving antenna was installed at a distance of 1.50 m. The typical measurement frequency is in the range 10 kHz–1 GHz, and can be measured up to 3 GHz when possible [12]. This experiment aimed at measuring the SE of multi-waveguides against electromagnetic waves penetrating the passage through which the fluid moved, rather than measuring the SE of the shielding wall as a whole. Therefore, frequencies of 200 MHz–1 GHz were measured within the measurement range of the horn antenna. The sampling frequency interval was 2 MHz, and the output of the transmitting antenna was 10 dBm. In addition, a preamplifier was utilized to achieve a wide dynamic range. As shown in Fig. 4, vertically polarized electromagnetic waves were emitted by the transmitting antenna connected to the signal generator when the experimental equipment and the test specimen were arranged, and the measured signals were quantified by the spectrum analyzer connected to the receiving antenna. Let the measured signal be Vm . Further, in Fig. 4, the signal is measured when the shielding wall and the specimen are removed, i.e., free space. Let the measured signal be Vc . Generally, the shielding effect SEdB is 20log(Vc / Vm) , which is the SE of both the shielding wall and the test specimen when measured as mentioned above. Therefore, in this study, which considers only the pure SE of a circular waveguide, the SE of the shielding wall and other SE values that may be a result of the experimental method should be corrected as follows:

Experimental apparatus Preparation of test specimen (test body) To investigate the effects of waveguide diameter and dielectric type on the EMP SE, waveguides of the type shown in Fig. 2 were manufactured. Fig. 2(a) shows a general pipe with a diameter of 250 mm and a length of 250 mm. Fig. 2(b) and 2(c) show the results obtained by inserting small pipes of inner diameters (thickness: 1 mm) 50 mm and 30 mm into the general 250 mm pipe. Fig. 2(b) and 2(c) show the multiwaveguides with lengths and diameters of 250 mm and 50 mm, and 150 mm and 30 mm, respectively. These test specimens were connected to the middle of a rectangular plate with 48 holes—with each hole having a diameter of 4 mm—to attach to the shielding wall of the EMP shielding room, as shown in Fig. 3(a). As shown in Fig. 3(b), transparent stopper caps for the fluid Table 1 Debye parameters of pure water. Temperature

εs

ε∞

τ (ps)

σdc (S/m)

25 ℃

78.2

5.5

8.1

10-5

3

Results in Physics 16 (2020) 102946

S. Yoon, et al.

Fig. 3. (a) Coupling of waveguide and shielding facility and (b) Coupling of waveguide and transparent plastic caps.

Experimental results and analysis

Table 2 Shielding effectiveness test equipment. Equipment name

Manufacturer

Model Name

Spectrum Analyzer Signal Generator Horn Antenna Cable S/W

Rohde & Schwarz Rohde & Schwarz Electro‐Metrics Gore KTI

ESIB40 SMR20 EM6960 0R01R0 KTISE103

SE measurements and analysis This study addresses the SE of three different types of fluids in three circular waveguides of different diameters. When air, oil, and water with dielectric constants of 1, 2, and 78.2 are present in circular waveguides with diameters of 250 mm, 50 mm, and 30 mm, respectively, the cutoff frequency fCTE11 can be obtained as shown in Table 3 using Eq. (1). The MIL-STD specifies that the cutoff frequency of the waveguide should be at least 1.5 GHz, so that the circular waveguide with a diameter of 250 mm, and the water-filled circular waveguides with diameters of 50 mm and 30 mm do not meet the shielding criterion. A circular waveguide with a diameter of 250 mm does not meet the shielding criterion because its cutoff frequency is low; however, to validate the theory, three types of fluids were tested. A circular waveguide test specimen with a diameter of 250 mm (shown in Fig. 2(a)) was installed on the shielding wall, the experimental equipment was placed as shown in Fig. 4, and the SE was measured for the following three cases: a) only when air was present, b) when oil was filled, and c) when water was filled. The measured values were corrected using Eq. (14) and compared with the theoretical values obtained using Eq. (3), Eq. (5), Eq. (7), and Eq. (9). Figs. 5–7 show the comparison of theoretical and experimental values for the pure SE of a circular waveguide with a diameter of 250 mm filled with air, oil, and water. In the case of the air-filled circular waveguide (Fig. 5), the experimental and the theoretical values generally exhibited similar slope variations, but the two values were slightly different. In the range of 200–490 MHz, the experimental value was higher than the theoretical value by 18 dB, and the experimental value was lower than the theoretical value by 6 dB on average at 490 MHz to cutoff frequency. In the case of the oil-filled circular waveguide (Fig. 6), the change in pattern was almost the same as that of the air-filled circular waveguide, and the difference between the experimental and the theoretical values was slightly smaller than that of air. In the range of 200–416 MHz, the experimental value was higher than the theoretical value by 17 dB, and lower than the theoretical value by 4 dB at 416 MHz to cutoff frequency. In the case of the water-filled circular waveguide (Fig. 7), the SE

Fig. 4. Shielding effectiveness measurement setup.

Circular wave guide pure SEdB=20log(Vc / Vm) − Shielding wall SEdB − Other SEdB

(14)

Shielding wall SEdB is the SE by the shielding wall (thickness: 4 mm) when the specimen is removed, as shown in Fig. 3(a) & Fig. 4; this means the pure SE by a circular waveguide with a diameter of 250 mm and length of 4 mm. According to the calculation using Eq. (9), this value is about 0.49 dB at 200 MHz, following which it gradually decreases to the cutoff frequency. The other SEdB is of two types: one is the additional SE by the dielectric in the transparent stopper caps (each with a length of 50 mm) of the fluid at both ends of Fig. 3(b), and the other is the transmission loss caused by the reflections that occur when the electromagnetic wave enters and leaves the specimen. The additional transmission loss due to reflection is given by Eq. (15) [18],

η ⎤ ⎡ Transmission = 10log10 ⎢|T 2| Re ⎛⎜ 0∗ ⎞⎟ ⎥ η ⎝ 1 ⎠⎦ ⎣

Table 3 Cutoff frequencies in circular waveguides.

(15)

where η0 and η1 are the intrinsic impedances of the incident medium and the transmission medium, respectively, and T is the permeability coefficient given by T = 2η1/(η0 + η1) . The experimental values to be analyzed in the next section are pure SE experimental values of the circular waveguide corrected by Eq. (14).

Cutoff frequency

Fluid(dielectric constant)

Waveguide 250 mm

Diameter 50 mm

30 mm

air(εr =1)

703

3,516

5,861

oil(εr =2) water(εr =78.2)

497 79.5

2,486 398

4,144 663

fnTEn (MHz)

4

Results in Physics 16 (2020) 102946

S. Yoon, et al.

Fig. 8. Field intensities measured for air-filled circular multi-waveguides with diameters of 30 mm and 50 mm.

Fig. 5. SEdB of an air-filled circular waveguide with a diameter of 250 mm.

The SE of the circular multi-waveguides with diameters of 30 mm and 50 mm was measured, and the SE was measured in air using the same measuring method employed in the circular waveguide with a diameter of 250 mm after the test specimen was manufactured. As shown in Fig. 8, the experimental results indicate that only two types of circular multi-waveguides have signals similar to noise. According to the theoretical calculation shown in Fig. 9, the pure SE of circular multiwaveguides with diameters of 30 mm and 50 mm for supplying air and oil in the range of 200 MHz to 1 GHz is higher than 150 dB at the least. Therefore, the circular multi-waveguides with diameters of 30 mm and 50 mm for supplying air and oil satisfy the shielding standards. However, this exceeds the scope because of the measurement limit that cannot be achieved even if the transmission power and the preamplifier are maximized, and other attenuation factors are minimized.

Fig. 6. SEdB of an oil-filled circular waveguide with a diameter of 250 mm.

Optimal multi-waveguides and review For the cut-off frequency, the criterion is satisfied only for air- or oilfilled circular multi-waveguides with diameters of 30 mm and 50 mm, as shown in Table 3. But the cutoff frequencies of the water-filled circular multi-waveguides with diameters of 30 mm and 50 mm are 663 MHz and 398 MHz, respectively, which do not satisfy the shielding standard. Further, as shown in Fig. 10, the SE of these waveguides increases slightly because of the electromagnetic wave attenuation caused by the dielectric; however, the waveguide with a diameter of 30 mm does not meet the 80 dB SE standard above 600 MHz, and that with a diameter of 50 mm does not meet the 80 dB SE standard above 400 MHz. Therefore, it can be concluded that the circular multi-waveguides with diameters of 30 mm and 50 mm do not satisfy the two shielding standards for supplying water. The circular multi-waveguides with diameters of 30 mm and 50 mm

Fig. 7. SEdB of a water-filled circular waveguide with a diameter of 250 mm.

increased with the increase in frequency, unlike the cases of air and oil. The experimental and the theoretical values exhibited a similar changing pattern; however, a significant difference existed between the two values. Compared with the theoretical value, the experimental value was higher than the theoretical value by 10–25 dB, except for the region in the range 200–1000 MHz. One of the biggest causes of the difference between the experimental and the theoretical values is that the electromagnetic wave mode considered in the theoretical value differs from the electromagnetic wave mode used in the experiment. In the theoretical case, only the TE11 single mode with the lowest cutoff frequency was used. However, because the multimode was used instead of the single mode in the measurement, more electromagnetic attenuation occurs in the modes except TE11. As a result, the SE of the theoretical value is higher than that of the theoretical value. The frequency range in which the experimental value is lower than the theoretical value is slightly lower than that of the cutoff frequency, and it is considered that this difference is caused mainly by the resonance phenomenon.

Fig. 9. SEdB of air- and oil-filled circular multi-waveguides with diameters of 30 mm and 50 mm. 5

Results in Physics 16 (2020) 102946

S. Yoon, et al.

circular multi-waveguides shown in Table 4 are the minimum standards satisfying the shielding criterion , and the optimum diameter and the length of the circular multi-waveguides are smaller for the diameter and longer for the length. The standard of the circular waveguide proposed for the air feeder is a diameter of less than 100 mm and a length of 500 mm or higher, which satisfy the criteria in provided in Table 4. Conclusion In the case of large-scale protection facilities, large-scale pipes capable of supplying air, water, and oil are generally installed. To ensure the continuous operation of such large-scale piping facilities, EMP protection measures must be prepared. To prevent EMP penetration through a piping system, a metal multi-waveguides pipe can be installed in a certain part of the piping facility. To obtain the optimum diameter and length, the theoretical equation for the circular multiwaveguides SE is obtained. Further, to validate the theoretical equations, EMP SE was measured for three different circular waveguides (air, oil, water) with a diameter of 250 mm. The maximum diameter and the minimum length of a circular multi-waveguides satisfying the minimum shielding criteria were obtained from the theoretical equations and are summarized in Table 4. In conclusion, the circular multi-waveguides for supplying air, oil, and water used in large piping facilities should be smaller in diameter and longer in length than those specified in Table 4. In addition, comparing the SE with the theoretical values presented in this study by fabricating a circular waveguide of diameter and length specified in Table 4 is important. Finally, in this study, we applied the shielding standard mentioned in MIL-STD, which specifies that the cut-off frequency must be higher than 1.5 GHz, and the SE must be higher than 80 dB at 1 GHz; however, it is known that the frequency generated by an e-bomb is currently in the range 1–10 GHz. Therefore, studying the minimum shielding criterion and the shielding method for an e-bomb that generates EMP of a frequency band higher than the HEMP would be meaningful and beneficial.

Fig. 10. SEdB of water-filled circular multi-waveguides with diameters of 30 mm and 50 mm.

Table 4 Maximum diameter and minimum length of a circular multi-waveguides that satisfy the requirements of SE. Use

Air supply

Oil supply

Water supply

Maximum diameter (mm) Minimum length (mm)

117.2 393.3

82.8 277.7

13.2 44.1

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 11. SEdB of circular waveguides of diameters and lengths that satisfy the minimum requirement of SE.

Acknowledgement

used for the analysis provided shielding effects greatly higher than the required shielding standards when used for supplying air and oil, and did not meet the shielding criteria when used for supplying water. Therefore, we suggest the theoretical equations for supplying air, oil, and water through multi-waveguides of proper diameter and length to satisfy the shielding standard defined in MIL-STD. The first standard of the shielding criteria is that the cut-off frequency should be higher than 1.5 GHz. This condition is determined only by the diameter of the circular waveguide and the dielectric constant of the fluid, as can be seen from Eq. (1). The maximum diameter of the circular waveguide is calculated using Eq. (1), and the fluid permittivity at 1.5 GHz is provided in Table 4. The second shielding criterion is that the SE should be greater than 80 dB at 1 GHz. The length of each circular multi-waveguides with an SE of 80 dB at 1 GHz is obtained from Eq. (11), and the results are provided in Table 4. The SE of a circular multi-waveguides of diameter and length given in Table 4 can be obtained using the theoretical formula, and the graph is shown in Fig. 11. The SE graphs for air and oil are almost similar, and at 1 GHz and higher, they closely match the minimum shielding standard. The SE graph for water is almost similar to those of air and oil at 1 GHz or less. In addition, at 1 GHz or higher, the SE is further reduced by dielectric attenuation. Therefore, the diameter and the length of the

This work is supported by a grant (NRF-2016K1A4A39-14113) from the National Research Foundation of Korea (NRF) and in part by the Kwangwoon University 2019, Korea. References [1] Mumtaz S, Lim J-S, Ghimire B, Lee S-W, Choi J-J, Choi E-H. Enhancing the power of high-power microwaves by using zone plate and investigations for the position of virtual cathode inside the drift tube. Phys Plasmas 2018;25:103113https://doi.org/ 10.1063/1.5043595. [2] Mumtaz S, Lamichhane P, Lim J-S, Yoon S-H, Jang J-H, Kim D-Y, et al. Enhancement in the power of microwaves by the interference with a cone-shaped reflector in an axial vircator. Results Phys. 2019:102611https://doi.org/10.1016/j. rinp.2019.102611. [3] Mumtaz S, Bhartiya P, Kaushik N, Adhikari M, Lamichhane P, Lee S-J, et al. Pulsed high-power microwaves do not impair the functions of skin normal and cancer cells in vitro: a short-term biological evaluation. J Adv Res 2019. [4] Foster Jr JS, Gjelde ME, Graham WR, Hermann RJ, Kluepfel MH, Lawson RL, et al. Report of the commission to assess the threat to the United States from electromagnetic pulse (EMP) attack. Volume 1. Executive Report 2004. [5] Manjung TA-S, Seongwon TA-C, Youngjin TA-K, Woo-Chul TA-P, Ho-Jae TA-K, Chang-Su TA-H. Electromagnetic wave shielding effectiveness measurement method of EMP. Prot Facility 2014;25. https://doi.org/10.5515/KJKIEES.2014.25.5.548. [6] Yeon-Chun TA-J, Jongkyung TA-L, Suk-Tai TA-K. Comparison of SE Evaluation Methods for HEMP. Shelters 2014;25. https://doi.org/10.5515/KJKIEES.2014.25.

6

Results in Physics 16 (2020) 102946

S. Yoon, et al.

[12] Pulse H-AE. Protection for Ground-Based C4I Facilities Performing Critical, TimeUrgent Missions Part 1 Fixed Facilities. 1998. [13] Balanis CA. Encyclopedia of RF and Microwave Engineering. In: K. Chang E-H, editor. 1st ed., NJ: John Wiley and Sons; 2005, p. 643–653. [14] Pozar DM. Microwave Engineering. New York: Microw. Eng. Second; 2012. p. 132–41. [15] Bereuter WA, Chang DC. Shielding effectiveness of metallic honeycombs. IEEE Trans Electromagn Compat 1982;EMC- 24:58–61. https://doi.org/10.1109/TEMC. 1982.304013. [16] Klein L, Swift C. An improved model for the dielectric constant of sea water at microwave frequencies. IEEE Trans Antennas Propag 1977;25:104–11. https://doi. org/10.1109/TAP.1977.1141539. [17] Stoudt DC, Peterkin FE, Hankla B. Transient RF and microwave pulse propagation in a debye medium (water). Interact Note 2011;622:1–43. [18] Jiang S, Georgakopoulos S. Electromagnetic wave propagation into fresh water. J Electromagn Anal Appl 2011;3:261.

11.1197. [7] Faust Scott W, Aloi Daniel N. Analysis, modeling, and measurement of shielding effectiveness for a cylindrical waveguide with a hexagonal insert structure. ACES J 2018;33(8). [8] US Army Corps of Engineers, “The effect of fluids on waveguides below cutoff penetrations as related to electromagnetic shielding effectiveness”, Technical Report M-354, July, 1984. [9] Kim S-B, Baek S-H, Lee J-H, Lee S-H, Baek J. A study on protective design standards for major military facilities. Seoul Minist. Natl. Def. 2012. [10] Dehkhoda P, Tavakoli A, Moini R. An efficient and reliable shielding effectiveness evaluation of a rectangular enclosure with numerous apertures. IEEE Trans Electromagn Compat 2008;50:208–12. https://doi.org/10.1109/TEMC.2007. 911922. [11] Lee K-W, Cheong Y-C, Hong I-P, Yook J-G. Prediction of shielding effectiveness in honeycomb structure using the modified design equation. J Korean Inst Electromagn Eng Sci 2005;16:862–71.

7