Microstrip Bandstop Filter for Preventing Conduction Electromagnetic Information Leakage of High-Power Transmission Line

The research of transient electromagnetic pulse emission monitoring technology (TEMPEST) protection is essential. Based on microstrip bandstop ﬁlter (MSBSF), a method to prevent electromagnetic information leakage of high-power transmission lines is proposed in this paper. The MSBSF with a high insertion loss and carrying large current in the frequency of 2.40–2.49GHz is designed, fabricated, and experimentally measured. The fabricated MSBSF with the insertion loss of 38 dB at 2.41–2.49GHz can carry a current greater than 10A and withstand a voltage of 1.7 kV. Compared with the traditional electromagnetic interference (EMI) ﬁlter, the MSBSF has the advantages of preventing conducted electromagnetic information leakage in the high-frequency bands, carrying greater current subject to higher voltage, and possessing lighter weight. The MSBSF may be used to prevent the leakage of high-frequency electromagnetic information of high-power transmission lines.


Introduction
With the rapid development of information and communication technology, electromagnetic information leakage associated with telecommunication devices, such as transmission lines, brings increasingly strong impacts on information security. Transient electromagnetic pulse emission monitoring technology (TEMPEST) is known for using reverse technology to collect and restore the transmitting source signals and preventing potential security threats of information equipment [1]. Nowadays, many scholars have begun to actively research and develop TEMPEST [2][3][4][5][6], especially TEMPEST protection. Many methods of TEM-PEST protection have been achieved, for example, low-radiation equipment [7][8][9][10], noise interference [11], electromagnetic shielding [12][13][14], filtering technology [15], and optical fiber technology [16,17]. Low-radiation equipment, noise interference, electromagnetic shielding, and optical fiber technology mainly prevent electromagnetic information leakage caused by electromagnetic radiation. Filtering technology is the only way to prevent electromagnetic information leakage caused by conduction.
With the development of high-speed communication technology, the leakage of electromagnetic information in high-frequency band should be widely concerned. In 2005, the electromagnetic information leakage up to 3 GHz was measured by K. Hoffmann and Skvor [18]. Traditional electromagnetic interference (EMI) filters at their operation frequency can block information only in the frequency range of 150KHz-30 MHz and carry a current of 10 A and withstand a voltage of 250 V. What is worse, EMI filter designed by using lumped components has greater volume and weight and cannot prevent information leakage of information equipment in high-frequency band (∼GHz). e microstrip filter made of PCB has the advantages of light weight and small volume, which has been studied by many scholars. In addition, based on a triple path signal interference mechanism, Song et al. [19] proposed a compact broadband bandstop filter with the insertion loss of 30 dB for use in modern wireless systems. A broadband commonmode filter with the insertion loss of 10 dB was designed for solving electromagnetic interference in high-speed differential digital systems and discussed by Chan et al. [20]. Phudpong and Hunter [21] projected a prototype of a singleresonator bandstop limiter with an insertion loss of 32 dB, a resonant frequency of 2 GHz, and a limiting bandwidth of approximately 200 MHz. e dual-wideband bandstop filters, which are used in multi-wideband wireless systems, were demonstrated by Feng et al. [22]. A planar high insertion loss bandstop filter within a frequency band from 2.79 to 3.21 GHz was developed by Wu et al. [23]. However, those microstrip filters are used in low-power communication circuits and devices. To the authors' best knowledge, microstrip filters for preventing conduction electromagnetic information leakage of high-power transmission lines are rarely reported.
Because of the importance of TEMPEST protection, a method to prevent electromagnetic information leakage of high-power transmission lines based on the microstrip bandstop filter (MSBSF) is proposed in this paper. e MSBSF with high insertion loss at 2.40-2.49 GHz is designed, fabricated, and measured. e results show that it can carry the current greater than 10A, withstand the voltage of 1.7 kV, and have the insertion loss of 38 dB at 2.41-2.49 GHz. Compared with the EMI filter, the MSBSF can more effectively prevent the conduction electromagnetic information leakage of high-power transmission lines in high-frequency band apart from carrying greater current subject to higher voltage, having lighter weight and smaller volume. e MSBSF has a good application in preventing conducted electromagnetic information leakage in high frequency. Figure 1, when transmitting electromagnetic signal or energy over a long distance, the transmission line of an information device may carry electromagnetic information, and the information leakage will be caused by the process of probe detection and spectrum analysis. To prevent information leakage, the filter should be connected before the information device is connected to the transmission line.

2.2.
eory Analysis. Transmission lines of information devices such as power lines and telephone lines can be regarded as parallel two conductors because the length of transmission line is very long. In the situation of transmitting high-frequency electromagnetic waves, Kirchhoff's law cannot be applied to the calculation of voltage and current on a transmission line. Transmission line is divided into many very short transmission lines, the length of each segment is shorter than the wavelength of high-frequency electromagnetic waves, and Kirchhoff's law can be used for each segment of the transmission line. As shown in Figure 2, each segment of the transmission line can be equivalent to loss resistance, distributed inductance, conductivity, and distributed capacitance due to the electromagnetic coupling effect. e resistance R 0 , inductance L 0 , capacitance C 0 , and conductance G 0 are denoted as where µ 1 is the permeability of two conductors, σ 1 is the conductivity of two conductors, ε 1 is the dielectric constant of the medium between two conductors, µ is the permeability of the medium between two conductors, σ is the conductivity of the medium between two conductors, D is the distance between two conductors, and d is the diameter of the two conductors. Based on Kirchhoff's low, each segment transmission line is expressed as zi(x, t) zx i(x, t) � i max cos(wt + yi(x)) where u(x) is the voltage at position x on the transmission line; i(x) is the current at the position x on the transmission line. Equations (6) and (7) can be derived from the combined equations (2)- (5).
e derivative of equations (6) and (7) with respect to x, respectively, gives us the following: where c is the propagation coefficient of voltage and current on the transmission line. By solving (8) and (9), the general solutions (10) and (11) can be obtained.
When the voltage and current at x � 0 on the transmission line are known, the voltage and current at any part of the transmission line can be obtained. e equation is described as follows: When the voltage or current on the transmission line is detected by a probe, information leakage will occur. e schematic diagram of the filter is shown in Figure 3. It can be found that filter is a two-port network, the input voltage and current of port 1 are u 1 and i 1 , respectively, and the output voltage and current of port 2 are u 2 and i 2 , respectively. When the filter ports match, the forward voltage transmission of the filter can be expressed as [24] When the filter is a bandstop filter, the filter can reduce the voltage and current containing information on the  International Journal of Antennas and Propagation 3 transmission line. Hence, the bandstop filter can prevent information leakage before the information device is connected to the transmission line. Based on the previously mentioned analysis, the bandstop filter is important to prevent the transmission line conducting electromagnetic information leakage. erefore, it is worthy to research bandstop filter and the design of bandstop filter is mainly introduced in the following paper.

Filter Structure
e structure diagram of the traditional bandstop filter is shown in Figure 4(a). It is composed of connected and open stubs. e filter with connected and open stubs can achieve high insertion loss, but the physical width of connected stubs is too narrow to carry a high current. According to [23], the structure of the traditional bandstop filter can be transformed into that of Figure 4(c) by using the structure of Figure 4(b), and the unequal width of the coupled structure can be used in the structure of Figure 4(c), which enables the connected stubs to carry a high current. Hence, Figure 4(c) is the structure of the proposed MSBSF for preventing conduction electromagnetic information leakage of highpower transmission line in this paper.
As is shown in Figure 4(c), the proposed MSBSF consists of n parts, and each part consists of a parallel coupling line with an open stub. To realize a high insertion loss (>40 dB) and has a simple structure, the MSBSF consists of only five parts. e reason MSBSF consists of only five parts is shown in Figure 5. It can be found that the MSBSF consists of only part1, or part1 + part2, or part1 + part2 + part3, or part1 + part2 + part3 + part4 cannot realize high insertion loss, so the structure of the filter is composed of five parts, and the structure diagram in shown in Figure 2(d). e odd and even mode characteristic impedances of the parallel coupling lines are Z o1 and Z e1 , Z o2 and Z e2 , Z o3 and Z e3 , Z o4 and Z e4 , Z o5 and Z e5 , respectively. e characteristic impedances of the open stubs are Z 1 , Z 2 , Z 3 , Z 4 , Z 5 , respectively. e characteristic impedances of the port are Z 0 , Z 0 � 50 Ω. e electrical lengths of the parallel coupling lines and open stubs all are θ. θ � π/2 at the center frequency of the stopband.
e MSBSF is symmetrical around part 3 for the convenience of design and analysis. Because the MSBSF is symmetrical around part 3,

eory Analysis.
According to the structure diagram of the MSBSF with five parts, the ABCD matrix of each part can be derived as [21] where k i is the coupling coefficient and is defined as (19), Since each part is connected in cascade, the ABCD matrix of MSBSF can be described as Based on formula (17), when S 21 � 0, the transmission zeros of MSBSF can be calculated.

Simulation
Analysis. e influence of MSBSF's characteristic impedance on its insertion loss and stopband bandwidth is explored by using HFSS (3D electromagnetic simulation software). By simulation, the results are shown in Figure 6(a), and it can be found that the insertion loss increases with the increase of the characteristic impedance of Z 1 (Z 5 ) and Z 3 , and decreases with the increase of the characteristic impedance of Z 2 (Z 4 ). When Z 1 (Z 5 ) is greater than 90 Ω, Z 2 (Z 4 ) is less than 80 Ω, and Z 3 is greater than 80 Ω, the insertion loss of MSBSF is greater than 50 dB. e relationship between Z 1 (Z 5 ), Z 2 (Z 4 ), Z 3 , and bandwidth at an insertion loss of 50 dB is shown in Figure 6(b), respectively. When the insertion loss is greater than 50 dB, the bandwidth at 50 dB decreases with the increase of characteristic impedance Z 1 (Z 5 ) and Z 3 and increases with the increase of characteristic impedance Z 2 (Z 4 ). Figure 7(a) shows the relationship between the coupling coefficient and maximum insertion loss. It can be found that when coupling coefficient k 1 (k 5 ) is greater than 0.45, the insertion loss of MSBSF is greater than 50 dB, and when the coupling coefficients k 2 (k 4 ) and k 3 are 0.3 to 0.45, the insertion loss is greater than 50 dB. When the insertion loss is greater than 50 dB, the relationship between the coupling coefficient and the bandwidth at insertion loss 50 dB is shown in Figure 7(b). It can be found that the bandwidth at insertion loss 50 dB increases with the increase of the coupling coefficient of k 2 (k 4 ) and k 3 and decreases with the increase of the characteristic impedance of k 1 (k 5 ).
rough the previously mentioned simulation analysis, the design process of the MSBSF can be summarized as follows: (1) Specify the insertion loss and the bandwidth of the designed MSBSF (2) According to Figures 6 and 7, the coupling coefficient and characteristic impedance are determined (3) Finally, optimization is done using HFSS

External Microstrip Line and PCB Analysis
In this section, in addition to the width of the external microstrip line, which can carry a current of 10A, the relationship between the width and the characteristics of PCB is also discussed. e cross-section structure of PCB is shown in Figure 8, where M is the thickness of the external microstrip line, H is the thickness of the substrate, and W is the width of the external microstrip line. To enable the MSBSF to be used in high-power transmission lines, the current carrying capacity of the external microstrip line must be considered. According to the IPC-2221standard, when the materials of the external microstrip line and ground plane are copper, the current that an external microstrip line can carry is as follows [25]:

International Journal of Antennas and Propagation
where I is the maximum current and dT is the temperature rise above ambient. e relationship between the current that an external microstrip line can carry and the width of the line is shown in Figure 9. e higher the current, the larger the width required for the external microstrip line. When the current is 10 A, the width of the external microstrip line is 3.58 mm.
us, if the external microstrip line carries a current more than 10 A, the width of the line needs to be larger than 3.58 mm.
According to [26][27][28], when the characteristic impedance of the external microstrip line is a fixed value, the width of the line is related to the dielectric constant and thickness of the substrate. e specific relationship between the width of the external microstrip line and the dielectric constant and the thickness of the substrate is as follows [24]: where Z is the characteristic impedance of the external microstrip line and ε is the dielectric constant of the substrate.
To clearly show the relationship between the width of the external microstrip line and the dielectric constant and the thickness of the substrate, the relevant data calculated by line calculation software are shown in Figures 10(a) and 10(b). Figure 10(a) reveals the relationship between the dielectric constant of the substrate and the width of the external microstrip line. With the increase of the dielectric constant of the substrate, the width of the external microstrip line decreases. When the dielectric constant of the substrate is less than 3.1, the width of the external microstrip line is larger than 3.58 mm, and the current flowing through the external microstrip line is greater than 10A. e relationship between the thickness of the substrate and the width of the external microstrip line is plotted in Figure 10(b). With the increase of the substrate thickness, the width of the external microstrip line also increases. When the thickness of the substrate is greater than 1.35 mm, the width of the external microstrip line is greater than 3.58 mm, and the current flowing through the external microstrip line is greater than 10A.
By analyzing the performance of the external microstrip line passing through the current, it can be found that when the characteristic impedance of the external microstrip line is 50 Ω and the external microstrip line can carry a current of 10A, the PCB with a substrate thickness of 1.5 mm and a dielectric constant of the substrate less than 3.1 can be used, or the PCB with a dielectric constant of 2.65 and a thickness of substrate greater than 1.35 mm can be used.
Several commonly used PCBs for fabricating MSBSF are shown in Table 1. It can be found that the dielectric constant and substrate thickness of the PCBs meet the requirements, but the cost of F4B is less than that of Rogers.

Experiment and Discussion
Based on the analysis in Sections 4 and 5, a MSBSF with an insertion loss of 50 dB in the frequency range of 2.4-2.49 GHz is designed. According to Figures 6 and 7, 3 Ω, k 1 � k 5 � 0.48, k 2 � k 4 � 0.37, and k 3 � 0.46. After simulation and optimization by HFSS, the final characteristic impedance parameters of the MSBSF are Z 0 � 50 Ω, Z 1 � Z 5 � 91.33 Ω, 24 Ω, and Z e3 � 108 Ω. According to Section 5, considering the low cost, the MSBSF is made of F4BM265, and its dielectric constant, thickness of the substrate, and thickness of external microstrip line are 2.65, 1.5 mm, and 0.07 mm, respectively. To make the transmission line of the MSBSF pass through the current of 10A, the unequal width of the coupled structure [23,29] is used. e final layout and the dimensions of the MSBSF are shown in Figure 11(a) and Table 2, respectively. e manufactured MSBSF is presented in Figure 11(b).
To verify the filtering effect of the MSBSF, a vector network analyzer is utilized. e experimental equipment for measuring the filtering effect of the MSBSF is shown in Figures 12(a) and 12(b). e electromagnetic (EM) simulation and measurement results of the MSBSF are plotted in Figure 13. e bandstop filter designed in this paper can produce transmission zeros, but there are no zeros at the center frequency, so the bandwidth of the stopband is narrow. e EM simulation results show that an insertion loss more than 50 dB can be achieved in the frequency range  International Journal of Antennas and Propagation 7 of 2.40-2.49 GHz, and three transmission zeros can be realized in the stopband, which are located at 2.41 GHz, 2.44 GHz, and 2.48 GHz, respectively. Impedance matching must be considered when the microstrip filter is connected to the transmission line to prevent the transmission line conducting electromagnetic information leakage, but impedance matching is not the main research in this paper. erefore, for matching the impedance, the coaxial     To verify that the MSBSF can pass through a current greater than 10A, a current tester and an electromagnet are used. e current tester provides current, the electromagnet is the load resistance, and the MSBSF is connected in series with the power transmission lines. e experimental equipment for measuring the current is shown in Figure 14(a). It can be found that the MSBSF can carry a current greater than 10A. To test the withstand voltage performance of the MSBSF, a high-voltage generator is applied.
e experimental equipment is shown in Figure 14(b). It can be found that MSBSF can withstand a voltage of 1.7 kV.
rough the previously mentioned experiments, it can be found that the MSBSF can achieve an attenuation of 38 dB to block 2.41-2.49 GHz high-frequency signal and can be used in transmission lines with a power of at least 17544 W. e comparison among the MSBSF in this paper and the reported filters is provided in Table 3. It can be found that the insertion loss of MSBSF is better than that given in [19,20], and the current and voltage of the filter are analyzed and verified. e comparison between the MSBSF and the EMI International Journal of Antennas and Propagation 9 filter is provided in Table 4. It can be found that the MSBSF withstands higher voltage and carries greater current than EMI filters in addition to being used in high-frequency bands.

Conclusion
In this paper, a high insertion loss microstrip bandstop filter (MSBSF), which can be applied to high-power transmission lines, is designed. According to the structure of the traditional branch load band stop filter, the equivalent structure and the unequal-width technology of the parallel coupled microstrip line are used to realize the filter can carry large current and have high insertion loss by increasing the filter order. e MSBSF is composed of n parts, and each part consists of a parallel coupling line with an open stub. To produce the MSBSF with a high insertion loss, an analysis is carried out, and it is found that the MSBSF consisting of five parts can realize an insertion loss more than 40 dB and has a simple structure. en, the MSBSF is analyzed from both theory and simulation aspects, and the design process is summarized. Besides, the width of the external microstrip line which can carry a current of 10A is analyzed, and the characteristics of PCB which can carry a current of 10A are summarized. Finally, according to the design process of MSBSF, the requirements of the external microstrip line carrying a current of 10A, and the characteristics of PCB which can carry a current of 10A, MSBSF with an insertion loss of 50 dB in the frequency range of 2.40-2.49 GHz is designed and fabricated with low cost F4BM265. To test the performance of MSBSF, a vector network analyzer, a current tester, and a high-voltage generator are used. It is found that the attenuation of MSBSF is 38 dB at 2.412.49 GHz, and the MSBSF can carry a current greater than 10A and withstand the voltage of 1.7 kV. Hence, the designed MSBSF may be used in preventing high-frequency conduction electromagnetic information leakage of high-power transmission lines. Microstrip filter has good performance in microwave frequency band, and because of its unique structure, it is convenient to connect with transmission line and has unique advantages in preventing transmission line conduction electromagnetic information leakage. In order to better filter electromagnetic information in transmission line, the filter used to prevent transmission line conduction electromagnetic information leakage should have high insertion loss. erefore, the research of filter with high insertion loss has research value in preventing transmission line conduction electromagnetic information leakage and should become one of the performance improvements of band stop filter in the future. Although the range of work frequency of the filter that we discuss in this paper is very narrow (2.41-2.49 GHz), it is a part of our current research work. e range of work frequency of this filter has the possibility of broadening, but it requires further simulation and verification, which is also what we will do in the future.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.