Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures

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1 Research Article Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures Takanori TSUTAOKA* 1,, Uswatun HASANAH*, Aiko TSURUNAGA* 1, Takashi UMEDA* 1, Kinya SHIMIZU* * 1 Department of Science Education, Graduate School of Education, Hiroshima University * Division of Natural Science, Graduate School for International Development and Cooperation, Hiroshima University Abstract Reflection and transmission of electromagnetic (EM) waves by a Metal Wire Array (MWA) composite structure were studied for a demonstration of electromagnetic shielding. Metal wire array composites have a layered structure consisting of a cross section paper, an array of thin copper wires and Polyethylene Terephthalate (PET) film. A simple experimental apparatus was constructed for the measurement of the reflection coefficient Γ and the transmission one T of the MWA composites with different intervals of metal wires. The electromagnetic shielding effect was evaluated by the calculation of Γ and T using the transmission line theory. The experimental results indicated a good agreement with the theoretical calculation; it can be considered that the microwave transmission and reflection experiments using MWA composite structures can be utilized as an experimental teaching material to learn radio wave shielding. Key words: electromagnetic shielding, transmission and reflection coefficients, metal wire array I. Introduction Pollution by electromagnetic radiation is becoming more and more serious with increasing use of electrical devices such as TVs, computers, mobile phones, and radios in our daily lives. It is increasingly affecting the normal life of people, animals, and has imperceptible side effect on animals and humans (Bhattacharjee, S., 14). Metals are the most common materials for Electromagnetic Interference (EMI) shielding. They function mainly by reflection due to the free electrons in them. Metal sheets are bulky, so metal coating made by electroplating, electroless plating, or vacuum deposition are commonly used for shielding (Chung, D.D.L., ). The use of electromagnetic shielding technology to eliminate or reduce the hazards of electromagnetic radiation has gained significant meaning (Yan-Jun, S. et al., 15; Tipa, R.S. et al., 8). In science education, the properties of electromagnetic waves such as reflection, refraction, diffraction etc. are taught in secondary physics education or undergraduate physics class at university level using microwaves as well as those of light. Furthermore, the reflection or absorption of EM waves is a topic in engineering education from the view point of the Electromagnetic Compatibility (EMC) technology as described above. Hence the electromagnetic shielding effect by metals or some other materials can be an issue for science and engineering education. In our daily lives, the microwave oven or Induction heater (IH) are home electronics products; the EM shielding is utilized in the protection from microwave radiation. As microwave shielding materials, we have selected the Metal Wire Array (MWA), which is used as the metal wire grid in the transparent front door of a microwave oven. To prevent the emission of harmful microwaves from the radiation source, very low transmission and high reflection or absorption characteristics are required for electromagnetic shielding materials. In this study, we investigated the reflection and transmission of a microwave through Metal Wire Array (MWA) composite structures composed of thin metal wires and a Polyethylene Terephthalate (PET) film to prepare experiments

2 Vol. No. for the demonstration of the EM shielding with metal wire array or grid and its composite structures. The complex transmission and reflection coefficients, T and Γ have been calculated as a function of wire distance by transmission line theory; the experiments were carried out using the 1.5 GHz microwave. In this report, the experimental and theoretical results of the transmission and reflection of the EM wave through the MWA composite structure will be presented; the effect of EM shielding by the MWA composite will be discussed from the viewpoint of physics or engineering education. II. Electromagnetic properties of metal wire array and its composite structure We consider a simple model of a thin metal wire array having a diameter d and the distance among the wires x in the configuration as shown in Fig. 1. We assume that a plane wave having the electric field E and magnetic field H propagates along the wave vector k. When the distance x is smaller than several cm, radio waves with a frequency up to several 1 MHz cannot pass through the MWA; the radio wave can be shielded by the metal wire structures (array or grid) even when there exists a free space between wires. When the radio waves arrive at the MWA, the polarization of free electrons is induced by the electric field E along the metal wire. This polarization produces an electric field E i along the opposite direction of E; E will be canceled out by E i. This concept is taught in high school or undergraduate physics class as well as undergraduate engineering education. As is known, since the transmission and reflection characteristics of EM waves through the MWA will change depending on the polarization direction of the incident wave, it is required to consider the polarization axis in the theoretical and experimental treatments. The electromagnetic properties of metal wire array can be described by transmission line theory using equivalent circuit analysis; the reflection or transmission of the plane waves through an electromagnetic medium can be presented by the following matrix representation (Pozer, D.M., 5), E A BE1 H C D H. (1) 1 where, E 1, H 1 and E, H are the electric and magnetic fields at the anteroposterior position of the medium, respectively. The matrix [A, B, C, D] is called transmission matrix. When a plane wave passes through the medium with the thickness d, the formula (1) can be written as cosh d Zsinh d E E1 H 1 sinh d cosh d H, () 1 Z where γ and Z are the propagation constant and impedance of the medium, respectively. In the case of metal wire array as shown in Fig. 1, the Fig. 1. Structure of the Metal Wire Array. The inset indicates the equivalent circuit.

3 Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures equivalent circuit can be described by an inductance L which is inserted parallel to the transmission line as shown in the inset of Fig. 1; the transmission matrix can be described by (Decker, M.T., 1959; Lewis, E.A. et al., 195) A B C D 1 1 j j Y. (3) 1 1 wg L X 1 g X g indicates the reactance of the metal wire array. When we set d and x as the diameter of metal wires and the distance between them, respectively, the X g is given as follows (Rotman, W., 196), X g Z x x ln, (4) d where λ and π are the wavelength of the plane wave in the free space and the ratio of the circumference of a circle to its diameter, respectively. In the matrix (3), the element C can be represented by the admittance of the wire array Y wg, Ywg j X g 1. (5) The transmission and reflection coefficients of the medium can be calculated from the input impedance of this medium Z in ; the Z in is given by, Z in B A E AE1 BH1 Z. (6) H CE DH D 1 1 C Z Hence the reflection coefficient Γ and the transmission one T are written as and T B A CZ D E Z Z Z E B i Zin Z A CZ D Z r in E E Et E E E t i i Z 1, (8) AZ B B A CZ D Z (7) where E i, E r and E t are the electric field of incident, reflected and transmitted EM waves, respectively. By using (3) to (5) we can get the Γ and T of the metal wire array as follows, ZY wg Z Y and T Z Y wg wg (9). (1) The calculated absolute values of the reflection and transmission coefficients Γ and T are shown in Fig. ((a) reflection coefficient, (b) transmission one) as a function of the distance x..45 GHz is the operating frequency of a commercially available microwave oven; 1.5 GHz indicates the operating frequency of this study. The microwave generator at 1.5 GHz can be fabricated using a gun diode; commercially available microwave experimental equipment can be used in the physics class (Microwave Optics Systems, 16). The thickness of wires is.1 mm. As is shown in Fig. (b), T is zero up to about x=.1 cm indicating the both microwaves cannot pass through the MWA. Γ is unity in this distance range; it is indicated that the microwaves at.45 and 1.5 GHz can be shielded by the metal wire array or grid having 1 mm distance. Γ starts to decrease from about x=. cm and becomes zero at about x=1 cm for the.45 GHz wave. Simultaneously, T increases with x in this distance range. For the 1.5 GHz microwave, the shielding frequency shifts to a smaller distance. In this study, we made a laminated structure consisting of the MWA and a PET film as shown in Fig. 3(a). The parallel straight copper wires were aligned with the y-axis as shown in Fig. 3(b). The distance between wires is defined by x. The two types of copper wire with diameter d of.1 and.3 mm were used. The equivalent circuit of this composite structure is shown in Fig. 4. The MWA is sandwiched between a PET film with the thickness of b and a cross section paper. The metal wire array part can be represented by the Z with the thickness of d/ and the lumped admittance Y wg. In this structure, the PET film has a larger permittivity than that of vacuum, ε ; the permittivity value of the cross section paper is almost the same as ε. Hence the impedance of PET film Z w should be taken into account.

Vol. No. Fig.. Theoretical calculation results of reflection (a) and transmission (b) coefficients for the metal wire array as a function of the distance between wires x at different frequencies,.

4 Vol. No. Fig.. Theoretical calculation results of reflection (a) and transmission (b) coefficients for the metal wire array as a function of the distance between wires x at different frequencies,.45 and 1.5 GHz. Fig. 3. Schematic diagram of the metal wire array composite structure (a) and a photograph of the metal wire array on the cross-section paper (b). Fig. 4. Structure of the metal wire array composite (MWAC) and the equivalent circuit.

5 Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures The element C of the transmission matrix (3) of the metal wire array composite structure (MWAC) can be represented by (Hatakeyama, K. et al., 15; Yamamoto, S. et al., 15) C Ywg j, (11) Z b d m where ε m is the electric permittivity of the PET film. Δb and Δd are defined by, b and d b (1) d. (13) The transmission and reflection coefficients of the MWAC can be calculated from (7) and (8) using the transmission matrix with C as (11); Γ and T are given by ZY Z Y and T with Z Y (14) (15) The f p is called plasma frequency; the ε r becomes negative below this frequency. In the formula (17), if ε m is real number (ε r =: no loss), ε r has only the real part ε r. Since the magnetic permeability of the MWA and MWAC is positive in the whole frequency range, EM waves with a frequency below f p cannot propagate in this medium. The shielding effect of the MWA is attributed to the plasmonic property of metal grids which is produced by the plasma oscillation of conduction electrons in metal wires (Rotman, W., 196), the effective permittivity becomes negative in the plasmonic state below plasma frequency f p. This is the physical reason that the EM waves can be shielded by the plasma medium. The frequency dependence of ε r then can be represented by mb d fp r ' b d 1 f. (19) When the frequency is fixed, ε r can be obtained as a function of wire array distance x from (17). The variation of ε r with frequency for the MWA at several wire array distances has been studied so far; good agreements between theoretical calculations and experimental results for the MWA structure were reported (Tsutaoka, T. et al., 4). Furthermore, it has been reported that the plasma frequency f p of MWA shifts to a higher frequency range as x decreases. The calculated electric permittivity ε r of the MWAC is shown in Fig. 5 as a function of the wire distance x at several fixed frequencies. Y Ywg j. (16) Z b d m Furthermore, we can consider the electric permittivity of this MWA composite structure; by use of the given parameters, the relative permittivity ε r of the MWAC can be represented by (Yamamoto, S. et al., 15) mb d 1 r r' jr". (17) bd xb d x ln d Since the wavelength λ can be represented by λ =c/f (c is the speed of light), the frequency f p at which the ε r crosses zero from negative to positive is given by 1 c 1 fp. (18) x mbd x ln d Fig. 5. Relative permittivity ε r of the metal wire array as a function of distance x at several frequencies.

Vol. No. We can define the plasma distance x p at which the ε r becomes negative below this distance for the MWA or MWAC. At 1.5 GHz, the x p locates at about 5.6 cm. III.

6 Vol. No. We can define the plasma distance x p at which the ε r becomes negative below this distance for the MWA or MWAC. At 1.5 GHz, the x p locates at about 5.6 cm. III. Experimental apparatus and setup In this study, basic research was carried out for the development of experimental teaching materials to observe electromagnetic wave shielding by metal wire array. A metal wire grid having an array of thin metal lines as shown in Fig. 3(b) was prepared using thin copper wires with a diameter d of.1 mm. The distance between wires x was adjusted to be several values from 1. to 3 mm. As described above, to keep the MWA in the right position, a composite structure in which the MWA was sandwiched by the two layers of cross section paper and PET film was prepared. The thickness of the PET film is 1 μm with a relative permittivity value ε m =3.. We constructed simple experimental setups for the transmission and reflection measurements. A schematic diagram of the transmission experiment for the MWA is shown in Fig. 6(a). As the transmitter of the 1.5 GHz microwave, a commercially available microwave experimental apparatus (Uchida TE-4) was used. This type of microwave experimental apparatus is common in commercially available teaching materials for physics education. This equipment contains the signal generator and antenna in a box. The Fig. 6. Experimental setup for the transmission coefficient measurement.

Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures transmitted wave was detected by the receiver which contains the antenna and detecting circuits inside.

7 Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures transmitted wave was detected by the receiver which contains the antenna and detecting circuits inside. The appropriate distance between transmitter and MWA or MWAC and receiver was selected considering the apertural area of antenna and receiver where the microwave becomes the far field plane wave at the sample position. In this experiment, we set z 1 as the distance from the top surface of the transmitting antenna to the metal wire array and z as the distance between the MWA and the antenna of the receiver; z 1 and z are 48.8 cm and 37.4 cm, respectively. A photograph of the transmission experiment setup is shown in Fig. 6(b). The EM absorber was placed beside the sample to eliminate the diffracted waves. The direction of the electric field E from the transmitter was set to be parallel to the metal wire array direction; the plane wave with the wave vector k was made incident vertically to the MWAC plane. The amplitude of the received signal was recorded using a voltmeter. The transmission coefficient was determined from the amplitude ratio in which the obtained signal voltage V was normalized by the voltage V which is measured without the sample between transmitter and receiver as V T, () V where T changes from zero (no transmission) to unity (1% transmission). In the reflection measurement, transmitter and receiver were placed on the same side from the MWAC sample; the microwave was diagonally entered into the surface of the MWAC sample as shown in Fig. 7. The incident angle θ was set to 4. The reflection coefficient Γ was determined by the same procedure using the normalized voltage V as the reflected signal by a metal copper plate, Γ=V/V. In this case, Γ changes from unity (1% reflection by the metal plate) to zero (no reflection). IV. Results and discussion The experimental results of the transmission and reflection coefficients, T and Γ, of the metal wire array composite structure are shown in Fig. 8 ((a) transmission coefficient T and (b) reflection one Γ) as a function of the distance x. In Fig. 8(a), the solid lines are the theoretical curves of T calculated from (15) for the MWA only and the MWA+PET film (MWAC), respectively. The solid circles indicate the experimental results of the setup for the transmission measurement. When the interval x is 1 mm or above, the transmission coefficient T is close to unity indicating that the MWAC is transparent to the 1.5 GHz microwave. Meanwhile, T changes significantly when x is between 5 and 1 mm; the microwave cannot pass through the MWAC below about 1 mm. Hence it is shown that the distance between wire s should be at least 1. mm to shield the 1.5 GHz microwave with the MWAC structure. As shown in this figure, the calculation curve for the MWAC has a good agreement with the current measurement. The reflection coefficient Γ of the MWAC and the MWA is Fig. 7. Experimental setup for the reflection coefficient measurement.

8 Vol. No. Fig. 8. The reflection coefficient Γ and transmission coefficient T of the metal wire array composite at 1.5 GHz as a function of the wire distance x. Solid lines indicate the theoretical calculation results and solid circles show the experimental results. shown in Fig. 8(b) as a function of the distance x. The solid lines are the calculated curves; the solid circles indicate the experimental results. When the interval among the metal wires x is below about.1 cm, the reflection coefficient Γ is almost unity; Γ rapidly decreases between x=.1 and 1. cm. A fairly good agreement between the calculated results of the MWAC and the measurements was obtained. The difference of the calculated transmission coefficient T between the MWA and the MWAC is not so large compared to that for the reflection coefficient. This result indicates that the transmission characteristic of the microwave through the MWAC is mainly determined by the metal wire array; the PET film does not have an important role in the transmission. On the other hand, the PET film decreases the shielding effect of the microwave; since Γ at the 1. cm distance is.4 for the MWA, Γ at the same x is almost zero. The plasma distance x p is also indicated in the figures. At the x p, T is unity and Γ is zero; % shielding can be achieved at the plasma distance. From the results indicated above, it can be concluded that the characteristics of the microwave shielding by metal wire array or grid can be demonstrated by a simple experiment using a metal wire array composite combined with numerical calculations by the transmission line theory. As an example for a practical application, it can be shown that the EM wave from the microwave oven can be shielded by the metal wire grid with several mm of aperture; hence the object in the oven is visible through the front door. Simultaneously, a low frequency plasmonic state which can be achieved by the metal wire array composite structure can be demonstrated from the view point of material science as well. V. Conclusions We investigated the demonstration of microwave shielding by a simple experimental setup using metal wire array sheets as well as a theoretical calculation of the transmission and reflection coefficients. A commercially available experimental apparatus using a 1.5 GHz microwave was employed as transmitter and receiver. The transmission and reflection coefficients of the metal wire array composite structure were measured as a function of the distance x; good agreements between theoretical and experimental results for the transmission and reflection coefficients have been obtained. Hence this experiment can be utilized as an experimental teaching material to observe microwave shielding by metal wire array in undergraduate physics or engineering education as well as advanced study for secondary physics education. Acknowledgements This work was supported by a grant-in-aid for scientific research (A) No. 17H8) from the Japan Society for the

9 Demonstration of Electromagnetic Shielding Using Metal Wire Array Composite Structures Promotion of Science and the Indonesia Endowment Fund for Education Scholarship (LPDP). References Bhattacharjee, S. (14): Protective Measures to Minimize the electromagnetic Radiation, Research India Publications, 4, Chung, D. L. (): Materials for Electromagnetic Interference Shielding Materials Engineering and Performances (Springer). Decker, M. T. (1959): Transmission and Reflection by a Parallel Wire Grid, Journal of Research of the National Bureau of Standards - D. Radio Propagation, 63D, Hatakeyama, K., Tsutaoka, T., Kanemoto, T., Yamamoto, S., Iwai, T. (1): Reflection and Transmission Characteristics of EM-Waves with the Wire-Grid and Its Use as a Back Layer of EM-Wave Absorber, IEICE TRANSACTIONS on Communications, J93-B, (in Japanese). Microwave Optics Systems (16): prodcompare/microwave-optics-systems/index.cfm. Lewis, E. A., Casey, J. P. (195): Electromagnetic Reflection and Transmission by Gratings of Resistive Wires, Journal of Applied Physics, 3, Pozar, D. M. (5): Microwave Engineering, 3rd edn (Jone Wiley & Sons). Rotman, W. (196): Plasma simulation by artificial dielectrics and parallel-plate media, IRE Transactions on Antennas and Propagation, 1, Tipa, R. S., Baltag, O. I. (8): Study on A Model of Bragg Diffraction Using Microwaves, Romania Journal Physics, 53, Tsutaoka, T., Hirashiba, M., Kasagi, T., Hatakeyama, K., Fujimoto, K. (4): A Left-Handed Material Combined YIG and Thin Metal Wire Array, Proceedings of the 9th International Conference on Ferrites (ICF 9), Yamamoto, S., Okita, M., Hatakeyama, K., Tsutaoka, T. (15): HF Characteristics of laminated structure consisting with negative permittivity and high permittivity materials, Proceedings of the Joint IEEE International Symposium on EMC Europ, Dresden, Yamamoto, S., Hatakeyama, K., Tsutaoka, T. (15): Reflection and transmission characteristics of laminated structures consisting a dipole array sheet and wire grid and dielectric layer, IEICE TRANSACTIONS on Communications, E98-B, Yan-Jun, S., Hao, C., Song-hang, W., Yan-Bing, L., Li, W. (15): Study on Electromagnetic Shielding of Infrared/Visible Optical Window, Canadian Center of Science and Education, 9, (Received July 6, 17; Accepted September 11, 17) Takanori TSUTAOKA Department of Science Education, Graduate School of Education, Hiroshima University Kagamiyama, Higashi-Hiroshima , Japan

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