Optimized Shielding Pattern of RF Faraday CageNaoki Ohmura and Satoshi Ogino

Microwaveabsorbers Inc5-4-5, Asakusabashi, Taito-Ku, Tokyo, Japan

Email: ohmura@mwa.co.jp

Yoshinobu OkanoTokyo City University

1-28-1, Tamazutsumi, Setagaya-Ku, Tokyo, JapanEmail: y-okano@tcu.ac.jp

Abstract—The application of UHF radio-frequency identifi-cation (RFID) systems is rapidly increasing. However, becauseband near the frequency for the UHF-RFID system is also usedwith the cellular phone, the influence on UHF-RFID system isworried. Prepare the Faraday cage is easy to solve this problem,but generally it shields external noise frequency by broadband.So, It makes difficult to using wireless communications in theinside and outside of Faraday cage. On the other hand, it’s easyto control target frequency for self resonance pattern. This paperdescribes Faraday cage with special pattern which shields mainlytarget frequency, and it has suitable shielding effectiveness whichenables wireless communication in other frequency band.

I. INTRODUCTIONIn Japan, 952-954MHz was allocated as frequency band

for UHF-RFID system. Recently, UHF-RFID system wouldbe moved to international standard frequency band (915-920MHz) by reorganizing the frequency of the cellular phone.However, a lot of UHF-RFID equipments used in Japan areoperated by conventional frequency band until now. Therefore,there is a possibility of serious interference outbreak betweena part of cellular phone frequency band (940-960 MHz)and conventional UHF-RFID equipment.[1][2] To solve thisproblem, we should use a RF Faraday cage which shieldsmobile frequency. [3] In this Faraday cage, existing UHF-RFID system is isolated out of between 940 - 960 MHz.However in this cage, there is a possibility that usual mobilecommunication or Wi-Fi communication out of tent cannot beused by this isolated area. Therefore we invent an RF Faradaycage which shields target frequency such as 952 - 954MHz inorder that in this cage we can use existing UHF-RFID systemsand mobile phone/Wi-Fi communication simultaneously.

II. DESIGN OF SHIELDING PATTERNThis section describes the condition of numerical simulation

and measurement of Shielding Effectiveness(SE). The technol-ogy that controls the microwave’s pass band by the conductorelement array (hereafter, it is called shielding pattern) hasalready been reported[4]. However, it is difficult to achievehigh SE in broadband frequency.On the other hand, the mesh-conductive-cloth is a conve-

nient material which can control SE without the frequencydependence. But it’s also difficult to generate resonance point.The technology that can control both the frequency pass bandand SE with layered shielding pattern and mesh-conductive-cloth (it is named united shielding pattern (USP)) is proposedin this report.

Fig.1 shows that outline of actual Faraday cage withUSP. The Faraday cage has the dimensions 1.8m×1.8m×1.8mand the window frame it’s attached USP has the diminsions0.6m×0.6m(see Fig.1 (b)). Around margin of the window,it’s covered with doubles mesh-conductive-cloth. On the otherhand, the window frame area uses one layer mesh-conductive-cloth. So, it’s expected that electromagnetic wave mainlypass through from window frame area. Therefore, the targetfrequency band 940-960MHz is mainly shielded for USP. Theshielding pattern’s arm length is set to half of target’s wave-length λ (when the target frequency is set to 953 MHz, λ isestimated with 314 mm.) as shown in Fig 2. Each specificationused to calculation by finite-difference-time-domain(FDTD)method is shown in Table.I. Fig.3 shows the model usedfor parameter analysis by the FDTD method. The cell sizeis set to 2.0 mm in X, Y and Z directions. The time stepis 3.852 ps based on the Courant condition [5][6]. As theabsorbing boundary condition, a Higdon s condition is used.Experimental results of SE for USP are given in next Section.

TABLE IEACH PARAMETER OF THE CALCULATION

Structure Specification Abilityframe length W 200 mm

Shield pattern shielding pattern’s arm length ls 157 mmshielding pattern’s arm width ws 10 mm

material PECDielectric substrate thickness ds 40 mm

material styrofoamrelative permittivities 1.5

conductivity 5.0×10−4 S/mmesh-conductive-cloth thickness 0.5 mm

material coated Agconductivity σm 200 S/m

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Fig. 1. (a)

Fig. 2. (b)

Fig. 3. Faraday cage with united shielding pattern (a)Outline of actual Faradaycage (b)Dimension of the Faraday cage

Fig. 4. Structure of USP and each length of shielding pattern

Fig. 5. FDTD analytical model of USP

III. MEASUREMENT RESULT OF SHIELDINGEFFECTIVENESS

Measurement set up of shielding pattern is shown in Fig.4and Table.II lists that the specification of the measurementequipment. The size of USP is prepared 600mm×600mm anddistance of spacer between shielding pattern and shieldingmesh ds is fixed 40 mm. The distance between the antenna andthe USP is fixed to 1m and changed the degree of incidenceangle θ = 0◦ to 45◦. Time-domain measurements of the vectornetwork analyzer were performed in the SE measurement. Thereflection interference of inside Faraday cage was excluded bydetecting only the transmitting pulse in the time domain. Themeasurement and calculation result of SE only by the shieldingpattern layer with Faraday cage is shown in Fig. 5. Thesharp characteristic of SE as the function of the frequency isconfirmed on both the calculation and measurement data. Thefrequency of maximum SE shows good agreement between thenumerical prediction and measured one though the measuredmaximum SE is 50dB. Fig.6. shows that result of SE in caseof the length of shielding pattern ls is changed. The length lsis longer, the resonance point shifts low frequency.

Fig. 6. Measurement setup of shielding pattern

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TABLE IISPECIFICATION OF THE MEASUREMENT EQUIPMENT

Equipment used Specification AbilityLength 5 m

Cable Characteristic impedance 50Loss 2.5 dBtype log-periodic dipole array antenna

Antenna Frequency range 300-3000 MHzVoltage standing wave ratio 2.0 or less

Gain 4-7 dBiAntenna factor 23-38 dB/m

The SE measurement data for (a) shielding pattern layeruse, (b) mesh-conductive-cloth layer use and (c) USP useare shown in Fig. 6 respectively. The resonance target fre-quency(950MHz) does not change compared with in caseof using shielding pattern. Furthermore, SE is raising about70dB at 950MHz and other frequency band is covered about20dB. In addition, Fig.7 shows that result of SE in case ofthe distance of spacer ds is changed. Both of calculationand measurement data, ds = 40 is best value for SE atresonant point. Decreasing of distance ds, the resonant pointis disappeared and SE goes down.Fig.8 shows that measurement result of SE in case of USPwhich changed the degree of oblique incidence angle θ. SEpeaks in θ = 0. On the other hand, when incidence angleexceeds θ > 15◦, the peak of SE becomes dull. However,even when the incidence angle θ = 45◦, the value of SE is50dB at 950MHz which is +30dB larger compared to onlyuses mesh-conductive-cloth.

Fig. 7. Result of SE in case of shielding pattern

Fig. 8. Result of SE in case of shielding pattern ls

Fig. 9. Result of SE in case of USP

Fig. 10. Result of SE in case of USP which changed the distance of spacerds

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Fig. 11. Result of SE in case of USP which is changed the degree of obliqueincidence angle θ

IV. CONCLUSIONIn this paper, we propsed the new structure of isolation

system which solve the interference problem for UHF-RFIDsystem. The new Faraday cage which attached the USPshields well to target frequency 940-960MHz ,and enable touse wireless communication in other frequency. The USP isconsisting of a certain shielding pattern and mesh-conductive-cloth which has resonance frequency and shields broadband.We found out that there exists a certain optimized distanceof spacer in this condition. In optimized condition, this USPshields over 70dB in target frequency 950MHz, and also SE isabout 20dB in other frequency band between 300-3000GHz.In general, shielding pattern which has resonance point isdifficult to shields except for vertical incidence. However,this USP shields over 50dB even when the oblique incidenceangle 15◦ − 45◦. In the future, we intend to investigate theactual communication test uses existing UHF-RFID systemsand mobile phone/Wi-Fi communication simultaneously.

REFERENCES[1] http://www.tele.soumu.go.jp/j/ref/portal/index.htm[2] http://www.softbankmobile.co.jp/ja/info/public/900mhz/[3] M.Gleaves, Understanding Shielding Performance of Screened

Rooms , in proc.3rd International Symposium on Digital ObjectIdentifier.,2002,pp.748-751.

[4] S.Yamamoto , K.Suezaki, K.Hatakeyama and T.Iwai, Reflection andTransmission Characteristic of Laminated Structures Using Metal WireArray Sheets and Dielectric Material ,The Institute of Electron-ics , Information and Communication Engineers (IEICE) TechnicalReport,EMCJ2013-25(2013-6),pp.75-80

[5] A. Taflove and M. E. Brodwin, Numerical solution of steady stateelectromagnetic scattering problem using the time dependent Maxwells equation, IEEE Trans. on Microwave Theory and Tech., vol. 23, pp.623-630, Aug. 1975.

[6] K. S. Kunz and R. J. Lubbers, Finite difference time domain method forelectromagnetics., Boca Raton, FL: CRC Press, 1993.

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