Asian Journal of physics Vol 17, No 2 (2012) 1-6
Metamaterial Based Electromagnetic Shielding Sougata Chatterjee#1, Shantanu Das$2. #1 $2
NCRA-TIFR (Tata Institute for Fundamental Research)-Pune, India.
Reactor Control Division BARC (Bhabha Atomic Research Center) Mumbai, India. email-id:
[email protected],
[email protected].
Metamaterial based electromagnetic shields may be an future option for shielding electronic equipments for RS (Radiative Susceptibility) and RE (Radiative Emmissibility) compatibility of Electromagnetic Interference/Electromagnetic Compatibility (EMI/EMC) standards. Metamaterial is a composite material whose properties are not found in nature. Changing the macroscopic properties of ordinary substrate by inclusion of metallic structures we artificially create properties for negative permittivity and negative permeability. Now if one of these properties of artificial structures, becomes negative and other remaining positive will then make the refractive index (n) as imaginary. In this paper we report if the refractive property (n) is imaginary then we can used this material as a absorber perpose-as an electromagnetic shield. Wire array can give negative permittivity and below plasma frequency (Drude model) and its nature is high pass filter type, while the SRR (Split Ring Resonetor) can give negative permeability (Lorentz model), its nature is of band pass stop filter type . Our objective of this paper is to create negative permeability medium by using NBSR (Non Bianisotropic Spiral Resonator) and try to analyze its response in the reactive zone (that is near field) and radiative zone (that is far field region) of radiation; thereafter to give a theoretical aspect for making microwave absorber without using any ferrite material. This technique is useful for making light weight microwave absorber where we want to stop certain band of frequency there this type of absorber is very useful. Keyword: - MTM (Metamaterial), MNG (Mue Negative), SNG (Single Negative ), NBSR (Non Bianisotropic Spiral Resonator)
1.Introduction Metamaterials exhibits counterintuitive phenomena like a reversal of Snell's law, reversed Doppler effect and reversed Cerenkov radiation. The theoretical aspect for making metamaterial is first reported by V.G.Veselago[1] where the material permittivity and permeability become negative. The possibility of realization of negative permittivity at microwave frequency was reported [3] in 1996 using thin metallic wire array, while the realization of negative permeability was subsequently reported by the same researcher J. B. Pendry et al. [4], in 1999 using Split Ring Resonator (SRR) and Swiss Roll (SR). The experiment of negative index of refraction (NRI) was first demonstrated by D. R. Smith et al [5] in 2000 and started a new field of structured- material which shows a negative refractive index (in a particular frequency band). Metamaterial’s exhotic properties attracted many of the researchers for making miniaturized filter, coupler, also high gain and high directivity antenna [6-10]. Some of the researchers proposed and experimentally proved, making of metamaterial based absorber[11-13]. In this paper we give a description of making microwave absorber for absorbing the microwave signal for some specific frequency. Our first part of the paper, will discuss the experimental realiztion of MNG material unitcell , and experiments in free space and inside parallel plate wave guide to characterize the system. After that we will give a descrption of its responses in free at different zones of radition viz, very near, near, and farfield regions and try to established a microwave absorber system, for EMI/EMC shielding applications. 2.MNG Material Experimental characterization in different methods The unit cell of NBSR is given in Fig.1. in this diagram we describe the electric, magnetic fields, and its propagation directions. The experimental characterization of MTM or SNG materials are done by using free spce or parallel plate waveguide method where we can make TM0 mode which behaves like a TEM mode carrier or a plane wave (guide) [14]. The numerical simulations are usually done by using TMM (Transfer Matrix Method) or commercially available simulation software. We have selected the second method and designed the two structures (NBSR) for the MNG material experiment. The numerical simulation techniques are taken from reference [15], for the unit-cell simulation where we take NBSR (unit cell) and place it inside a parallel plate waveguide and its boundaries are PEC
(Perfect Electrical Conductor) and PMC (Perfect Magnetic Conductor) respectively (as given in figure-1a). The detailed descrption is not included in this paper because this not intension here. For parallel plate waveguide experiment, the magnetic -inclusion structures mounted in parallel plate waveguide is having top and bottom surfaces covered with perfect electrical conductor (PEC) to eliminate the possible interferences, and the side walls being open; thus emulating a perfect magnetic conductor (PMC); the schematic diagram of the parallel plate experiment is given in Figure-2 .
Fig.1. Schematic of Unit Cell of NBSR.
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(b) Fig.2. (a) Schematic representation of Unloaded and Loaded (MNG material) for parallel plate waveguide experiments.(b)Practical experimental setup for the characterization of MNG material.
Fig.3. Practical experimental setup for characterization of MNG material.
With these two different methods for characterization the fabricated MNG material, we give the result in Fig.4 where we have plotted the data of the two method (parallel plate and free space experiment), along with the unit cell simulation results.
Fig. 4. Experimental results for X-Band NBSR (MNG) material in unit cell (simulated), parallel plate waveguide, and free space.
From the figure-4 we observe that in unit cell simulation result, parallel plate wave guide result and free space experiment result; the stop band response is in the same frequencyband around 9.50 GHz . In the free space experiment, the maximum dip is around -90dB. Therefore, one can say that this is a nothing but the ‘very high’ absorption of EM radiation in this frequeny band. In the next part we will discuss the free space experiment with different region, and also fundamental work on making an absorber (shield). Proposed Micowave absorber From the discussion in the previous sections, we conclude that free space experiment gives better stop band as compared to other methods (that is parallel plate waveguide method). Now we place our designed MNG metamaterial in the three regions of radiation viz inductive, Fresnel and Fraunhfor zone and do the experiment. The experimental arragment is given in Fig.5 and the result is given in Fig.6.
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(b)
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Fig. 5. (a) Experimental arrangement for Inductive zone X-Band NBSR (MNG) material in free space.(b) Experimental arrangement for Fresnel zone X-Band NBSR (MNG) material in free space.(c) Experimental arrangement for Fraunhfor zone X-Band NBSR (MNG) material in free space.
Fig. 6. Experimental results for X-Band NBSR (MNG) material in free space in different zone.
From the Fig.5. we observe that in the inductive zone the stop band response is not very sharp as compared to the other two zones. The reason behind it is that in the inductive zone, the the ratio of magnitude of electric field to the magnitude of magnetic is not equal to 377 ohm; and also the field lines are spherical in nature . In the Fresnel zone, the field pattern tends forming a plane wave so that shows the stop band response of the MNG material. This manifestation is from spherical to plane wave formation is given in figure-7.
Fig.7. Schematic representation Electric and magnetic field pattern in different region
For fabricating the MNG meta-material, we used RT Duroid 5880, 2.2 and for the experiment purpose we used 10x5 array of NBSR, in z direction, where 10 sheets of 5 unit cell of NBSRR are placed, for all the three experiments. Our anlylitical and numerical study shows that if we increase the radius (r) then systems response shifts towards lower frequnecy while the distance between the ring (d) controls the dip of stop-band. The gap length (g) controls the band width of negative permeability region. Using the NBSR structure is due to two reasons; first it gives more negative permeability region compare to other MNG material and secondly it is having operating frequency higher compared to other SR (spiral resonator). MNG material based Absorber: From the observations and discussions of the previous sections we conclude that our analytical, numerical study and experimental observations, show that MNG material of NBSR type has a stop band nature which is nothing but the nature of absorber. If we change the dimension (r, d, g) of the unit cell then the systems stop band response will be at other frequency. Now the question is how can we make broad band absorber? The answer, is if we make multiple number of resonator where one response it tending to stop while the other stop band starts, so that the overall response looks like a contionus stop band in cascaded mode. The diagramatic representation is given in Fig.8. In this diagram we represent that, if we place four different radii of NBSR’s, keeping the other dimensions same, in a single board of RT Duroid, with each unit cell having its own raesonating frequency, so that they produce different stop bands, thus giving the overall stop band large as 1 GHz.
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Fig.8.(a) Four different NBSR create four stop band.(b)Those four stop namd will create a signle stop band.
For making of this type of a the absorber the vertical lattice constant is kept at the lowest order resonator (highest frequency) ,because if it is operated in lowest order then it will be operating in also in low frequency. The other issue of this absorber is that cross polarization effects. Due to use of different sized resonators in a single unit cells with operating frequencies in different regions, there is less chance of this cross polarization effect. Thus, if we use this four different NBSR resonators to make a single unit cell, and then if we use multiple number of board then system behaves like a broad band absorber where there is no ferrite material. Fig 9.gives the diagramatic resprenation of MNG NBSR based absorber.
Fig.9.Four different NBSR in a single unit cell formed a sigle unit cell and work as a combind MNG absorber for sheliding purpose.
In our free space experiment we have seen that the medium is little bit lossy, and that its transmission response starts at -20dB . In the microstrip based circuits, the main loss is created by the dielectric and the metal. Thus for making this MNG material we have etched the bottom portion of the dielectric, so that there main loss is in the dielectric part. If we use further low loss dielectric material, this loss of the metamaterial will decreese further. Conclusion In this paper we report the response of MNG material designed that is NBSR; inside parallel plate wave guide then in the free space and there after that we discussed how MNG material reacts in free space, in different zones of radiation. Finally we state that the four different NBSR structures of unitcells can make as ‘single unit cell’ and operate as an absorber of electromagnetic radiation with broad band. Therefore metamaterial based electromagnetic shields may be an option in future for electronic equipment shielding for RS and RE compatibility of EMI/EMC. Acknowledgement: - This work is fully funded by “Board of Research in Nuclear Science”(BRNS), Department of Atomic Energy (DAE) India, for a sanctioned project called Left Handed Maxwell (LHM) Systems; being carried out in SAMEER (Govt. Of India) Kolkata Centre along with collaboration with BARC. We are thankful to
Dr A L Das Director SAMEER and also special thanks to Mr. Arijit Majumder, Poulomi Sarkar, and Prof Subal kar for there support and encouragement for this project. Reference:[1] [2]
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