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A Study of the Anechoic Performance of Rice Husk-Based, Geometrically Tapered, Hollow Absorbers

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A Study of the Anechoic Performance of Rice Husk-Based, Geometrically Tapered, Hollow Absorbers
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  Research Article  A Study of the Anechoic Performance of Rice Husk-Based,Geometrically Tapered, Hollow Absorbers MuhammadNadeemIqbal, 1 Mohd.FareqMalek, 2  YengSengLee, 1 LiyanaZahid, 2 andMuhammadShafiqMezan 2  School of Computer & Communication Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra Campus, Arau,    Perlis, Malaysia  School of Electrical System Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra Campus, Arau,    Perlis, Malaysia Correspondence should be addressed to Muhammad Nadeem Iqbal; mr.nadeemiqbal@gmail.comReceived    August   ; Accepted    November   ; Published    January    Academic Editor: Ananda Sanagavarapu MohanCopyright ©    Muhammad Nadeem Iqbal et al.   is is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work isproperly cited.Although solid, geometrically tapered microwave absorbers are preferred due to their better performance, they are bulky andmust have a thickness on the order of     or more.   e goal of this study was to design lightweight absorbers that can reduce theelectromagneticre fl ectionstolessthan −  dB.Weusedaverysimpleapproach;twowastematerials,thatis,ricehusksandtiredustin powder form, were used to fabricate two independent samples. We measured and used their dielectric properties to determineand compare the propagation constants and quarter-wave thickness.   e quarter-wave thickness for the tire dust was   mm lessthan that of the rice husk material, but we preferred the rice-husk material.   is preference was based on the fact that our goalwas to achieve minimum backward re fl ections, and the rice-husk material, with its low dielectric constant, high loss factor, largeattenuation per unit length, and ease of fabrication, provided a better opportunity to achieve that goal.   e performance of theabsorbers was found to be better (lower) than  −  dB, and comparison of the results proved that the hollow design with   % lessweight was a good alternative to the use of solid absorbers. 1. Introduction Electromagnetic interference (EMI) is a serious threat toelectronics-basedcivilandmilitaryinfrastructures[  ,  ].Var-ious types of natural and man-made EMI sources have beenidenti fi ed that can lock, upset, damage, and cause malfunc-tions in sensitive electronic components in extremely com-plex and mission-critical systems [  –  ]. Electronic devicesmust follow the electromagnetic compatibility (EMC) stan-dards which impose certain conditions on the electronicdevices before they can be marketed [  –  ]. ComprehensiveEMC testing of these devices is conducted to determine theiremissions and susceptibility levels within specially designed,shielded, re fl ectionless facilities, that is, anechoic chambers.In these chambers, geometrically tapered, synthetic, carbon-impregnated foams with thicknesses on the order of     orgreater are used [  ].   ese absorbers are made from  fl exible,polyurethane foam, which is a heterochain polymer, synthe-sized by the reaction of isocyanate (–NCO functional group)compounds and polyether polyol alcohol (–OH functionalgroup)[  ,  ].Isocyanatesarehazardouscompoundsandcancause signi fi cant respiratory problems, such as asthma anddecreased lung function [  ].   e main cause of these healthhazardsistheinhalationofthefoamdustthatcontainstracesof isocyanates and contaminated  fi bers.Agricultural waste, especially rice husks, is a nonhaz-ardous natural source of lossy carbon [  ,   ] that can beused as a  fi ller in a polymer matrix to absorb the EMInoise. Rice husks are a waste material produced by the ricemilling industry, and this waste material is used along withits by-product, that is, rice husk ash, in the cement industry,power production equipment, and furnaces [  ]. Rice husk material is a low-cost, low-density raw biomass material thatis readily available for use in manufacturing value-added, Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2014, Article ID 498767, 9 pageshttp://dx.doi.org/10.1155/2014/498767    International Journal of Antennas and Propagationsilicon-based products [  ]. Rice-husk waste amounts to  –  % of the harvested product [  ], and its use in theproductionofcheaper,value-addedproductsalsoreducestheenvironmental pollution associated with burning rice husks,which has been a common practice for many years [  ].In previous research [  –  ], the main focus of theresearchers was to identify the rice-husk material as a new,microwave-energy-absorbing material, develop simulationdesigns, and evaluate its ability to re fl ect electromagneticnoise. Most such research involved the use of simulationtools to investigate solid forms that were shaped as pyramidsor wedges, and only a few of the studies presented experi-mental results indicating how well the material performed.In addition, the range of frequencies investigated was very narrow, and no data were provided concerning the hollow structures fabricated from the rice-husk material. Similarly,the extent of the attenuation of the microwave signal perunit length of the rice-husk and tire-dust material was notinvestigated even though it is a very important parameterthat is required to determine the minimum thickness of theabsorber. Obviously, hollow structures will be bene fi cial inreducing the overall weight of the absorbing material usedwithin an anechoic facility.In this study, a very simple method was used to fabri-cate and characterize the rice husk-based, hollow-structure,microwaveabsorbers.First,wemeasuredthecomplexdielec-tric properties of the two waste materials and used theseproperties to determine and compare attenuation constants.Rice-husk material was preferred over the tire-dust materialfor the fabrication of the hollow structures because of itshigh loss factor and comparatively large, spatial decay rate.  e hollow absorbers that we designed were investigated todetermine their anechoic performance over the broad rangeof frequencies from    to   GHz. 2. Geometric Tapering of Lossy Material  .  . Solid Pyramidal Structure.  Physicalorgeometricaltaper-ing of the lossy material is a type of broadband impedance-matching technique to minimize the re fl ections at theinterfaces in the required pass band. In this concept, theabsorber is modelled as a single structure composed of anin fi nite number of thin sheets (impedance transformers).However, the volume of the successive sheets increasesgradually with a constant ratio from the tip of the pyramidto the bottom, as shown in Figure   . Due to the grad-ual change in the volume of the material, each successivelayer of the pyramidal absorbing structure exhibits di ff  erentdielectric properties and characteristic impedances. Accord-ing to transmission-line theory, the number of impedancetransformers must be in fi nite to smooth the impedancediscontinuities at the interfaces. Adding an in fi nite numberof layers to the absorbing structure improves the impedancematching at the cost of large material thickness in termsof the wavelength to achieve minimum re fl ections at theair-absorber interface. Impedance at the   th interface canbe found by using the information of the propagationconstant (   ) and characteristic impedance (     and    +1 )of each layer according to the transmission line theory asfollows: Ζ   =      +1  +    tanh        +   +1  tanh      .  (  )  .  .HollowPyramidalStructure.  Hollowpyramidalabsorbercan be considered as a diverging structure having an in fi -nite number of square loops (apertures).   e area of eachsuccessive square loop is greater than the preceding loop.Above-mentioned theory  Section   .   is valid only for thethick lossy material (solid absorbers), which can be modeledas an in fi nite-section transmission line. In case of a hollow pyramidal structure, the minimum re fl ections at the bound-aries do not guarantee the attenuation of the signal withinthe absorbing pyramid without considering the loss per unitthickness of the walls of the absorbing pyramid. Hence, thewallsofahollowpyramidalabsorbermustbelossysothattheincident EM energy can be dissipated into heat by the Jouleheating principle without re fl ections [  ].  epropagationcharacteristicsoftheincidentEMsignalthrough the lossy medium  (  e ff   =    +     ̸ =0)  usually are described by the wave propagation constant,    =  (  e ff   +   ) =   +  ,where  ( NP / m ) and  ( rad / m ) aretheattenuationandphaseconstants,respectively,ofthewavepropagating through the lossy medium.   e exact equationsfor these two constants are expressed as [  ,   ]   =     12  1 +  e ff     2 − 1  1/2 ,   =     12  1 +  e ff     2 + 1  1/2 ; (  )here,   ,    , and   e ff   are the complex magnetic permeability,real part of complex permittivity, and e ff  ective conductivity of the lossy medium.  .  .Metal-BackedLayerofaLossyMaterial.  Asinglelayerof a metal-backed lossy material (e ff  ective in suppressing thelow frequency signals) can be modeled as a short-circuitedtransmission line. In this case, we can transform (  ) as,     =    tanh (     ) andthere fl ectionlosscanbecalculatedbyusingthefollowingequationtoevaluatetheanechoicperformance:RL ( dB ) = 20 log    − 0    +  0  .  (  )To achieve the best anechoic performance (less echoes), it isclear that it is very important to know the propagation char-acteristics within the lossy material.  International Journal of Antennas and Propagation   Incident waves    M  e   t  a    l    l   i  c  w  a    l    l Transmitted wavesReflected waveTip reflected waves (a) Incident EM plane wave    M  e   t  a    l   p    l  a   t  e Z w  = EH= 120  󰂷 Ω      V      i  ,            i  ,            i      V      i    +     1  ,            i    +     1  ,            i    +     1 V i+1 V i = const  i+1  >   i    G  r  a   d   u  a    l    i  n  c  r  e  a  s  e    i  n     l  o  s  s   a  n   d    v  o    l   u  m  e (b) F   : Illustration of wave-absorber interactions: (a) possible interactions of incident EMI signal with the wall of the anechoic chamberthat is lined with a periodic array of pyramids; (b) sketch of a unit cell of the array of solid pyramids, showing a gradual increase in volumeand loss factor along the axis of the wave propagation. 3. Research Methodology   .  . Sample Fabrication and Characterization Method.  Twoindependent samples were fabricated by using simple, lab-oratory-scale methods, as described in [  ]. In one of thesamples, we mixed the rice husks (  .  % by weight) with un-saturated polyester resin UPR (  .  % by weight) and methylethyl ketone peroxide MEKP (  .  % by weight). In the secondsample, we used tire dust (  % by weight) with UPR (  %by weight) and MEKP (  % by weight). We used UPR as apolymer matrix to bind the  fi llers, that is, rice husks and tiredust, and MEKP was used as a catalyst to cure the UPR.In this study, we used an open-ended, coaxial probe(Figure   (a)) to measure the dielectric properties of the fab-ricated samples.   is method provided broadband, nonde-structive, rapid experimental means to measure both com-ponents of complex permittivity [  –  ]. We performed a“threestandard”calibrationoftheprobebeforebeginningthemeasurements,andwater,air,andaload,whichareshowninFigure   (b), were used as calibration standards. Figure   (c)shows a typical experimental setup for the measurement of the dielectric properties in the frequency range of    –  GHz.  e apparatus used for the dielectric characterization in thisstudyincludedtheAgilent  BHigh-TemperatureDielec-tric Probe, Agilent Vector Network Analyzer, and Agilent   so  ware.   e measurement procedure is very simpleand easy; the end of the probe is brought into the contactwith the  fl at surface of the sample, and the probe sensesthe signal that is re fl ected from the sample [  ].   en, thisre fl ected signalisdisplayed onthe networkanalyzer by usingthe so  ware.  .  . Frequency Spectrum of Dielectric Properties.  Figure   shows the frequency spectrum of the relative complex per-mittivity   (    =     −    )  for the two samples composed of two wastes.   e measured values of the real part of therelative complex permittivity for the two samples are greaterthan that for air, which shows their ability to be polarizedto a greater extent than air.   e overall pattern of the realpart  (   )  decreased in values as the frequency increased.   eoverall dielectric constant for the tire-dust sample for theentire frequency spectrum (  –  GHz) was in the range of   .  –  .  ,whichwasgreaterthantherangefortherice-husk material (  .  –  .  ).   is was due to the presence of the highdielectricconstantmaterial,thatis,rubber,whichisthemajorcomponent in the tire dust.   e results showed  fl uctuations(ripples),andthedeviationsoftheamplitudesoftheserippleswere within the range of    .  –  .  .  ese  fl uctuations were due to the nonuniform nature of the material and also to the presence of discontinuities inthe impedance, which caused re fl ections at the interface of the probe and the samples at high frequencies. In addition,it also can be seen in Figure    that the low-frequency regime    International Journal of Antennas and Propagation Coaxial probe (a) Load (b) Open end of the probe Inner conductor Field fringing due to gapRadial fieldMaterialPort  1 of network analyzer (c) F   : Equipment and setup for the measurement of dielectric properties of the two samples. (a) Cylindrical, open-ended, and coaxialprobe [  ]. (b) Load for the calibration of the open-ended probe [  ]. (c) Schematics of the experimental setup, showing  fi eld infringementat the open end of the probe due to abrupt change in impedance. 4E + 09 8E + 09 1.2E + 10 1.6E + 10 2E + 10 00.511.522.533.544.55    C  o  m   p    l  e  x   p  e  r  m   i   t   t   i  v   i   t  y Frequency (Hz)Real part of rice husk Real part of tire dustImaginary part of rice husk Imaginary part of tire dust F   : Frequency spectrum of the complex permittivity of thetwo samples fabricated using rice husks and tire dust. (  –  GHz) has smaller  fl uctuations than the high-frequency regime. At high frequencies, the dimensions of the gap (airpocket), which is formed due to the lose contact betweenthe surface of the sample and the  fl at end of the probe,becomeaverycriticalissuebecauseitismucheasierforsmallwavelength signals to leak from the small gaps compared tolarge wavelength signals. Figure    also shows that the samplecomposed of rice husks had the greatest loss factor (rangingfrom   .   to   .  ), whereas the sample composed of tire dusthadthesmallestvaluesoflossfactor(  .  to  .  ).Adetailedcomparison of the measured complex dielectric properties 0100200300400500600700800 900 0102030405060708090100Frequency (Hz) Rice husk sample Tire dust sample Rice husk sample Tire dust sample     P    h  a  s  e  c  o  n  s   t  a  n   t ,         4E + 09 8E + 09 1.2E + 10 1.6E + 10 2E + 10    A   t   t  e  n  u  a   t   i  o  n  c  o  n  s   t  a  n   t ,       F   : Frequency spectrum of the propagation constants of thetwosamples,thatis,ricehusksandtiredustbasedontheirmeasureddielectric properties. that were measured up to two decimal places in multiplefrequency bands is presented in the Table   .  .  . Attenuation Constant and Quarter-Wave   ickness of theTwo Waste Materials.  In considering microwave absorption,theimaginarypart (    ) isthemostimportantofthedielectricproperties, because it has a direct relation with the dissi-pation of the microwave energy within the material. How-ever, the dielectric storage capability of the medium, that  International Journal of Antennas and Propagation   T   : Measured range of dielectric properties of the two fabricated samples in di ff  erent frequency bands.Sample Properties C-band(  –  GHz)X-band(  –  GHz)   -band(  –  GHz)Rice husk       .  –  .   .  –  .   .  –  .      .  –  .   .  –  .   .  –  .  Tire dust      .  –  .   .  –  .   .  –  .      .  –  .   .  –  .   .  –  .  02468101214Frequency (Hz)Rice huskTire dust         i  c    k  n  e  s  s  e  q  u   i  v  a    l  e  n   t   t  o  q  u  a  r   t  e  r  -  w  a  v  e    l  e  n  g   t    h    (  m  m    ) 4E + 09 8E + 09 1.2E + 10 1.6E + 10 2E + 10 (a) 01234 5 Frequency (Hz)    E       ff   e  c   t   i  v  e  c  o  m   p    l  e  x   p  e  r  m   i   t   t   i  v   i   t  y 4E + 09 8E + 09 1.2E + 10 1.6E + 10 2E + 10   e ff    e ff  (b) F   : (a)   ickness equivalent to the quarter-wavelength based on the measured dielectric properties for rice-husk and rubber tire-dustmaterial. (b) Calculated e ff  ective dielectric properties for hollow pyramidal absorber for wall thickness of    mm and  fi ll factor   = 0.32 . is, its capacitance, a ff  ects the speed at which the signal ispropagated. Figure    shows the propagation parameters, thatis, attenuation  (  )  and phase constant  (  ) , based on themeasured dielectric properties (    and     ) for the rice-husk and tire-dust materials. All the results show resonances athigh frequencies, but the resonances in case of     are moreprominent than those in the case of    .   is occurs because   has a strong dependence on the loss factor,     .   ere aremanyfactorsthatcana ff  ectthelossfactor    ofthepyramidal-shaped, composite material, but the most important factoris the inhomogeneous mixing of the chemical constituents.If we ignore these resonances for the sake of analysis, it isclear that the two parameters,    and   , have overall positiveslopes, that is,    (  ) > 0  and    (  ) > 0 , and they increasecontinually as frequency increases. However, the values fortheattenuationconstantofthetiredustaresmallerthanthosefor the rice husk, which con fi rms that the rice-husk materialis lossier than the tire-dust material.   is behavior is dueto the frequency dependence (dispersion) of the constitutiveparameters  (  e ff  ,   ,    )  of the two materials. Tire dust has alowdielectriclossfactorandahighdielectricconstant,whichexplainsthefactthatitsattenuationperunitlengthissmallerthan that of the rice husks.   e phase constant (  ) values forthe tire-dust material were greater than those for the rice-husk material over the entire frequency range because of thehigh dielectric constant of the tire dust.  e higher value of the dielectric constant of the tiredust indicated that the signal was propagated at a slow speeddue to the large refractive index of the medium.   is showsthat the wavelength of the transmitted wave inside the tire-dust material decreases more rapidly than that of the rice-husk material; hence the thickness equivalent to the quarterwavelength (  /  ) of thewave is less for the tire-dust material.Figure    shows the decrease in the thickness equivalent tothe quarter-wavelength of the two materials with increasein the frequency.   e maximum thickness of the tire-dustmaterial, equivalent to the quarter-wavelength at the lowest   = 197 NP/m, is    mm for the lowest operating frequency of    GHz, whereas the rice-husk material required   mm at  GHz for its lowest   = 142 NP/m.However, for the same thickness of the two materials,the attenuation of the amplitude of the wave per unit lengthfor tire-dust is less than that of the rice husk material. Wepreferred the rice-husk material for the fabrication of thehollow, pyramidal, microwave absorbers on the basis of itshigher loss factor and attenuation per unit length, whichmake it possible to attenuate microwave energy within a thinlayer.E ff  ective dielectric properties for the hollow absorber forwall thickness of    mm are also shown in Figure   (b).   ese values were calculated by using the actual bulk dielectricproperties (Figure   ) and fraction of the volume ( fi ll factor)occupied by the rice husk material in hollow pyramidaccordingto[  ].However,ithasbeenmentionedin[  ]thate ff  ectivelayermodelisadequateonlyfor   <   ,where  istheperiodoftheperiodicstructure.Wewereinterestedtodesignalightweightandbroadbandsmallabsorber;henceavalueof    = 50 mm was preferred on the basis of our previous study for solid pyramid design [  ].  .  . Determination of Optimum Wall    ickness.  We per-formed a parametric study by using commercially availableCST Microwave Studio so  ware to determine the optimum
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