Free Space Measurement Technique on Dielectric Properties of ...

II.METHODOLOGYA. Equipment and <strong>Measurement</strong> SetupThe basic free space measurement system consists of: (1) anetwork analyzer, in this case is an Agilent PNA networkanalyzer, (2) a sample holder, (3) two horn antennas, and (4)Agilent 85071E software. Two horn antennas will be used asthe transmitter and receiver, respectively, and the agriculturalwaste material boards will be placed in between them as thematerial under test (MUT) [1]. The horn antenna is shown inFigure 1. The antenna’s dimension used was 309 mm x 238.5mm x 294 mm, with operating capability between 2.2 GHz to3.3 GHz with VSWR is less or equal to 1.225.Figure 1: Horn antennaThe Network Analyzer provided S- parameters such asand , and was installed with Agilent Technologies 85071E,the material measurement software. This software has thecapability to determine the intrinsic electromagnetic propertiesof many dielectric and magnetic materials. The 85071Esoftware controlled the network analyzer and calculated thecomplex permittivity ε’ (or dielectric constant) andpermeability µ’, including the loss factor. Results weredisplayed as a function of frequency, with 1 to 2% accuracy(typical). Depending on the Agilent network analyzer andfixture used, test frequencies can also be extended to 325 GHz[3]. Figure 2 shows PNA network analyzer that used in themeasurement.Figure 2: PNA Network Analyzer<strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong> system setup is shown in Figure3. The data gathered through this measurement will be able todetermine a set of dielectric constant and loss factor, in orderfor characterization of these agricultural materials. Theseresults are also important to investigate and study theproperties of the materials which are planned to be used forthe further RF microwave design. In addition to this, resultsgraphs can also be viewed through the measurement softwarethat was available as an option in PNA Network Analyzer –the Agilent 85071E <strong>Measurement</strong> software.Figure 3: <strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong> SystemRice Husk, Rice Straw, and Kenaf material boards werethe Material under Test (MUT) for the <strong>Free</strong> <strong>Space</strong><strong>Measurement</strong> <strong>Technique</strong>. These boards were placedperpendicularly with the bottom plane that was used to supporthorn antennas and agricultural material boards. Sampleholders were used to hold the material boards such that it isnon-contacting with the horn antennas. Without sampleholder, it will cause the material board to be deviate to somedegrees. <strong>Measurement</strong>s without the sample holder haveproduced some deviation, which in turn will lead toinaccuracies [3]. The tested material boards are shown inFigure 4.(a) (b) (c)Figure 4: Rice Husk, Rice Straw and Kenaf material boardsB. Calibration<strong>Measurement</strong> errors such as systematic, random and drifterrors are most probably due to imperfections in the analyzer,and instrument noises (source phase noise, sampler noise, IFnoise) [7]. These undesired errors must be removed by two portcalibration, for both reflection and transmission measurements.Two-port calibration is done on four known standards (shortopen-loadthrough or SOLT) [8]. In this calibration, an Agilent85052 D calibration kit was used.C. Time Gating SettingThe material board samples were placed in between thehorn antennas and the time domain gating feature of the PNAwas used to determine the actual placement distance ofmaterial under test [8]. The setting of time domain gatingfeature can be seen in Figure 5. The average difference mustbe more or equal to 40 dB when measuring a non-metal plate,compared to measuring a metal plate. The ‘a’ symbol showsthe plot of the measurement with metal plate and non- metalplate. The two responses appeared at ‘b’ and ‘c’ as shown infigure 5 were the transmitting and receiving horn antennaresponses. This was done across the frequency range of 2.2GHz and 3.3 GHz, frequencies where the use of the hornantennas was optimized.2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 184

acfrom that, a software simulation setup was also introduced toprovide an avenue of comparison of S- Parameter betweenhardware measurement and software simulation. Dielectricproperties results that obtained through measurement werecalculated averagely from 2.2 GHz until 3.3 GHz.Figure 5: Time Gating SettingD. SimulationComputer Simulation Technology (CST) simulationprogram was then used in order to study the variation of the<strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong> setup, which is shown in Figure 6.The measurement setup was remodeled in this softwareprogram along with the agricultural material board of exactdimensions. Waves were excited through the horn ports, andthe scattering parameters characterizing the material wereobtained. The parameters that needed to be specified in thissimulation were the dielectric properties of the agriculturalmaterial boards (measured through free space measurementtechnique), range of frequencies in hardware measurement,actual dimensions of the equipment and it material under test.46 mm294 mm(a)309 mm(b)215 mmFigure 6: <strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong> System in CST29.5 mmIII. RESULT AND DISCUSSIONSeveral variations of MUT were tested to investigate thedegree of distinction that the <strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong><strong>Technique</strong> was able to obtain. These MUTs include 1.Dielectric properties of rice husk with the effect of differentresin Phenol Formaldehyde, PF and Urea Formaldehyde, UF.2. Dielectric properties of three type agricultural wastematerial boards with the effect of different percentage of resinPhenol Formaldehyde, PF. 3. Dielectric properties of threetype agricultural waste material boards with the distant effectbetween horn antennas with material under test (MUT). ApartA. Dielectric Properties of Rice Husk with the effect of bothresin PF and UF.Both Urea Formaldehyde (UF) and Phenol Formaldehyde(PF) are strong adhesives, which are popularly used to createstrong bonds between adjacent material layers [9], thus waschosen as the agricultural waste material bonding agent. FromTable I, the dielectric constant, ε’, of the rice husk bondedusing PF composite was found to be higher than the UFcomposite. This trend was also observed in differingpercentage (20% and 30%) contents of both resins. Highermoisture level in PF will lead to higher dielectric constant datacompare to low moisture level contain in UF [11-12].TABLE I. DIELECTRIC PROPERTIES OF RICE HUSK WITH EFFECTOF PF AND UFResin Percentage, %Dielectric Propertiesε’ ε”PF10 3.236 0.27420 3.405 0.26830 3.681 0.439UF10 2.891 0.22220 3.275 0.32930 3.581 0.272One of the reasons to this was due to UF, being less waterabsorbent, while its original liquid form is also densercompared to PF [9]. As can be seen across all percentages, theloss factor of the rice husk particle boards have been found tobe increasing and declining unsteadily. This is due to theuneven distribution of the resin on the raw rice husk. Somepart of the rice husk board was covered with a high amount ofresin, while some other parts with a low volume of resin. Thusthe loss factor for the part that high resin concentration wasfound to be higher compared to areas of low resinconcentration [10].B. Dielectric Properties of three types agricultural wastesmaterial boards with the effect of different percentage ofresin Phenol Formaldehyde, PFThe agricultural waste material boards were fabricatedwith resin Urea Formaldehyde (UF) concentration varyingfrom 10 % to 50% by volume and the thicknesses were 8 mm.Table II gives the ε’ and ε” values for three types ofagricultural waste material boards and it was observed that ε’increases as the concentration of PF was increased from 10%,30%, and 50% respectively. This is due to higher volumefraction of the chemical resin in the composite. Water ischaracterized by high dielectric values and thus explains thehigher dielectric constant values at elevated moisture contents2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 185

[11-13]. Thus, it showed that for similar measurementdistances for all three materials, the dielectric constant, ε’increased linearly with the increase in percentage of bothresins. The inaccuracies of ε” measurement in free spacetechnique are due to two main sources of errors; namely (1)the diffraction effects at the edges of agricultural residuesmaterial specimen and (2) multiple reflection during signaltransmission occurring between the two horn antennas andmode transitions via surface of the sample. These propagationeffects had lead to low signal strength, as well as a lengthenedtime for signal to reach the MUTs [14].TABLE II. DIELECTRIC PROPERTIES OF AGRICULTURALWASTESWITH EFFECT OF DIFFERENT PERCENTAGE OF RESIN PFMaterial Percentage, % Dielectric Propertiesε’ ε”RH10 3.139 0.47730 3.473 0.61750 4.491 0.484RS10 1.891 0.21530 1.974 0.37250 2.639 0.211K10 2.101 0.05430 2.141 0.06950 2.382 0.132C. Dielectric Properties of three types agricultural wastematerial boards with the effect of different sampledistancesThis part of the investigation was carried out to identifyrelationships between the dielectric properties and the threedifferent distances established between horn antennas to theMUTs. Table III shows the variations of dielectric constant, ε’and loss factor, ε” versus resin percentage (UF and PF)between the frequency range of 2.2GHz to 3.3GHz, at roomtemperature (27ºC). As can be seen in the table III, thedielectric constant, ε’ values dropped constantly from aninitial measurement distance (215 mm between horn antennato agricultural waste sample board) compared to largermeasurement distances of 377 mm as well as measurementdistances at 475.5 mm.antennas. The results can be seen in Table III. This is due tothe electromagnetic penetration into the material boards,which was less for larger distances and vice versa for theshorter distances during measurement [14]. Hence, thematerial boards have undergone more energy loss than it wasabsorbing. The reason of inconsistent value of ε” was similarto observation in the previous measurement [16].D. S- Parameter simulation vs measurementFinally, a comparison between the hardware measurementand software simulation was carried out, generating a list of S-parameters ( and ). For the hardware measurement,and can be calculated automatically by using the NetworkAnalyzer and software, while for the software simulation, thedielectric constant data from the hardware measurement wasused to produce the and results via simulation usingComputer Simulation Technology (CST) software.Scattering parameter of and of the kenaf boardswith different composite percentage of Phenol Formaldehyde,PF resin, showed good agreement with the similar of graphobtained from hardware measurement, within the frequencyband of 2.2 GHz to 3.3 GHz, as shown in Table IV andFigures 7,8, and 9.TABLE IV. and MEASUREMENT AND SIMULATION OF KENAFAT 10%, 30%, AND 50%PFPF%S- Parameter(measurement) (simulation) (measurement) (simulation)10 -12.129 -13.680 -0.100 -0.86130 -12.127 -16.344 0.000 -0.18850 -10.504 -15.694 -0.013 -0.439TABLE III. DIELECTRIC PROPERTIES OF THREE TYPESAGRICULTURAL WASTE MATERIAL BOARDS WITH DIFFERENTSAMPLE DISTANCEMaterialsRiceHuskRiceStrawKenafResinSample Distance (mm)volume 215 377 475.5(%) ε’ ε” ε’ ε” ε’ ε”10 3.236 0.274 3.139 0.477 2.885 0.49630 3.581 0.439 3.473 0.617 3.249 0.65150 4.558 0.216 4.491 0.484 4.031 0.53910 1.906 0.135 1.891 0.215 1.814 0.24630 2.012 0.123 1.974 0.211 1.907 0.23450 2.736 0.224 2.639 0.372 2.485 0.39210 2.101 0.054 2.029 0.124 2.003 0.127830 2.141 0.069 2.109 0.168 2.029 0.28450 2.382 0.133 2.310 0.240 2.103 0.210Figure 7: and measurement and simulation of kenaf 10% PFThe decreasing trend of dielectric constant was found whenall the three type agricultural waste material boards (rice husk,rice straw, and kenaf) were placed further from the hornFigure 8: and measurement and simulation of kenaf 30% PF2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 186

Malaysian Ministry of Science, Technology and Innovation(MOSTI) (e-ScienceFund Grant 9005-00016) which enabledthe production of this article.Figure 9: and measurement and simulation of kenaf 50% PFHowever, some uncertainty was detected between andwhen comparing between hardware result and softwaresimulation, which is typically less than 50%. This can be seenin Table V. This was caused due to several uncertainty factorssuch as instrumentation, dimensional and geometricaluncertainty of MUTs, roughness and conductivity of theconduction surfaces [15]. Additional variations may also havebeen caused by the systematic uncertainties in the particularinstrumentation, calibration standards and the dimensionalimperfections of the implemented test fixture [16-18].IV. CONCLUSIONThe <strong>Free</strong> <strong>Space</strong> <strong>Measurement</strong> was found to be a suitablemethod in determining the dielectric properties, especiallywhen a non-destructive measurement of flat materials isdesired. This method is simple, fast and offers superioraccuracy, as materials were placed between antennas for anon-contacting measurement. The free space method worksbest for large flat solid materials, while granular and powderedmaterials can also be measured in a fixture. Calibration andgating techniques performed in the network analyzer can beused to reduce measurement errors. Dielectric constant, lossfactor, and S-parameters were determined and tabulated forrice husk, rice straw, and kenaf boards at specified moisturecontents or bonding structure of resins for the frequency rangefrom 2.2 GHz to 3.3 GHz at room temperature of 27 ºC. Fromthe result obtained, rice husk was found to possess a highdielectric constant compared to rice straw and kenaf, due tothe natural properties of rice husk. The large surface area ofrice straw has provided the ability to absorb moreelectromagnetic signal.V. ACKNOWLEDGEMENTVI.REFERENCES[1] M.S. Venkatesh and G.S.V. Raghavan, “An overview of dielectricproperties measuring techniques”, Canadian Biosystems Engineering,2005[2] B.-K. Chung, “Dielectric Constant <strong>Measurement</strong> For Thin Material AtMicrowave Frequencies,” Progress In Electromagnetics Research, PIER75, 239–252, 2007.[3] Agilent Technologies, “Basic of Measuring the Dielectric PropertiesMeterials – Application notes”, 26 June 2006[4] C.A. 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Tabassum Afzal, Bruce Colpitts, Kurt Galik, “Dielectric Propertiesof Softwood Species Measured with an Open-ended Coaxial Probe”, 8thInternational IUFRO Wood Drying Conference – 2003[16] Kamal Sarabandi and Fawwaz T. Ulaby “<strong>Technique</strong> for Measuring theDielectric Constant of Thin Materials”, IEEE Transactions OnMicrowave Theory and techniques,VOL. 31, NO. 4, December 1988[17] P. Kumar, P. Coronel, J. Simuvic, V.D. Truong, And K.P. Sandeep“<strong>Measurement</strong> of Dielectric Properties of Pumpable Food Materialsunder Static and Continuous Flow Conditions” Food Engineering andPhysical Properties[18] J. Wang, T. Schmugge, and D. Williams “Dielectric Constant Of SoilsAt Microwave Frequencies “NASA Technical Paper 1238The authors would like to acknowledge UniversitiMalaysia Perlis (Short Term Grant 9001-00110) and2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 187