EpsiMu : A New Microwave Materials Measurements Kit. - SEE

EpsiMu : A New Microwave Materials Measurements Kit.Pierre SABOUROUX, Pascal BOSCHIUniversité de ProvenceInstitut Fresnel – UMR CNRS 6133Centre Scientifique de Saint Jérôme - Case 162Avenue Escadrille Normandie Niémen13 397 MARSEILLE cedex 20phone : 33- 4 91 28 83 53e-mail : pierre.sabouroux@fresnel.frKeywords: Coaxial cell, microwave, dielectricconstant, complex permittivity, complexpermeability, real time method.1. IntroductionSince several decades, measurements ofdielectric constant (permittivity) and permeabilityof materials in the microwave domain have been ofgreat interest for stealth issues involving RAM(Radar Absorbing Materials) [1]. Thesemeasurements are still important, not only inmilitary applications, but also in the civil arenawith EMC/EMI problems in aviation, automotive,telecommunications, etc. Today, many people needto know permittivitty for a variety applications.There are several well-known techniques based onmeasurements in free space, in guidedconfiguration and in resonant cavities. We proposea new kit (called EpsiMu) with an extremelysimple coaxial cell using propagating guidedwaves, together with its associated software. Themost important characteristic of this new kit is thatit provides measurements in real time over a widerange of frequencies, and requires only onemeasurement to obtain all information onelectromagnetic characteristics of materialsamples.In the first part of this article, we give a briefhistory of permittivity measurements to explain theimportance of this measurement method. In thesecond part, we describe the new kit and presentsome illustrative results.2. Why EpsiMu ?For an isotropic media, the complex relativepermittivity ε r and permeability µ r depend onfrequency:ε r = ε’(f)-jε’’(f) and µ r = µ’(f)-jµ’’(f).It is commonly known that the determinationof the complex dielectric permittivity and thecomplex permeability in microwave range isquiet difficult. As already said, several differenttechniques are used in different laboratories[2,3]: free space techniques or guide linetechniques (rectangular, circular or coaxialguide line). In general, all commonexperimental techniques are difficult to use:after calibrating the vectorial network analyser(VNA) to fix reference planes of phases, it isnecessary to recalibrate the cell each time beforeintroducing a different material sample. After astudy of the performance of different methods,we have adopted a reflexion/transmissiontechnique in a coaxial line [4]. This methodallows us to make wide-band measurements[5,6] in real time by using the de-embeddingtechnique [7].This de-embedding is possible because wecharacterise the cell in terms of fixedparameters: the electrical length and linearattenuation, they form an Identity Card off thecell. With this method, we do not need torecalibrate the cell every time. Only thecalibration of the VNA is necessary.EpsiMu is a new method, because only onemeasurement of the [S] matrix of the sample issufficient to determine both dielectricpermittivity and permeability. Consequently,from the same data set, it is possible to calculateother quantities of interest such as tanδ oradditional diagrams (for example a Cole-Colediagram).3. Description of EpsiMu kit3.1. Measurement CellEpsiMu kit is composed of three parts: aspecial coaxial cell, a associated software and ameasurements guide.The cell is manufactured in our laboratoryand could therefore be modified to address avariety of physical problems (liquids, powders,author: P. SabourouxETTC 2005 – European Test & Telemetry Conference

and studies versus temperature, etc). The standardcell is around 144 mm long. The outer diameter is7 mm. Connectors are of the type PC7. Thedimensions determine the frequency range, thehighest frequency is over 19 GHz. We fixfrequency range from 1GHz to 18GHz.Two coaxial cells and a sample of a testmaterial are shown in the first photograph. One ofthe cells is taken apart to view all the componentsof the cell.Photograph 2: EpsiMu kitOuter conductorInner conductorPhotograph 1: Coaxial cellThis cell is characterised with its electricaldistances from the reference phase planes and withits attenuation. These characteristics form theIdentity Card. Each cell must be measured todetermine its ''Identity Card''. Knowing thesecharacteristics, the de-embedding technique can beperformed.3.2. Associated SoftwareThe software of EpsiMu is not restricted to aparticular VNA. In fact, any analyser could beused with EpsiMu. Tests were performed withAgilent 8510 and 8720 family, Anritsu MS462xfamily and Rodhe & Schwarz ZVR or ZVM.However, it is assigned to one particular cellversus its Identity Card. This sowftare have twoparts: the first part performs measurements in realtime. This is the “Epsilon-meter”. The second partis used post-treatments, for example, visualisation,comparisons of several results, etc.Nowadays, the software is in its first version,but a new one is under preparation to proposegreater possibilities, like a linearization of results,or visualisation of Cole-Cole diagram, etc.3.3. EpsiMu KitTest samplesWe show the complete kit in the secondphotograph. It contains one measurement cell,some accesories to make and verify materialssamples and one CD, containing the associatedsoftware, the measurement guide and some data toverify the kit. In addition, a sample of thereference material is in the kit.43214. ResultsTo illustrate the performance of this new kit,we chose several particular materials: adielectric material (polyethylene), a magneticmaterial (Emerson and Cuming SF10) and airsample.At first, we can verify if the measurementswith EspiMu, are correct by using thevisualisation of the S-parameters of the sample.In particular, we can determine if the sample istoo long or manufactured not perfectly.Second, we can proceed to measurements inreal time. The result of these measurements areshown in the next figures.4.1. Dielectric Material: PolyethyleneThe thickness of the sample is 1.85mm. Fourcurves are plotted on these graphics presentingpermittivity and permeability, two for the realparts (forward and reverse measurements withVNA) and two for the imaginary parts.Dielectric permittivity00.00 5.00 10.00 15.00frequency (GHz)ε''Figure 1:Polyethylene : permittivityε'author: P. SabourouxETTC 2005 – European Test & Telemetry Conference

10µ''0.00.00 5.00 10.00 15.00frequency (GHz)864202.01.51.00.532.521.510.50Permeability4.2. Magnetic Material: SF10This material is magnetically-loaded,electrically non-conductive silicone sheet. It ismanufactered by Emerson and Cuming. The truethickness of the test sample is 1.8mm (the nominalthickness given by the manufactured is 1.4 mm).We can verify in figure 2 that this material ismagnetic because the real part of its permeabilitydiffers from unity. The black curves are obtainedwith foward measurements with VNA (S 11 and S 21 )and the grey curves are obtained using reversemeasurements (S 22 and S 12 ).µ'Figure 2: Polyethylene: permeabilityµ''Permeabilityµ'ε'ε''0 5 10 15frequency (GHz)Figure 3: SF10 : permittivityµ''Dielectric permittivityµ'ε'0 5 10 15frequency (GHz)ε''Figure 4: SF10 : permeabilityepsilon'epsilon'4.3. AirTo test the kit, we measure an empty celllike an air-line. The thickness is not defined. So,we can choose any values to demonstrateseveral phenomena. In the same figure 3 a samegraphic (figure 3), four curves are ploted withfour arbitrary values of thickness: 3, 4, 5 and6mm. All results are superposed, as could beexpected.The results for the other three parameters areidentical (ε”≈ 0, µ’ ≈1 and µ’’ ≈0).1.510.50Dielectric permittivity8 10 12 14 16 18frequency (GHz)Figure 5: test with empty cellThe choice of sample thickness is veryimportant because, a jump will appear when thethickness is higher than λ mat /2. And indeed, for athickness equal to 9mm of the same virtual airsample, the jump appears at 16.6GHz (figure 4).21.510.50-0.5-13mm4mm5mm6mm3mm9mm8 10 12 14 16 18frequency (GHz)Because of this jump, when using the firstversion of EpsiMu, the thikness of samples mustbe lower than λ mat /2. In the next version of thesoftware, we shall take into account thesejumps.The accuracy of measurements is around±3% and ±10% depending on values ofpermittivity and permeability constants and onthe nature of material.ε'ε'Figure 6: Choice of thickness (test with anempty cell)author: P. SabourouxETTC 2005 – European Test & Telemetry Conference

5. ConclusionEpsiMu is a new kit for measurements ofdielectric constant and permeability and is easy touse. The results are obtained in quasi real timebecause only one measurement is necessary toobtain simultaneously all the data. EpsiMu iscompatible with any vectorial network analyser.The materials analysed in this article aredifferent from the reference material included inthe kit, which is a sample of APU10 made bySIEPEL, a company specialised in manufacturingshielded rooms, anechoic chambers, absorbers,reverberation chambers, stripline antennas,turntables, antenna masts and shielded cabinets.SIEPEL is associated with Institut Fresnel tocommercialise EpsiMu.Today, EpsiMu is a name which is reserved tothis kit [8]. Today this kit is avaibale on the openmarket. EpsiMu presents a true evolution in thetechnique of electromagnetic characterisation ofmaterials. The kit can be used with each kind ofvectorial network analyser. Several new versionswill be available in the future time.[6] Measuring Dielectric Constant with the HP8510 Network Analyzer.Hewlett Packard - product note n°8510-3.[7] De-embedding and embedding S-parameter network using a Vector NetworkAnalyzer.Agilent - Application note n°1364-1.[8] Enveloppe SOLEAU n°191097-160304–INPI - France[9] Website of Emerson and Cuming :www.eccosorb.com/catalog/eccosorb/SF.pdf[10] Website of Institut Fesnel : www.loe.u-3mrs.fr/labo/sabouroux/EpsiMu_avril2004.pdf6. References :[1] Radar-Absorbing Material: A Passive Role inAn Active Scenario.Richard N. JohnsonThe International Countermeasure Handbook,11th Edition[2] Measuring the Permittivity and Permeabilityof Lossy Materials :Solids, Liquids, Metals,Building Materials and Negative-IndexMaterials.J. Baker-Jarvis, M.D. Janezic, B.F. Riddle,R.T. Johnk, P. Kabos, C.L. Holloway,R.G.Geyer and C.A. GrosvenorNIST Technical Note 1536 – Decembre 2004[3] Extracting µ r and ε r of solids from one-phasornetwork analyzer measurements.H. Brian Sequeira, Martin MariettaLaboratories, MML TR 89-83, Sep. 1989.[4] Intercomparaison of permittivitymeasurements using theTransmission/Reflection method in 7-mmcoaxial transmission lines.E. Vanzura, J. Baker-Jarvis, J. Grosvenor, M.Janesik, IEEE Trans MTT, vol.42, n°11 –nov. 1994.[5] Techniques de mesures en vue de lacaractérisation de structures absorbanteslarges bandes.P. Sabouroux et J.P. ParneixThe Journal of Wave-Material Interaction,Vol. 7, N° 1, Janvier 1992author: P. SabourouxETTC 2005 – European Test & Telemetry Conference