Patent Publication Number: US-8994386-B2

Title: Method and device for measuring permittivity and/or permeability

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage entry under 35 USC 371 for: PCT/EP2010/052605, filed on Mar. 2, 2010, which claims the benefit of the Mar. 13, 2009 priority date of French application 0951598. The contents of both the foregoing application are incorporated herein by reference. 
     The invention pertains to a method and device for measuring the permittivity and/or the permeability of a sample of non-conductive material. 
     Herein, barring indications to the contrary, the term “permittivity” designates relative dielectric permittivity and the term “permeability” designates relative magnetic permeability. 
     There are methods for measuring the permittivity of samples of non-conductive material comprising steps of:
         a) measuring a variable representing an admittance Y testco  solely from the amplitude and phase of electromagnetic waves reflected on an interface between the sample and one end of a first waveguide, this end having at least one core separated from a conductive sheath by a layer made of dielectric material in which the electromagnetic waves are propagated.       

     For example, a method of this kind is disclosed in the French patent FR 2 651 580. 
     Herein, a sample of material is considered to be non-conductive if its conductivity is below 10 −2  S·m −1  and preferably below 10 −3  S·m −1 . 
     These prior-art methods are also known as methods of “electromagnetic characterization in reflection” because they only use the reflection of the magnetic waves on the sample to measure the permittivity. 
     These methods of measurement by reflection have many advantages. For example, they make it possible to: 
     carry out measurements on liquid, solid or soft materials (powders, elastic materials etc), 
     carry out non-destructive measurements because it is not necessary to machine the solid sample to give it a specific shape other than a plane shape, and 
     characterize the material on a wide range of frequencies (for example 0 MHz to 20 GHz). 
     However, prior-art methods have the drawback of not being capable of implementation on samples having permeability different from 1. Thus, with these prior-art methods, it is not possible, for example to measure the permittivity of a ferrite. 
     Furthermore, it is only permittivity that can be measured with these prior-art methods, and not magnetic permeability. 
     To overcome these drawbacks, methods for measuring permittivity and permeability have been developed. These methods use, in addition to reflection, the transmission of electromagnetic waves through the sample. A method of measurement by reflection and transmission of this kind is described in the patent application US 2007 09 08 46. 
     These methods of measurement by reflection and transmission necessitate the machining or conformation of the sample to enable it to be housed within an electromagnetic waveguide. This is a drawback. 
     To overcome this drawback, the US patent application 2008/191711 proposes to use two waveguides placed one beside the other on the same side of the sample. Electromagnetic waves are sent out in the first waveguide. A part of these waves is reflected on the sample at the interface with the sample and another part gets propagated through the sample up to the second waveguide. Then, the permittivity or the permeability is measured from the reflected and transmitted electromagnetic waves. However, the fact that a part of the measurements are made in transmission imposes major constraints. For example, the distance between the two waveguides must be known with precision. Furthermore, this approach always dictates the simultaneous use of two waveguides. 
     The invention is aimed at overcoming at least one of these drawbacks. 
     An object of the invention therefore is a method for measuring the permittivity and/or permeability of a sample of non-conductive material comprising:
         b) the measurement of a magnitude or variable representing an admittance Y testcc  solely from the amplitude and the phase of the electromagnetic waves reflected on the interface between the sample and the end of a second waveguide having at least one conductive core separated from a conductive sheath by a layer of dielectric material in which the electromagnetic waves get propagated, the second waveguide also comprising a short-circuit between the central core and the sheath made at the interface with the sample so that the phase and/or the amplitude of the electromagnetic waves reflected on the sample are different from those obtained when there is no short-circuit, and   c) the computing of the permittivity of the sample from the variables representing values of admittance Y testco  and Y testcc  and/or computing the permeability of the sample from the variables representing the values of admittance Y testco  and Y testcc .       

     The above method can be used to measure the permittivity of samples having a permeability equal to 1 as well as of samples having a permeability different from 1. For example, the permeability of ferrite can be measured by means of the above method. 
     The above method also enables the measurement, purely by reflection of electromagnetic waves on the sample, of both the permeability and the permittivity of this sample. 
     These results are achieved without using any measurement in transmission. More specifically, the use of a short-circuit, here below called a “partial” short-circuit, to modify the phase and/or amplitude of the waves reflected by the sample advantageously replaces the commonly used method of making measurements in transmission. The problems related to the performance of measurement by transmission are therefore averted. Such a partial short-circuit can be distinguished from the known short-circuits used during the calibration phase. Known short-circuits are used to measure the characteristics of the waveguides. These known short-circuits are not implemented to measure characteristics of the sample. On the contrary, these known short-circuits are used to isolate the waveguide from the sample so that they are sensitive only to the characteristics of the waveguide. 
     Finally, the above method also preserves all the advantages related to measurement solely by reflection as compared with the methods of measurement by reflection and transmission. 
     The embodiments of this method may comprise one or more of the following characteristics:
         between the steps a) and b), the method comprises the switching over of a switch electrically connected between the core and the sheath to pass from the first waveguide where the core and the sheath are electrically insulated from each other to the second waveguide where the core and the sheath are electrically short-circuited;   the measurements of the variables representing admittance values Y testco  and Y testcc  are made with electromagnetic waves of a same frequency.       

     These embodiments of the method furthermore have the following advantage: 
     the switching over of a switch to make it pass from the first waveguide to the second waveguide enables a speedy change from the measurement of the admittance Y testco  to the measurement of the admittance Y testcc  and vice-versa. 
     An object of the invention is also an information-recording medium comprising instructions for the implementing of the above method when the instructions are executed by an electronic computer. 
     An object of the invention is also a device for measuring the permittivity and/or permeability of a sample of non-conductive material, this device comprising: 
     an apparatus for measuring admittance values solely from the amplitude and the phase of the electromagnetic waves reflected on an interface between the sample and an end of a guide of these electromagnetic waves, 
     a first waveguide having a first end capable of being applied to the sample and a second opposite end capable of being connected to the measuring apparatus to measure a variable representing an admittance Y testco , this first waveguide being equipped with at least one core separated from a conductive sheath by a layer made of dielectric material in which the electromagnetic waves are propagated 
     a second waveguide having a first end capable of being applied to the sample and a second opposite end capable of being connected to the measuring apparatus to measure a variable representing an admittance Y testcc , this second waveguide being equipped with at least one conductive core separated from a conductive sheath by a layer of dielectric material in which the electromagnetic waves get propagated, the second waveguide also comprising a short-circuit between the central core and the sheath made at the interface with the sample so that the phase and/or the amplitude of the electromagnetic waves reflected on the sample are different from those obtained when there is no short-circuit, and 
     a computer capable of computing the permittivity of the sample from the variables representing values of admittance Y testco  and Y testcc  and/or computing the permeability of the sample from the variables representing the values of admittance Y testco  and Y testcc . 
     The embodiments of this device may comprise one or more of the following characteristics:
         the core, the sheath and the dielectric material of the first and second waveguides are common, and the device comprises a switch electrically connected between the core and the sheath to pass from the first waveguide where the core and the sheath are electrically insulated from one another to the second waveguide where the core and the sheath are short-circuited;   the first end of the second waveguide comprises a metal plate extending from the central core to the sheath to short-circuit the core and the sheath and obstruct only one part of the section of the dielectric material in which the electromagnetic waves reflected on the sample get propagated;   the second waveguide is identical to the first waveguide except that it comprises said metal plate;   the sheath entirely surrounds the core.       

     These embodiments furthermore have the following advantage: 
     having first and second waveguides that are almost identical makes it possible to have working frequency ranges for these guides that are also almost identical. 
    
    
     
       The invention will be understood more clearly from the following description given purely by way of a non-restrictive example and made with reference to the drawings of which: 
         FIG. 1  is a schematic illustration of a device for measuring the permittivity and the permeability of a sample of non-conductive material, 
         FIG. 2  is a schematic illustration in perspective of the end of a waveguide of the device of  FIG. 1 , 
         FIGS. 3 and 4  are schematic illustrations of the end shown in  FIG. 2  in an open-circuit configuration and in a partial short-circuit configuration as well as an illustration of the corresponding electrical models, 
         FIG. 5  is a flowchart of a method for measuring the permittivity and the permeability of a sample by means of the device of  FIG. 1 , 
         FIG. 6  is a schematic illustration of another embodiment of a device for measuring the permittivity and the permeability of a sample, and 
         FIG. 7  is a schematic illustration in perspective of one end of a waveguide of the measuring device of  FIG. 6 . 
     
    
    
     In these figures, the same references are used to designate the same elements. 
     Here below in this description, the characteristics and functions well-known to those skilled in the art are not described in detail. 
       FIG. 1  shows a device  2  for measuring the permittivity and the permeability of a sample  4  made of non-conductive material. As an illustration, the sample  4  is a sample made out of a material whose permeability is different from 1. For example, the sample  4  is a ferrite. The sample  4  may be a part of an object or the totality of this object. 
     The device  2  has an apparatus  6  for measuring admittance at the interface between the sample  4  and a probe  8  forming the end of an electromagnetic waveguide  10 . The admittance, denoted as Y, is the reciprocal of the impedance. 
     Another end of the guide  10  is electrically connected to the apparatus  6 . 
     Here, the apparatus  6  is capable of generating electromagnetic waves which are directed by the waveguide  10  up to the plane face  12  of the sample  4 . To this end, the probe  8  is directly applied to the face  12 . The apparatus  6  measures the amplitude and phase of the waves reflected at the interface with the sample  4 . 
     For example, the apparatus  6  is a vectorial network analyzer (VNA). These analyzers are known and shall not be described herein in greater detail. 
     By way of an illustration, the apparatus  6  is connected to an electronic computer  20  capable of computing the permittivity and the permeability of the sample  4  from values of admittance measured by the apparatus  6 . For example, the computer  20  is a programmable electronic computer capable of executing instructions recorded on an information-recording medium. To this end, the computer  20  is connected to a memory  22  comprising the instructions needed to execute the method of  FIG. 5 . 
     The waveguide  10  is formed by a conductive sheath  14  which extends around a central conductive core  16 . The sheath  14  and the core  16  are for example made of metal. The sheath  14  and the core  16  are separated from each other by a dielectric material in which the guided electromagnetic waves get propagated. For example, the waveguide  10  is a coaxial cable. 
     The computer  20  is also connected to a man-machine interface  24  to communicate the measured values of permittivity and permeability to a user. For example, the interface  24  is herein formed by a screen  26  and a keyboard  28 . 
       FIG. 2  shows the probe  8  in greater detail. 
     The probe  8  has a clamp  30  designed to be lean directly on the face  12  of the sample  4 . This clamp  30  surrounds the sheath  14 . 
     Between the sheath  14  and the core  16 , there is housed a dielectric material  32  that electrically insulates the core  16  from the sheath  14 . Here, substantially in the plane of the clamp  30 , the core  16  is electrically connected to the sheath  14  by means of a controllable switch  34 . In its closed position, the switch  34  short-circuits the core  16  and the sheath  14 . In its open position, on the contrary, the core  16  and the sheath  14  are electrically insulated from each other. The dimensions of this switch  34  as well as those of the electrical conductors  36  and  28  that connect them, respectively, to the core  16  and the sheath  14  are chosen so that, in the closed position, a part of the electromagnetic wave gets reflected all the same on the sample  4 . To this end, when the switch is closed, the short-circuit blocks only one part of the cross-section of the dielectric material  32 . This short-circuit modifies the phases of the electromagnetic waves reflected on the face  12  relatively to the case where the switch  34  is in its open position. Thus, in closed position, this short-circuit is a partial short-circuit. The dimensions of the switch  34  and of the conductors  36  and  38  can also be chosen so that the closing of the switch  34  also gives rise to a modification of the amplitude of the electromagnetic waves reflected on the face  12  in relation to the case where the switch  34  is in its open position. For example, these modifications of the phase and/or of the amplitude are at least greater than 1% or 10% of the phase and/or of the amplitude obtained when the switch  34  is in its open position. 
     Preferably, in its closed position, the dimensions of the switch  34  and of the conductors  36  and  38  are chosen in such a way that the short-circuit created corresponds also to an electromagnetic short-circuit capable of introducing an 180° phase shift relatively to the situation where the switch  34  is in its open position. 
     Many different embodiments are possible for the switch  34 . For example, the switch  34  can be made by means of: 
     semiconductor components such as diodes, transistors or thyristors, 
     mechanical microsystems (MEMS), or again: 
     optoelectronic components (phototransistors). 
     For example, in  FIG. 2 , the switch  34  is made by means of a diode. 
     The device  2  also has a unit  36  for controlling the switch  34 . This unit  36  is capable of controlling the switch-over of the switch  34  between its open and closed positions. For example, the switch-over between the open and closed positions is activated manually by a user. 
     In the case of a diode, the unit  36  has, for example, a DC voltage source  38  connected between the sheath  14  and the core  16 . This source  38  is series-connected with a selector switch  40 . When the selector switch  40  is in a closed position, a DC voltage is applied between the core  16  and the sheath  14 . This DC voltage is chosen so as to turn the diode  34  on. When the selector switch  40  is in its open position, no DC voltage is applied between the sheath  14  and the core  16  so that the diode  34  is in its open position and therefore does not let current pass through. 
     When the switch  34  is in its closed position, it allows the AC current at the working frequency to get propagated from the core  16  up to the sheath  14  by conductivity. Conversely, when the switch  34  is in its open position, this AC current at the working frequency cannot get propagated by conductivity of the core  16  up to the sheath  14 . 
     The selector switch  40  is switched over between its open and closed positions manually by an operator. For example, the selector switch  40  is a push-button. 
       FIG. 3  is a schematic cross-section view of the probe  8  when the switch  34  is in its open position. This probe is then said to be in an open circuit. In this state, the interface between the end  8  and the sample  4  can be modeled with an electrical model  44 . Here, this electrical model  44  comprises an inductor  46  series-connected with a capacitor  48 . A capacitor  50  is also parallel-connected with the terminals of the capacitor  48 . The value μL 0co  of the inductor  46  is proportional to the permeability μ of the sample  4 . The capacitance C fco  of the capacitor  48  is independent of the permittivity and the permeability. The capacitance ∈C 0co  of the capacitor  50  is proportional to the permittivity ∈ of the sample  4 . 
       FIG. 4  is a schematic cross-section view of the probe  8  when the switch  34  is in its closed position. The probe  8  is said then to be in a short-circuit. In this figure, the short-circuit between the core  14  and the sheath  16  is represented by a bold line  52 . 
     The probe in short-circuit is modeled by means of a model  54 . For example, the model  54  comprises, successively connected in parallel, one inductor  56  and three capacitors  58  to  60 . The value μL 0cc  of the inductor  56  is proportional to the permeability μ of the sample  4 . The capacitance C fcc  of the capacitor  58  is independent of the permittivity and the permeability. The capacitances ∈C 0cc , and ∈ 2 C 1cc  respectively of the capacitors  59  and  60  are a function of the permittivity ∈ of the sample  4 . 
     From the model  44 , it is possible to estimate the admittance Y CO  measured by the apparatus  6  in an open circuit. This estimation is given by the following relationship: 
     
       
         
           
             
               
                 
                   
                     Y 
                     CO 
                   
                   = 
                   
                     
                       jω 
                       ⁡ 
                       
                         ( 
                         
                           
                             C 
                             fco 
                           
                           + 
                           
                             ɛ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               C 
                               
                                 0 
                                 ⁢ 
                                 co 
                               
                             
                           
                         
                         ) 
                       
                     
                     
                       1 
                       - 
                       
                         
                           
                             ω 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 fco 
                               
                               + 
                               
                                 ɛ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   C 
                                   
                                     0 
                                     ⁢ 
                                     co 
                                   
                                 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         μ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           L 
                           
                             0 
                             ⁢ 
                             co 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where: 
     ω is the pulsation (ω=2πf, where f is the working frequency in Hertz), 
     j is the complex number such that j 2 =−1, and 
     L 0co , C fco , and C 0co  are parameters of the model  44  whose values are independent of the permittivity ∈ and of the permeability μ. 
     Similarly, from the model  54 , it is possible to estimate the admittance Y CC  measured by the apparatus  6  in a short-circuit. This estimation is given by the following relationship: 
     
       
         
           
             
               
                 
                   
                     Y 
                     CC 
                   
                   = 
                   
                     
                       jω 
                       ⁡ 
                       
                         ( 
                         
                           
                             C 
                             fcc 
                           
                           + 
                           
                             ɛ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               C 
                               
                                 0 
                                 ⁢ 
                                 cc 
                               
                             
                           
                           + 
                           
                             
                               ɛ 
                               2 
                             
                             ⁢ 
                             
                               C 
                               
                                 1 
                                 ⁢ 
                                 cc 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       1 
                       
                         jωμ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           L 
                           
                             0 
                             ⁢ 
                             cc 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where:
         L 0cc , C fcc , C 0cc , and C 1cc  are parameters of the model  54  whose values are independent of permittivity ∈ and of the permeability μ.       

     The working of the device  2  shall now be described with reference to the method of  FIG. 5 . 
     The method starts with a phase  70  for calibrating the device  2 . Initially, at the beginning of this phase  70 , at a step  72 , the apparatus  6  is calibrated. This step  72  is a conventional step and shall not be described in greater detail. 
     Then, a step  74  for calibrating the probe  8  in an open circuit is performed. This step  74  is aimed at determining the value of the parameters C fco , C 0co  and L 0co . For this step  74 , the invention uses standard samples. The permittivity and the permeability of these standard samples are known in advance. For example, the step  74  uses three standard samples chosen from the group formed by de-ionized water, Teflon (RM), air and a magnetic-dielectric material whose permittivity and permeability are equal to 5. 
     More specifically, in an operation  76 , the first standard sample is placed instead of the sample  4 . In other words, the probe  8  is applied to a plane face of this standard sample. Then, under these conditions, the admittance Y ref1co  is measured by the apparatus  6 . Then, the operation  76  is repeated for the second and third standard samples so as to measure the admittance values Y ref2co  and Y ref3co  respectively. 
     Then, in an operation  78 , the computer  20  determines the values of the parameters C 0co , C fco  and L 0co  from the measurements Y ref1co , Y ref2co  and Y ref3co  as well as from the known values of permittivity ∈ ref1 , ∈ ref2  and ∈ ref3  and of permeability μ ref1 , μ ref2  and μ ref3 , respectively of the first, second and third standard samples. For example, to this effect, the following system of equations is resolved: 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             
                               jω 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     
                                       ɛ 
                                       
                                         ref 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
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                                       2 
                                     
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                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
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                                 ⁢ 
                                 
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                                     ref 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
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                                 ⁢ 
                                 
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                                     0 
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                               + 
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                           = 
                           
                             Y 
                             
                               ref 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               ⁢ 
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                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     
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                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
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                                     ⁡ 
                                     
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                                               ⁢ 
                                               
                                                   
                                               
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                                     ref 
                                     ⁢ 
                                     
                                         
                                     
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                               + 
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                           = 
                           
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                               ref 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
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                               ⁢ 
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                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     
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                                         ref 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         3 
                                       
                                     
                                     ⁢ 
                                     
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                                         0 
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                                     C 
                                     fco 
                                   
                                 
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                                   - 
                                   
                                     
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                                     ⁡ 
                                     
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                                               ⁢ 
                                               
                                                   
                                               
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                                     ref 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     3 
                                   
                                 
                                 ⁢ 
                                 
                                   L 
                                   
                                     0 
                                     ⁢ 
                                     co 
                                   
                                 
                               
                               + 
                               1 
                             
                           
                           = 
                           
                             Y 
                             
                               ref 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               3 
                               ⁢ 
                               co 
                             
                           
                         
                       
                     
                   
                   } 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The value of the parameters C 0c0 , C fco  and L 0co  depends on the working frequency of the electromagnetic wave and therefore on the frequency of the excitation current flowing in the core  16  used to measure the admittance. Consequently, the operations  76  and  78  are preferably repeated for a large number of different frequencies evenly distributed in the range of possible working frequencies. For example, this range of possible working frequencies extends from 0 MHz to 20 GHz. Preferably, the range of working frequencies is from 100 MHz to 10 GHz. 
     Once the step  74  is completed, a step  80  is performed for calibrating the probe  8  in short-circuit. To this end, the switch  34  is switched over into its closed position and the admittance of four standard samples is measured by means of the device  2  in an operation  82 . As above, the known values of permittivity and permeability of these standard samples are denoted respectively as ∈ ref1 , ∈ ref2 , ∈ ref3  and ∈ ref4  while the known values of permeability are denoted as μ ref1 , μ ref2 , μ ref3  and μ ref4 . The standard samples are for example chosen out of the same group as the one described with reference to the step  74 . 
     In an operation  82 , the respective admittance values Y ref1 , Y ref2 , Y ref3  and Y ref4  of each of these standard samples are measured by means of the apparatus  6 . 
     Then, in an operation  84 , the values of the parameters C fcc , C 0cc , C 1cc  and L 0cc  are computed from the measured admittance values Y ref1 , Y ref2 , Y ref3  and Y ref4  as well as from the known values of permeability and permittivity of the standard samples. For example, at the operation  84 , the following system of equations is resolved: 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             Y 
                             
                               ref 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           = 
                           
                             
                               jω 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     C 
                                     fcc 
                                   
                                   + 
                                   
                                     
                                       ɛ 
                                       
                                         ref 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                     ⁢ 
                                     
                                       C 
                                       
                                         0 
                                         ⁢ 
                                         cc 
                                       
                                     
                                   
                                   + 
                                   
                                     
                                       ɛ 
                                       
                                         ref 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                       2 
                                     
                                     ⁢ 
                                     
                                       C 
                                       
                                         1 
                                         ⁢ 
                                         cc 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                             + 
                             
                               1 
                               
                                 
                                   jωμ 
                                   
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     The operations  82  and  84  are reiterated for several working frequencies evenly distributed in the range of working frequencies. Preferably, the same working frequencies as those used in the step  74  are used in the step  80 . 
     Once the device  2  has been calibrated, the phase  70  is terminated and a phase  90  for measuring permittivity and permeability of the sample  4  can be activated. 
     At the start of the phase  90 , in a step  92 , the probe  8  is placed so as to lean on the face  12  of the sample  4  for which the permittivity ∈ test  and the permeability μ test  have to be measured. 
     Then, at a step  94 , the switch  30  is switched over to its open position and the admittance Y testco  is measured by means of the apparatus  6 . Preferably, the step  94  is reiterated for the same working frequencies as those used in the step  74  so that several measurements of admittance Y testco  are obtained. 
     Then, at a step  96 , the switch is switched over to its closed position. 
     Then, at a step  98 , the admittance Y testcc  is measured by the apparatus  6  for the different working frequencies used. 
     At a step  100 , the computer  20  computes the permittivity ∈ test  and the permeability μ test  for each of the working frequencies used during the steps  94  and  98 . The permeability ∈ test  and the permeability μ test  are each obtained from the admittance values Y testco  and Y testcc  measured at the same frequency. For example, the permittivity ∈ test  and the permeability μ test  are computed by resolving the following system of equations: 
                   {               y   testco     -       jω   ⁡     (       C   fco     +       ɛ   test     ⁢     C     0   ⁢   co           )         1   -         ω   2     ⁡     (       C   fco     +       ɛ   test     ⁢     C     0   ⁢   co           )       ⁢     μ   test     ⁢     L     0   ⁢   co               =   0                   y   testcc     -     jω   ⁡     (       C   fcc     +       ɛ   test     ⁢     C     0   ⁢   cc         +       ɛ   test   2     ⁢     C     1   ⁢   cc           )       -     1       jωμ   test     ⁢     L     0   ⁢   cc             =   0           }           (   5   )               
where:
 
     C fco , C 0co , L 0co , L 0cc , C fcc , C 0cc  and C 1cc  are the values of the parameters measured or interpolated on the basis of the measurements made during the phase  70 , 
     Y testco  and Y testcc  test are the admittance values measured in open circuit and in closed circuit for this working frequency. 
     The above system of equations can be resolved analytically. However, it has three solutions. To choose the right solution, the solution having the permittivity closest to 1 is chosen. For example, to this end, this system of equations is resolved by means of a Newton-type resolution initialized by taking the permittivity value μ test  equal to 1. 
     Once the permittivity ∈ test  and the permeability μ test  have been computed at the step  100 , they are presented during a step  102  to the user through the man-machine interface or recorded in a file. 
       FIG. 6  shows another embodiment of a device  110  for measuring the permittivity and the permeability of the sample  4 . This device  110  is identical to the device  2  except that the waveguide  10  is replaced by two waveguides  112  and  114 . The waveguides  112  and  114  respectively have ends  116  and  118  that are to lean directly on the face  12  when a measurement of admittance is made. The end  116  forms an open-circuit probe. For example, the end  116  is identical to the probe  8  except that the switch  34 , the conductors  36  and  38  and the control unit  36  have been omitted. Thus, the probe  116  has only an open-circuit configuration. 
     Conversely, the end  118  forms a probe having only a partial short-circuit configuration. This probe  118  is shown in greater detail in  FIG. 7 . This probe  118  is for example identical to the probe  8  except that the switch  34  and the conductors  36  and  38  are replaced by a thin metal plate  120  which extends from the core  16  to the sheath  14 . The dimensions of this plate  120  are sufficient to modify the phase and/or the amplitude of the electromagnetic waves reflected by the face  12  as compared with the case where the plate is omitted. For example, this modification of the phase and/or of the amplitude is at least greater than 1% or 10% of the phase and/or of the amplitude obtained when the plate  120  is omitted. For example, the width of the plate  120  is equal to the diameter of the core  16 . 
     The other end of each of the waveguides  112  and  114  is equipped respectively with connectors  122  and  124 . These connectors  122  and  124  enable the connection and, alternately, the disconnection of the waveguides  112  and  114  to/from the apparatus  6 . 
     The working of the device  110  is identical to the one described with reference to the method of  FIG. 5  except that to pass from the probe in open circuit to the probe in closed circuit, it is necessary here to replace the waveguide  112  by the waveguide  114  and vice versa. 
     Many other embodiments are possible. For example, the section of the waveguides is not necessarily circular as in the case of coaxial cables. For example, the sections of the sheath and/or of the core may be rectangular. 
     The waveguide may have several cores. 
     In the device  110 , the condition by which the first and second waveguides are structurally identical with the exception of the metal plate  120  is not necessary. For example, the dimensions of the waveguides  112  and  114  may differ. For example, the dimensions of the sheath and of the core (for example their respective diameters) may be different between these two waveguides. 
     As a variant, the computer computes only the permittivity from the measured admittance values Y testco  and Y testcc  without delivering the value of the permeability μ test  to the user. This makes it possible to measure the permittivity of material for which the permeability is different from 1 (for example ferrite). Conversely, the computer can also compute only the permeability μ test  from the admittance values measured Y testco  and Y testcc  without delivering the permittivity value ∈ test  to the user. 
     Electrical models other than those represented here can be used provided that they show the permittivity and the permeability of the sample. These other models may be obtained from equations of the radiation of the electromagnetic waves. In particular, more complex models may be used instead of those shown. 
     The end of each waveguide does not necessarily take the form of a probe. For example, the end of the waveguide may be directly applied to the sample without having any clamp or similar means. 
     Here, the devices  2  and  110  have been described in the particular case where the measurements of admittance are directly transmitted to the computer by means of a wire link. As a variant, this wire link is omitted and the measurements made by the apparatus  6  are entered through the keyboard  28  in order to communicate them to the computer  20  which is responsible for making computations of permittivity and permeability. 
     Finally, the devices  2  and  110  and the different systems of equations have been described in the particular case where the variables measured are admittance values. However, the devices  2  and  110  can be adapted to measuring not admittance values directly but a variable representing admittance. The term “variable representing an admittance” designates a physical variable that can be measured by means of an apparatus and from which it is possible to deduce the value of the admittance without any complementary measurements. For example the impedance and the coefficient of reflection of the electromagnetic waves on the face  12  are variables representing admittance. Should a variable representing admittance other than admittance itself be used, then the systems of equations described here above are adapted in order to take account of this difference.