Principles of the radiofrequency and microwave characterization of materials



according to the proprietes of your materials


Free-space characterization test bench: paraxial beam



Listing of the team publication
Permeability tensor of magnetized ferrites



Description
This method enables a broad-band frequency and temperature measurement of the complex permittivity and permeability of isotropic materials in liquid, powder, soft or solid form.

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Principle
This characterization method is based on the determination of the reflection coefficient of a sharp discontinuity between a coaxial line and the material to be investigated. Using a network analyzer this coefficient is measured in the frequency range: 200 MHz-20 GHz.. The schematic description of the discontinuity from an equivalent electric diagram enables one to simply determine the permittivity of the material under test.

Analysis of measured data
The electromagnetic analysis of the coaxial line/material discontinuity is based on electrical modeling by localized elements A data analysis software computes the complex permittivity from the measured reflection coefficient (S11). The validity range of this model extends up to the frequency at which the first upper mode occurs, that is around 20 GHz. A calibration procedure is required to compensate for the errors on measurements due to defects in the cell, i.e. Plexiglas-made seal, impedance mismatch..., as well as in the junction cable. It is validated further to another measurement made on reference materials of known permittivity, e.g. ethanol and distilled water for liquids, Teflon and polyethylene.

Results

  • Frequency range used: 200 MHz - 20 GHz (network analyzer HP8720A).
  • Temperature range used: 0 - 60°C.
  • Type of material tested: dielectrics (e'< 100) with mean to high losses, i.e. tan d > 0.05.
  • Accuracy: relative errors < 5%.
  • Network analyzer calibration: open-circuit, short-circuit, distilled water.
  • Dimensions of the tested samples: liquids and powders: 5 cl; diameter > 20 mm; thickness < 20/ (l er * l)0.5

Measurement of distilled water permittivity versus frequency.


Comments
The experimental data obtained on several types of liquids and solids with our cell fit well with those reported in the literature [1]. The relative error on measured permittivity in liquids as well as in solids is less than 5%. The test device allows measurement at low or high temperatures, 0 or 60°C, respectively; it has been developed within the framework of a contract with MOULINEX company.

1. D. S. ENGELDER and C. R. BUFFLER "Measuring dielectric properties of food products at microwave frequency ", Microwave World , Vol. 12, No. 2, Summer 1991.