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Description
This method allows a broad-band measurement of the complex permittivity and permeability of solid and isotropic materials. The samples to be tested are either rectangular plates or thin films put (or mounted) on a dielectric holder.
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Principle
This method is based on the determination of the distribution parameters, Sij, of a 3-plate transmission microstrip line that contains the material to be tested (Top figure). These Sij parameters are measured using a vector network analyzer. The rectangle-shaped sample consists in either a plate or a thin film. The method enables one to get rid of sample machining problems since the latter does not fully fill in the cross-section of the cell. This constitutes an important asset, especially in the characterization of ferromagnetic materials: indeed, such materials are sometimes fragile and their machining quite difficult because of their granular nature. In addition, the mapping of electromagnetic fields associated to the fundamental mode of the 3-plate line is well suited to the characterization of rectangular and low thickness samples (thin films).
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Analysis of measured data
The electromagnetic analysis of the cell is based on a quasi-static approach [1]. It consists in displaying the transverse section of the 3-plate line that contains the sample under test by an equivalent cross-section made of a homogeneous medium of effective permittivity, eff, and effective permeability, µeff. Then, the experimental data are analyzed in two steps: first, the effective permittivity and effective permeability of the line section in which the sample under test has been placed are calculated versus the S-parameters of the measuring cell; then, the complex permittivity and permeability of the material are calculated from eeff and µeff issued from Sij measurement. The procedure used is thus a dichotomic one.e
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Results
- Type of material tested: dielectric, ferrimagnetic and ferrocomposite materials with mean to high losses, i.e. (tan d > 10-3).
- Accuracy: relative errors < 5%.
- Network analyzer calibration: SOLT.
- Dimensions of the tested samples:
length = at least 10 mm; width = a few millimeters; thickness = some micrometers to 1.8 mm.
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Measurement of a ferrocomposite complex permeability versus frequency under various polarizing field values
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Comments
To validate the data analysis software associated to the 3-plate cell, materials of known properties have been measured between 100 MHz and 8 GHz. The expected permittivy and permeability of reference materials have been experimentally measured. The above figure illustrates the experimental data obtained on ferrocomposite materials made of several ferromagnetic layers and insulating films in alternation. The method used is being published in a peer-reviewed journal [2].
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1. R. Crampagne, M. Ahmadpanah, J-L. Guiraud, "A Simple Method for determining the Green's Function for a Large Class of MIC Lines having Multilayered Dielectric Structures", IEEE Trans. Microwave Theory Tech.,vol. MTT-26, pp. 82-87, Feb. 1978.
2. E. Salahun, P. Quéffélec, M. Le Floc'h and Ph. Gelin, "A broad band permeameter for in situ measurements of plates samples", IEEE Trans. On Magn., to be published July 2001.
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