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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 analyzed are torus-shaped; they have been machined in order their geometrical dimensions fit well those of the coaxial line used.
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Principle
This characterization method enables one to simultaneously calculate the permittivity and complex permeability of a given material from the measurement of the S-parameters of a coaxial line that contains it (Top Figure); they are measured using a vector network analyzer.
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Analysis of measured data
The electromagnetic analysis of the cell is based on the line theory: it assumes that only one TEM (transverse electromagnetic) mode is propagating along the line [1]. According to this hypothesis, the material must totally fill in the line cross section. The torus-shaped sample must have been extremely precisely machined in order to avoid the presence of air gaps between the sample and the line walls. The electromagnetic analysis of the coaxial line enables one to analytically calculate the complex permeability from the measured reflection and transmission parameters.
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Results
- Frequency range used: 10 MHz - 20 GHz (network analyzers HP 8753ES and HP8720A).
- Type of material tested: dielectric, ferrimagnetic and ferrocomposite materials with mean and high losses, i.e.(tan > 10-3).d
- Accuracy: relative errors < 5%.
- Network analyzer calibration: SOLT.
- Dimensions of the tested samples:
- inner diameter = 3.04 mm;
- outer diameter = 7 mm;
- thickness = 2 to 4 mm.
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Measurement of a ferricomposite permeability and permittivity versus frequency
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Comments
The above method was used to characterize several types of dielectrics, ferrites and ferro or ferri composites; the comparison of our experimental results with those reported in the literature allowed us to validate them. When the sample thickness is within 2 and 4 mm, the relative error on permittivity and permeability is less than 5%. This method is, thus, meant for easily-machined or pressed materials The coaxial cell has been selected to define a standard for the characterization of dielectric and magnetic materials.
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1. J. BAKER-JARVIS, E. J. VANZURA and W. A. KISSICK "Improved technique for determining complex permittivity with the transmission/reflection method ", IEEE Transactions on Microwave Theory and Techniques Vol. 38, No. 8, pp. 1096-1103, August 1990.
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