Low Frequency Material Characterization of Thin Substrates in a Coaxial Transmission Line
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH GRADUATE SCHOOL OF ENGINEERING AND MANAGEMENT
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This research analyzes the complex permittivity and permeability of thin material samples at low frequencies in the presence of large outer air gaps. It uses the modal method developed in previous research to account for the errors introduced to the system by the presence of higher order modes excited by the air gaps. The theoretical scattering parameters returned by the modal method are compared to the measured scattering parameters and the difference between the two is minimized using a complex root search algorithm. The method is tested with samples of a dielectric material and the magnetic material WaveX which respectively fill 22 and 12.5 of the distance between the inner and outer conductors. The method accurately determined the complex permittivity of the dielectric samples, appearing to converge after the inclusion of 10 higher order modes. The method performed poorly for the WaveX material, particularly for complex permittivity. Assumptions of an axially symmetric field environment in the sample region and a linear, isotropic, homogeneous material were made. Upon evaluation, the axially symmetric environment is likely disrupted due to discontinuities arising from the material wrapping around the inner conductor. The material also exhibited strong anisotropic tendencies upon further evaluation.
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