Design and implementation of a two-port microstrip test fixture for complex permittivity characterization and near field imaging of biological materials up to 50 GHz.

摘要

Interaction of electromagnetic fields with human tissues, particularly the brain, is of great interest in omnipresent wireless communications. A field analysis (e.g., FDTD) requires knowledge of the complex permittivity of tissues exposed to the electromagnetic radiation. However, broadband complex permittivity values of biological tissues above 20 GHz obtained from direct measurements have not been reported in the literature.; This thesis presents design and measurement results of a two-port microstrip test fixture for complex permittivity measurements of biological tissues up to 50 GHz. The test fixture uses aperture coupling for interaction with the tissue. Performance of the test fixture is modeled using a commercial finite element package. The simulated S parameters of the test fixture, obtained for various complex permittivity values, are fitted to a function similar to the pole-zero expansion of the transfer function of a linear system. The fitting procedure is computationally more efficient than the previously developed fitting methods.; A two-port TRL method is selected to remove the measurement systematic errors. An Anritsu 37397C network analyzer is used for measurements from 5 to 50 GHz and the complex permittivity of white and grey brain matters is obtained for the first time up to 50 GHz. The results show very good agreements with previously reported results at 35 GHz. They also suggest that modifications to the available Cole-Cole models, extrapolated from the measured results below 26.5 GHz, are necessary.; Small artifacts are seen due to the lack of repeatability of coaxial to microstrip launchers. Two correction methods, which utilize water (i.e., a known material) for correction, are introduced and successfully employed for removing these artifacts. Finally, the corrected results are fitted to a single term Cole-Cole relation for representing γ-dispersion of white and grey matters up to 50 GHz.; The test fixture design allows it to be used for near field imaging of brain slices. Such an image of a 1 mm thick sample slice immersed in saline is prepared. Qualitative (i.e., S₂₁ and quantitative (i.e., ϵ ′ and ϵ′′, images are presented, and their resolution and contrast at various frequencies are discussed.