Browsing by Author "Bingle, Marianne"
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- ItemThe finite difference time domain modelling of liquids with dispersive dielectric properties(Stellenbosch : Stellenbosch University, 1995) Bingle, Marianne; Cloete, J. H.; Davidson, D. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The dielectric properties of materials are of great importance in fundamental and applied research. A study of dielectric materials, their microscopic behaviour and their influence on macroscopic fields leads to the various material models for dispersive dielectric materials. A short review is given of the electromagnetic description of dielectric behaviour. A complete measurement system for measuring dispersive dielectric liquids is described. The system consists of a short-circuited coaxial cavity fed by a coaxial connector. The research for this thesis is concerned with the measurement and analysis of the coaxial cavity system filled with a dispersive dielectric of known properties. The goal is to obtain a model of the measuring system which accurately predicts the reflection measurements for the known dielectric. Measurements are conducted over the frequency range of the network analyser, 30MHz to 6GHz. The measurement system involves a stepped discontinuity in the coaxial geometry. The transition from the coaxial connector to the cavity is achieved by two step discontinuities, in the inner and outer conductors respectively, offset with a distance δ. The effect of the amount of offset on reflection in the coaxial system is investigated, both with the body of revolution finite difference time domain method and through measurement to find the optimum value of δ for minimum reflection over the frequency range of interest. Reflection measurements were done with a network analyser over the frequency range 30MHz to 6GHz. The results from the numerical simulations show good agreement with the measurements. The well-matched transition that resulted from this investigation led to simple calibration and modelling of the measurement system. The system is analysed in the time domain with the body of revolution finite difference time domain method and in the frequency domain with transmission line theory. The body of revolution finite difference time domain method solves for the time-dependent Maxwell equations; the discretised convolution integral is solved by recursion. The analysis of the measurement system is tested by determining the reflections from the cavity when filled with purified water and comparing it to measurements. In both methods of analysis, the permittivity of the water is described by a Debye model. The body of revolution finite difference time domain simulation, after transformation to the frequency domain, and the transmission line analysis are in good agreement. The measured reflection coefficient for a water-filled cavity is compared to the results of the analyses over the 30MHz to 6GHz frequency band. Water leaking into the transition region significantly influenced the measured reflection coefficient in the upper frequency band. Prevention of water leaking into the transition region improved the agreement between the measured and predicted reflection coefficient of the analysis. Higher order modes did not prove to be a problem in the highly symmetrical measurement system.
- ItemThe role of chirality in synthetic microwave absorbers(Stellenbosch : Stellenbosch University, 1998) Bingle, Marianne; Cloete, J. H.; Davidson, D. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Claims were made during the late 1980's that synthetic chiral materials can yield microwave absorbers with significantly improved electromagnetic and physical properties. Papers appeared in scientific literature and a number of patents were issued. These claims stimulated wide interest in microwave chiral absorbers. In 1992 Bohren et al. challenged the claims about chirality by asking the question, "Microwave absorbing chiral composites: is chirality essential or accidental?" They concluded that "although helices have some excellent properties in the design of composite absorbing materials, the resulting chirality is accidental and not essential." An experimental study by Cloete et al. seems to support the findings of Bohren et al. However, we are also of the opinion that fundamentally the role of chirality in the interaction between an electromagnetic wave and an absorbing medium is not yet fully understood, especially in the regime where the chiral inclusions are resonant. Hence this theoretical and experimental study of closely related uniaxial crystals of conducting wire structures of resonant length in the microwave regime, embedded in a dielectric host. Unit cells of synthetic material are designed to fit into a rectangular or square waveguide to allow accurate measurements of reflection and transmission coefficients. The three segment wire hook was chosen as basic element, instead of the helix, for the simple geometrical relation between the chiral enantiomorphs and the non-chiral structures. Four uniaxial crystals are described, and classified according to their point group symmetry: a chiral unit cell, consisting of four hooks of the same handedness; a non-chiral unit cell of four staples; a non-chiral unit cell of four cranks; and a racemic unit cell of enantiomorphous chiral hooks, two of each handedness. A finite difference time domain code was developed for the full-wave numerical analysis in a rectangular waveguide of the conducting thin wire structures, embedded in an absorbing host. The code uses a subcell thin wire formulation for conducting wires, including the effective material properties of the wires. In order to validate the FDTD code, wire crystals were manufactured and measured in S-hand (2 4 GHz) rectangular waveguide. The FDTD subcell thin wire formulation for highly conducting wires was found to predict the scattering with respect to magnitude, phase and absorption, with good accuracy. Numerical experiments were performed in a square waveguide on the wire crystals to study their absorption properties. The following observations were made. The inclusion of wire structures in a microwave absorber can improve the absorption interaction of an electromagnetic wave with the host, provided that the inclusions are good conductors and of resonant length. This is observed whether the inclusions are chiral, or non-chiral. The unit cell of non-chiral cranks couples the incident field to the loss mechanisms of the host as effectively as the unit cell of chiral hooks and the racemic unit cell of enantiomorphous hooks. Chirality is a geometrical requirement for optical activity. However, no evidence in these experiments suggests that chirality is a geometrical requirement for absorption by synthetic microwave materials.