James R. Baker-Jarvis

Improved technique for determining complex permittivity with the transmission/reflection method

The transmission/reflection method for complex permittivity and permeability determination is studied. The special case of permittivity measurement is examined in detail. Robust algorithms for permittivity determination that eliminate the ill-behaved nature of the commonly used procedures at frequencies corresponding to integer multiples of one-half wavelength in the sample are presented. An error analysis yielding estimates of the errors incurred due to the uncertainty in scattering parameters, length measurement, and reference plane position is presented. Equations for determining complex permittivity independent from reference plane position and sample length are derived. >

A dielectric resonator for measurements of complex permittivity of low loss dielectric materials as a function of temperature

An application of a mode dielectric resonator is described for precise measurements of complex permittivity and the thermal effects on permittivity for isotropic dielectric materials. The Rayleigh-Ritz technique was employed to find a rigorous relationship between permittivity, resonant frequency, and the dimensions of the resonant structure, with relative computational accuracy of less than . The influence of conductor loss and its temperature dependence was taken into account in the dielectric loss tangent evaluation. Complex permittivities of several materials, including cross-linked polystyrene, polytetrafluoroethylene, and alumina, were measured in the temperature range of 300-400 K. Absolute uncertainties of relative permittivity measurements were estimated to be smaller than 0.2%, limited mainly by uncertainty in the sample dimensions. For properly chosen sample dimensions, materials with dielectric loss tangents in the range of to can be measured using the mode dielectric resonator.

Analysis of an open-ended coaxial probe with lift-off for nondestructive testing

The open-ended coaxial probe with lift-off is studied using a full-wave analysis, and an uncertainty analysis is presented. The field equations for the following terminations are worked out: (1) the sample extends to /spl infin/ in the positive axial direction, (2) the sample is backed by a well-characterized material, and (3) the sample is backed by a short-circuit termination. The equations are valid for both dielectric and magnetic materials. The model allows the study of the open-ended coaxial probe as a nondestructive testing tool. The analysis allows a study of the effects of air gaps on probe measurements. The reflection coefficient and phase are studied as a function of lift-off, coaxial line size, permittivity, permeability, and frequency. Numerical results indicate that the probe is very sensitive to lift-off. For medium to high permittivity values and electrically small probes, gaps on the order of fractions of a millimeter strongly influence the reflection coefficient. In order for the field to penetrate through the air gap, larger size coaxial line or higher frequencies need to be used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included. >

Dielectric characterization of low-loss materials a comparison of techniques

Measurements on low-loss materials using closed and open cavity resonators, and dielectric resonator methods are presented. Results indicate that consistent measurement results can be obtained with a number of well-characterized fixtures. Uncertainties associated with each method are addressed. Measurements also were performed on materials used in previous intercomparisons.

Complex permittivity measurements of common plastics over variable temperatures

In this paper, we present complex permittivity data at microwave frequencies (approximately 10 GHz) for many common plastics over a temperature range of 122 to 375 K. The measurements were made with a TE/sub 01/spl delta// dielectric resonator placed inside an environmental chamber. Data are presented for the following materials: acrylonitrile butadiene styrene, polytetrafluoroethylene, cross-linked polystyrene, tetrafluorethylene-perfluorpropylene, polypropylene, polysulfone, polymethylmethacrylate, polyvinyl chloride, polycarbonate, high-density polyethylene, polyoxy-methylene (acetal homopolymer), and polyamide.

Uncertainty of complex permittivity measurements by split-post dielectric resonator technique

Abstract Split-post dielectric resonators operating at frequencies 1.4–5.5 GHz were used to measure complex permittivity of single crystal standard reference dielectric materials with well known dielectric properties previously measured by other techniques. Detailed error analysis of permittivity and dielectric loss tangent measurements has been performed. It was proved both theoretically and experimentally that using split post resonators it is possible to measure permittivity with uncertainty about 0.3% and dielectric loss tangent with resolution 2×10 −5 for well-machined laminar specimens.

Applicability of effective medium theory to ferroelectric/ferrimagnetic composites with composition and frequency‐dependent complex permittivities and permeabilities

High‐frequency (1 MHz–1 GHz) transmission line measurements were used to determine the composition and frequency‐dependent complex permittivities and complex permeabilities of ferroelectric/ferrimagnetic (barium titanate and a magnesium‐copper‐zinc ferrite) composites. The effective medium rules of Maxwell–Garnett give both lower and upper bounds for the effective permittivities and permeabilities and yield accurate estimates of the bulk electric and magnetic properties at low volume fill fraction of either component provided the proper host matrix is chosen. Bruggeman theory yielded the best predictive values for the permittivity and permeability over the entire composition range. In all cases these complex quantities were shown to be constrained by Bergman–Milton bounds.

Full-wave analysis of a split-cylinder resonator for nondestructive permittivity measurements

This paper presents a full-wave analysis of the split-cylinder resonator. We outline a model where the fringing fields are rigorously accounted for and the resonance condition is derived. Using this model, a method for nondestructively measuring the complex permittivity of materials is examined. Measurements of the complex permittivity for low-loss dielectric materials using the split-cylinder resonator agree well with measurements made in a cylindrical cavity. An uncertainty analysis for the complex permittivity is also provided.

A nonlinear least-squares solution with causality constraints applied to transmission line permittivity and permeability determination

A technique for the solution of one-port and two-port scattering equations for complex permittivity and permeability determination is presented. Using a nonlinear regression procedure, the model determines parameters for the specification of the spectral functional form of complex permittivity and permeability. The method is based on a nonlinear regression technique and uses the fact that a causal, analytic function can be represented by poles and zeros. The technique allows the accurate determination of many low- and high-permittivity dielectric and magnetic materials in either the low- or high-loss range. The model allows for small adjustments, consistent with the physics of the problem, to independent variable data such as angular frequency, sample length, sample position, and cut-off wavelength. The model can determine permittivity and permeability for samples where sample length, sample position, and sample holder length are not known precisely. The problem of local minima is discussed. >

High-frequency dielectric measurements

The demands on dielectric material measurements have increased over the years as electrical components have been miniaturized and device frequency bands have increased. Well-characterized dielectric measurements on thin materials are needed for circuit design, minimization of crosstalk, and characterization of signal-propagation speed. Bulk material applications have also increased. For accurate dielectric measurements, each measurement band and material geometry requires specific fixtures. Engineers and researchers must carefully match their material system and uncertainty requirements to the best available measurement system. Broadband measurements require transmission-line methods, and accurate measurements on low-loss materials are performed in resonators. The development of the most accurate methods for each application requires accurate fixture selection in terms of field geometry, accurate field models, and precise measurement apparatus.