Calibration and measurement of dielectric properties of finitethickness composite sheets with open-ended coaxial sensors

The application of open-ended coaxial sensors for dielectric measurement of finite thickness composite sheets is studied. Expressions for calculation of the complex aperture admittance for two geometries are presented. These expressions are used to calculate the dielectric constant of infinite half-space as well as finite thickness slabs. A more efficient method of such calculations, using a personal computer, for low to medium loss dielectrics is demonstrated. The question of when a dielectric layer may be considered as infinitely thick is also addressed, and examples are presented. A different calibration technique (compared to the conventional ones) is described and successfully implemented. This calibration technique utilizes a dielectric sheet with known dielectric properties and thickness. Measurements for different airgaps between the open-ended coaxial line and the dielectric sheet are used to perform and enhance the calibration. The results of this calibration technique and several subsequent measurements are presented and discussed     

Measuring and fitting the MOS transistor at high frequencies

A novel small-signal model of a MOS transistor is presented, which is valid at frequencies around the unity gain frequency. As a major advantage compared with earlier models, this novel model takes into account the nonquasistatic and transmission-line effects of the transistor. By using S-parameter measurements, computer-controlled calibration techniques of the test setup and network analyzer, mathematical transformations and fit routines, all the AC parameters can be extracted from the measured data. The obtained model parameters are used in the design of a high-frequency circuit to prove the validity of the model as well as the measurement method     

A Quasi-Optical Free-Space Measurement Setup Without Time-Domain Gating for Material Characterization in the W-Band

In this paper, a new free-space measurement setup at millimeter waves for material characterization is presented. Using specific Gaussian optics lens antennas and a thru, reflect, and line calibration, the setup provides the free-space four S-parameters over the W-band of planar dielectric slabs without time-domain gating. An efficient optimization procedure is implemented to extract complex permittivity from the four S-parameters of homogeneous dielectric materials. Nonhomogeneous materials can also be tested, and measurements are presented. Very good agreement is observed between simulated and measured four S-parameters of various dielectric plates. Thanks to this new specific calibration and measurement procedure, automation of the test bench is easily achieved     

Modal transmission-line theory of three-dimensional periodic structures with arbitrary lattice configurations

The scattering of waves by multilayered periodic structures is formulated in three-dimensional space by using Fourier expansions for both the basic lattice and its associated reciprocal lattice. The fields in each layer are then expressed in terms of characteristic modes, and the complete solution is found rigorously by using a transmission-line representation to address the pertinent boundary-value problems. Such an approach can treat periodic arbitrary lattices containing arbitrarily shaped dielectric components, which may generally be absorbing and have biaxial properties along directions that are parallel or perpendicular to the layers. We illustrate the present approach by comparing our numerical results with data reported in the past for simple structures. In addition, we provide new results for more complex configurations, which include multiple periodic regions that contain absorbing uniaxial components with several possible canonic shapes and high dielectric constants.     

A 2.5-ns Timing Standard

The design, construction, and calibration of a passive transmission-line network that provides 2 pulses separated by 2.5 ns ± 1 ps, when excited by a single pulse, is described. The timing standard is useful for calibrating time window durations in time-domain measurement.     

The idea of constructing microwave devices for the uniform heating of sheet materials

The idea of producing a uniform temperature distribution over the thickness of sheet dielectric materials with different electrical parameters in microwave devices, constructed using a combination of a waveguide-type system and slow-wave structures, is considered. The results of measurements of the temperature distribution over the thickness of materials in traveling-wave-type microwave devices are presented. Translated from Izmeritel’naya Tekhnika, No. 3, pp. 58–59, March, 2009.     

Low insertion loss, wideband bandpass filters with sharp rejection characteristics

A general circuit configuration using two transmission-line sections is presented to design a sharp-rejection bandpass filter. Sharp rejection characteristic is obtained by creating two transmission zeros across the filter passband by interfering signals propagating through the transmission lines. A simple lossless transmission-line model is used for filter analysis. If an impedance variation of 20-130 ? is considered, the present method is most suitable for 10-50% 3-dB fractional bandwidth variation. Two prototype bandpass filters having approximately 29% fractional bandwidth at the midband frequency 2.9 GHz are fabricated in microstrip line. The filter structures are simple and easy to fabricate. Theoretical, simulated and measured results are in good agreement.     

Multiphase microwave dielectrics

Several types of multiphase ceramic materials with excellent microwave (MW) dielectric properties have been considered. Studied materials were based on complex niobates and titanates with various crystal structures in which very high values of the MW quality factors (Q) can be attained. Slight compositional changes in the complex oxide systems have been shown to induce rather combined effect on the cation arrangement of separate crystal phases as well as on the microstructure and phase composition of multiphase ceramic materials. As a consequence, the ways to tailor MW dielectric parameters of the ceramics through a proper adjustment of both structure and distribution of enclosed phases have been presented and discussed. The examples of the multiphase MW dielectrics with enhanced properties have also been presented.     

Scanning force microscope as a tool for studying optical surfaces

The scanning force microscope (SFM) is used to study the characteristics of optical surfaces, such as polished and precision-machined surfaces and thin-film structures. Previously unreported images of raised surface scratches and clumpiness on the surface of extremely smooth dielectric films are presented. The characteristics of SFM's that are important in studying optical surfaces are discussed. They include the effects of tip geometry, surface charging, particulate contamination, scanner artifacts, and instrument calibration.     

Microwave detection and depth determination of disbonds in low-permittivity and low-loss thick sandwich composites

Nondestructive evaluation (NDE) of disbonded low-permittivity and low-loss dielectric multilayered composite media is of considerable interest in many applications. The ability of microwaves to penetrate inside dielectric materials makes microwave NDE techniques very suitable for interrogating structures made of multilayered dielectric composites. Additionally, the sensitivity of microwaves to the presence of dissimilar layers in such materials allows for accurate detection of a disbonded layer. In a multilayered composite, a disbond may occur between any two (or more) layers. The potential of utilizing microwave NDE techniques for the detection and depth estimation of disbonds in a thick sandwich composite is investigated. This study utilizes a theoretical model developed for investigating the interaction of microwave radiation from an open-ended rectangular waveguide sensor with ann-layer dielectric composite medium. The influence of the standoff distance between the sensor and the medium and the operating frequency on the sensitivity of disbond detection and depth estimation is studied to obtain an optimum set of parameters for enhanced detection sensitivity. Results of the theoretical study are presented with a discussion on the optimization process for a thick sandwich composite composed of 13 dielectric layers.     

Loss Angle and Permittivity Determination of a Dielectric Rod Using a Three-Wire Cage-Shape System

A simple innovative measurement system is proposed for the simultaneous determination of the loss angle and permittivity of a cylindrical dielectric rod at high-frequency regimes (megahertz band). The measurement method, based on multiconductor transmission-line (MTL) theory, explores the frequency-domain resonant features of the new proposed structure, which consists of three equally spaced wires where the cylindrical rod is tightly caged inside. The wires are bonded together at the far end of the MTL structure, while at the sending end, only two are bonded. The input impedance of the transmission line is measured against the frequency, with sharp peaks being observed when the length of the structure comes close to an odd multiple of a quarter wavelength. A theory is developed showing how the propagation parameters of the MTL device depend on the dielectric rod properties, and based on that, a measuring technique is devised for retrieving the loss angle and permittivity of the rod from the critical resonance frequencies and input impedance maximum values. Experimental results are presented to show the effectiveness of the method.      

Free-space, broadband measurements of high-temperature, complexdielectric properties at microwave frequencies

A free-space microwave measurement system that is used for the high-temperature measurement of dielectric constants and loss tangents of homogeneous materials and that is applicable to composite materials as well is discussed. The system is capable of operating in the 5.85-40-GHz frequency range and ambient to 850°C temperature range. A computer is used to control and coordinate furnace temperature, network analyzer functions, and data storage. Dielectric constants and loss tangents of the materials are calculated from the measured values of S₂₁. The measurement system, including the high-temperature furnace and the calibration technique, is described. Dielectric constants and loss tangents are presented for fused quartz and boron nitride grade HP in the frequency range 13.0-17.4 GHz and the temperature range from ambient to 850°C     

Transmission-line matrix solution of waveguides with dielectric losses

The transmission-line matrix method is a time-domain numerical method for solving wave problems. This paper describes how a minor change in the computer program for loss-free dielectrics extends the method to include dielectric losses. Results are given in terms of the wave impedance of a waveguide with losses, and also in terms of power decay due to losses in the dielectric of a resonant cavity.     

On the Characterisation of Foam and Micro‐lattice Materials used in Sandwich Construction1

Abstract: The paper gives an overview of issues relating to the characterisation of the progressive collapse of core cellular materials used in sandwich construction. The specific structural application addressed is foreign object impact, and in this case the core cellular material is subject to multi‐axial stresses, progressive collapse and possible rupture. The paper gives an overview of various theoretical and modelling issues, which are then related to experimental materials and structural tests for model development, calibration and validation. Most discussion concerns polymeric crushable foam, metal foam and metallic lattice structures.     

Non destructive evaluation of adhesively bonded composite structures using high frequency dielectric spectroscopy

Over the last ten years, the application of high frequency dielectric techniques for the assessment of adhesively bonded structures has been investigated. The technique has been used for the study of adhesively bonded aluminium structures and its application to carbon fibre reinforced plastic (CFRP) bonded structures forms the basis of this paper. The electrical conductivity of the carbon fibres in the CFRP composite materials is sufficiently high for adhesively bonded structures to exhibit the properties of a wave-guide. The non-conductive adhesive behaves as a dielectric. The time domain data allows the integrity of the structure to be explored and is sensitive to the orientation of the fibres at the adherent-adhesive interface. Furthermore, a good correlation is shown between time domain dielectric spectroscopy and gravimetric results. This study indicates that the success obtained in the application of high frequency dielectric measurements to adhesively bonded aluminium structures is also applicable to CFRP bonded structures. The dielectric studies not only indicate a new way to assess the state of such a structure but also are producing new insight into the application of dielectric time domain response (TDR) measurement to non-isotropic materials.     

Optical electromagnetic vector-field modeling for the accurate analysis of finite diffractive structures of high complexity

An electromagnetic vector-field model for design of optical components based on the finite-difference time-domain method and radiation integrals is presented. Its ability to predict the optical electromagnetic dynamics in structures with complex material distributions is demonstrated. Theoretical and numerical investigations of finite-length surface-relief structures embedded in polymer dielectric waveguiding materials are presented. The importance of several geometric parameter dependencies is indicated as far-field power distributions are rearranged between diffraction orders. The influences of the variation in grating period, modulation depth, length, and profile are investigated.     

New density-independent calibration function for microwave sensingof moisture content in particulate materials

Microwave techniques have been considered for a long time for moisture sensing in many food processing and agriculture-related industries. They are suitable for on-line real-time monitoring and control. However, with particulate materials, bulk density fluctuations cause significant errors in moisture content determination. To overcome this shortcoming, density-independent calibration functions are needed. In this paper, a new approach is presented in which both bulk density and moisture content are determined directly from measured microwave dielectric properties. A simple relationship between bulk density and the dielectric properties is identified, and a new density-independent function for moisture content prediction, exclusively dependent on the dielectric properties of the material under test (ϵ', ϵ"), is proposed. The validity and applicability of this function are demonstrated with an extensive data set obtained from measurements on a granular material (wheat), over wide ranges of frequency (11-18 GHz), temperature (-1°C-42°C), moisture content (10.6%-19.2%, wet basis), and bulk density (0.72-0.88 g/cm³). Explicit calibration equations for moisture prediction at different frequencies and temperatures are provided. Although data obtained by a transmission microwave measurement technique were used, this new approach remains valid in general for other techniques, provided that ϵ' and ϵ" are determined accurately     

Dielectric plug-loaded two-port transmission line measurementtechnique for dielectric property characterization of granular andliquid materials

There are numerous dielectric property characterization techniques available in the microwave regime each with its own uniqueness, advantages and disadvantages. The two-port completely-filled waveguide (transmission line) technique is a robust measurement approach which is well suited for solid dielectric materials. In this case, the dielectric material can be relatively easily machined to fit inside the waveguide and the subsequent measurement of the scattering parameters of this two-port device renders the dielectric properties of the material filling the waveguide. However, this technique is not well suited for measuring the dielectric properties of granular and liquid materials. These materials are used in the production of various composites which are increasingly replacing the use of metals in many environments. If this technique is directly applied to these types of materials, several approximations either in the measurement apparatus or the formulation must be made. To overcome this problem, this paper describes a modification to this measurement technique utilizing two dielectric plugs which are used to house the granular or the liquid dielectric material. In this approach no approximation to the measurement apparatus is made while the presence of the plugs are fully accounted for in the derivations. Using this technique, the dielectric properties of cement powder, corn oil, antifreeze solution and tap water, constituting low- and high-loss dielectric materials (granular and liquid) were measured. In addition, the important issue of measurement uncertainty associated with this technique is also fully addressed. The issue of optimal choice of various measurement parameters is also discussed as it relates to the measurement uncertainty     

Calibration of adhesion models based on Bayesian inference

The use of composite materials in a myriad of applications fostered the development of reliable procedures to connect components with adhesives. This led to a demand for reliable adhesion models to be used in engineering designs that are based on computer simulations. This paper presents a strategy to be used for calibration of adhesion models. The proposed methodology is built on the formalism of Statistical Inverse Problems. Uncertainties about the unknowns are inferred using Population-Based Markov Chain Monte Carlo and Adaptive Metropolis. It is proposed to perform model assessments based on the analysis of a validation metric. Realizations of the validation metric are computed with the posterior densities of model parameters that are provided by the calibration process. The analysis of the validation metric allows for model selection to be performed. Some numerical experiments are presented with noise-contaminated data. The calibration strategy proved effective when dealing with both the nonlinearity and nondifferentiability of the adhesion constitutive equation.     

R.F. sputtering system for the preparation of multilayer optical filters

Thin film multilayer optical filters are usually prepared by vacuum evaporation of successive layers. The deposition of thin films by sputtering has many advantages over the vacuum evaporation technique and hence it is employed for the fabrication of better quality optical filters. However, the filters may be more time consuming to prepare if a single-target sputtering system is used because two different materials are required for alternate layers. For the preparation of multilayer optical filters, a special r.f. sputtering jig arrangement with two target holders is fabricated. Using this the successive layers can be deposited in one pumpdown. A simple coupling arrangement used to match the r.f. generator output impedance to the load is explained. With all the sputtering parameters such as the pressure, the temperature and the power maintained constant except the time, control of the thickness of the multilayers is achieved. A calibration chart of sputtering time versus thickness of the materials to be deposited is prepared for TiO₂ and SiO₂ for the above purpose. Using this calibration chart, the deposition time of different layers of any optical filters based on these dielectric materials can be fixed to obtain optical filters of acceptable quality and reproducibility.