Measurements of the Electromagnetic Properties of Materials with Low UHF Losses by Dielectric Resonator Methods

Two methods for measuring the electromagnetic properties of materials at ultrahigh frequencies based on a dielectric resonator are analyzed. Combination of the methods makes it possible to reduce errors for measurements of material dielectric permittivity and magnetic permeability. Measurement errors are evaluated.     

Complex permittivity measurements of ferroelectrics employing composite dielectric resonator technique

Composite cylindrical TE(0n1) mode dielectric resonator has been used for the complex permittivity measurements of ferroelectrics at frequency about 8.8 GHz. Rigorous equations have been derived that allowed us to find a relationship between measured resonance frequency and Q-factor and the complex permittivity. It has been shown that the choice of appropriate diameter of a sample together with rigorous complex angular frequency analysis allows precise measurements of various ferroelectric. Proposed technique can be used for materials having both real and imaginary part of permittivity as large as a few thousand. Variable temperature measurements were performed on a PbMg(1/3)Nb(2/3)O3 (PMN) ceramic sample, and the measured complex permittivity have shown good agreement with the results of measurements obtained on the same sample at lower frequencies (0.1-1.8 GHz).     

A tunable split resonator method for nondestructive permittivity characterization

The split cylinder resonator method is improved for nondestructive and accurate measurement for low permittivity materials at multiple frequency points. The dielectric constants of flat substrate materials are calculated based on a rigorous mode match analysis of the TE/sub 011/ mode. The loss tangent is also approximately calculated. The dielectric properties of two commercial substrates have been measured at multiple frequencies. The results demonstrate that this technology is capable of accurately characterizing the dielectric properties of flat substrate materials versus frequency in a nondestructive way.     

Computations of Unloaded Q 0-Factor and Resonant Frequency of the TE011 Mode Hakki–Coleman Dielectric Resonators with Coupling Structures Using ANSOFT HFSS

Microwave characterization of materials using dielectric resonators is based on measurements of the Q-factor of the resonator containing a sample under test and on the loss equation for the test structure. The loss equation contains geometrical factors, which are calculated assuming an ideal metallic cavity of the resonator. In this paper a rigorous analysis of cylindrical parallel plate dielectric resonators has been performed to assess the influence of the presence of coupling holes and cables on the unloaded Q-factor and resonant frequency. Calculations have been done for the TE011 mode resonators with differing cavity to dielectric diameter ratio, conductivity of the cavity material, loss tangent and relative permittivity of the dielectric rod, position of the coupling loops and size of the coupling cables. Results have shown that Qo-factors calculated for real resonators were smaller than Qo-factors for ideal resonators. Also this paper presents a brief history of analysis of dielectric resonators.     

Computations of Unloaded Q0-Factor and Resonant Frequency of the TE011 Mode Hakki–Coleman Dielectric Resonators with Coupling Structures Using ANSOFT HFSS

Microwave characterization of materials using dielectric resonators is based on measurements of the Q-factor of the resonator containing a sample under test and on the loss equation for the test structure. The loss equation contains geometrical factors, which are calculated assuming an ideal metallic cavity of the resonator. In this paper a rigorous analysis of cylindrical parallel plate dielectric resonators has been performed to assess the influence of the presence of coupling holes and cables on the unloaded Q-factor and resonant frequency. Calculations have been done for the TE011 mode resonators with differing cavity to dielectric diameter ratio, conductivity of the cavity material, loss tangent and relative permittivity of the dielectric rod, position of the coupling loops and size of the coupling cables. Results have shown that Qo-factors calculated for real resonators were smaller than Qo-factors for ideal resonators. Also this paper presents a brief history of analysis of dielectric resonators.     

Millimeter-Wave Measurement of Complex Permittivity by Perturbation Method Using Open Resonator

This paper explains how better accuracy can be achieved in measurements of the complex permittivity (relative dielectric constant $varepsilon_{r}^{prime}$ and dielectric loss tangent $tandelta$) of thin films having a thickness in the range of 10–100 $muhbox{m}$. Specimens to be measured using a conventional open-resonator method need to have a thickness that is an integral multiple of a half wavelength, so specimens thinner than this half wavelength cannot be measured with an open resonator. However, the use of the perturbation method made it possible to obtain values of complex permittivity of ultrathin specimens of materials such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and PTFE including glass fiber used for printed circuit boards identical to those obtained for thick specimens.      

Dielectric and Magnetic Measurements: A Survey of Nondestructive,Quasi-Nondestructive,and Process-Control Techniques

Abstract

A review of the most common methods for nondestructive permittivity and permeability measurements is presented. Transmission-line techniques, coaxial apertures, open resonators, surface-waves, and dielectric resonator methods are examined. Measurements on bulk, thin materials, and thin films are addressed. Measurement fixtures that can be used as sensors are highlighted. The frequency range of applicability and typical uncertainties associated with each method are addressed.     

Millimeter-Wave Transmittance Measurements on Ferrites Near Ferromagnetic Resonance

This paper presents complex dielectric permittivity and magnetic permeability measurements of pure and diluted barium ferrites in the millimeter-wave frequency range. A broadband quasi-optical millimeter-wave spectroscopy with backward-wave oscillators as high-power tunable sources of coherent radiation has been utilized. The real and imaginary parts of the dielectric permittivity and magnetic permeability as functions of frequency are computed from the transmittance spectra. A magneto-optical approach in millimeter waves has been successfully employed for the separation of the dielectric and magnetic effects and the simultaneous determination of the magnetic and dielectric parameters of barium ferrite materials near ferromagnetic resonance.     

Standard-independent estimation of dielectric permittivity with microdielectric fringe-effect sensors

Microdielectric spectroscopy with planar fringe-effect (FE) interdigital sensors is a useful method for noninvasive characterization of the interfacial properties of the materials. Unfortunately, obtaining an accurate dielectric spectrum is difficult because of the complexity of the probing electrical field created by the FE sensor and the contribution of the sensor substrate and stray elements to the overall measurements. Previously, quantitative microdielectric spectroscopy required the calibration of the FE sensor with standard materials that are known to be dielectrically similar to an unknown sample of interest. This limitation complicates the application of microdielectric spectroscopy, particularly in cases where the monitored sample undergoes a transformation that changes its dielectric permittivity. A standard-independent method for quantitative FE microdielectric measurements is proposed in this paper. The developed method is based on comparison of the theoretically predicted admittance of the FE sensor with the sample of known dielectric properties and the measured sensor admittance. Comparison of the theoretical predictions with the admittance measurements reveals the contribution of the unknown stray elements. The measurements with an unknown sample are then adjusted for the strays. The contribution of the sensor substrate to the sensor measurements is removed using the theoretical model derived from the electroquasistatic approximation of Maxwell equations. The dielectric permittivity of the material being tested is calculated by successively solving the system of complex nonlinear equations for each frequency at which the sensor admittance is measured. The developed method is illustrated by applying it to the dielectric measurements of several dissimilar samples. The results are in excellent agreement with those obtained using the gold standard parallel-plate measurement method over the entire range of frequencies.     

测量介质材料复介电常数的准光腔法

针对介质材料,介绍了在毫米波段和亚毫米波段能够准确测量其复介电常数的准光学谐振腔法。准光学谐振腔具有高Q值、使用简便、样品放置容易等许多优点,能够有效地完成介质材料电介质参数精密测量的任务。这种谐振腔为半球型并由一个平面镜和一个凹面铜镜组成,采用固定腔长法或固定频率法进行测量。另外,还介绍了准光学谐振腔的测试系统和最新的研究成果及改进方法,例如测量多层薄膜及在更高频段的测量方法。     

尖晶石结构磁介电材料的研究进展

随着通信技术的发展,对无限通信设备的集成度有了更高的要求,天线小型化成为目前重要的研究方向。等磁介电材料是一种既具有磁导率又具有介电常数,且磁导率和介电常数几乎相等的材料,使用等磁介电材料作为天线的基板,能有效的减小天线的尺寸,提高带宽,增加辐射效率。铁氧体是由Fe2O3和一种或多种金属氧化物复合而成,具有较高的磁导率和介电常数,由于其同时具有磁特性和介电特性,是一种潜在的等磁介电材料。综述了近几年尖晶石结构磁介电材料的国内外研究进展,着重讨论了掺杂改性对烧结温度、磁导率、介电常数、直流电阻等电磁特性的影响。最后指出目前研究中存在的问题,并展望了该材料在未来发展的方向。     

Whispering-gallery mode technique applied to the measurement of the dielectric properties of Langasite between 4 K and 300 K

We report new measurements of dielectric properties of Lanthanum gallium silicate (Langasite or LGS) conducted with the whispering-gallery mode technique at microwave frequencies and between 4.2 K and 300 K. The real part of the permittivity tensor of LGS presents two components having temperature coefficients of opposite sign. This unique property enables the design of a temperature compensated resonator that may be useful in building stable microwave oscillators or filters. We report also the first measurements of the two independent components of the imaginary part of the permittivity tensor. It appears LGS is a relatively high-loss dielectric material compared with sapphire or quartz.     

A methodology for determining complex permittivity of construction materials based on transmission-only coherent,wide-bandwidth free-space measurements

An integrated methodology for determining the unique combination of complex permittivity based on measured transmission coefficient and time difference of arrival (TDOA) information in free-space measurements is proposed. The methodology consists of an estimation procedure of the real part of complex permittivity based on TDOA, and a root-searching procedure based on parametric system identification (SI) together with an error sum of squares (SSE) criterion. Generally, non-unique combinations of dielectric constant and loss factor are encountered when lossy or low-loss materials are measured and the proposed methodology is aimed at the determination of unique combinations of dielectric constant and loss factor for such materials. The proposed methodology is validated by measurements of several materials with known dielectric properties. The estimated complex permittivity values for Teflon, Lexan, Bakelite, and concrete are in good agreement with those reported in the literature. The method has potential for in-situ measurement of dielectric properties for construction materials. Applicability and limitations of the methodology are discussed.     

Low temperature microwave characterisation of lithium fluoride at different frequencies

Precise knowledge of dielectric properties of materials is required to implement the material in high frequency devices and circuits. At microwave frequencies complex permittivity (dielectric constant and loss tangent) are the two mandatory parameters prior to any design. We have identified Lithium Fluoride as a potential candidate, which can be used in conjunction with superconducting and non-superconducting parts of several microwave communication devices. Even though dielectric constant of LiF is known at room temperature there only limited data presented at cryogenic temperatures. We have used a dielectric post resonator for the microwave characterisation of the rod shaped LiF crystal. In this paper, we have reported the dielectric constant (perpendicular component of the real part of complex permittivity) and loss tangent of two LiF crystals as a function of temperature (15–290 K) at frequencies of 8 and 16.5 GHz. We have also studied and reported the temperature coefficient of frequency and permittivity. The concept of using temperature coefficient of frequency as a standard is proved to be wrong in this paper. Microwave properties of other Fluorides are also compared with the LiF crystal.     

Dielectric and magnetic measurements: A survey of nondestructive,quasi-nondestructive,and process-control techniques

A review of the most common methods for nondestructive permittivity and permeability measurements is presented. Transmission-line techniques, coaxial apertures, open resonators, surface-waves, and dielectric resonator methods are examined. Measurements on bulk, thin materials, and thin films are addressed. Measurement fixtures that can be used as sensors are highlighted. The frequency range of applicability and typical uncertainties associated with each method are addressed. U.S. Government work not protected by U.S. copyright.     

Computer-Aided Permittivity Measurements of Moistened and Pyrolized Materials in Strong RF Fields (Part I)

This paper presents the results of computer-aided permittivity measurements (CAPM) on moistened paper samples, exposed to strong RF fields inside a microwave cavity. Permittivity measurements, sample weight, and RF power absorbed by the sample are monitored in real time by a microcomputer. From the CAPM, the permittivity values and the moisture content of the material are calculated. The effects of paper pyrolization on the permittivity measurements are examined, and a reversal in the measured dielectric parameters is observed at the 0-percent moisture level. This CAPM method is useful to examine the dielectric properties of materials critically dependent on the moisture content, for example, in food products and in the telephone wire industry where the insulating material is obtained from dried paper pulp. In addition, this CAPM technique can be applied to study the effects of thermal and RF shocks in dielectric materials such as used in high-power microwave windows or in studying the biological effects of microwave radiation using both short and long exposure cycles.     

A miniature filter on a suspended substrate with a two-sided pattern of strip conductors

A miniature bandpass filter of new design with original stripline resonators on suspended substrate has been studied. The proposed filters of third to sixth order are distinguished for their high frequency-selective properties and mush smaller size in comparison to analogs. It is shown that a broad stopband extending above three-fold central bandpass frequency is determined by weak coupling of resonators at resonances of the second and third modes. A prototype sixth-order filter with a central frequency of 1 GHz, manufactured on a ceramic substrate with dielectric permittivity ε = 80, has contour dimensions of 36.6 × 4.8 × 0.5 mm³. Parametric synthesis of the filter, based on electrodynamic 3D model simulations, showed quite good agreement with the results of measurements.     

Measurement and analysis for stripline material parameters usingnetwork analyzers

The characterization of material parameters for dielectrics used in the construction of stripline and related structures is presented. The measured scattering parameters of transmission-line samples are used to predict the characteristics of the dielectric materials used in the construction in addition to effects due to rough dielectric-conductor interfaces. The measurements have been performed using a network analyzer with the `thru-reflect-line' calibration technique. The calibration process, the structure of the transmission-line sample, and the frequency-domain model are reviewed. Measured results are presented with predicted values for the dielectric constant and loss tangent of dielectric materials common in thick-film structures     

The reflectivity discrepancy method for the determination of thepermittivity and permeability of complex materials

The possibility of distinguishing whether the losses in a material under investigation are due to either its dielectric or magnetic properties, or both, is tested by treating a single spectrum of a reflection simulated by computer. To determine the frequency bandwidth and range of sample thickness sufficient to measure both the permittivity and the permeability at any frequency, the reflection spectrum of a sample with magnetic and dielectric properties is analyzed. If the reflectivity is measured over a frequency band several times wider than the absorption line of the sample and the sample is semitransparent in the middle of the line, the magnetic and dielectric spectra may be determined. Insertion of an air gap between the sample, and the plane of the short circuit has been found to facilitate the measurements. The validity of this approach is shown with permittivity and permeability measurements of a composite substance containing iron     

New method for measurement of complex magnetic permeability in themillimeter-wave range, part II: hexaferrites

Magneto-optical methods were applied for the first time in millimeter wavelength range for characterization of anisotropic ferrites. The principles of the free-space magneto-optical method are presented and the new experimental procedures leading to the determination of millimeter-wave permeability and permittivity in anisotropic ferrite materials are described. The measurements were performed with a computer-controlled W-band (70-120 GHz) quasi-optical-waveguide bridge. A backward-wave oscillator was used as a source of tunable millimeter wave radiation. The oriented Sr-hexaferrite ceramic was selected for the verification of millimeter-wave magneto-optical method. The magneto optical measurements in transverse configuration revealed strong anomalous dispersion in the millimeter-wave refractive index spectrum for Sr hexaferrite, mainly due to the frequency variation of magnetic permeability. Computer simulations revealed a good agreement of measured parameters with known data for Sr hexaferrite. It was shown that the free-carrier absorption (σ~0.05 Ω^-1 cm^-1) and magnetic permeability contribute to the relatively high millimeter-wave losses in hexaferrite ceramics. The frequency dependence of Faraday rotation in the millimeter-wave range was measured, and the experimental results are discussed. The results presented in this paper demonstrate that this new magneto-optical method is capable of providing accurate dielectric and magnetic data in the millimeter-wavelength range