The results of measurement and monitoring of reflection and transmission properties of cement-based specimens (blocks of mortar, concrete) obtained by using a simple and an inexpensive measurement system at microwave frequencies (X-band) are presented. Dependencies of the reflection and transmission coefficients on water-to-cement (w/c) ratio, preparing and curing conditions of the specimens are demonstrated. It is shown that the amplitudes of reflection and transmission coefficients, together with thickness of the specimens, determine the complex dielectric permittivity of the hardened cement-based specimens. The expected applications of the results for the determination of physical properties of cement-based materials are discussed. The causes and effects of measurement errors and uncertainties are also discussed.

/pdf/measurement-and-monitoring-of-microwave-reflection-and-8lp54jj2ps.pdf

Measurement and monitoring of microwave reflection and transmission properties of cement-based specimens

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

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

This paper presents a new free-space millimeter-wave method based on the transverse magnetooptical effect, which provides the complete and accurate characterization of the ferrites for the first time in the millimeter-wave range. The described data analysis and processing methods lead to the separation of dielectric and magnetic effects and to the simultaneous determination of both millimeter-wave permittivity and permeability. It is demonstrated that this new method can be used for the characterization of soft ferrites as well as hard anisotropic ferrites. The magnetooptical measurements were realized using a millimeter-wave spectrometer combined with the electromagnet providing a transverse magnetic field up to 1 T. The backward-wave oscillators were used as a source of tunable coherent millimeter-wave radiation. It was shown that the freespace method applying the polarized Gaussian beams generates precise transmission data and enables the accurate determination of the optical constants of ferrites in the entire millimeter-wave range.

Millimeter-wave magnetooptics: New method for characterization of ferrites in the millimeter-wave range

The transmission-reflection method is modified for measuring constitutive parameters of thin high-loss materials used as radar absorbers. The method uses a two-layer structure, consisting of a layer of thin flexible unknown material supported by a thicker rigid known material. The analysis and measurements focus on nonmagnetic samples of a high dielectric constant and loss factor and on the waveguide configuration in the X-band. A nonlinear least-squares optimization is used to obtain the complex permittivity from the measured scattering parameters. The uncertainty analysis presented facilitates selection of the support layer thickness. Simulations with the finite-difference time-domain method explore the effects of sample imperfections. Accuracy of a few percent can be achieved for a sample thickness of a fraction of a millimeter, provided that the thickness of the support dielectric is close to optimum and sample has only small surface imperfections.

Modified transmission-reflection method for measuring constitutive parameters of thin flexible high-loss materials

The aim of this series of two papers is to propose an original and low-cost tool dedicated to industrial applications and based on the reflection ellipsometry technique for in situ characterization of dielectric materials at microwave frequencies. In this first paper, different theoretical developments are presented that concern first a specific numerical method for calculating the complex permittivity of a single-layer sample from the measured parameters. Based on contour line charts, this method allows obtaining simultaneously the relative uncertainties on the real and imaginary parts of the complex permittivity. Secondly, for experimental comparisons with the classical Fresnel method, a numerical data processing method based on contour line charts has also been developed, which aims at the determination of the reflection coefficients in both parallel and perpendicular polarizations of the material.

/pdf/in-situ-measurements-of-the-complex-permittivity-of-43or6gb9p5.pdf

In situ measurements of the complex permittivity of materials using reflection ellipsometry in the microwave band: experiments (Part II)

A free-space system operating in the 75 GHz to 95 GHz range for measurement of the complex permittivity of matter is presented. The complex transmission coefficient of a parallel polarized quasiplane wave through a planar sample is determined for different incidence angles. The quasiplane wave is realized by a collimated Gaussian beam. The complex permittivity is subsequently calculated from the measured data by applying a nonlinear least-squares method. Errors induced by multiple reflections between the transmitting and receiving horn antennas are removed by a time-domain gating technique. The design of the quasioptical measurement setup is based on Gaussian beam optics. A dielectric lens collimates the beam of the transmitting horn antenna for minimization of diffraction effects at the edges of the sample. In order to verify the Gaussian beam and thin lens approximations at millimeter wavelengths, the far-field pattern of the horn antenna and the dielectric lens system has been measured. The performance of the measurement setup has been tested using glass samples.

G.L. Friedsam

Papers

A broadband free-space dielectric properties measurement system at millimeter wavelengths