Ugur Cem Hasar

A Broadband and Stable Method for Unique Complex Permittivity Determination of Low-Loss Materials

Transmission-reflection methods suffer from the increasing uncertainty in the phase of reflection scattering ( S-) parameter measurements of low-loss materials. In addition, transmission S -parameter measurements produce multiple solutions for the complex permittivity. In this paper, we propose a broadband and stable method for unique complex permittivity determination of low-loss materials by eliminating these problems. For elimination of the phase uncertainty problem, we utilize only the amplitudes of reflection S -parameters and complex transmission S-parameters. In order to avoid multiple solutions, we express multivalued terms, which result in multiple solutions, in terms of single-valued terms. The method can work very well in limited frequency-band applications or for dispersive materials since it is based on point-by-point (or frequency-by-frequency) extraction. We measured the complex permittivity of two low-loss dielectric materials by different methods for validation of the method.

A New Calibration-Independent Method for Complex Permittivity Extraction of Solid Dielectric Materials

Microwave nonresonant methods generally require some sort of calibration before conducting the measurements. Calibration-independent nonresonant methods are very attractive since they eliminate this need. In the literature, different calibration-independent methods for complex permittivity determination of materials have been proposed. While some of them use uncalibrated S-parameter measurements of two identical samples with different lengths, the others utilize the same measurements of one sample. The advantage of the latter methods is that they eliminate any impurity and/or inhomogeneity present in the second sample and avoid any thickness uncertainty that can arise from using the second sample. In the literature, the proposed approaches in latter methods, however, require precise location of the sample inside its cell (a waveguide or coaxial-line section) or exact shifting distance of the sample inside its cell. This letter proposes a method to eliminate these requirements using uncalibrated S-parameter measurements of an extra cell (empty) and the cell, in which the sample is arbitrarily located.

An Accurate Complex Permittivity Method for Thin Dielectric Materials

A promising microwave method has been proposed to accurately determine the complex permittivity of thin materials. The method uses amplitude-only scattering parameter measurements at one frequency for this purpose. It resolves the problems arising from any offset of the sample inside its cell in complex reflection scattering parameter measurements and from any uncertainty in sample thickness in transmission scattering parameter measurements. The method determines unique permittivity since, for thin samples, multi-valued trigonometric terms can be linearized. It uses higher order approximations to extract highly accurate permittivity values. It works very well in limited frequency-band applications or for dispersive materials since it is based upon point-by-point (or frequency-byfrequency) measurements. For validation of the method, we measured the complex permittivity of two thin polytetrafluoro-ethylene (PTFE) samples.

A Microwave Method for Noniterative Constitutive Parameters Determination of Thin Low-Loss or Lossy Materials

A noniterative transmission-reflection method is proposed for instant measurement of constitutive parameters of thin samples attached to a sample holder. It uses reflection-only measurements for constitutive parameters determination thanks to the asymmetric nature of the structure (holder sample). In addition, the method can also measure the complex permittivity of the sample holder from transmission and reflection measurements. The disadvantage of the method, however, is that it requires different reflection-only measurements, and hence, a suitable selection of holder thickness and permittivity combination. In case similar reflection properties of the structure are measured, the noniteratively measured constitutive parameters by the proposed method can be utilized as an initial guess in a search algorithm, which uses transmission and reflection measurements. In this way, the proposed method provides a suitable initial guess for electrical properties of materials under test with no prior information.

Retrieval Approach for Determination of Forward and Backward Wave Impedances of Bianisotropic Metamaterials

A simple approach is proposed for retrieving the forward and backward wave impedances of lossless and lossy bianisotropic metamaterials. Compared with other methods in the literature, its main advantage is that forward and backward wave impedances can be uniquely and noniteratively extracted. It has been validated for both lossless and lossy bianisotropic metamaterials by performing a numerical analysis. The proposed approach can be applied for checking whether the metamaterial structure shows the bianisotropic property by monitoring forward and backward wave impedances, since the forward and backward wave impedances of a metamaterial structure depend on difierent polarizations of the incident wave.

UNIQUE PERMITTIVITY DETERMINATION OF LOW-LOSS DIELECTRIC MATERIALS FROM TRANSMISSION MEASUREMENTS AT MICROWAVE FREQUENCIES

A non-resonant microwave method has been proposed for accurate complex permittivity determination of low-loss materials. The method uses two measurement data of the magnitude of transmission properties of the sample. While the flrst datum must correspond to a frequency point resulting in a maximum magnitude of transmission properties, the other can be any datum at a frequency difierent than the flrst datum and not far distant from the flrst datum. Two closed-from expressions are derived for a good initial guess using the above data. The limitations of each expression are discussed. The method has been validated by transmission measurements at X-band (8.2{12.4GHz) of a low-loss sample located into a waveguide sample holder.

Permittivity Determination of Fresh Cement-Based Materials by an Open-Ended Waveguide Probe Using Amplitude-Only Measurements

An open-ended waveguide probe has been adapted for complex permittivity determination and hence mechanical property inspection of cement-based materials. The probe uses amplitude-only re∞ection measurements at difierent frequencies for this goal, which is suitable for industrial based applications when cost and ease of use are important considerations. We have derived expressions by taking into account of the wave-material interaction. The reference plane for measurements is set inside the waveguide to measure solely the re∞ected signal of the dominant mode. It is shown that the measurement results are in good agreement with the theory.

A NEW MICROWAVE METHOD BASED ON TRANSMISSION SCATTERING PARAMETER MEASUREMENTS FOR SIMULTANEOUS BROADBAND AND STABLE PERMITTIVITY AND PERMEABILITY DETERMINATION

A new microwave method has been proposed for simultaneous broadband and stable complex permittivity and complex permeability determination of magnetic and nonmagnetic materials. The method utilizes complex transmission scattering measurements at difierent frequencies. For a change in constitutive parameters determination, we considered zero-order and higher- order approximations. We have verifled the proposed method from measurements of two medium- and low-loss materials with another method and available reference data in the literature.

A New Microwave Method for Electrical Characterization of Low-Loss Materials

A microwave method is proposed to effectively determine the complex permittivity of low-loss dielectric materials. The method uses raw scattering parameter measurements of an empty cell and a measurement cell (a waveguide or coaxial-line section) with one sample for its two configurations. There are three advantages of the proposed method. First, it does not necessitate any calibration to carry out the microwave measurements. Second, it eliminates the measurement errors arising from sample thickness and inhomogeneity present in the second sample, which is required by other methods. Third, it uses transmission-only scattering parameters of the sample in the theoretical formulations so that it may decrease the measurement errors happening from the phase uncertainty in reflection scattering parameters. The measurement results of two low-loss dielectric materials confirm the accuracy of the proposed method.

Stepwise technique for accurate and unique retrieval of electromagnetic properties of bianisotropic metamaterials

Metamaterials (MMs) are artificial materials that have received attention recently because their built-in features create collective electromagnetic effects that are otherwise impossible, such as negative refraction, and because of their exotic electromagnetic applications, namely, perfect lens and invisibility cloaks. Depending on wave propagation characteristics, MMs possessing normally weak magneto-electric coupling coefficients start to exhibit stronger bianisotropic effects. Therefore, accurate electromagnetic characterization of these MMs is important. In this study, we adapt a stepwise method based on the Nicolson–Ross–Weir technique for accurate and unique retrieval of electromagnetic properties of bianisotropic MM slabs. For this goal, we have derived explicit expressions for unique retrieval of electromagnetic properties of these slabs and compared these expressions with those in the literature in the retrieval process. From the comparison, we note that derived expressions are appropriate for unique determination of electromagnetic properties of bianisotropic MM slabs. In the performance analysis of the stepwise method for different measurement scenarios, we considered different bianisotropic MM cell configurations (split-ring and Omega-shaped resonators as well as the same resonators with wire strips) and extracted their electromagnetic properties when measured/simulated scattering parameters have some thermal noise. We note that for most of the frequencies, the stepwise method retrieves correct electromagnetic properties even when a relatively higher normally distributed noise with zero mean value and with standard deviations of 0.015 is present. In addition to the influence of thermal noise on performance of the stepwise method, we also analyzed the effect of both increasing length slab and the frequency band on retrieved electromagnetic properties of the analyzed various bianisotropic MM slabs.