Plamen I. Dankov

Bio: Plamen I. Dankov is an academic researcher from Sofia University. The author has contributed to research in topics: Dielectric & Anisotropy. The author has an hindex of 6, co-authored 33 publications receiving 102 citations.

Utilization of 3D simulators for characterization of dielectric properties of anisotropic materials

Abstract: The principles for utilization of the electromagnetic 3D simulators for determination of dielectric constant and loss tangent of anisotropic materials by microwave cavity resonators are described. Two-resonator method (described elsewhere) based on TE/sub 011/-mode and TM/sub 010/-mode resonators with disk-shape sample is applied for separate measurement of the longitudinal and transversal dielectric parameters. Results for isotropic material (polycarbonate Lexan/spl reg/ Exell D sheets) and for anisotropic material (RO4003 substrates) are presented in the paper in order to compare the accuracy of three different cavity models - perturbation models, analytical dispersion equation models and 3D simulator models. The applicability of the measurement method is discussed and some limitations are formulated concerning the separate determination of longitudinal and transversal parameters of anisotropic substrates.

Characterization of microwave substrates with split- cylinder and split-coaxial-cylinder resonators

Abstract: Two types of split-cylinder resonators are used for characterization of the longitudinal dielectric parameters of reinforced RF substrates. The novelty of the method is the introduced theoretical 3D model of the measurement cavities, which allow determination of the parameters with the help of arbitrary 3D electromagnetic simulators. Two known isotropic and anisotropic materials are measured with circular and split-cylinder resonators and the results are compared. One of the considered structures - the split-coaxial resonator with movable inner post cylinder - allows substrate measurements at lower frequencies compared with the resonance frequency of the ordinary split-cylinder resonator.

Analysis of Dielectric Properties of Polydimethylsiloxane (PDMS) as a Flexible Substrate for Sensors and Antenna Applications

Abstract: In this paper, a complete analysis of the complex dielectric constant of a flexible substrate from the silicon-based polymer- Polydimethylsiloxane (PDMS) is performed, and the obtained results are discussed. Two experimental methods are applied in this research. The first Two-resonator method is based on resonance measurements by excitation of two types of TE- and TM-mode cylinder resonators with PDMS disks, ensure an accurate determination of the dielectric constant and dielectric loss tangent in both parallel and perpendicular directions (e.g., $\varepsilon _{\textit {par}}$ and $\varepsilon _{\textit {perp}}$ ). The second method is based on the tight coverage of planar microstrip ring resonators with non- metalized PDMS samples gives reliable information for the equivalent dielectric parameters (e.g., $\varepsilon _{\textit {eq}}$ , tan $\delta _{\varepsilon \textit {eq}}$ ). The obtained results show that PDMS substrates have relatively weak but measurable uniaxial anisotropy and well-expressed frequency dependencies of the extracted dielectric parameters in the range 1–40 GHz, namely $\varepsilon _{\textit {par}} \sim 2.82$ -2.7; $\varepsilon _{\textit {perp}} \sim 2.73$ -2.52 and $\varepsilon _{\textit {eq}} \sim 2.75$ -2.64, tan $\delta _{\varepsilon eq} \sim 0.017$ -0.048. The results are confirmed by several other complementary methods. The considered pair of methods are also applied in the temperature interval–40/ $+ 70^{\circ }\text{C}$ ; the measured temperature dependencies on the dielectric parameters turn out to be relatively strong. The possible origin of the measured PDMS uniaxial anisotropy has been discussed; in fact, it appears mainly in the temperature range–30/ $+ 40^{\circ }\text{C}$ .

Characterization and Modeling of Microwave Absorbers in the RF and Antenna Projects

Abstract: Some practical problems concerning the characterization and modeling of microwave absorbers are discussed in this paper. First, a number of measurement methods are considered for determination of the relative and absolute absorbing ability of the most popular absorbing materials - foams, rubber sheets, coatings and thin films. Next, several more complicated methods for characterization of the complex dielectric parameters of the absorbers are presented and discussed. Finally, examples for modeling of microwave absorbers by 3D simulators are given. they are looking for absorber materials with appropriate attenuation in a given frequency range. In this case the users have a necessity to verify the absolute or even the relative absorbing ability of the used materials, and they can pass over the problem of knowing the complex permittivity and permeability. Thus, the values of the attenuation in dB/mm through a given absorber layer in the frequency range of interest are fully enough to characterize the absorbing abilities of the materials. We summarize and compare in this paper several measurement methods, which can be used for this purpose. Lately, another need has appeared in the RF designer work, namely to have possibilities to simulate the whole devices or antennas by 3D simulators, when the absorbing layers are incorporated in the structures. Now they have to know the complex permittivity and permeability of the used materials. Unfortunately, most of the producers do not give enough information for the absorber material parameters. Therefore, users must measure the complex permittivity and permeability of the samples in their laboratories. The determination of the material parameters (especially of the permeability) is considerably more difficult than the measurement of the absorbing abilities of the same materials. We describe here several simple methods for this purpose. Finally, we present examples for simulation of simple structures with microwave absorbers.

Characterization of Dielectric Properties, Resultant Isotropy and Anisotropy of 3D Printed Dielectrics

Abstract: In this paper we perform a set of numerical and experimental investigations of the resulting isotropy/anisotropy of the dielectric properties of the modern artificial dielectrics, produced by 3D printers applying different unit cells. As in the cases of other artificial dielectrics with antenna and microwave applications, e.g. reinforced substrates, multilayer radomes, textile fabrics, some metamaterials, etc., the 3D printed materials can be considered as mixtures between two or more dielectrics or metals and we estimate their anisotropy using unit cells with different shapes (spheres, cubes, prisms, hexagons, etc.) and materials with different dielectric parameters. In this first our paper in the area of bi-axial anisotropy of 3D printed dielectrics we investigate the influence of the type and orientation of the “building blocks” in the unit cells over the resultant dielectric constant and dielectric loss tangent of the samples. Experimental data have been presented for simple 3D printed samples with unit cells as hexagons and triangles from PLA (polylactic acid). Special attention is devoted for determination of the resulting isotropy of dielectrics for 3D printed Luneburg lens antennas, fabricated by applying of unit cells with different symmetry.

Two-resonator method for measurement of dielectric anisotropy in multilayer samples

Abstract: A two-resonator method, based on TE/sub 011/-mode and TM/sub 010/-mode resonance cavities with a multilayer disk sample, has been developed for measurements of the longitudinal and transversal dielectric constant and dielectric loss tangent of each layer (if the other ones have known parameters) or in the whole sample averaging over the layers contribution. Dispersion equations for the considered modes in both types of cavities with three-, two-, or one-layer samples are obtained. The measurement sensitivity and errors in the dielectric constant are discussed. Analytical expressions for the computation of the dielectric loss tangent of the unknown layer in the two directions are presented for each of the considered cavities. The proposed method is applicable in simple laboratory conditions and allows an estimation of the dielectric anisotropy of multilayer materials in many practical cases. The measuring errors for one-layer artificial substrates with thicknesses of 0.25-0.5 mm are approximately 3%-6% for dielectric constants in the interval of 2.0-4.5 and 10%-15% for dielectric loss tangents in the interval of 0.002-0.010. The obtained pair of longitudinal and transversal dielectric parameters can be used in modern structure simulators for more realistic simulations of microwave components, radiating elements, antenna radomes, etc. Three practical examples for three-layer antenna radomes are given for an illustration of the dielectric anisotropy characterization of multilayer samples.

Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

Abstract: The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the |μ| = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent first-order probe for spherical near-field antenna measurements at low frequencies.

A Low-Cost Ka -Band Circularly Polarized Passive Phased-Array Antenna for Mobile Satellite Applications

Abstract: This paper presents a low-profile, high-performance, and low-cost Ka -band circularly polarized (CP) passive phased-array antenna (PAA) system. A $1\times 4$ passive subarray operating at the Ka -band frequency band is investigated, developed, and tested as a possible building block for mobile satellite communication systems. In particular, a CP antenna integrated with a high-performance passive phase shifter is presented and experimentally verified. A new low-cost and low-profile magnetic actuator is proposed to precisely control the phase state of the phase shifter. A 3-D printed polymer package is utilized to enclose the phase shifter and reduce the environmental effects. The proposed CP passive PAA achieves high-performance right-hand CP radiation with high cross-polarization discrimination over a wide scan angle from 0° to ±38° over the frequency band (29.5–30.5 GHz). An axial ratio (AR) < 3 dB is demonstrated over the entire scanning range. Moreover, the measured radiation efficiency is 60% at boresight. The gain drops by 1 dB at ±38° scanning angles.

Spectral Signatures for Identifying Explosives With Wideband Millimeter-Wave Illumination

Abstract: Millimeter-wave imaging systems used in airports, government buildings, and other facilities for personnel screening use advanced imaging technology (AIT) to detect explosives and weapons concealed under clothing. Additional information in the imaging data can be applied to identify the composition of the detected objects. The method described here demonstrates that material data in the form of the dielectric constant can be derived from the variation of reflectivity in millimeter waves over a range of frequencies from 18 to 40 GHz. By fitting the reflectivity to an optical model, the thickness and dielectric constant, including attenuation, can be computed. The method is applied to samples of inert substances and a military sheet explosive to show that detected anomalies can be distinguished as distinct materials through their dielectric constants. For absorptive materials, a frequency band of lower frequencies, 2–18 GHz, can be applied to detect the dielectric, as is demonstrated in the case of a commercial explosive. Used with AIT, the technique can facilitate the evaluation of threats at personnel checkpoints.

A Broadband and Parametric Model of Differential Via Holes Using Space-Mapping Neural Network

Abstract: This letter presents a novel broadband and completely parametric model of differential via holes by virtue of the space-mapping neural network technique. This model consists of a neural network and an equivalent circuit that is utilized to account for various EM effects of differential via holes. The neural network is trained to learn the multi-dimensional mapping between the geometrical variables and the values of independent circuit elements in the equivalent circuit. Once trained with the EM data, this model provides accurate and fast prediction of the EM behavior of differential via holes with geometry parameters as variables. Experiments in comparison with measurement data and EM simulations are included to demonstrate the merits of this new model in both the frequency and time domains.