Rostyslav Dubrovka

Antennas and propagation for on-body communication systems

On-body communication channels are of increasing interest for a number of applications, such as medical-sensor networks, emergency-service workers, and personal communications. This paper describes investigations into channel characterization and antenna performance at 2.45 GHz. It is shown that significant channel fading occurs during normal activity, due primarily to the dynamic nature of the human body, but also due to multipath around the body and from scattering by the environment. This fading can be mitigated by the use of antenna diversity, and gains of up to 10 dB are obtained. Separation of the antennas performance from the channel characteristics is difficult, but results show that for many channels, an antenna polarized normal to the bodys surface gives the best path gain. Simulation and modeling present many challenges, particularly in terms of the problems scale, and the need for accurate modeling of the body and its movement.

On-body path gain variations with changing body posture and antenna position

Variability of an on-body transmission channel at 2.45 GHz is investigated. The propagation path gain was measured for a number of antenna positions and at a number of static human body postures as well as during arbitrary movements. Dependence of the path gain on conformal antenna separation is investigated and probability distributions of the path gain during arbitrary movement are derived

Sub-terahertz spectroscopy reveals that proteins influence the properties of water at greater distances than previously detected.

The initial purpose of the study is to systematically investigate the solvation properties of different proteins in water solution by terahertz (THz) radiation absorption. Transmission measurements of protein water solutions have been performed using a vector network analyser-driven quasi-optical bench covering the WR-3 waveguide band (0.220-0.325 THz). The following proteins, ranging from low to high molecular weight, were chosen for this study: lysozyme, myoglobin, and bovine serum albumin (BSA). Absorption properties of solutions were studied at different concentrations of proteins ranging from 2 to 100 mg/ml. The concentration-dependent absorption of protein molecules was determined by treating the solution as a two-component model first; then, based on protein absorptivity, the extent of the hydration shell is estimated. Protein molecules are shown to possess a concentration-dependent absorptivity in water solutions. Absorption curves of all three proteins sharply peak towards a dilution-limit that is attributed to the enhanced flexibility of protein and amino acid side chains. An alternative approach to the determination of hydration shell thickness is thereby suggested, based on protein absorptivity. The proposed approach is independent of the absorption of the hydration shell. The derived estimate of hydration shell thickness for each protein supports previous findings that protein-water interaction dynamics extends beyond 2-3 water solvation-layers as predicted by molecular dynamics simulations and other techniques such as NMR, X-ray scattering, and neutron scattering. According to our estimations, the radius of the dynamic hydration shell is 16, 19, and 25 Å, respectively, for lysozyme, myoglobin, and BSA proteins and correlates with the dipole moment of the protein. It is also seen that THz radiation can serve as an initial estimate of the protein hydrophobicity.

Magnification of subwavelength field distributions using a tapered array of metallic wires with planar interfaces and an embedded dielectric phase compensator

We report the magnification of subwavelength field distributions using a tapered array of metallic wires with planar front and back interfaces through numerical simulations and experiments. It is demonstrated that subwavelength images with a resolution of one-fifteenth of a wavelength can be transferred to a distance of three wavelengths with a threefold magnification. We also propose embedding a dielectric phase compensator in the tapered array to compensate the phase differences introduced by the different lengths of wires and significantly improve the operational bandwidth of the image transmission and magnification device.

Terahertz spectral domain computational analysis of hydration shell of proteins with increasingly complex tertiary structure.

Solvation dynamics of biomolecules and water in a hydration shell have been studied by different methods; however, a clear picture of this process is not yet established. Terahertz (THz) studies of molecular behavior in binary solutions present unique information on the picosecond motions of molecules. A complete mechanical interpretation of THz absorption spectra associated with solvated biomolecules remains a challenging task. The Gromacs molecular dynamics (MD) simulation package is used here to examine the spectral characteristics of water molecules in close proximity to biomolecules using vibrational density of states (VDOS). Systematic simulation of solvation dynamics of proteins of different size and tertiary structure are presented. The following have been selected for analysis. They range from less to more complex tertiary structure (corresponding to an increased number of secondary structure elements): TRP-cage13-20 peptide, TRP-cage, BPTI, and lysozyme. The initial study predicts that the depth of the hydration shell, determined by VDOS of water, extends to approximately 10 Å and does not depend on protein size. Furthermore the integral perturbation coefficient of the whole solvation layer is found to be increased for larger proteins due to a higher retardation rate of water molecules in their shells. Differences in solvation dynamics among the proteins considered originate primarily from the water molecules buried in the interior of the protein and not from the on-surface molecules.

Revised metrology for enhanced accuracy in complex optical constant determination by THz-time-domain spectrometry

THz time-domain spectrometry (TDS) probes the complex polarization response of materials. Various analytical procedures are applied by many to extract the associated material optical constants. This has commonly been done by iteratively varying material parameters in order to achieve a match between experiment and a theoretical transfer function (TF). The poly root behavior of a TF is emphasized for measurements with reflections in the time domain. This study provides a comprehensive analysis of the influence of the initial guesses on the accuracy of extracted material parameters. In addition, various ways of representing multiple reflections inside the sample (a Fabry-Perot-like effect) are compared and their contribution to the uncertainty of material parameters is analyzed. Experimental evidence is provided where appropriate to support theoretical statements. Furthermore, this paper offers a basis for data comparison between different THz-TDS systems in transmission mode. Finally, a clear distinction is made between a commonly used basic analysis and an enhanced one, in terms of associated uncertainties in determination of the real and imaginary parts of the complex refractive index.

Recent progress in development of multiband feed horns (Review)

This paper proposes overview of the multiband feed system for reflector-type antennas. Coaxial, multimode Potters type structures, multiband corrugated and dielectric loaded feed horns is discussed below.

Near-Field Antenna Radome Based on Extremely Anisotropic Metamaterial

We demonstrate experimentally that a block of extremely anisotropic metamaterial formed by an array of metallic wires embedded into a dielectric material is a near-field radome. The radome is intrinsically multiband and can operate, for instance, in both GSM frequency bands. The structure both hides an antenna from external mechanical and environmental influence and transmits its near-field onto an outer interface in contrast to conventional radomes that alter antenna characteristics and have to be taken into account during design process.

Radiation and Matching Characteristics of a Novel Dual-Band Dielectric Loaded Coaxial Horn

Copolar and crosspolar radiation as well as matching characteristics of a novel dual-band coaxial hybrid-mode feed horn with partial dielectric loading [1] for reflector antennas have been numerically and experimentally investigated. In comparison with dual-band corrugated horns this horn has higher crosspolar level, but provides larger band separation and rather good electrical performances over a wide frequency range (20% and more) within each operational frequency band. Furthermore, the main concept of the coaxial feed system can be easily extended for multi-band cost-effective applications in reflector antennas with polarization diversity.

Single-mode subwavelength waveguides with wire metamaterials

We study the guided modes of a slab of wire metamaterial and reveal that such a structure supports deep-subwavelength propagating modes exhibiting the properties of single-mode waveguides at any fixed frequency below the plasma frequency of metal wires. We compare our analytical results with the dispersion relations extracted from the experimental measurements.