• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.30 no.2
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• pp.124-131
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• 2019

This study presents an imaging algorithm that can provide an image of a moving target in a multiple-input-multiple-output radar environment where multiple transmitting and receiving radars are fixed on the ground. The proposed algorithm, which is based on polar format processing using plane wave approximation, is shown to provide an unaliased image by using multiple transmitting radars even when the distances between the receiving radars are relatively large. We derive the conditions necessary to deploy the transmitting radars by which the resolution of the reconstructed image can be improved, while simultaneously reducing aliasing artifacts. Moreover, we offer a means of separating out each transmitting radar target echo. Finally, the performance of the proposed system is verified through a simulation.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.185.2-185.2
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• 2014

We have studied the atomic and electronic structure of graphene nanoribbons (GNRs) on a hexagonal boron nitride (h-BN) sheet with intercalated atoms using first-principles calculations. The h-BN sheet is an insulator with the band gap about 6 eV and then it may a good candidate as a supporting dielectric substrate for graphene-based nanodevices. Especially, the h-BN sheet has the similar bond structure as graphene with a slightly longer lattice constant. For the computation, we use the Vienna ab initio simulation package (VASP). The generalized gradient approximation (GGA) in the form of the PBE-type parameterization is employed. The ions are described via the projector augmented wave potentials, and the cutoff energy for the plane-wave basis is set to 400 eV. To include weak van der Waals (vdW) interactions, we adopt the Grimme's DFT-D2 vdW correction based on a semi-empirical GGA-type theory. Our calculations reveal that the localized states appear at the zigzag edge of the GNR on the h-BN sheet due to the flat band of the zigzag edge at the Fermi level and the localized states rapidly decay into the bulk. The open-edged graphene with a large corrugation allows some space between graphene and h-BN sheet. Therefore, atoms or molecules can be intercalated between them. We have considered various types of atoms for intercalation. The atoms are initially placed at the edge of the GNR or inserted in between GNR and h-BN sheet to find the effect of intercalated atoms on the atomic and electronic structure of graphene. We find that the impurity atoms at the edge of GNR are more stable than in between GNR and h-BN sheet for all cases considered. The nickel atom has the lowest energy difference of ~0.2 eV, which means that it is relatively easy to intercalate the Ni atom in this structure. Finally, the magnetic properties of intercalated atoms between GNR and h-BN sheet are investigated.

• Journal of the Korean Magnetics Society
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• v.21 no.4
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• pp.127-131
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• 2011

In this paper, we study the electronic and magnetic properties of Cr substituted SiTe in the rock-salt structure compound using the full potential linearized augmented plane wave method within the generalized gradient approximation to the exchange correlation potential. Two stoichiometries are studied: $CrSi_3Te_4$ with 25 %, and $CrSiTe_2$ with 50 % Cr substitution. We found, from the total energy calculations, that the equilibrium lattice constant for cubic $CrSi_3Te_4$ is 11.64 a.u. and a = 7.89 a.u. and c = 11.13 a.u. for tetragonal $CrSiTe_2$. The integer value of the calculated magnetic moment per unit cell, $4{\mu}_B$ for $CrSiTe_2$ suggests that this compound is halfmetallic. The magnetic moment per unit cell for $CrSi_3Te_4$ is slightly larger than $4{\mu}_B$. The magnetic moment on Cr atoms are 3.61 and $3.62{\mu}_B$ in the $CrSi_3Te_4$ and $CrSiTe_2$, respectively. The presence of Cr atoms causes that the other atoms become slightly magnetized in both compounds. The electronic properties and the magnetism are discussed with the calculated spin-polarized density of states.

• The Journal of the Acoustical Society of Korea
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• v.36 no.5
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• pp.305-313
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• 2017

In this paper, sound absorption of thin elastic plates installed in a rigid duct is discussed using an analytic method. The number of plates can be one or two, and each plate might have micro-perforation. Vibration of the plates and sound pressure fields inside the duct and air cavity are expressed in terms of an infinite series of modal functions. Under the plane wave assumption, a low frequency approximation is derived by including the first few plate modes. It is found that the sound absorption coefficient of the plates without micro-perforation shows sharp peaks at resonance frequencies, and due to the interaction between the plates and air cavity, the resonance frequencies move as the cavity depth changes. For the case of micro-perforated plates, it is found that the sound absorption is mainly affected by the perforation ratio. When the perforation ratio is order of few percent, the sound absorption is almost independent of plate vibration.

• The Journal of the Acoustical Society of Korea
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• v.37 no.4
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• pp.181-187
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• 2018

In this paper, sound absorption of micro-perforated elastic plates installed in an impedance tube of a circular cross-section is discussed using an analytic method. Vibration of the plates and sound pressure fields inside the duct are expressed in terms of an infinite series of modal functions, where modal functions in the radial direction is given in terms of the Bessel functions. Under the plane wave assumption, a low frequency approximation is derived by including the first few plate modes, and the sound absorption coefficient is given in terms of an equivalent impedance of a single surface. The sound absorption coefficient using the proposed formula is in excellent agreement with the result by the FEM (Finite Element Method), and shows dips and peaks at the natural frequencies of the plate. When the perforation ratio is very small, the sound absorption coefficient is dominated by the vibration effect. However, when the perforation ratio reaches a certain value, the sound absorption is mainly governed by the rigid MPP (Micro-Perforated Plate), while the vibration effect becomes very small.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.19 no.1
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• pp.46-53
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• 2008

In general, the cassegrain dual offset reflector is used for the satellite communication antenna, but is analyzed as the reflector system for the CPTR(Compact Payload Test Range) facility in here. The near-field at the test zone of the CPTR is obtained by using the physical optics approximation. The CPTR has to provide a uniform plane wave with the minimum amplitude and phase ripple and the low cross polarization. Therefore, in this paper, the near-field pattern are calculated, and the ripple and taper of the field and the cross polarization are investigated with the variation of the reflector geometry and the position of the test region. Especially, the cross polarization of the antenna axis direction which is not found in the satellite reflector antennas is investigated.

• The Journal of the Acoustical Society of Korea
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• v.35 no.4
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• pp.253-260
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• 2016

In this paper, derivation of the STL (Sound Transmission Loss) of the double plates installed in an impedance tube is discussed using an analytic method, where an air cavity exists between the plates. Vibration of the plates and sound pressure field inside the tube are expressed in terms of infinite series of modal functions. Under the plane wave assumption, it is shown that consideration of the first few modes yields sufficiently accurate results, and locations of peaks and dips are investigated. It is determined that the peak frequencies of the double plates coincide with those of each single plate. When the two plates are identical, the STL of the double plates as well as that of the single plate become zero at the natural frequencies of the single plate. The location and amplitude of the dips are investigated using an approximation solution when the cavity depth is very small.

• Journal of Magnetics
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• v.12 no.3
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• pp.97-102
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• 2007

We have investigated the electronic structures and magnetism of a full Heusler alloy $Co_2CrGa(001)$ surface by using the all-electron full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA). We considered two types of different terminations: the Co-terminated (Co-Term) and the CrGa-terminated (CrGa-Term) surfaces. From the calculated layer-projected density of states (LDOS), we found that the surface of the CrGa-Term shows nearly half-metallic character while that of the Co-Term is far from the half-metallic. For the Co-Term, the surface Co atom moves down to the bulk region by $0.05{\AA}A$, while the subsurface Cr and Ga atoms move up to the surface layer by 0.05 and $0.01{\AA}$, respectively. For the CrGa-Term, there is a large inward relaxation of the surface Ga atom $(0.07{\AA})$, but the relaxation of the surface Cr atom is very small $(0.01{\AA})$. The relaxations affect not much to the overall shapes of DOS for both terminations, but make the surface states of the surface Cr and Ga atoms for the CrGa-Term shift to higher energy that enhances the nearly half-metallic character of the CrGa-Term. The magnetic moments of the surface $Cr(2.98{\mu}_B)$ in the CrGa-Term and the surface $Co(1.17{\mu}_B)$ in the Co-Term were much increased compared to those of the inner-layers $(1.79\;and\;0.77{\mu}_B)$, respectively, while that of the subsurface Cr atom in the Co-Term was decreased to $1.19{\mu}_B$.

• Journal of the Korean Institute of Telematics and Electronics
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• v.26 no.4
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• pp.16-26
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• 1989

Diffraction patterns of electromagnetic fields for the incidence of E-polarized plane wave on both interfaces of an arbitrary-angle dielect wedge are obtained by sum of geometric optics term and the edge diffracted fields. The diffraction coefficients of the edge diffracted fields are evaluated by employing the physical optics approximation and then correcting its error with the multipole line source at the dielectric edge. For the wedge angle $120^{circ}$, the incident angle $60^{circ}$, the relative dielectric constant of the dielectric wedge, 2, 5, and 10, and the observation distance from the tip of the wedge, 5 and 10 wavelength, the diffraction coefficients and the diffraction patterns corresponding to geometric optics, physical optics, and the solution corrected by the multipole line source are plotted, respectively. While the corrected solutions presented in this paper are valid only in the far-field region, these asymptotic solutions show to satisfy the boundary condition on the dielectric interfaces.

• Nuclear Engineering and Technology
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• v.51 no.5
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• pp.1373-1380
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• 2019

Besides promising implications as fertile nuclear materials, thorium carbonitrides are of great interest owing to their peculiar physical and chemical properties, such as high density, high melting point, good thermal conductivity. This paper reports first-principles simulation results on the structural, electronic and magnetic properties of cubic thorium carbonitrides $ThC_xN_{(1-x)}$ (X = 0.03125, 0.0625, 0.09375, 0.125, 0.15625) employing formalism of density-functional-theory. For the simulation of physical properties, we incorporated full-potential linearized augmented plane-wave (FPLAPW) method while the exchange-correlation potential terms in Kohn-Sham Equation (KSE) are treated within Generalized-Gradient-Approximation (GGA) in conjunction with Perdew-Bruke-Ernzerhof (PBE) correction. The structural parameters were calculated by fitting total energy into the Murnaghan's equation of state. The lattice constants, bulk moduli, total energy, electronic band structure and spin magnetic moments of the compounds show dependence on the C/N concentration ratio. The electronic and magnetic properties have revealed non-magnetic but metallic character of the compounds. The main contribution to density of states at the Fermi level stems from the comparable spectral intensity of Th (6d+5f) and (C+N) 2p states. In comparison with spin magnetic moments of ThSb and ThBi calculated earlier with LDA+U approach, we observed an enhancement in the spin magnetic moments after carbon-doping into ThN monopnictide.