• Journal of the Korean Magnetics Society
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• v.15 no.3
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• pp.202-206
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• 2005

The present investigation provides an electromagnetic radiation absorptive composition which comprises silver-coated ferrite microspheres dispersed in silicon rubber matrix for the aim of thin microwave absorber in GHz frequencies. Ni-Zn ferrite spheres with $50{\mu}m$ size in average were prepared by spray-drying and sintering at $1130^{\circ}C$. Conductive silver layer was plated on ferrite spheres by electroless plating. Conductive Ni-Zn ferrite sphere with uniform silver layer were obtained in the concentration of 10 g/L $AgNO_3$ per 20 g ferrite spheres. For this powder, electrical resistance is reduced as low as $10^{-2}\~10^{-3}\;\Omega$. The most sensitive material parameters with silver plating is real and imaginary parts of complex permittivity. The conductive Ni-Zn ferrite spheres have large values of dielectric constant. Due to this high dielectric constant of microspheres, matching thickness is reduced to as low as 2 mm at the frequency of 7 GHz, which is much thinner than conventional ferrite absorbers.

• Journal of the Korean Magnetics Society
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• v.15 no.5
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• pp.282-286
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• 2005

Ni-Cu-Zn ferrites were prepared by the co-precipitation and ferrite microwave absorbers on low temperature sintering were investigated in this work. The properties of its microwave absorbing and physical were analyzed into variations of Ni addition, calcination temperature, sintering temperature. From the analysis of X-ray diffraction patterns, we can see that all the particles have only a single phase spinel structure. In addition, the powders particle size distribution obtained the nano size. By increasing the Ni additive, the permeability of the powders was decreased and the loss factor increased at sintering temperature $1100^{\circ}C$. Also, we considered that it can used high frequency rage. We found that the $(Ni_{0.7}Cu_{0.2}Zn_{0.1}O)_{1.02}(Fe_{2}O_3)_{0.98}$ appeared microwave absorbing properties better than other composition.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.4
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• pp.344-349
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• 2009

To improve performance of the main landing gear for helicopters, a semi-active control landing gear is introduced in this paper. An MR damper based on commercial finite element electromagnetic field analysis of an electromagnet has been adapted the shock absorber. Force control algorithm (which maintains constantly the sum of air spring force and damping force as internal forces) which keep the sum of air spring force and damping force constant during landing, has been used for the controller, applied to control the semi-active landing gear. A series of drop simulations using ADAMS has been done with the passive, sky-hook control type, and force control type landing gears. The result of each simulation has been compared to evaluate the landing performance of the proposed force control type landing gear.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.235.1-235.1
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• 2015

Artificially-engineered materials, whose electromagnetic properties are not available in nature, such as negative reflective index, are called metamaterials (MMs). Although many scientists have investigated MMs for negative-reflective-index properties at the beginning, their interests have been extended to many other fields comprising perfect lenses. Among various kinds of MMs, metamaterial absorbers (MM-As) mimic the blackbody through minimizing transmission and reflection. In order to maximize absorption, the real and the imaginary parts of the permittivity and permeability of MM-As should be adjusted to possess the same impedance as that of free space. We propose a dual-wide-band and polarization-independent MM-A. It is basically a triple-layer structure made of metal/dielectric multilayered truncated cones. The multilayered truncated cones are periodically arranged and play a role of meta-atoms. We realize not only a wide-band absorption, which utilizes the fundamental magnetic resonances, but also another wide-band absorption in the high-frequency range based on the third-harmonic resonances, in both simulation and experiment. In simulation, the absorption bands with absorption higher than 90% are 3.93 - 6.05 GHz and 11.64 - 14.55 GHz, while the experimental absorption bands are in 3.88 - 6.08 GHz and 9.95 - 13.84 GHz. The physical origins of these absorption bands are elucidated. Additionally, it is also polarization-independent because of its circularly symmetric structures. Our design is scalable to smaller size for the infrared and the visible ranges.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.12
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• pp.1089-1094
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• 2007

This paper is concerned with a new concept for the damper without neither a coil spring nor fluid. The new damper concept consists of the permanent magnets and the cylinder of the conducting material. The opposite pole magnets produces the repulsive forces and this is substituted for the coil spring. The relative motion between the magnets and conducting cylinder produces eddy currents thus resulting in the electromagnetic force, which turns out to be the damping force and is substituted for a damping fluid. This damper is called the eddy current damper(ECD). The important advantage of the proposed ECD is that it does not require any damping fluid and any external power and is non-contacting and relatively insensitive to temperature. In the present study, the proposed ECD was constructed and the experiments were performed to investigate its dynamic characteristics. The experiments shows that the proposed ECD has the excellent damping ability.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.21 no.11
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• pp.2206-2212
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• 2017

When it comes to naval surface warfare, the probability of detection is an important factor in survivability and the Radar Cross Section(RCS) is a major parameter. In this paper, the RCS reduction technology of the Radar Absorbing Material(RAM) method is carried out for the general frequency range for naval warfare. We set the analysis model with the simplified ship model and the wide band metamaterial which is high-tech radar absorbing materials is selected for the RAM method. The modeling of the wide band metamaterial composed of an MIK surface which has the wide band resonant properties and flexible substance and the electromagnetic absorptions and reflections of the wide band metamaterial has been simulated to explore the performance. Also, the wide band metamaterial is compared with the paint absorber to analyze RCS reduction in terms of RCS values.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.18 no.11
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• pp.1217-1223
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• 2007

Advanced microwave absorbers for wide oblique incidence angles are required in many applications including wireless communication or vehicle identification in ITS(Intelligent Transport System) where 5.8 GHz DSRC(Dedicated Short Range Communication) system is applied. In this study, two-layered microwave absorber(with a laminate structure of dielectric/magnetic composites) has been designed for the achievement of low reflection coefficient over wide incidence angles at 5.8 GHz. Iron flake particles are used as the filler in the absorbing layer, and the magnetic composite sheet exhibits high magnetic loss due to ferromagnetic resonance in gigahertz frequencies. The surface layer of low dielectric constant containing small amount of carbon black is used as the impedance transformer. On the basis of transmission line theory, the reflection loss has been calculated for the two-layer structure with variation of incident angles for both TE(Transverse Electric) and TM(Transverse Magnetic) polarizations. At the optimum thickness of the composite layers, a low value of reflection loss(less than -10 dB) has been predicted for wide incidence angles up to $55^{\circ}$ which is in good agreement with the measured value determined by free-space measurement.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.17 no.3 s.106
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• pp.239-248
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• 2006

In this paper, the cryogenically cooled low noise amplifier module for radio telescope receiver front-end using pHE-MT MMIC is designed and fabricated. In the selection of MMIC, the MMIC fabricated with the pHEMTS providing successful cryogenic operation are chosen. They are mounted in the housing using the thin film substrate. In the design of the housing, the absorber and the elimination of the gap between the carrier and the housing as well removed the unnecessary oscillations by its structure. The mismatch is improved by ribbon-tuning to provide the best performance at room temperature. The fabricated module shows the gain of $35dB{\pm}1dB$ and the noise figure of $2.37{\sim}2.57dB$ at room temperature over $21.5{\sim}23.5GHz$. In the cryogenic temperature of $15^{\circ}K$ cooled by He gas, the measured gain was above 35 dB and flatness ${\pm}2dB$ and the noise temperatures of $28{\sim}37^{\circ}K$.

• Journal of the Korean Magnetics Society
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• v.18 no.3
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• pp.120-125
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• 2008

In this study, NiCoZn ferrites with the variation of sintering temperature and chemical composition were prepared by the coprecipitation. Microstructures Crystal structure of NiCoZn ferrites were analyzed by XRD and their electric magnetic characteristics were analyzed by LCR meter and their morphology observed by SEM. We identified that these powders have a typical NiCoZn spinel structure and nanoparticles average size of 40 nm. The impurity, the initial permeability and the Q factor value are the lowest of sintered NiCoZn ferrite at $1250^{\circ}C$. Also, we measured S-parameter for $(Ni_{0.4}Co_{0.1}Zn_{0.5})Fe_2O_4$ which showed a maximum reflection loss of -3.1 dB at 6 GHz for the 2 mm thick sample. From this result, we found that the NiCoZn ferrite can be used in ferrite microwave-absorbing application at a higher frequency region (> 6 GHz).