• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.17 no.7 s.110
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• pp.623-628
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• 2006

This paper presents the characteristics of an antenna factor of two kinds of EMI dipole antennas with a coaxial cable balun used in the frequency range between 30 and 1,000 MHz. The integral equation for unknown current distribution is solved by the Galerkin's method of moments with piecewise sinusoidal functions. An antenna factor for EMI dipole antennas with the coaxial cable balun is derived by using the power loss concepts. We can realize two kinds of EMI dipole antennas with appropriate antenna factors in the frequency range from 30 to 1,000 MHz: 150-cm dipole length($30{\sim}300 MHz$) and 30cm dipole length($300{\sim}1,000 MHz$). To check th ε validity of the theoretical analysis, the complex antenna factor was measured using by reference antenna methods. It is shown that the calculated complex antenna factor is good agreement with experimental results.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.21 no.6
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• pp.553-563
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• 2010

This paper presents a tapered slot-antenna(TSA) for optimal impulse-signal transmission in ultra-wide band(UWB). The proposed TSA provides radiates in end-fire direction, which meets an impulse-radio UWB(IR-UWB) system demands(e.g., low loss, thus less error throughout the UWB band). In order to minimize the pulse distortion, we used an wideband impedance transformer and a microstrip slotline. The pulse fidelity characteristics was evaluated with finite-difference time-domain(FDTD) analysis technique and pulse fidelity correlation equation. Approximately 93.89 % pulse fidelity was obtained between the two antennas in 0.5 m range. Additionally, derived chirp Z-transform algorithm enables us to utilize the zoom-in option on the pulse signal in few nano-seconds below. Thus, it is possible to analyze the pulse signal distortion, delay or dispersion characteristics.

• The Journal of the Acoustical Society of Korea
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• v.15 no.5
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• pp.44-52
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• 1996

Acoustical characteristics of internal pipe structures and a loudspeaker of the thermoacoustic refrigerator are analyzed by using the transfer matrix method. The resonator system is dismantled into verious basic acoustic elements, and then linearized transfer matrices are serially combined with the dynamical system of linearized loudspeaker model, that the total system of thermoacoustic refrigerator can be analyzed in terms of frequency characteristics and acoustic field shape. Additionally, by using equations for energy flow through the capillary stack, the temperature distribution over the stack is numerically estimated. After expressing the acoustic work flow, thermoacoustic flow, and energy loss per unit length in a single capillary duct by using the transverse functional variations, overall energy flow rate and energy balance are obtained for the whole capillary stack. The final expression for energy flow through the stack is numerically evaluated by varying physical parameters obtained from the sound field analysis. After confirming good agreements between predicted and experimental results for the interior sound field of a refrigerator model, the thermoacoustic characteristics of Hofler's apparatus is analyzed by the proposed method and it is observed that the results agree well with Hofler's experimental results.

• Journal of Advanced Marine Engineering and Technology
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• v.38 no.3
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• pp.285-291
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• 2014

This paper is related to the improvement of efficiency for high performance centrifugal pumps by reducing leakage loss, which is achieved by applying the grooved seal as a non-contact seal to the pumps. Various combinations of grooved seal types, including the spiral and the parallel groove in the rotor and/or in the stator, were tested by the experiment. And the corresponding hydraulic performance and the magnitude of axial thrust were measured and calculated for ten cases. From the results, the type with the spiral groove(spiral angle : $0.98^{\circ}$) in both the rotor and the stator was found to be most effective. In this case, the head and the efficiency were improved from the original design by 2.1% and 2.3% respectively at design capacity($340m^3/h$), and the axial thrust was decreased by 10%.

• Proceedings of the Korea Water Resources Association Conference
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• 2007.05a
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• pp.517-521
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• 2007

최근 위성영상의 해상도가 증가함에 따라, 하천을 대상으로 한 연구 사례가 많이 보고되고 있다. 하지만 대부분이 환경 생태학적 분야에서 이루어진 연구 결과들이고, 수자원 관리 분야에서의 위성영상 활용 사례는 아직 그 예를 찾아보기가 어렵다. 갑천은 1988년 이후 급속도로 도시화가 진행되어 현재는 잘 정비되어 있는 전형적인 도시하천의 모습을 갖추고 있다. 그러나 아직까지 하천 내에는 많은 양의 초지나 사주가 분포하고 있으며, 이러한 초지나 사주의 존재는 자연형 하천으로서의 역할을 담당하지만 이수 및 치수차원에서는 많은 양의 저수량 손실을 야기하는 문제도 함께 공존해 있다. 본 연구에서는 도시하천을 효율적으로 관리하기 위한 방안으로서 영상을 이용한 수자원 관리 기법을 제안하였다. 다시 말해 본 연구에서 수행한 "도시하천에서의 저수량 평가"는 보와 보사이의 흐름에 의한 유량변화가 전체 저수량에 크게 영향을 미치지 않는다는 가정 하에 다음의 세 단계로 이루어진다. 첫째, 현하천을 사주구역, 초지구역 및 그 외 구역으로 구분한다. 둘째, 현장조사를 실시하여 사주나 초지가 수면과 이루는 경사각을 조사하고, 적절한 통계적 기법을 이용하여 대상 하천의 대표 경사 값을 구한다. 또한 현장조사 시 사주나 초지로부터 충분히 떨어진 지점들의 수심을 측량하여 그 외 지역의 평균 수심을 구한다. 셋째, 영상을 해석할 수 있는 도구를 이용하여 구분된 각 구역의 면적을 계산한 후, 두 번째 단계에서 구한 경사각과 수심을 이용하여 저수량을 산정한다. 본 연구에서는 도시하천을 효율적으로 관리하기 위한 방안으로서 영상을 이용한 수자원 관리 기법을 제안하였다. 특히 도시하천에서의 저수량 산정을 위한 기초연구로서 사주 및 초지가 수면과 이루는 방위별 경사각을 조사하였으며, 그 결과를 분석하여 사주와 초지 각각의 경우에 대한 거리와 수심 간의 회귀곡선식을 구하였다. 하지만 세 번째 단계인 "저수량 산정"은 영상의 확보가 어려워, 본 연구에서는 수행되지 못하였다. 향후, 대상지역 뿐만 아니라 전국 어느 곳이라도, 하루빨리 고해상도의 위성영상이 저가에 보급되어 지기를 기대한다.

• Journal of Satellite, Information and Communications
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• v.5 no.2
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• pp.57-63
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• 2010

Many efficient frequency sharing methods are issued in present because of increasing users with various wireless communication terminals. In the satellite communications, the service coverage is generally very wide so frequency sharing with terrestrial system is essentially needed, and the research is progressing dynamically related on this frequency sharing method. But if we adopt the terrestrial system which is commonly used, it can't avoid the interference from terrestrial service to satellite service. Therefore, this paper will introduce methods for reducing the interference from terrestrial station to earth station using cognitive radio system Satellite system is guaranteed with decreasing interference from terrestrial stations using Genetic Algorithm based power control method. Furthermore, terrestrial systems can have increased QoS because the frequency reuse factor in proposed method is higher than existing methods.

• Journal of Korea Water Resources Association
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• v.33 no.3
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• pp.365-374
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• 2000

This paper is to present the determination of the optimal Joss rate parameters and urnt bydrographs from the observed single rainfall-runoff event using optimization models coupled with a stochastic technique for the global solution. Two kinds of the linear program models are formulated to derive the optimal unit hydrographs and loss rate parameters for gaged basins; one mimmizes the summation of the absolute residual between predlCted and observed runoff ordinates and the other, the maximum absolute residuaL Multistart algorithm which is one or stochastic techniques for the global optimum is adopted to perturb the parameters of the loss rate equations. Multistart efficiently searches the feasIble region to identify the global optimlUll for loss rate parameters, which yields the optimal loss rate parameters and unit hydrograph for Kostiakov's, Plulip's, and Horton's equation. The unique unit hydrograph ordinates for a gIven rainfall-runoff event iS exclusrvely obtained WIth $\Phi$ index, but unit hydrograph ordinates depend upon the parameters [or each loss rate equations. The parameters of Green-Ampt's are determined through a trial and error method. In this paper the single rainfall-nmoff event observed from a watershed is considered to test the proposed method. The optimal unit hydrograph herein found has smaller deviations than the ones reported previously by other researchers.

• Korean Journal of Optics and Photonics
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• v.20 no.4
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• pp.211-216
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• 2009

Much research into two-dimensional (2-D) photonic crystal (PC) structures has been conducted for realization of ultrasmall optical integrated circuits. A 2-D silicon (Si) PC slab structure with air cladding (n=1) is one of the representative structures in 2-D PCs. While air-clad Si PC slab structures have good optical characteristics, their suspension in air can lead to mechanical weakness, making integration with some optical devices difficult. In this paper, we propose improving the mechanical robustness of PC structure by developing a 2-D Si PC structure with symmetric silica cladding (n=1.44) and comparing its optical properties to that of the air-clad structure. First, we investigate the optical properties of a 2-D Si PC slab structure with air cladding by using a 3-D finite difference time domain method. We determined that a photonic bandgap of 330 nm and a non-leaky propagating bandwidth of 100 nm in the optical communication range are possible. Next, we investigate the optical properties of 2-D Si PC slab structures with silica cladding. Even though the refractive index of the silica cladding is higher than that of air, we developed a silica-clad structure with good optical properties: a photonic band gap of approximately 230 nm and a non-leaky propagating bandwidth of 90 nm, comparable to that of the air-clad PC structures.

• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.270-278
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• 2020

In this paper, sound transmission of Micro-Perforated Plates (MPPs) installed in an impedance tube with a circular cross-section is described using an analytic method. Vibration of the plates is expressed in terms of an infinite series of modal functions, where modal function in the radial direction is given by the Bessel function. Under the plane wave assumption, a low frequency approximation is derived, and a formula for the sound transmission coefficient of multi-layered MPPs is presented using the transfer matrix method. The Sound Transmission Losses (STLs) of single and double MPPs are computed using the proposed method and compared with those done by the Finite Element Method (FEM), which shows an excellent agreement. As the perforation increases, the STL is degraded, since the STL becomes dominated by the perforation ratio rather than by vibration of the plate. The STL shows dips at natural frequencies as well as at the mass-spring-mass resonance frequency. The proposed model for the STL prediction in this study can be applied to an arbitrary number of MPPs, where each MPP may or may not have a perforation.

• The Journal of the Acoustical Society of Korea
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• v.20 no.5
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• pp.69-75
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• 2001

In this paper, a fast and easy impedance matching method, which could give the impedance matching component for the general 1 or 2-port network was introduced. First, the entire network structure was defined which consists of the network part to be matched and the impedance matching part composed of inductors and capacitors. Next, the transmission matrix and input and output impedances of the entire network from the terminal impedance conditions were calculated, then the exact solutions for the matching components were obtained. To verify the efficiency of this method, this method was applied to the CDMA If band withdrawal weighted SAW transversal filter, and investigated the effects of the impedance matching before and after, through the simulation and experiment. As the result, the performance of a fractional bandwidth of 1.2%, insertion loss of 29 dB, and VSWR of 80 have improved to a factional bandwidth of 1.8％, insertion loss of 9 dB, VSWR of 3 at 85.38 MHz center frequency. The result shows that this impedance matching method could be used in the SAW devices and other types of 1 or 2-port network.