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Design of Wideband Microwave Absorbers Using Reactive Salisbury Screens with Maximum Flat Reflection
Sanghoek Kim,Bomson Lee,Gunyoung Kim 한국전자파학회JEES 2019 Journal of Electromagnetic Engineering and Science Vol.19 No.2
This paper presents a design methodology for wideband single-layered microwave absorbers with arbitrary absorption at the design center frequency using reactive Salisbury screens. The bandwidth of the absorber increases when the flatness of the reflection response at the design center frequency is maximized. Based on this observation, closed-form design formulas for wideband absorbers are derived. As they are scalable to any design frequency, wideband reactive screens can be systematically realized using two-dimensional periodic crossed-dipole structures patterned on a resistive sheet. Based on this method, a single-layered absorber with a 90% bandwidth improved to 124% of the design center frequency is presented. For the purpose of physical demonstration, an absorber with a design center frequency of 10 ㎓ is designed and fabricated using a silver nanowire resistive film with a surface resistance of 30 Ω/square. The measured absorption shows a good agreement with both the calculation and the electromagnetic simulation.
A transistor based on 2D material and silicon junction
Kim, Sanghoek,Lee, Seunghyun 한국물리학회 2017 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol. No.
<P>A new type of graphene-silicon junction transistor based on bipolar charge-carrier injection was designed and investigated. In contrast to many recent studies on graphene field-effect transistor (FET), this device is a new type of bipolar junction transistor (BJT). The transistor fully utilizes the Fermi level tunability of graphene under bias to increase the minority-carrier injection efficiency of the base-emitter junction in the BJT. Single-layer graphene was used to form the emitter and the collector, and a p-type silicon was used as the base. The output of this transistor was compared with a metal-silicon junction transistor (i.e. surface-barrier transistor) to understand the difference between a graphene-silicon junction and metal-silicon Schottky junction. A significantly higher current gain was observed in the graphene-silicon junction transistor as the base current was increased. The graphene-semiconductor heterojunction transistor offers several unique advantages, such as an extremely thin device profile, a low-temperature (< 110 degrees C) fabrication process, low cost (no furnace process), and high-temperature tolerance due to graphene's stability. A transistor current gain (beta) of 33.7 and a common-emitter amplifier voltage gain of 24.9 were achieved.</P>
Control of Power Distribution for Multiple Receivers in SIMO Wireless Power Transfer System
Gunyoung Kim,Seunghyun Boo,Sanghoek Kim,Bomson Lee 한국전자파학회JEES 2018 Journal of Electromagnetic Engineering and Science Vol.18 No.4
A method to control the power distribution among receivers by the load values in a single-input, multiple-output (SIMO) wireless power transfer (WPT) system is investigated. We first derive the value of loads to maximize total efficiency. Next, a simple, but effective analytical formula of the load condition for the desired power distribution ratio is presented. The derived load solutions are simply given by system figure of merits and desired power ratios. The formula is validated with many numerical examples via electromagnetic simulations. We demonstrate that with the choice of loads from this simple formula, the power can be conveniently and accurately distributed among receivers for most practical requirements in SIMO WPT systems.
김동훈(Donghoon Kim),박정훈(Junghoon Park),김상혁(Sanghoek Kim) 한국통신학회 2021 한국통신학회 학술대회논문집 Vol.2021 No.2
본 논문은 항공기에서 측정 가능한 레이다 클러터 신호 생성 및 생성된 신호에 대한 신호처리 및 탐지 시뮬레이션을 다루었다. 항공기 레이다는 항공기의 움직임과 레이다 빔 운용기법, 탐지 기법, 추적 기법 등 역동적인 환경에 놓이게 된다. 그리고 이러한 전술 환경 및 운용기법들은 해상 및 지상 그리고 날씨에 대한 환경적인 변수가 개입하게 되는데 이러한 영향은 클러터 신호로 나타나게 된다. 기존 논문들은 이러한 클러터 신호에 대해 정적인 환경에서의 신호 모사가 가능하게 개발되어 이러한 환경에 대한 영향을 고려하기가 어려운 점이 있다. 이러한 점들을 극복하기 위해 본 논문에서는 패치신호 합 기법을 이용하여 클러터 신호를 이용하여 클러터를 생성하였다. 패치 합 신호 기법 또한 기존 논문들에서 제시하는 단순한 랜덤 복소 가우시안 신호의 합으로 표현 하는 것이 아닌, 문헌상에서 제시하고 있는 클러터들의 PSD 특성 [1-11]을 이용하여 생성하였다. 또한 기존 많은 문헌에서 언급되고 있는 공대공 탐지 모드에 대한 알고리즘 [12-13]을 적용하여 클러터의 영향을 확인하였다.
Sungyoun Hwang,Sanghoek Kim,Bomson Lee,Sejin Kim 한국전자파학회JEES 2018 Journal of Electromagnetic Engineering and Science Vol.18 No.3
In this paper, the efficiency of single-input multiple-output (SIMO) wireless power transfer systems is examined. Closed-form solutions for the receiver loads that maximize either the total efficiency or the efficiency for a specific receiver are derived. They are validated with the solutions obtained using genetic algorithm (GA) optimization. The optimum load values required to maximize the total efficiency are found to be identical for all the receivers. Alternatively, the loads of receivers can be adjusted to deliver power selectively to a receiver of interest. The total efficiency is not significantly affected by this selective power distribution. A SIMO system is fabricated and tested; the measured efficiency matches closely with the efficiency obtained from the theory.
Power Link Optimization for a Neurostimulator in Nasal Cavity
Lee, Seunghyun,Kim, Jonghoek,Kim, Sanghoek Hindawi Limited 2017 International journal of antennas and propagation Vol.2017 No.-
<P>This paper examines system optimization for wirelessly powering a small implant embedded in tissue. For a given small receiver in a multilayer tissue model, the transmitter is abstracted as a sheet of tangential current density for which the optimal distribution is analytically found. This proposes a new design methodology for wireless power transfer systems. That is, from the optimal current distribution, the maximum achievable efficiency is derived first. Next, various design parameters are determined to achieve the target efficiency. Based on this design methodology, a centimeter-sized neurostimulator inside the nasal cavity is demonstrated. For this centimeter-sized implant, the optimal distribution resembles that of a coil source and the optimal frequency is around 15 MHz. While the existing solution showed an efficiency of about 0.3 percent, the proposed system could enhance the efficiency fivefold.</P>
자외선 레이저를 사용한 마찰대전 기반의 움직임 감지 유연센서
조현우(Hyunwoo Cho),김상혁(Sanghoek Kim),김대원(Daewon Kim) 한국통신학회 2022 한국통신학회 학술대회논문집 Vol.2022 No.2
본 논문은 자외선 레이저를 통해 물리 및 화학적으로 정전기 기반의 마찰대전 나노발전기의 전기적 출력을 향상하고, 움직임 센서로서의 활용성에 대한 연구 결과를 제시한다.
박성익(Sungik Park),이종헌,김상혁(Sanghoek Kim) 한국통신학회 2021 한국통신학회 학술대회논문집 Vol.2021 No.2
본 논문은 공동기 (Cavity Resonator)를 이용하여 전자기장 (Electromagnetic Feild)의 노출을 최소화한 견고한 효율의 무선 전력전송 시스템을 설계하였다. 공동기의 높은 quality factor를 통한 전자기장 노출의 최소화와 홀짝 대칭적 회로 (Parity-Tmes Symmetric Circuit)를 적용하여 넓은 수신 범위에도 일정한 무선 전력전송 효율을 가지는 시스템을 연구하였다. 송신 공동기와 수신 공동기가 동일한 경우와 수신 공동기가 sacled-down한 경우로 무선 전력전송 실험을 하였으며 공동기의 높은 Quality-factor를 통하여 공동기 간에 낮은 에너지 결합률 (Coupling factor)에도 높은 전력전송 효율을 가지게 된다.
이현건(Lee Hyeonkeon),박홍현(Park Honghyeon),김상혁(Kim Sanghoek) 한국통신학회 2021 한국통신학회 학술대회논문집 Vol.2021 No.2
본 논문은 혈당에 따라 복사하는 빛의 세기가 변하는 형광 센서와 결합하여 체내에 삽입되는, 무선으로 체내 혈당을 측정할 수 있는 회로를 제시한다. 무선전력전송과 저전력 후방산란 통신을 사용하여 배터리가 필요 없고 매우 작은 크기와 극히 적은 전력으로 동작한다.