A practical theorem on using interferometry to measure the global 21-cm signal

The sky-averaged, or global, background of redshifted $21$ cm radiation is expected to be a rich source of information on cosmological reheating and reionizaton. However, measuring the signal is technically challenging: one must extract a small, frequency-dependent signal from under much brighter spectrally smooth foregrounds. Traditional approaches to study the global signal have used single antennas, which require one to calibrate out the frequency-dependent structure in the overall system gain (due to internal reflections, for example) as well as remove the noise bias from auto-correlating a single amplifier output. This has motivated proposals to measure the signal using cross-correlations in interferometric setups, where additional calibration techniques are available. In this paper we focus on the general principles driving the sensitivity of the interferometric setups to the global signal. We prove that this sensitivity is directly related to two characteristics of the setup: the cross-talk between readout channels (i.e. the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g. absorbers or the ground) radiating into both channels. Thus in an interferometric setup, one cannot suppress cross-talk and correlated thermal noise without reducing sensitivity to the global signal by the same factor -- instead, the challenge is to characterize these effects and their frequency dependence. We illustrate our general theorem by explicit calculations within toy setups consisting of two short dipole antennas in free space and above a perfectly reflecting ground surface, as well as two well-separated identical lossless antennas arranged to achieve zero cross-talk.Comment: 17 pages, 6 figures, published in Ap

Signal Attenuation in V.H.F. Biotelemetry

In this paper, the change in loop resistance is examined when the loop is placed at the center of a spherical air cavity which is in turn surrounded by a sphere of physiological saline solution, as shown. The functional dependence of the magnitude of the dissipation resistance on the cavity radius will be studied. The validity of introducing a dissipation resistance will be experimentally tested by immersing a loop in a physiological saline solution and directly measuring the change in resistance with an r.f. bridge. Once the increase in resistance is established, the resultant reduction in radiated power will be mathematically calculated and compared with the reduction in radiated power caused by radiated power absorption, in an effort to determine which effect predominates. The analysis will begin with the derivation of the electro-magnetic fields in the space around the loop in air and in a lossy medium. An analysis of the fields in the cavity and in the saline solution will be made with the goal of obtaining a good approximation for the fields in the solution, from which the Joule heat liberated in the solution can be found and the dissipation resistance determined

방사 무선전력전송을 위한 무지향성 안테나 및 전송 효율 한계에 대한 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 남상욱.본 논문에는 방사하는 전자파를 이용한 무선 전력 전송에 대해 집중적으로 연구를 진행하였다. 보다 구체적으로는, 무지향성 안테나의 분석과 설계, 자유공간과 손실매질에서의 최적 송신 전류 분포, 전송 효율의 한계에 대한 연구를 기술하였다. 추가적으로, 전자파의 인체 영향에 대한 비교 및 이론적인 최적 전류 분포의 효과적 구현에 대한 연구를 진행하였다. 본 연구에서는 방사형 무선전력전송을 전원을 공급 유무에 따라 수동형과 능동형 무선전력전송으로 구분하고, 수동형 방사 무선전력전송부터 능동형 방사 무선전력전송까지의 연구를 순차적으로 기술하였다. 먼저, 수동형 방사 무선전력전송 연구에서는 전자파 에너지 하베스팅용 안테나에 대한 분석 및 설계에 대한 연구를 진행하였다. 수동 방사 무선전력전송의 상황을 고려하여 등방성 패턴, 전기적으로 작은 크기, 높은 효율 특성을 나타내는 안테나를 제안하였다. 전기적으로 소형이면서 등방성 패턴을 방사하는 SRR이 기본 구조로 활용되었다. SRR에 대한 이론적 분석을 진행하였고, 시뮬레이션 결과와 잘 맞는 것을 확인하였다. 분석에 기초하여 FSRR 안테나를 설계하였고, 측정을 통해 제안한 아이디어를 검증하였다. 수신 파워의 크기를 향상시키기 위해, 이중 대역 및 확장된 대역에서 동작하는 FSRR 안테나를 추가로 설계하였다. 제안된 구조는 선행연구와 비교하였을 때, 상대적으로 우수한 성능을 보여주었다. 한편 수동형 방사 무선 전력전송의 경우, 주변의 낮은 전력 밀도로 인해 수신 전력이 매우 낮은 한계점이 존재한다. 따라서, 송신 타워를 이용해 모바일 안테나로 무선 전력을 전송할 수 있는 능동형 방사 무선전력전송에 대한 후속 연구를 진행하였다. 능동형 방사 무선전력전송에서는, 송신 타워를 활용하여 모바일 기기에 효과적으로 무선전력전송을 수행하는 방법에 대한 연구를 진행하였다. 본 연구에서는 방사형 무선전력전송의 효율을 최대화 하는 송신 전류분포와 주어진 면적을 활용할 때 얻을 수 있는 최대 한계 효율을 이론적으로 도출하였다. 본 연구의 결과를 통해, 기존의 방식으로는 파악할 수 없었던 중거리 무선전력전송 효율의 최대 한계치와 송신 전류분포의 최적 형태를 파악할 수 있다. 연구의 결론에 따르면, 수신하는 안테나의 송신 방사 패턴이 효율을 결정함에 있어 중요한 역할을 하였다. 제안한 이론을 실제 안테나에 적용하여 선행연구와 비교를 하였고, 선행 연구로 파악할 수 없는 이론적 한계 효율을 도출하였다. 제안한 연구를 일반적인 상황으로 확장하기 위해 손실 매질 내부에서의 무선전력전송에 대한 추가 연구를 진행하였다. 손실 매질이 있는 경우에서도 최적 전류 분포와 효율의 최대 한계치를 도출하였다. 최적 송신 전류를 활용하여, 실제 안테나 어레이를 구현하고 인체 팬텀에 미치는 영향을 파악해보았다. 마지막으로, 앞서 도출한 이론적인 전류분포를 실제 안테나로 구현하는 방법에 대해 연구를 진행하였다. 선행 연구를 참고하여, 이상적인 전류분포를 thinned 배열로 구현하는 두가지 방법을 제안하였다. 이론적인 전류 분포에 유전 알고리즘을 활용한 방법과, density tapering을 응용한 방법을 적용하였다. 두 방법 모두 동일한 개수의 균등 어레이에 비해 성능이 개선되는 결과를 보였다. 특히 density tapering을 이용하면 같은 개수 및 같은 면적의 격자구조보다 비용, 무게, 효율 등에서 장점이 있으며, 수신기의 위치가 변할 때에도 더 높은 효율로 송신이 가능하다.In this thesis, research on the radiative-wireless power transmission (R-WPT) using radiated electromagnetic(EM) fields is presented. More specifically, the analysis and design of quasi-isotropic antennas, the analytical study on the optimal transmitting current, and the efficiency bounds are described. In addition, research on the comparison of the EM effects on the human phantom, and the effective implementation of the optimal current distribution are conducted. The research is described sequentially from the passive R-WPT to active R-WPT, which indicate the absence or presence of the power supplying base station. First, research is conducted on the analysis and design of the passive R-WPT antenna. Considering the ambient environment, an antenna with quasi-isotropic pattern, electrically small size, and high efficiency is proposed. A split-ring resonator (SRR) that radiate quasi-isotropic pattern with electrically small size is used as a basic structure. The analysis on the SRR is well matched with the simulation results. Based on the analysis, folded split-ring resonator (FSRR) is proposed and designed for the passive R-WPT antennas, and verified through the measurement. A dualband and wideband FSRR that can harvest more ambient power is designed as an extended work. The proposed antennas are compared with recent studies showing superior performances. On the other hand, the receiving power of the passive R-WPT is very low due to low power density of ambient field, a study on the active R-WPT, which can transfer wireless powers from the base station to the mobile antenna, is conducted as a next step. In the active R-WPT, a study on the way to effectively transfer wireless power to the mobile devices by using a transmitting tower is described. The optimal current distribution of the transmitting surface, and maximum power transfer efficiency (PTE) bounds when the transmitting area is limited are analytically derived. Through the results, it is possible to figure out the maximum efficiency bounds for the mid-range R-WPT and the optimal shape of transmission current distribution that could not be found by the conventional method. The results indicate that the optimum current distribution on the transmitting surface and the maximum efficiency of radiative WPT depend on the radiating field pattern of the mobile antenna. To generalize the proposed theory, an additional analysis in lossy environment is carried out. The optimal transmitting current and efficiency bound in lossy media is found for a couple of examples. The results are compared with the previous works to verify the proposed theory. Based on the results in lossy media, the EM effects on the human body is investigated. Lastly, research on the effective implementation of the theoretical current distribution as practical antenna arrays is described. Based on the previous research, two techniques that can effectively realize the ideal current are proposed in designing a thinned array. An optimization using genetic algorithm, and deterministic density tapering are applied to sample the theoretical current distribution. As a results, the proposed thinned arrays show improved performance compared to the same number of densely arranged regular arrays. In particular, the use of density tapering has advantages in cost, weight, efficiency than the same number of the regular array. In addition, it is possible to transmit wireless power with better efficiency even when the position of the receiver changes.Chapter 1. Introduction 1 1.1. Classification of Wireless Power Transmission 1 1.2. Separation of Regions 3 1.3. Passive and Active Radiative-Wireless Power Transmission 6 1.4. References 14 Chapter 2. Passive: RF Energy Harvesting Antenna 18 2.1. Motivation 18 2.2. Analytical Study on RF Energy Harvesting Antenna 19 2.2.1. Previous Research 19 2.2.2. Analysis on Split-Ring Resonator 21 2.2.3. Analysis on the Symmetric Folded Split-Ring Resonator 25 2.2.4. Analysis on the Asymmetric Folded Split-Ring Resonator 30 2.3. Design of RF Energy Harvesting Antenna 34 2.3.1. Antenna Design 37 2.3.2 Results and Discussion 44 2.4. Design of RF Energy Harvesting Antenna with Dual-band Operation 45 2.4.1. Motivation 45 2.4.2 Antenna Design 45 2.4.3. Results and Discussion 48 2.5. RF Energy Harvesting Antenna with Wide-band Operation 53 2.5.1. Motivation 53 2.5.2 Antenna Design 54 2.5.3. Results and Discussion 57 2.6. Conclusion 65 2.7. References 69 Chapter 3. Active: Radiative-WPT in Lossless Medium 73 3.1. Motivation 73 3.2. Previous Research 73 3.3. Theoretical Approach 77 3.3.1. Power Transfer Efficiency 77 3.3.2. Optimum Transmitting Current 80 3.3.3. Minimizing Transmitting Area 84 3.4. Numerical Examples 86 3.3.1. Dipole Antenna 88 3.3.2. Patch Antenna 90 3.3.3. Horn Antenna 91 3.5. Results and Discussion 93 3.5. Conclusion 98 3.6. References 99 Chapter 4. Active: Radiative-WPT in Lossy Media 103 4.1. Motivation 103 4.2. Previous Research 103 4.3. Theoretical Approach 106 4.3.1. Problem Formulation 108 4.3.2. Maximum Power Transfer Efficiency 110 4.4. Practical Examples 114 4.4.1. Planar Inverted-F Antenna 116 4.4.2. Half-Mode Cavity-Backed Antenna 120 4.5. Electromagnetic Human Exposure in Radiative WPT System 125 4.5.1. Motivation 125 4.5.2. Simulation Results 126 4.6. Conclusion 132 4.7. References 134 Chapter 5. Active: Implementation of Optimal Transmitting Current Distribution 138 5.1. Motivation 138 5.2. Theoretical Approach 139 5.2.1. Radiation Pattern Matching 139 5.2.2. Optimal Excitation Coefficient 141 5.2.3. Thinning of Transmitting Array 141 5.3. Implementation of the Optimal Current Sheet 145 5.3.1. Array Thinning using Genetic Algorithm 145 5.3.2. Results and Discussions 148 5.3.3. Array Thinning using Density Tapering 151 5.3.4. Results and Discussions 154 5.4. Conclusion 158 5.5. References 159Docto

Design Tools for Small Implantable antennas

Implantable applications have been increasing in number in an exponential manner since the turn of the millennium. This is largely due to the increasing availability of ultra low power consumption electronics, enabling the emergence of healthcare services using implantable or swallowable sensors. Other fields of applications exist, e.g. in the domains of military, security or sports. A large number of antennas for these implantable or swallowable capsules have been designed, and can be found in the literature. They are however always presented for a specific capsule and application, and are thus very difficult to compare in terms of antenna characteristics. In this paper, we show results obtained for simple canonical implantable radiation sources, obtained using a specifically developed numerical tool presented earlier. These canonical results will allow us to obtain some simple but useful figures of merit that enable an easy assessment of the quality of the radiation characteristic of a specific antenna design in a specific capsul

Fundamental Limits for Implanted Antennas: Maximum Power Density Reaching Free Space

Medical implants with communication capability are becoming increasingly popular with today’s trends to continuously monitor patient’s condition. This is a major challenge for antenna designers since the implants are inherently small and placed in a communication-wise very lossy environment. Our goal is to determine the fundamental limitations of such antennas when placed inside human bodies and to develop guidelines for most efficient design. We base our findings on in-house analysis tools based on spherical and cylindrical wave expansion applied to simplified spherical and cylindrical body models respectively. These give us insight into wave propagation and show the maximum power density levels that can be reached just outside the body. Based on the obtained limits we can propose a useful upper bound for more complex scenarios

Wireless powering efficiency assessment for deep-body implantable devices

Several frequency-dependent mechanisms restrict the maximum achievable efficiency for wireless powering implantable bioelectric devices. Similarly, many mathematical formulations have been proposed to evaluate the effect of these mechanisms as well as predict this maximum efficiency and the corresponding optimum frequency. However, most of these methods consider a simplified model, and they cannot tackle some realistic aspects of implantable wireless power transfer. Therefore, this paper proposed a novel approach that can analyze the efficiency in anatomical models and provide insightful information on achieving this optimum operation. First, this approach is validated with a theoretical spherical wave expansion analysis, and the results for a simplified spherical model and a bidimensional human pectoral model are compared. Results have shown that even though a magnetic receiver outperforms an electric one for near-field operation and both sources could be equally employed in far-field range, it is in mid-field that the maximum efficiency is achieved, with an optimum frequency between 1-5 GHz, depending on the implantation depth. In addition, the receiver orientation is another factor that affects the efficiency, with a maximum difference between the best and worst-case scenarios around five times for an electric source and over 13 times for the magnetic one. Finally, this approach is used to analyze the case of a wirelessly powered deep-implanted pacemaker by an on-body transmitter and to establish the parameters that lead to the maximum achievable efficiency

Doctor of Philosophy

dissertationThe invention of antennas has revolutionized the world by enabling fast communications over long distances. Since their invention in the 1800s, antennas have been extensively studied at RF and microwave frequencies. The high conductivity of metals coupled with operation in a relatively loss-free environment has fueled the rapid development of antennas designed to operate at particular frequency ranges. Evolution of science and technology has now led to the study of lossy antennas. The ability to create nanometer scale structures has allowed for the creation of antennas for operation at very high (optical) frequencies. At these frequencies, metals are no longer excellent conductors, and their lossiness must be considered in order to effectively design and create resonant antennas. In the RF, the fast development of applications for antennas has led to an artificial allocation of frequency bands to prevent interference. In the nanoworld, electromagnetic spectrum allocation is more natural and is dictated by the particular application. In both cases, the ability to apply antennas relies upon control of their resonances. Lossy materials contribute an additional variable to the creation and control of resonant structures. The purpose of this research is to study the effect of material losses in RF and optical antennas. The investigation is done via the study of implantable spiral antennas at RF frequencies and crescent nanoantennas at optical wavelengths

The admittance of the infinite cylindrical antenna in a lossy, isotropic, compressible plasma

Numerical values for admittance of infinite cylindrical antenna in lossy, isotropic, compressible plasm