| 1. |
- Algaba Brazalez, Astrid, 1983
(författare)
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Gap Waveguide Technology- Electromagnetic Packaging and Transitions
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The scarcity of available frequency bands for new wireless communication systems, as well as the growing need for higher capacity, motivates the current interest in moving upwards in frequency. The millimeter wave frequencyrange is very attractive to allocate potential commercial opportunities such as short range wireless communications that allow high-speed data/video transmission, and automotive radars for both driver assistance and increasedsafety.The shift towards higher frequencies involves significant challenges. For instance, the mechanical tolerance requirements become much stricter, which provokes the need for high precision fabrication and assembly processes.Moreover, in order to achieve highly functional and densely packed electronic modules, new waveguide technologies that can meet the demands of high interconnectivity and ensure optimum electromagnetic packaging, need to be developed. The gap waveguide technology constitutes a new type of guiding structure that shows strong potential to become a suitable millimeter wave approach, and overcome the limitations of conventional planar transmissionlines and hollow waveguides.This thesis presents recent investigations on gap waveguide technology, concerning packaging aspects and design of millimeter wave transitions. The first part of the thesis deals with an investigation of the electromagnetic packagingcapabilities of the gap waveguide. This study addresses the performance of a Ku-band microstrip coupled line filter when it is packaged in different ways: unpackaged situation, and packaged by using a: i) metal cover, ii) metal cover with absorbing material (i.e. the conventional approach), iii) Perfect Magnetic Conductor (PMC), and iv) gap waveguide packaging realized by using a lid of nails. The obtained results show that the best filter performanceis achieved when the microstrip circuit is packaged with a PMC, and a lid of nails. The PMC packaging condition is an ideal situation and can only be simulated, but it is still of great interest since it enables us to reduce simulation time compared to other packaging approaches. Also, the PMCrepresents a good approximation of the practical lid of nails. The lid of nails solution has been properly validated by both simulations and measurements.The second part of the thesis focuses on the design of millimeter wave transitions between different types of gap waveguide and classical waveguide or transmission lines. In order to achieve high integration between passivecomponents, active components and antennas in a gap waveguide module, good compatibility between gap waveguides and other technologies is critical. One of the reasons is that for integrating gap waveguide circuits with MonolithicMicrowave Integrated Circuits (MMIC), we need to develop suitable interconnections to microstrip and coplanar waveguide since the MMICs are substrate based. On the other hand, the interfaces of millimeter wave measurement instruments are either standard coaxial connectors, coplanar waveguide probes or rectangular waveguides. The transitions are therefore necessary in order to enable measurements of gap waveguide prototypes at highfrequencies. We present two different F-band transition designs between ridge gap waveguide and coplanar waveguide/microstrip, and a V-band transition from inverted microstrip gap waveguide to rectangular waveguide. Duringthe experimental validation of these transitions, we have identified different mechanical limitations involved in the assembly process of the gap waveguide prototypes. The assembly has an impact on the gap waveguide performance,and we have analyzed methods to mitigate the effect on the measurements.
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| 2. |
- Akiyama, Eiji, et al.
(författare)
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SPIRAL STRUCTURE AND DIFFERENTIAL DUST SIZE DISTRIBUTION IN THE LkH alpha 330 DISK
- 2016
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 152:6
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Tidskriftsartikel (refereegranskat)abstract
- Dust trapping accelerates the coagulation of dust particles, and, thus, it represents an initial step toward the formation of planetesimals. We report H-band (1.6 mu m) linear polarimetric observations and 0.87 mm interferometric continuum observations toward a transitional disk around LkH alpha 330. As a. result, a pair of spiral arms were detected in the H-band emission, and an asymmetric (potentially arm-like) structure was detected in the 0.87 mm continuum emission. We discuss the origin of the spiral arm and the asymmetric structure. and suggest that a massive unseen planet is the most plausible explanation. The possibility of dust trapping and grain growth causing the asymmetric structure was also investigated through the opacity index (beta) by plotting the observed spectral energy distribution slope between 0.87 mm from our Submillimeter Array observation and 1.3 mm from literature. The results imply that grains are indistinguishable from interstellar medium-like dust in the east side (beta = 2.0 +/- 0.5) but are much smaller in the west side beta = 0.7(-0.4)(+0.5), indicating differential dust size distribution between the two sides of the disk. Combining the results of near-infrared and submillimeter observations, we conjecture that the spiral arms exist at the upper surface and an asymmetric structure resides in the disk interior. Future observations at centimeter wavelengths and differential polarization imaging in other bands (Y-K) with extreme AO imagers are required to understand how large dust grains form and to further explore the dust distribution in the disk.
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| 3. |
- Akiyama, Kazunori, et al.
(författare)
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First Sagittarius A * Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 106 M☉, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 103-105 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.
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| 4. |
- Akiyama, Kazunori, et al.
(författare)
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First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of similar to 50 mu as, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.
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| 5. |
- Akiyama, Kazunori, et al.
(författare)
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First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a "variability noise budget" in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of similar to 50 mu as, consistent with the expected "shadow" of a 4 x 10(6) M (circle dot) black hole in the Galactic center located at a distance of 8 kpc.
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| 6. |
- Kim, Jeong-Lim, et al.
(författare)
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Fish consumption and school grades in Swedish adolescents: a study of the large general population.
- 2010
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Ingår i: Acta Paediatrica. - 1651-2227. ; 99, s. 72-7
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Tidskriftsartikel (refereegranskat)abstract
- AIM: To study the associations between fish intake and academic achievement as cognitive parameter among Swedish adolescents. METHODS: In 2000, a questionnaire including respiratory items, socioeconomic conditions and dietary information was mailed to all schoolchildren (n = 18 158), aged 15 and living in Västra Götaland region of Sweden. The questionnaire was returned by 10 837 subjects. One year later, the total school grades for each subject who had completed the questionnaire and who included their full personal identification number were obtained from the national registers. Multiple linear regression models were applied to evaluate the association between fish intake and academic grades among 9448 schoolchildren, while adjusting for potential confounders, e.g. parents' education. RESULTS: Grades were higher in subjects with fish consumption once a week compared with subjects with fish consumption of less than once a week (reference group) [increment in estimate 14.5, 95% confidence interval (CI) 11.8-17.1]. Grades were even higher in subjects with fish consumption of more than once a week compared with the reference group (increment in estimate 19.9, 95% CI 16.5-23.3). In the model stratified for parents' education, there were still higher grades among subjects with frequent fish intake in all educational strata (p < 0.01). CONCLUSION: Frequent fish intake among schoolchildren may provide benefits in terms of academic achievement.
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| 7. |
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| 8. |
- Georgiadis, Georgios, 1981, et al.
(författare)
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Multi-agent distributed method for dynamic power system rebalancing
- 2015
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Ingår i: IEEE PES Innovative Smart Grid Technologies Conference Europe. ; 2015-January:January
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Konferensbidrag (refereegranskat)abstract
- Global efforts aim at utilizing the available information technology in order to enhance the power grid with resilience properties, precise accounting and new services. Towards this end, we provide methods to support an adaptive power grid and study the relative changes in supply and/or demand while in a quasi-steady state. Acknowledging that the absolute amount of the power demand/supply is not always the limiting factor, the proposed algorithm focuses on the deviations in the generated and/or demanded resources, as well as the location of these changes in the grid. Through our experimental evaluation study we show that the algorithm leads to a faster reconvergence to a new steady state operating point when compared to convergence from initial conditions after changes complete. We also show that changes can be resolved in a distributed and local manner, needing only limited and fast communication with nodes that are far from the locus of change.
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