Microwavephysics and Atmospheric Physics
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Last update: 15.06.2017
HS 2015: Seminare über Microwavephysics and Atmospheric Physics
Friday 10-12
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Mittwoch, 09.09.2015

Promotionsvortrag
The diurnal variation in middle-atmospheric ozone: simulation and intercomparison

Zeit: 10:00 Uhr
Hörsaal: 119
 
Ansgar Schanz
Institute of Applied Physics
University of Bern

 
Freitag, 18.09.2015

Development of Low-Noise and Power Amplifiers for Millimeter Wave and Terahertz Array Receivers

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Mikko Kotiranta
Fraunhofer-Institut für Angewandte Festkörperphysik
Freiburg im Breisgau, Germany

Heterodyne array receivers are employed in radio astronomy to reduce the observing time needed for mapping extended sources. For attaining the lowest receiver noise temperature at observation frequencies up to about 100 GHz the signal is amplified with a high electron mobility transistor (HEMT) low-noise amplifier (LNA) before being converted to lower frequencies. Monolithic microwave integrated circuit (MMIC) technology is ideal for array receivers with a large number of pixels as it allows manufacturing of LNAs in large numbers with a uniform performance.

At frequencies higher than 100 GHz the best receiver performance is obtained by placing a SIS-mixer as the first active component in the receiver chain. One of the main factors limiting the number of pixels in receivers at terahertz frequencies is the difficulty of generating a sufficient amount of local oscillator (LO) power. Another challenge is efficient diplexing and coupling of the LO and signal power to the mixers.

First, the performance of a state-of-the-art metamorphic HEMT MMIC low-noise amplifier for the band 60-120 GHz is presented. Then the problems related to local oscillator power are addressed by proposing the application of a vacuum electronic terahertz power amplifier for the amplification of the LO-signal and by introducing a method combining ray-tracing simulations and measurements for finding the defects in a quasioptical diplexer.

 
Freitag, 02.10.2015

Ein Tag unter Strom

Zeit: 00:00 Uhr
Hörsaal:
 
Fachexkursion Grimsel

 
Freitag, 16.10.2015

Equatorial measurements of CO2, CH4 and other major greenhouse gases in the Total Carbon Column Observing Network (TCCON)

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Dietrich Feist
Max Planck Institute for Biogeochemistry
Jena, Germany

In 2012, the first equatorial station of the Total Carbon Column Observing Network (TCCON) was established on Ascension Island. During daytime, this station provides continuous total-column averaged dry-air mole fraction of all major greenhouse gases like H2O, CO2, CH4, CO, N2O, and others.

The station is located strategically between Africa and South America. The total-column measurements cover the whole atmosphere from the ground to the top of the mesosphere. Since Ascension Island is a small island with only sparse vegetation and negligible anthropogenic emissions, most of the observed changes in trace gas columns are expected to be the result of long-range transport of air masses from central Africa (most of the time) or the Amazon region (occasionally). At the same time, Ascension Island is one of the few locations where satellite glint observations by GOSAT or OCO-2 can be validated on a regular basis.

Numerous technical problems had to be solved before a continuous data stream of high quality could be provided. The time series is now long enough to provide a first evaluation of short-term events like biomass burning as well as seasonal patterns in tropical atmospheric transport.

 
Freitag, 23.10.2015

Numerical Analysis and Optimization of Conical Calibration Targets for Microwave Radiometers

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Arne Schröder
Institute of Applied Physics
University of Bern

This talk presents results from numerical studies of conical calibration targets for the Sub-millimeter Wave Instrument (SWI) on board the Jupiter Icy Moon Explorer (JUICE). The use of well-designed calibration targets in radiometers is of particular importance as the target's microwave emissivity directly impacts the accuracy of calibration. Therefore, analysing the emissivity plays a key role in the design process. In the presented work, we've evaluated different numerical approaches to assess the electromagnetic performance of conical targets. As the application of full-wave approaches, such as the finite element method, is restricted to comparatively low frequencies, we developed an efficient asymptotic technique which makes fast electromagnetic investigations feasible even at high frequencies. Based on the introduced approaches, a novel way for improving the emissivity of conical targets has been proposed. It is demonstrated that cone targets with advanced profiles exhibit a significant higher emissivity than conventional cone targets while having similar dimensions.

 
Freitag, 30.10.2015

Characterization of the SWI 530 - 625 GHz Receiver Chain for the Jupiter Mission JUICE

Zeit: 10:15 Uhr
Hörsaal: A97
 
Karl Jacob
Institute of Applied Physics
University of Bern

This talk will give an overview of the current development status of the Submillimetre Wave Instrument (SWI) for the Jupiter Icy Moons Explorer mission (JUICE) of the European Space Agency (ESA). The SWI is a passive radiometer, in the current baseline configuration consisting of two orthogonally polarized heterodyne receivers which are independently tunable over a frequency range from 530GHz to 625GHz. Its main scientific objectives are the mapping of the chemical composition, temperatures and winds in the stratosphere of Jupiter and the exosphere of its Galilean moons Callisto, Europa and Ganymede. The SWI receivers will be passively cooled to about 150K with a cold space radiator in order to improve their sensitivity. In particular this talk will focus on the SWI 600GHz receiver chain and present results of the first performance measurements covering the entire SWI bandwidth at room and cryogenic temperature operation. In this context the difficulties with the highly variable local oscillator power within the bandwidth, which needs a careful tuning of the bias voltages of the multipliers in order to optimize the noise temperature and to avoid damage of the mixers are pointed out.

 
Freitag, 06.11.2015

WIRA-C: the new wind radiometer for campaigns
(Master's thesis presentation)

Zeit: 10:15 Uhr
Hörsaal: A97
 
Jonas Hagen
Institute of Applied Physics
University of Bern

 
Freitag, 13.11.2015

The solar rotational cycle detection in the stratosphere and mesosphere

Zeit: 10:15 Uhr
Hörsaal: A97
 
Timofei Sukhodolov
PMOD/WRC
Physikalisch-Meteorologisches Observatorium
Davos

Analysis of the middle atmospheric response to solar irradiance variability at rotational time scales benefits from the availability of observational data covering large number of cycles. This facilitates a robust statistical analysis and therefore can contribute to understanding the mechanisms behind the solar influence on the atmosphere at longer timescales. The response of the short-lived species in the tropical mesosphere and stratosphere to solar UV irradiance variability can be clearly identified in both satellite observations and model simulations. However, the stratospheric ozone and temperature response is not so robust, since it is highly modulated by internal variability modes. In my talk I will present the analysis of observational data as well as the results of modelling experiments covering different aspects of the middle atmosphere variability on the intra-annual time scale.

 
Freitag, 27.11.2015

Cloud radiative effect depending on cloud type and cloud fraction at three sites in Switzerland

Zeit: 10:15 Uhr
Hörsaal: A97
 
Christine Aebi
PMOD/WRC
Physikalisch-Meteorologisches Observatorium
Davos, and Institute of Applied Physics
University of Bern

Radiative transfer of energy in the atmosphere and the influence of clouds on the radiation budget remain the greatest sources of uncertainty in the simulation of climate change. Small changes in cloudiness and radiation can have large impacts on the Earth’s climate. The role of clouds on the surface radiation budget is studied in order to quantify the longwave and shortwave cloud radiative effect (CRE) depending on atmospheric composition, cloud cover and cloud type. This study is performed for three different sites in Switzerland (Davos, Payerne and Jungfraujoch). Data are mainly taken from visible all-sky cameras, pyranometers, pyrgeometers and a ceilometer. My talk will be divided into two parts: In a first part I will present a climatology of the longwave and shortwave CRE and its sensitivity to different parameters for the three aforementioned stations separately. In a second part I will talk about a new instrument developed in-house at PMOD/WRC, the all-sky thermal infrared cloud cam (IRCCAM). This new instrument will enable nighttime measurements and analyses of clouds as well.

 
Freitag, 04.12.2015

Simulating Supercells in Midlatitudes using WRF
(Master's thesis presentation)

Zeit: 10:15 Uhr
Hörsaal: A97
 
Sascha Boetzel
Institute of Applied Physics
University of Bern

 
Freitag, 11.12.2015

Presentation of Master projects: Molecular conductance studied by the MCBJ and STMBJ techniques

The electrodeposition of nanowire arrays made of the n-type semiconductor Bi2.00Te2.70Se0.30 for the fabrication of a thermoelectric microdevice

Zeit: 10:15 Uhr
Hörsaal: A97
 
David Reber und Patrik Schürch
Departement für Chemie

The analysis of single molecular conductance features is an important step towards understanding the processes underlying charge transport in molecular electronics. The countless possibilities of adequate design and synthesis of molecules allow the investigation of an abundance of processes and phenomena. Employing the mechanically controllable- and the electrochemically gated scanning tunnelling microscope break junction techniques (MCBJ and EC STMBJ), we were able to show that intermolecular interactions can have a great influence on molecular conductance. For certain molecular systems stacked junctions exhibited much higher conductance features than would have been expected for single molecules of that type. Also the influence of a nitrogen heteroatom on molecular conductance in an aromatic compound has systematically been studied on a benzene/pyridine system. The experimental results perfectly reflect the trend expected from transmission calculations. They demonstrate how, depending on its position in the molecule, a heteroatom can have a strong influence on single molecular conductance. Lastly, effective photochromic switching of a modified diarylethene type molecule was demonstrated platform-independently. A clear shift towards higher conductance was observed upon illumination with UV light.

Scientific interest in creating thermoelectric generators has been revitalized by the promise of the introduction of nanostructures in such devices. In this work, highly oriented, high density nanowire arrays made from the n-type semiconductor Bi2.00Te2.70Se0.30  were produced by electrodeposition. Thermoelectric measurement showed that nanowire arrays exhibited Seebeck coefficients of > 51.3 µV/K. Electrical resistance of the entire arrays were measured to be > 0:219Ohm with a peak power output of 380 nW.