Browsing by Department "Department of Radio Science and Engineering"
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Item The 2009 multiwavelength campaign on Mrk 421: Variability and correlation studies(2015) , MAGIC Collaboration; , VERITAS Collaboration; Metsähovi Radio Observatory; Department of Radio Science and Engineering; Anne Lähteenmäki GroupAims: We perform an extensive characterization of the broadbandemission of Mrk 421, as well as its temporal evolution, during thenon-flaring (low) state. The high brightness and nearby location (z =0.031) of Mrk 421 make it an excellent laboratory to study blazaremission. The goal is to learn about the physical processes responsiblefor the typical emission of Mrk 421, which might also be extended toother blazars that are located farther away and hence are more difficultto study. Methods: We performed a 4.5-month multi-instrumentcampaign on Mrk 421 between January 2009 and June 2009, which includedVLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC, and Whipple,among other instruments and collaborations. This extensive radio tovery-high-energy (VHE; E> 100 GeV) γ-ray dataset providesexcellent temporal and energy coverage, which allows detailed studies ofthe evolution of the broadband spectral energy distribution. Results: Mrk421 was found in its typical (non-flaring) activity state,with a VHE flux of about half that of the Crab Nebula, yet the lightcurves show significant variability at all wavelengths, the highestvariability being in the X-rays. We determined the power spectraldensities (PSD) at most wavelengths and found that all PSDs can bedescribed by power-laws without a break, and with indices consistentwith pink/red-noise behavior. We observed a harder-when-brighterbehavior in the X-ray spectra and measured a positive correlationbetween VHE and X-ray fluxes with zero time lag. Such characteristicshave been reported many times during flaring activity, but here they arereported for the first time in the non-flaring state. We also observedan overall anti-correlation between optical/UV and X-rays extending overthe duration of the campaign. Conclusions: Theharder-when-brighter behavior in the X-ray spectra and the measuredpositive X-ray/VHE correlation during the 2009 multi-wavelength campaignsuggests that the physical processes dominating the emission duringnon-flaring states have similarities with those occurring during flaringactivity. In particular, this observation supports leptonic scenarios asbeing responsible for the emission of Mrk 421 during non-flaringactivity. Such a temporally extended X-ray/VHE correlation is not drivenby any single flaring event, and hence is difficult to explain withinthe standard hadronic scenarios. The highest variability is observed inthe X-ray band, which, within the one-zone synchrotron self-Comptonscenario, indicates that the electron energy distribution is mostvariable at the highest energies.Appendix A is available in electronic form at http://www.aanda.orgThecomplete data set shown in Fig. 1 is only available at the CDS viaanonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/576/A126Item 94 GHz pulsed coherent radar for high power amplifier evaluation(SPIE, 2016) Robertson, Duncan A.; Hunter, Robert I.; Gallacher, Thomas F.; Department of Radio Science and EngineeringWe present the design and characterization of a 94 GHz pulsed coherent radar to be used for the evaluation and demonstration of novel wideband, high power vacuum tube amplifier technology. The radar is designed to be fully coherent and exploits a low phase noise architecture to maximize Doppler performance. We selected to use horn-fed Fresnel zone plate lens antennas (FZPs) with 4-level phase quantization as a low cost method of realizing large aperture (0.5 m) antennas. The measured performance of these FZPs agrees closely with the design predictions and exceeds that obtainable with a Cassegrain of an equivalent size.Item Aalto-1 -satelliitin spektrikamerateknologian validointi avaruusympäristöön(2013-05-20) Näsilä, Antti; Viherkanto, Kai; Department of Radio Science and Engineering; Radiotieteen ja -tekniikan laitos; Sähkötekniikan korkeakoulu; Hallikainen, MarttiTässä työssä suoritettiin Aalto-1 -nanosatelliitin spektrikameran pietso-aktuoidun Fabry–Perot Interferometrin avaruuskvalifiointi. Spektrikamera tulee olemaan Aalto-1 -satelliitin päähyötykuorma. Avaruuskvalifiointia varten suunniteltiin ja toteutettiin avaruustestikampanja, joka sisälsi lämpösyklauksen lämpökaapissa, tärinä- ja shokkitestit, taajuus- ja askelvasteiden mittaukset tyhjiössä. Lopuksi suoritettiin myös sarja testejä termovakuumikammiossa. Fabry–Perot Interferometrin suorituskyvylle asetettiin seuraavat spesifikaatiot: läpäisy 0.1–0.3, spektraalinen resoluutio 10–30 nm, spektrinen vakaus 0.2 nm/°C sekä säätönopeus < 30 ms. Tämän lisäksi vaaditiin myös selviytymistä lämpötila- (-45–65°C) ja laukaisuympäristöistä. Muutamien korjausten jälkeen Fabry–Perot -moduuli läpäisi kaikki kvalifikaatiotestit ja spesifikaatiot täytettiin, joten moduuli on kvalifioitu avaruusolosuhteisiin onnistuneesti.Item Aalto-1 nanosatelliitin S-kaistan lähetin(2013-06-10) Jussila, Jaakko; Holopainen, Jari; Department of Radio Science and Engineering; Radiotieteen ja -tekniikan laitos; Sähkötekniikan korkeakoulu; Viikari, VilleTässä työssä esitellään Aalto-1 satelliitin S-kaistan lähettävän radion suunittelu. Aalto-1 on CubeSat-standardiin perustuva kaukokartoitussatelliitti, jossa on hyötykuormina kolme tieteellistä instrumenttia: korkearesoluutioinen AaSI sepktrikamera, säteilymittari ja plasmajarru. Spektrikamera tuottaa informaatiota jopa useita kymmeniä megatavuja yhden kuvauksen aikana, joten lähettimeltä vaaditaan suurta tiedonsiirtonopeutta. Työssä määritellään vaatimukset, jotka perustuvat CubeSat-satelliittien tehorajoituksiin, avaruusolosuhteiden haasteisiin sekä linkkibudjettisimulaatioihin. Lähetin rakennetaan kaupallisista komponteista, jotka on valittu edellä määritettyjen rajoitteiden perusteella. Lähettimen topologia perustuu Texas Instruments:n CC2500 lähetin-vastaanottimeen, RFMD:n RF5602 tehovahvistimeen ja Texas Instuments:n MSP430 mikro-ohjaimeen. RF-sovituspiirit suunnitellaan käyttäen RF-piirisimulaattoreita ja toteutetaan mikroliuskatekniikalla. Lähettimelle suunniteltiin ensimmäinen prototyyppilevy sekä kehitettiin mikro-ohjaimen ohjelmiston ensimmäinen versio. Prototyypin ensimmäiset funktionaaliset testit osoittavat, että suunniteltu lähettimen rakenne pystyy täyttämään sille asetetut vaatimukset. RF-sovituspiirien testit osoittavat, että sovituspiirien suunnitelmat vaativat jatkokehitystä, jotta lähetin pystyy tuottamaan lähetyksessä tarvittavan tehotason.Item Active and passive dielectric rod waveguide components for millimetre wavelengths(Aalto-yliopiston teknillinen korkeakoulu, 2010) Pousi, Patrik; Lioubtchenko, Dmitri, Doc.; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Aalto-yliopiston teknillinen korkeakoulu; Räisänen, Antti, Prof.In this thesis, active and passive dielectric rod waveguide (DRW) components have been studied and developed for millimetre wavelengths. DRW antennas made of relatively high permittivity materials like silicon and sapphire are designed and simulated with HFSS (a commercial electromagnetic structure simulator based on Finite Element Method) at frequencies up to 325 GHz. A prototype antenna for D band (110-170 GHz) has been fabricated and measured. The location of the phase centre of an antenna is an important parameter in many applications, especially when the antenna is used as a feed for a reflector. This location of the phase centre for the DRW antenna has been studied with different methods from measurement results obtained with a planar scanner at W band. A high-permittivity DRW antenna is an interesting candidate for an antenna array element. According to W-band simulations and measurements the mutual coupling between the elements is relatively low even with the distance of ∼λ ⁄ 2 as the element spacing. This makes the DRW an appealing alternative as an element for densely packed arrays. However, mutual coupling studies for antenna arrays revealed a very strong coupling phenomenon when the distance between the elements is small. At some point the power is transferred completely from the excited waveguide to the neighbouring waveguide. This phenomenon is similar to cross-talk in optical fibres. In that regime cross-talk has been utilised in many applications, e.g. in directional couplers. In this thesis a frequency selective coupler based on that strong coupling has been developed. Frequency selectivity is based on the fact, that the waveguide length required for complete power transfer depends on the frequency. The length that is required for the power to transfer from one waveguide to the other and back is also called as the beat length. A prototype coupler has been simulated, manufactured and measured at W band. The results are very promising for this type of components made of DRWs for millimetre wavelengths. Different types of DRW junctions for power division have also been studied. Such junctions can be used also for monitoring the propagating power in a DRW. Junctions can be designed for both E and H planes depending on the application. Symmetric and asymmetric Y-type junctions have been designed and studied with simulations in both planes. Active components studied in this thesis include a travelling-wave amplifier based on GaAs/AlGaAs heterostructure and a DRW phase shifter designed for ferroelectric varactors fed through Au strips, both designed for W band. A DRW travelling-wave amplifier is based on the interaction between an electron drift and an electromagnetic wave travelling in a periodic structure. Electronic gain of 10 dB/cm at 150 V/cm has been measured at 70-80 GHz. The proposed novel prototype of millimetre-wave phase shifter includes a dielectric rod waveguide with a periodic printed array of electrically small dipoles loaded with ferroelectric varactors. Measurement results of a non-tunable phase shifter prototype show that optimally a phase shift of 60 deg/dB can be obtained at W band.Item All-dielectric reciprocal bianisotropic nanoparticles(2015) Alaee, Rasoul; Albooyeh, Mohammad; Rahimzadegan, Aso; Mirmoosa, Mohammad S.; Kivshar, Yuri S.; Rockstuhl, Carsten; Department of Radio Science and EngineeringThe study of high-index dielectric nanoparticles currently attracts a lot of attention. They do not suffer from absorption but promise to provide control of the properties of light comparable to plasmonic nanoparticles. To further advance the field, it is important to identify versatile dielectric nanoparticles with unconventional properties. Here, we show that breaking the symmetry of an all-dielectric nanoparticle leads to a geometrically tunable magnetoelectric coupling, i.e., an omega-type bianisotropy. The suggested nanoparticle exhibits different backscatterings and, as an interesting consequence, different optical scattering forces for opposite illumination directions. An array of such nanoparticles provides different reflection phases when illuminated from opposite directions. With a proper geometrical tuning, this bianisotropic nanoparticle is capable of providing a 2π phase change in the reflection spectrum while possessing a rather large and constant amplitude. This allows the creation of reflectarrays with near-perfect transmission out of the resonance band due to the absence of a usually employed metallic screen.Item Analyses of Impact of Needle Surface Properties on Estimation of Needle Absorption Spectrum: Case Study with Coniferous Needle and Shoot Samples(2016-07) Yang, Bin; Knyazikhin, Yuri; Lin, Yi; Yan, Kai; Chen, Chi; Park, Taejin; Choi, Sungho; Mottus, Matti; Rautiainen, Miina; Myneni, Ranga B.; Yan, Lei; Department of Built Environment; Department of Radio Science and Engineering; Geoinformatics; Boston University; Peking University; Beijing Normal University; University of HelsinkiLeaf scattering spectrum is the key optical variable that conveys information about leaf absorbing constituents from remote sensing. It cannot be directly measured from space because the radiation scattered from leaves is affected by the 3D canopy structure. In addition, some radiation is specularly reflected at the surface of leaves. This portion of reflected radiation is partly polarized, does not interact with pigments inside the leaf and therefore contains no information about its interior. Very little empirical data are available on the spectral and angular scattering properties of leaf surfaces. Whereas canopy-structure effects are well understood, the impact of the leaf surface reflectance on estimation of leaf absorption spectra remains uncertain. This paper presents empirical and theoretical analyses of angular, spectral, and polarimetric measurements of light reflected by needles and shoots of Pinus koraiensis and Picea koraiensis species. Our results suggest that ignoring the leaf surface reflected radiation can result in an inaccurate estimation of the leaf absorption spectrum. Polarization measurements may be useful to account for leaf surface effects because radiation reflected from the leaf surface is partly polarized, whereas that from the leaf interior is not.Item Anisotropy in electrostatics - Solutions for inclusions with canonical shapes(Aalto University, 2016) Rimpiläinen, Tommi; Wallén, Henrik, Dr., Aalto University, Department of Radio Science and Engineering, Finland; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Sihvola, Ari, Prof., Aalto University, Department of Radio Science and Engineering, FinlandInclusions that take a canonical shape have a special place in electrostatics. Their analysis involves methods with great mathematical subtlety and rigor. Where an arbitrarily shaped inclusion can only be solved with a method that involves approximation, some of the canonical shapes permit an approach that leads to an exact solution in a closed form. What makes canonical shapes special, is that they have a particularly simple presentation in an important class of coordinate systems. There are 11 orthogonal coordinate systems where the coordinate curves are defined by equations of second degree. In these 11 systems, the Laplace equation is separable. The particular canonical shapes that the thesis studies are spheres, spheroids, and ellipses. In their corresponding coordinate systems, they correspond to surfaces with a fixed coordinate. Because these three systems allow the Laplace equation to separate, much work can be done analytically before involving approximation or computation. Some of the presented solution in the thesis are exact and others are semi-analytical. Although solutions for inclusions with the three canonical shapes exist in the literature, the thesis goes beyond the existing work by generalizing the materials of the inclusions. In all of the enclosed articles, the material of the inclusion is assumed to be anisotropic and inhomogeneous. More specifically, it is assumed that the axes of anisotropy correspond to the coordinate unit vectors of the relevant coordinate system. The term radial anisotropy has been used in the literature to refer to anisotropy in the spherical coordinates when the tangential components are equal but possibly differ from the normal component. The thesis employs a similar concept in spheroidal and elliptic systems. In the spherical coordinate system, the thesis relaxes the assumption that the two tangential components must be equal. The resulting inclusions is referred to as the systropic sphere.Item Anomalous absorption, plasmonic resonances, and invisibility of radially anisotropic spheres(American Geophysical Union, 2015) Wallén, Henrik; Kettunen, Henrik; Sihvola, Ari; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThis article analyzes the response of a sphere with radially anisotropic permittivity dyadic (RA sphere), in both the electrostatic and full electrodynamic settings. Depending on the values and signs of the permittivity components, the quasistatic polarizability of the RA sphere exhibits several very different interesting properties, including invisibility, field concentration, resonant singularities, and emergent losses. Special attention is given to the anomalous losses that appear in the case of certain hyperbolic anisotropy values. We show that their validity can be justified in a limiting sense by puncturing the sphere at the origin and adding a small imaginary part into the permittivity components. A hyperbolic RA sphere with very small intrinsic losses can thus have significant effective losses making it an effective absorber. With different choices of permittivities, the RA sphere could also perform as a cloak or a sensor. The Mie scattering results by an RA sphere are used to justify the quasistatic calculations. It is shown that in the small parameter limit the absorption efficiency of an RA sphere is nonzero for certain lossless hyperbolic anisotropies. The absorption and scattering efficiencies agree with the quasistatic calculations fairly well for spheres with size parameters up to 1/3.Item An antenna model for the Purcell effect(2015) Krasnok, Alexander E.; Slobozhanyuk, Alexey P.; Simovski, Constantin R.; Tretyakov, Sergei A.; Poddubny, Alexander N.; Miroshnichenko, Andrey E.; Kivshar, Yuri S.; Belov, Pavel A.; Department of Radio Science and Engineering; Sergei Tretiakov Group; Kostantin Simovski GroupThe Purcell effect is defined as a modification of the spontaneous emission rate of a quantum emitter at the presence of a resonant cavity. However, a change of the emission rate of an emitter caused by an environment has a classical counterpart. Any small antenna tuned to a resonance can be described as an oscillator with radiative losses, and the effect of the environment on its radiation can be modeled and measured in terms of the antenna radiation resistance, similar to a quantum emitter. We exploit this analogue behavior to develop a general approach for calculating the Purcell factors of different systems and various frequency ranges including both electric and magnetic Purcell factors. Our approach is illustrated by a general equivalent scheme, and it allows resenting the Purcell factor through the continuous radiation of a small antenna at the presence of an electromagnetic environment.Item The atmosphere of comet 67P/Churyumov-Gerasimenko diagnosed by charge-exchanged solar wind alpha particles(2016-03-01) Simon Wedlund, C.; Kallio, E.; Alho, Markku; Nilsson, H.; Stenberg Wieser, G.; Gunell, H.; Behar, Etienne; Pusa, Janne; Gronoff, G.; Department of Radio Science and Engineering; Esa Kallio Group; Luleå University of Technology; Swedish Institute of Space Physics; Royal Belgian Institute for Space Aeronomy; SSAIContext. The ESA/Rosetta mission has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, measuring its dayside plasma environment. The ion spectrometer onboard Rosetta has detected two ion populations, one energetic with a solar wind origin (H+, He2+, He+), the other at lower energies with a cometary origin (water group ions such as H2O+). He+ ions arise mainly from charge-exchange between solar wind alpha particles and cometary neutrals such as H2O. Aims. The He+ and He2+ ion fluxes measured by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA) give insight into the composition of the dayside neutral coma, into the importance of charge-exchange processes between the solar wind and cometary neutrals, and into the way these evolve when the comet draws closer to the Sun. Methods. We combine observations by the ion spectrometer RPC-ICA onboard Rosetta with calculations from an analytical model based on a collisionless neutral Haser atmosphere and nearly undisturbed solar wind conditions. Results. Equivalent neutral outgassing rates Q can be derived using the observed RPC-ICA He+/He2+ particle flux ratios as input into the analytical model in inverse mode. A revised dependence of Q on heliocentric distance Rh in AU is found to be Rh -7.06Rh-7.06 between 1.8 and 3.3 AU, suggesting that the activity in 2015 differed from that of the 2008 perihelion passage. Conversely, using an outgassing rate determined from optical remote sensing measurements from Earth, the forward analytical model results are in relatively good agreement with the measured RPC-ICA flux ratios. Modelled ratios in a 2D spherically-symmetric plane are also presented, showing that charge exchange is most efficient with solar wind protons. Detailed cometocentric profiles of these ratios are also presented. Conclusions. In conclusion, we show that, with the help of a simple analytical model of charge-exchange processes, a mass-capable ion spectrometer such as RPC-ICA can be used as a "remote-sensing" instrument for the neutral cometary atmosphere.Item Balanced and optimal bianisotropic particles: maximizing power extracted from electromagnetic fields(2013) "Ra'di", Younes; Tretyakov, Sergei; Department of Radio Science and Engineering; Sergei Tretiakov GroupItem Becoming an electrical engineer: a mixed methods study of electrical engineers' studies and career(Aalto University, 2013) Keltikangas, Kirsti; Ropo, Eero, Professor, University of Tampere, Finland; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Sihvola, Ari, Professor, Aalto University, FinlandEngineers are regarded as analytic problem-solvers and experts in their specific technical fields. This substantive expertise is acquired during engineering studies, whereas personal and professional competences are largely adopted after graduation. The on-going changes in society require that such qualifications be learnt faster, and thereby technical universities need to ‘produce’ graduates with better qualifications for working life. Yet, there is not enough knowledge of how engineers gain the seeds in expertise and professional competences in higher engineering studies. Furthermore, universities need more research-based data on ways which professional competences could more effectively be integrated into the curriculum design. This research examines electrical engineers, their education and careers. In particular, it describes experiences that electrical engineers have gained from their studies, expertise, professional competences, and careers in working life. The data for the study were collected and analysed with a mixed methods approach. Quantitative data were collected by means of survey questionnaires. The first survey (N=99) was sent to engineers graduated from the Department of Electrical and Communications Engineering at Helsinki University of Technology. The students at the Department (N=120) whose studies had been delayed, comprised the sample in the second survey. Finally, 12 interviews of graduated electrical engineers were conducted and turned into narrative stories with a qualitative, narrative approach. The results show that a foundation in engineering knowledge is built during the university studies, when students in general adapt an engineering mindset and identity. Such learning draws from their scientific studies, for instance, in mathematics and physics. The findings suggest that the respondents in the surveys and in the narrative stories were generally satisfied with their studies at the Department. Still, they criticised mass lecturing and claimed that teaching should have been more interactive. They also expressed that students could be regarded more as individuals. The narrative stories reveal that electrical engineers in working life become experts but not necessarily in electrical engineering. They acquired new competences e.g. in financial management, HR, or budgeting, and currently manage a group of experts in their industry. The results indicate there is a need to enhance knowledge of pedagogies incorporating expertise and professional competencies more systematically into the engineering education.Item Bimodal radio variability in OVRO-40 m-monitored blazars(2017-06) Liodakis, I.; Pavlidou, V.; Hovatta, T.; Max-Moerbeck, W.; Pearson, T. J.; Richards, J. L.; Readhead, A.C.S.; Metsähovi Radio Observatory; Department of Radio Science and Engineering; University of Crete; Max-Planck-Institut für Radioastronomie; California Institute of TechnologyBlazars are known to show periods of quiescence followed by outbursts visible throughout the electromagnetic spectrum. We present a novel maximum likelihood approach to capture this bimodal behaviour by examining blazar radio variability in the flux-density domain. We separate quiescent and flaring components of a source's light curve by modelling its flux-density distribution as a series of 'off'- and 'on'-states. Our modelling allows us to extract information regarding the flaring ratio, duty cycle, and the modulation index in the 'off'state, in the 'on'- state, as well as throughout the monitoring period of each blazar. We apply our method to a flux-density-limited subsample from the Owens Valley Radio Observatory's 15 GHz blazar monitoring programme, and explore differences in the variability characteristics between BL Lacs and FSRQs as well as between gamma-ray detected and non-detected sources. We find that (1) BL Lacs are more variable and have relatively larger outbursts than the FSRQs; (2) unclassified blazar candidates in our sample show similar variability characteristics as the FSRQs and (3) gamma-ray detected differ from the gamma-ray non-detected sources in all their variability properties, suggesting a link between the production of gamma-rays and the mechanism responsible for the radio variability. Finally, we fit distributions for blazar flaring ratios, duty cycles, and on- and off-modulation indices that can be used in population studies of variability-dependent blazar properties.Item Breaking the black-body limit with resonant surfaces(2017) Valagiannopoulos, Constantinos A.; Simovski, Constantin R.; Tretyakov, Sergei A.; Nazarbayev University; Department of Radio Science and Engineering; Department of Electronics and NanoengineeringThe speed with which electromagnetic energy can be wirelessly transferred from a source to the user is a crucial indicator for the performance of a large number of electronic and photonic devices. We expect that energy transfer can be enhanced using special materials. In this paper, we determine the constituent parameters of a medium which can support theoretically infinite energy concentration close to its boundary; such a material combines properties of Perfectly Matched Layers (PML) and Double-Negative (DNG) media. It realizes conjugate matching with free space for every possible mode including, most importantly, all evanescent modes; we call this medium Conjugate Matched Layer (CML). Sources located outside such layer deliver power to the conjugate-matched body exceptionally effectively, impressively overcoming the black-body absorption limit which takes into account only propagating waves. We also expand this near-field concept related to the infinitely fast absorption of energy along the air-medium interface to enhance the far-field radiation. This becomes possible with the use of small particles randomly placed along the boundary; the induced currents due to the extremely high-amplitude resonating fields can play the role of emission ‘‘vessels’’, by sending part of the theoretically unlimited near-field energy far away from the CML structure.Item Broadband antireflective coatings based on two-dimensional arrays of subwavelength nanopores(2015) Baranov, Dmitry A.; Dmitriev, Pavel A.; Mukhin, Ivan S.; Samusev, Anton K.; Belov, Pavel A.; Simovski, Constantin R.; Shalin, Alexander S.; Department of Radio Science and Engineering; Kostantin Simovski GroupItem Broadband Reflectionless Metasheets: Frequency-Selective Transmission and Perfect Absorption(2015) Asadchy, Viktar S.; Faniayeu, Ihar A.; "Ra'di", Y.; Khakhomov, Sergei A.; Semchenko, Igor V.; Tretyakov, Sergei A.; Department of Radio Science and Engineering; Sergei Tretiakov GroupEnergy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin resonant absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper, we explore possibilities to realize a thin absorbing layer that produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a thin resonant absorber, invisible in reflection in a very wide frequency range, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cell, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create nonreflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultracompact wave filters and selective multifrequency sensors.Item Building blocks for fast circuit simulation(Aalto University, 2012) Honkala, Mikko; Roos, Janne, D.Sc. (Tech.), Aalto University, Finland; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Valtonen, Martti, Prof., Aalto University, FinlandModern electronic circuits are typically large, consisting of thousands of transistors and othercomponents. During the design process, there is a need to perform computationally demandingnumerical simulations to verify the functionality of the circuit. Thus, the need for fast andaccurate circuit simulation tools is obvious. Four approaches to improve the speed and the convergence of the numerical circuitsimulation are presented. The first approach utilizes efficient iteration methods for nonlinear DC analysis.Newton–Raphson (NR) iteration is the most used nonliner iteration method for nonlinearcircuit equations, but it lacks good global convergence properties. Some new variants ofnonlinear iteration methods are proposed to improve the convergence of DC analysis. In the second approach, the computing time is reduced by using parallel processing.Parallelization of harmonic balance (HB) analysis using multithreads is studied. Also, themodified multilevel NR method that has improved convergence properties is presented. The third approach concentrates on improving the convergence of iterative solvers for linearsystems using preconditioners. The emphasis is in the preconditioning of Jacobians of the HBmethod. It is shown how to use time-domain preconditioners with frequency-domainpreconditioners in order to benefit from both. The fourth approach to speed up the circuit simulation is to use model-order reduction(MOR), where the idea is to approximate complex circuit models with simpler ones. This thesisconcentrates on MOR methods for linear circuits or the linear parts of nonlinear circuits.Efficient partitioning-based MOR methods and a new global approach to projection-basedMOR are proposed.Item Characterization of Transponder Antennas Using Intermodulation Response(IEEE, 2015) Hannula, Jari-Matti; Rasilainen, Kimmo; Viikari, Ville; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThe intermodulation measurement technique enables measuring transponder antennas without any cable connections. This contactless technique exploits the inherent nonlinearity of the transponder to generate intermodulation products that can be measured. In this paper, we relate the transponder antenna properties to the intermodulation response, and use this relation to calculate the gain andimpedance matching of the transponder antenna. Additionally, we consider the limitations of the measurement method and present three different measurement geometries for measuring the radiation pattern. The presented equations and methods are experimentally verified by measuring an example harmonic transponder.Item Characterization, modeling, and design for applications of waveguide impedance tuners and Schottky diodes at millimeter wavelengths(Aalto University, 2011) Kiuru, Tero; Mallat, Juha; Närhi, Tapani; Radiotieteen ja -tekniikan laitos; Department of Radio Science and Engineering; Sähkötekniikan korkeakoulu; Räisänen, AnttiThis work contributes to two fields of research at millimeter wavelengths: waveguide impedance tuners and Schottky diodes. Three novel impedance tuning devices for frequencies between 75-220 GHz are presented and new modeling and characterization methods applicable to millimeter-wave and THz Schottky diodes are introduced. In addition, the use of waveguide impedance tuners and Schottky diodes is demonstrated in three different applications. Waveguide tuners are used for dielectric material characterization at 75-110 GHz and in a fundamental frequency mixer diode test platform at 183 GHz. Schottky diodes characterized in this work are used in a novel MMIC frequency tripler for 75-140 GHz and in the aforementioned 183 GHz mixer diode test platform. Waveguide impedance tuners are widely used in a variety of applications at millimeter wavelengths, ranging from device matching to load-pull and noise parameter measurements. The three impedance tuners presented in this work are a multiwaveguide-band backshort, waveguide EH-tuner for 140-220 GHz, and a double-stub E-plane tuner for 75-110 GHz. All impedance tuners are based on the dielectric backshort concept, which offers resonance-free operation, low losses, and tuning with higher resolution than is possible with traditional backshorts. For example, at the center frequency of the W-band, 92 GHz, the VSWR of the multiwaveguide-band backshort is larger than 165 for all positions of the backshort and the phase resolution as a function of the backshort movement is 0.0825 (deg)/10 micrometers, which is 10 times more accurate than with a traditional backshort. The Schottky diode is the workhorse in almost all room temperature mixer and frequency multiplier applications at 100-3000 GHz. Design of Schottky-based circuits at these frequencies relies on accurate models for the Schottky diode. In this work, a novel method is presented for simultaneous extraction of Schottky diode series resistance and thermal resistance. The method avoids the inaccuracies inherent in the traditional I-V extraction methods caused by the self-heating of the diode, which can result in too small a value for the extracted series resistance. In addition, a quantitative comparison of low-frequency (1 MHz) and microwave frequency (3-10 GHz) capacitance determination techniques is performed for millimeter-wave and THz Schottky diodes.
