Browsing by Author "Wallén, Henrik"

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  • Anisotrooppisen pallon sähköinen vaste
    (2011) Rimpiläinen, Tommi
     Master's thesis
    Työ käsittelee anisotrooppisten pallojen sähköistä vastetta staattisessa sähkökentässä. Tarkasteltavat pallot ovat anisotrooppisia siten, että väliaineen permittiivisyys pallon sisällä on dyadi, jonka komponentit ovat vakioita pallokoordinaatistossa. Kuvatunlaisia palloja kutsutaan työssä systrooppisiksi. Tilanteessa, jossa systrooppinen pallo on homogeenisessa väliaineessa, esitetään kenttäratkaisu. Ratkaisun etsimiseen käytetään menetelmää, joka on osin analyyttinen ja osin numeerinen. Kenttäratkaisua käytetään polarisoituvuusdyadin muodostamiseen.
  • Anomalous absorption, plasmonic resonances, and invisibility of radially anisotropic spheres
    (2015) Wallén, Henrik; Kettunen, Henrik; Sihvola, Ari
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This 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.
  • Antenna Diversity for Electrically Small Transmitter Modules — A Real-Time Location System Perspective
    (2024-08-19) Hiltunen, Paavo
     Sähkötekniikan korkeakoulu | Master's thesis
    Accurate and real-time location information is becoming increasingly important in many applications, such as improving process efficiencies and equipment utilization. However, the difficult indoor radio environments and the uncertain orientations of the target objects challenge the reliability of the positional data acquired from real-time location systems. Utilizing radiation pattern diversity in the small transmitters attached to the tracked object is a promising solution for improving system robustness as pattern diversity alleviates both of the aforementioned issues. In this thesis a systematic workflow for designing two-antenna pattern diversity systems was devised and tested for electrically small transmitters intended for a Bluetooth-based real-time location system. Additionally, key design choices were evaluated to provide guidelines for utilizing the formulated workflow. To begin, the necessary theoretical background for understanding the problems related to positioning accuracy and designing antenna diversity systems in an electrically small scale was explored. Afterwards, the design workflow and testing plan alongside with basic assumptions and evaluation criteria were defined. Finally, the design process was tested with four different ground planes and two different antenna designs according to the testing plan. All combinations were studied utilizing electromagnetic simulations and additionally a subset of the combinations was manufactured and measured. The simulation results show that the extremely diverse patterns could be achieved for all antenna and ground plane combinations and that the diverse patterns result in the predicted even average radiation pattern. These findings were validated by the measurements, although the radiation pattern results were affected by the measurement cables. These results show that the formulated design workflow can be used to synthesize radiation pattern diversity for electrically small transmitter systems with symmetric ground planes, however future work is required for precise analysis of the radiation, estimation of the effect on the location performance, and evaluation of the generality of the design method.
  • Blodets dielektriska egenskaper
    (2013-12-11) Lemström, Sofia
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Can a dielectric sphere emulate the behavior of a surface impedance sphere?
    (2019-05-01) Tzarouchis, Dimitrios C.; Wallén, Henrik; Ylä-Oijala, Pasi; Sihvola, Ari
     A4 Artikkeli konferenssijulkaisussa
    We theoretically investigate whether a dielectric sphere can mimic the behavior of an equivalent surface impedance sphere and we discuss some of the of emergent properties, such as cloaking (minimum scattering) and maximum absorption conditions.
  • Cloaking and magnifying using radial anisotropy
    (2013) Kettunen, Henrik; Wallén, Henrik; Sihvola, Ari
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This paper studies the electrostatic responses of a polarly radially anisotropic cylinder and a spherically radially anisotropic sphere. For both geometries, the permittivity components differ from each other in the radial and tangential directions. We show that choosing the ratio between these components in a certain way, these rather simple structures can be used in cloaking dielectric inclusions with arbitrary permittivity and shape in the quasi-static limit. For an ideal cloak, the contrast between the permittivity components has to tend to infinity. However, only positive permittivity values are required and a notable cloaking effect can already be observed with relatively moderate permittivity contrasts. Furthermore, we show that the polarly anisotropic cylindrical shell has a complementary capability of magnifying the response of an inner cylinder.
  • Design av mobilantenner genom analys av karakteristiska moder
    (2016-09-26) Von Zweygbergk, Robert
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Detektion av bröstcancer med närfältsavbildning
    (2011) Järveläinen, John
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Different homogenization methods based on scattering parameters of dielectric-composite slabs
    (2011) Qi, Jiaran; Kettunen, Henrik; Wallén, Henrik; Sihvola, Ari
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The dispersion of the effective permittivity of a dielectric-composite slab is analyzed in a quasi-dynamic range using the simulated transmission and reflection data from the slab illuminated by an obliquely incident plane wave. Based on the retrieval results, the procedure for finding the dynamic trust region of the quasi-static Lord Rayleigh estimate for the effective permittivities of such composites is then developed. According to this process, the upper frequency limit of this trust region is numerically determined by an interpolation function. The proposed function of the inclusion area fraction p and relative permittivity ɛi is demonstrated as a good predictor within the ranges 0.1 ≤ p ≤ 0.5 and 10 ≤ ɛi ≤ 60. It is further shown that within the above ranges the effective wavelength inside the material should be at least 33 times the edge length of the unit cell, in order to ensure that the defined relative difference between the retrieved effective permittivity and the quasi-static estimate is not larger than 1%.
  • Electrostatic resonances of a negative-permittivity hemisphere
    (2008) Kettunen, Henrik; Wallén, Henrik; Sihvola, Ari
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This article studies the electric response of an electrically small hemispherical object with negative permittivity by computing its polarizability which is determined by two orthogonal components, the axial one and the transverse one. A certain range of negative permittivity values is found where the mathematical determination of the polarizability becomes impossible due to an unlimited number of singularities. These singularities are due to surface plasmons, also referred to as electrostatic resonances, caused by the sharp edge of the hemisphere. It is also found that the planar surface of the hemisphere may support resonant surface modes. Furthermore, there exists a dipolar resonance determined by the overall geometry. In addition, it is shown that the resonances can be smoothened by introducing losses and, even more importantly, rounding the edge.
  • Elektromagnetiska osynlighetsmantlar av metamaterial
    (2010) Sundholm, Mathias
     Elektroniikan, tietoliikenteen ja automaation tiedekunta | Bachelor's thesis
  • Elförbrukningens elasticitet
    (2013-05-03) Strandberg, Jan
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Fasstyrda gruppantenner och deras användning inom radar
    (2008) Rodas, Michael
     Elektroniikan, tietoliikenteen ja automaation tiedekunta | Bachelor's thesis
  • Faster Conversion between Multipole Series and Plane Waves for Broadband MLFMA
    (2019-06-01) Järvenpää, Seppo; Wallén, Henrik
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A new efficient approach for converting multipole series coefficients to plane wave samples and back is presented in detail. The presented algorithm utilizes the fast Fourier transform in order to speed up the process. It can be used to develop an efficient implementation of a broadband multilevel fast multipole algorithm (MLFMA). The numerical tests demonstrate that the presented process also allows full numerical error control, which is an important factor in a practical implementation.
  • Gruppantenner och förstärkare i flerantennsystem
    (2021-05-07) Harmia, Martin
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Images and plane waves : efficient field computation in electromagnetics and acoustics
    (2006-06-06) Wallén, Henrik
     Doctoral dissertation (article-based)
    In some simple or canonical problems, analytical solutions offer the most efficient way to compute the electromagnetic or acoustic fields. For arbitrary geometries, efficient numerical methods are needed. This thesis contains new or improved solutions of classical electrostatic problems and some further developments of a variant of the fast multipole method (FMM). In the first part, the electrostatic problems of pairs of both orthogonally intersecting and non-intersecting conducting spheres are solved using Kelvin's image theory. A new efficient method for evaluating the polarizability of two non-intersecting spheres is presented. Novel analytical solutions, and also computationally efficient approximative solutions, are obtained by applying Kelvin's inversion to the electrostatic image solution of a conducting wedge. Integral equation methods are popular for both electrodynamic and acoustic scattering problems. However, to be able to use very large number of unknowns, fast iterative methods, such as the fast multipole method, must be used. In the second part of this thesis, a new broadband variant of the multilevel fast multipole algorithm (MLFMA) is described and used for both acoustic and electromagnetic scattering problems. In particular, the implementation overcomes the low-frequency breakdown of the MLFMA using a combination of the spectral representation of the Green's function and Rokhlin's translation formula.
  • Jämförelse av hjärntomografimetoder för diagnostiseringen av alzheimer
    (2017-04-18) Lewis, Emelie
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Jämförelse av olika tekniker inom resonant trådlös laddning
    (2022-09-02) Aspelin, Robert
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Lokalisation av epilepsifokus med hjälp av magnetencefalografi
    (2016-12-05) Björkman, Cecilia
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Lysdiodens väg från laboratoriet till vardagsbelysning
    (2012) Hovi, Kimmo
     Sähkötekniikan korkeakoulu | Bachelor's thesis