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  • 1.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Vazquez, Pedro A.
    Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 12, article id 123605Article in journal (Refereed)
    Abstract [en]

    This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. ["Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model," Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.

  • 2.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Electrohydrodynamic motion due to space-charge limited injection of charges in cyclohexane2014In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, 2014Conference paper (Refereed)
    Abstract [en]

    A self-consistent numerical model is presented in order to study the electrohydrodynamic (EHD) motion generated by a stationary, space-charge limited injection of charge in the point-plane geometry. In this multiphysics model, the continuity equations for charge carriers and Poisson's equation are coupled with Navier-Stokes equations and the heat equation. This model is used to study the EHD motion of cyclohexane in the negative point-plane geometry for sharp points with tip radius of 0.2 μm. It is shown that the injection of charges from a very sharp point electrode results in the formation of a thin plume with high liquid velocity. The results show large differences in the liquid velocity close to the point electrode compared to the average velocity estimated by the well-known electrohydrodynamic mobility. The difference between the width of the charged core and the hydrodynamic plume is analyzed and presented. It is shown that the local heating of the liquid is strongly reduced by the convective losses generated by EHD motion. Finally, it is found that the liquid temperature in cyclohexane in the vicinitiy of sharp points under space charge limited injection can reach temperatures slightly above boiling temperature, without generating bubbles.

  • 3.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Division for Electricity, Uppsala University.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the modeling of the production and drift of carriers in cyclohexane2013In: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, IEEE , 2013, p. 905-908Conference paper (Refereed)
    Abstract [en]

    The modeling of the mechanisms of generation, loss, multiplication and transport of charge carriers is vital for the simulation of the prebreakdown process in dielectric liquids. Unfortunately, there is a lack of suitable coefficients to describe the electron generation and transport of carriers in liquids, which hinders the development of numerical models with sufficient predictive power. In this paper, the drift-dominated continuity equations for electrons and ions are coupled with Poisson's equation in order to simulate the carrier production and drift in the liquid phase under positive and negative voltages in cyclohexane. The estimations of the model are compared with measurements of current-voltage characteristics and Trichel current pulses reported in the literature for needle-plane configurations. In the analysis, the electron generation mechanisms suggested for dielectric liquids are analyzed and discussed. It is found that estimations based on the Zener equation for field-dependent molecular ionization do not agree with measurements for negative sharp points. It is also shown that the proper estimation of the electric current in the liquid phase should consider a field-dependent attachment term as well as the electrohydrodynamic movement of the liquid.

  • 4.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Closed-Form Relation between the Scan Angle and Feed Position for Extended Hemispherical Lenses based on Ray-Tracing2016In: IEEE Antennas and Wireless Propagation Letters, ISSN 1536-1225, E-ISSN 1548-5757, Vol. 15, p. 1963-1966Article in journal (Refereed)
    Abstract [en]

    This letter presents a closed-form relation between the scan angle and feed position for extended hemispherical lenses. This relation is derived using ray tracing, and it is valid for both large and small scan angles, in excellent agreement with full-wave simulations. It is demonstrated that the relation is linear in the small-angle limit, and the effective focal length determining the scan angle is presented. It is also demonstrated that the scan angle only depends on the geometrical configuration, and that it is independent of the lens material. To demonstrate the applicability of this scan angle relation to the design of focal plane arrays (FPAs), we demonstrate that it can be used to determine the FPA spacing that results in -3-dB overlap between switched beams. A comparison with full-wave simulations of lenses with varying materials and FPA elements demonstrates a root-mean-square (rms) accuracy of 0.27 degrees for the scan angle estimation, and rms accuracy of 0.26 dB for the -3-dB overlap criterion between the central and adjacent beams. Finally, we present scaling rules, which show that the scan resolution is inversely proportional to the lens diameter, whereas the FPA spacing is independent of the total lens size.

  • 5.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Efficient Computation of the Near-Field Mutual Coupling Between Antennas on Vehicles2014Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    A near-eld generalization of Friis transmission equation has previously been proposed in the literature. Using this generalization, it is possible to calculatethe mutual coupling between two antennas as a weighted integral over the antenna far-fields. In this thesis, a change of variables is used to remove the singularity in the integrand and a normalization of the antenna far-field is suggested to take mismatch and thermal losses into account. The resulting non-singular integral has been implemented in a computer program that can be used to calculate the mutual coupling between two arbitrarily polarized antennas given the antenna far-fields and the geometrical separation between the antennas. The program has several advantages compared to previous programs based on the near-field generalization of Friis transmission equation. Firstly, this program can calculate the mutual coupling between two arbitrarily polarized and oriented antennas whereas previous programs could only be used for linearly polarized and polarization-matched antennas. Secondly, the advantage of the non-singular form is the improved numerical stability. The mutual coupling calculated using this program is demonstrated to agree well with results from full three-dimensional simulations of antennas located in each others near-fields using commercial software. Finally, we investigate for the first time if this integral relation can be used to calculate approximate values of the mutual coupling between antennas on an electrically large vehicle.

  • 6. Frid, Henrik
    On millimeter and submillimeter wave focal plane arrays implemented with MEMS waveguide switches2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis presents research towards enabling micromachined millimeter and submillimeter wave focal plane arrays (FPAs). The FPAs operate under the following principle: a switch network consisting of microelectromechanical (MEMS) switches, integrated with micromachined waveguides, is used to feed an array of antenna elements, located in the focal plane of a high-gain quasi-optical system. Hence, it is possible to switch between a set of narrow beams in different directions. Such beam steering systems are needed for future millimeter and submillimeter wave imaging and communication systems. The contributions to future MEMS-switchable FPAs presented here are organized in three papers, as described below.

    Paper I presents a criterion on the spacing between adjacent FPA elements which results in -3 dB overlap between the switched beams, for the special case when an extended hemispherical dielectric lens is used as the optical system. A key step towards this criterion is a closed-form relation between the scan angle and the FPA element's position, which results in an expression for the effective focal length of extended hemispherical lenses. A comparison with full-wave simulations demonstrates an excellent agreement with the presented theoretical results. Finally, it is shown that the maximum feasible FPA spacing when using an extended hemispherical lens is about 0.7 wavelengths.

    Paper II presents a numerical study of silicon-micromachined planar extended hemispherical lenses, with up to three matching regions used to reduce internal reflections. The effective permittivity of the matching regions is tailor-made by etching periodic holes in the silicon wafer. The optimal thickness and permittivity of the matching regions were determined using TRF optimization, in order to yield the maximum wide-band aperture efficiency and small side-lobes. We introduce a new matching region geometry, referred to as shifted-type matching regions, and it is demonstrated that using three shifted-type matching regions results in twice as large aperture efficiency as compared to using three conventional concentric-type matching regions.

    Paper III presents a submillimeter-wave single-pole single-throw (SPST) 500-750 GHz MEMS waveguide switch, based on a MEMS-reconfigurable surface inserted between two waveguide flanges. A detailed design parameter study is carried out to select the best combination of the number of horizontal bars and vertical columns of the MEMS-reconfigurable surface, for achieving a low insertion loss in the transmissive state and a high isolation in the blocking state. A method is presented to model the non-ideal electrical contacts between the vertical cantilevers of the MEMS surface, with an excellent agreement between the simulated and measured isolation. It is shown that the isolation can be improved by replacing an ohmic contact by a new, capacitive contact. The measured isolation of the switch prototype is better than 19 dB and the measured insertion loss is between 2.5 and 3 dB.

  • 7.
    Frid, Henrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Simulation of Microbubbles during the Initial Stages of Breakdown in Cyclohexane2013In: 2013 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, IEEE conference proceedings, 2013, p. 901-904Conference paper (Refereed)
    Abstract [en]

    The formation of a vapor microbubble has previouslybeen suggested to be the initial mechanism in the process ofdielectric failure of dielectric liquids. The bubble is generated bya rapid, highly localized heating of a volume close to a highlystressed electrode, caused by electric currents in the liquid athigh voltages. A numerical model is presented in order toinvestigate the dynamics of a single microbubble in the pointplanegeometry in cyclohexane. A condition for the formation ofa vapor bubble is discussed. Thereafter, a Computational FluidDynamics (CFD) model of two-phase flow with phase transition isused to study the dynamics of the bubble from generation tocollapse, under a highly divergent electrostatic field in asubcooled liquid. The amount of subcooling in the simulations is5 K, and it is found that convergence gets significantly weaker asthe amount of subcooling increases. The bubble dynamics is alsosimulated considering the electrohydrodynamic (EHD) processesin the liquid and vapor phases. Finally, it is shown how theelectrostatic forces on the dielectric will cause a bubble to detachfrom the electrode.

  • 8.
    Frid, Henrik
    et al.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Holter, Henrik
    Jonsson, B. Lars G.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    An Approximate Method for Calculating the Near-Field Mutual Coupling Between Line-of-Sight Antennas on Vehicles2015In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 63, no 9, p. 4132-4138Article in journal (Refereed)
    Abstract [en]

    Calculating the mutual coupling between antennas on vehicles using full-wave simulations requires a vast amount of computer resources due to the electrical size of the structures. We therefore propose an alternative and approximate method to determine mutual coupling between antennas on vehicles for the case where there is line-of-sight (LOS) between the antennas. The proposed method is based on approximating the mutual coupling between LOS antennas on vehicles as near-field transmission between antennas in free space. We begin the analysis with a brief review of four methods for calculating the near-field free-space transmission. Of the investigated methods, we demonstrate that a nonsingular form of the near-field transmission integral originally proposed by Yaghjian (1982) is the most suitable for LOS antennas on vehicles. We introduce a modification to this method, in order to only use the antenna far-fields and geometrical separation to determine the mutual coupling. The comparison with full-wave simulations indicates that the proposed method has a good accuracy for LOS antennas. This paper ends with a full-scale mutual coupling calculation for two monopoles on an aircraft under LOS conditions, demonstrating a root mean square (rms) accuracy of 6 dB for frequencies up to 5 GHz, as compared with full-wave simulations.

  • 9. Frid, Henrik
    et al.
    Jonsson, B. Lars G.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Compensation of radome effects in small airborne monopulse arrays by convex optimization2018In: IET Conference Publications, Institution of Engineering and Technology , 2018, no CP741Conference paper (Refereed)
    Abstract [en]

    The quality of small array antennas in airborne monopulse systems can be significantly reduced by the radome. We therefore present a convex optimization approach to minimize radome effects in monopulse arrays. This is achieved by using active element patterns in the optimization to determine the excitation weights. Simulation results for a BoR array with 48 elements and an extended hemispherical radome are presented. We demonstrate that it is possible to reduce the side-lobe level by 3.5 dB by taking radome effects into account in the optimization. This approach also results in an increased gain, particularly at large scan angles. Furthermore, the presented approach allows the monopulse slope to be indirectly specified as a design parameter. It is shown that the trade-off between the monopulse slope coefficient and the side-lobe level is approximately linear.

  • 10.
    Frid, Henrik
    et al.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Töpfer, Fritzi
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Bhowmik, Shreyasi
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Dudorov, Sergey
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Oberhammer, Joachim
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Optimization of Micromachined Millimeter-Wave Planar Silicon Lens Antennas with Concentricand Shifted Matching Regions2017In: Progress In Electromagnetics Research C, ISSN 1937-8718, E-ISSN 1937-8718, Vol. 79, p. 17-29Article in journal (Refereed)
    Abstract [en]

    This paper presents a study of planar silicon lens antennas with up to three stepped-impedance matching regions. The effective permittivity of the matching regions is tailor-made byetching periodic holes in the silicon substrate. The optimal thickness and permittivity of the matchingregions were determined by numerical optimization to obtain the maximum wideband aperture efficiencyand smallest side-lobes. We introduce a new geometry for the matching regions, referred to as shiftedmatching regions. The simulation results indicate that using three shifted matching regions results intwice as large aperture efficiency as compared to using three conventional concentric matching regions.By increasing the number of matching regions from one to three, the band-averaged gain is increasedby 0.3 dB when using concentric matching regions, and by 3.7 dB when using shifted matching regions,which illustrates the advantage of the proposed shifted matching region design.

  • 11.
    Malmström, Johan
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Henrik, Frid
    KTH.
    Jonsson, B.L.G.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Approximate methods to determine the isolation between antennas on vehicles2016In: Antennas and Propagation (APSURSI), 2016 IEEE International Symposium on, IEEE conference proceedings, 2016, p. 131-132Conference paper (Refereed)
    Abstract [en]

    The isolation between antennas needs to be considered when integrating antennas on vehicles, since poor isolation between antennas can cause interference between radio systems on-board. The electrical size of vehicles at gigahertz frequencies often limits the usage of full-wave methods. This paper therefore evaluates two efficient methods to approximate the antenna isolation; the non-singular transmission integral (NSTI) method and the geometric theory of diffraction (GTD). We present the first evaluation of NSTI for antennas outside line-of-sight (non-LOS). It is shown to provide a 6.5 dB RMS accuracy in the early non-LOS zone for an antenna position sweep on an aircraft at 2 GHz, and for frequency sweeps on a cube and cylinder, the latter only to 5 GHz. The GTD implementation gives a 4.4 dB RMS accuracy for cylinders and simplified aircraft models. Both investigated methods give remarkably accurate results, considering memory requirements and runtime, which makes them interesting for further investigations.

  • 12.
    Shah, Umer
    et al.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Reck, Theodore
    Decrossas, Emmanuel
    Jung-Kubiak, Cecile
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Chattopadhyay, Goutam
    Mehdi, Imran
    Oberhammer, Joachim
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    500-750 GHz submillimeter-wave MEMS waveguide switch2016Conference paper (Refereed)
    Abstract [en]

    This paper presents a 500-750 GHz waveguide based single-pole single-throw (SPST) switch achieving a 40% bandwidth. It is the first ever RF MEMS switch reported to be operating above 220 GHz. The switch is based on a MEMS-reconfigurable surface which can block the wave propagation in the waveguide by short-circuiting the electrical field lines of the TE10 mode. The switch is designed for optimized isolation in the blocking state and for optimized insertion loss in the non-blocking state. The measurement results of the first prototypes show better than 15 dB isolation in the blocking state and better than 3 dB insertion loss in the non-blocking state for 500-750 GHz. The higher insertion loss is mainly attributed to the insufficient metal thickness and surface roughness on the waveguide sidewalls. Two switch designs with different number of blocking elements are fabricated and compared. The overall switch bandwidth is limited by the waveguide only and not by the switch technology itself.

  • 13.
    Shah, Umer
    et al.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Reck, Theodore
    Decrossas, Emmanuel
    Jung-Kubiak, Cecile
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Chattopadhyay, Goutam
    Mehdi, Imran
    Oberhammer, Joachim
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Micromachined Waveguide Integrated RF MEMS Switch Operating between 500-750 GHz2016Conference paper (Refereed)
  • 14.
    Shah, Umer
    et al.
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Reck, Theodore
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Jung-Kubiak, Cecile
    Chattopadhyay, Goutam
    Mehdi, Imran
    Oberhammer, Joachim
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    A 500–750 GHz RF MEMS Waveguide Switch2017In: IEEE Transactions on Terahertz Science and Technology, ISSN 2156-342X, E-ISSN 2156-3446, Vol. 7, no 3, p. 326-334Article in journal (Refereed)
    Abstract [en]

    This paper reports on a submillimeter-wave 500-750 GHz micorelectromechanical systems (MEMS) waveguide switch based on a MEMS-reconfigurable surface to block/unblock the wave propagation through the waveguide. In the non-blocking state, the electromagnetic wave can pass freely through the MEMS-reconfigurable surface, while in the blocking state, the electric field lines of the TE10 mode are short-circuited that blocks the wave propagation through a WM-380 (WR-1.5) waveguide. A detailed design parameter study is carried out to determine the best combination of the number of horizontal bars and vertical columns of the MEMS-reconfigurable surface for achieving a low insertion loss in the non-blocking state and a high isolation in the blocking state for the 500-750 GHz band. Two different switch concepts relying either on an ohmic-contact or a capacitive-contact between the contact cantilevers have been implemented. The measurements of the switch prototypes show a superior RF performance of the capacitive-contact switch. The measured isolation of the capacitive-contact switch designed with an 8 μm contact overlap is 19-24 dB and the measured insertion loss in the non-blocking state is 2.5-3 dB from 500 to 750 GHz including a 400 μm long micromachined waveguide section. By measuring reference chips, it is shown that the MEMS-reconfigurable surface contributes only to 0.5-1 dB of the insertion loss, while the rest is attributed to the limited sidewall metal thickness and to the surface roughness of the 400 μm long micromachined waveguide section. Finally, reliability measurements in an uncontrolled laboratory environment on a comb-drive actuator with an actuation voltage of 28 V showed no degradation in the functioning of the actuator over one hundred million cycles. The actuator was also kept in the actuated state for ten days and showed no sign of failure or deterioration.

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